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Full text of "Electrodynamics of Continuous Media"

Landau 



Electrodynamics of 
Continuous Media 



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Per,:,:: man 



Course of Theoretical Physics 
Volume 8 



L. D. Landau (Deceased) and E. M. Lifshitz 

Institute of Physical Problems 
USSR Academy of Sciences 





MCMXCVn 



COURSE OF THEORETICAL PHYSICS 

Volume 8 



ELECTRODYNAMICS OF CONTINUOUS 

MEDIA 



COURSE OF THEORETICAL PHYSICS 

Vol. 1. Mechanics 

Vol. 2. The Classical Theory of Fields 

Vol. 3. Quantum Mechanics — Non-relativistic Theory 

Vol. 4. Relativistic Quantum Theory 

Vol. 5. Statistical Physics 

Vol. 6. Fluid Mechanics 

Vol. 7. Theory of Elasticity 

Vol. 9. Physical Kinetics 



ELECTRODYNAMICS OF 
CONTINUOUS MEDIA 



by 
L. D. LANDAU and E. M. LIFSHITZ 

INSTITUTE OF PHYSICAL PROBLEMS, U.S.S.R. ACADEMY OF SCIENCES 

Volume 8 of Course of Theoretical Physics 

Translated from the Russian by 
J. B. SYKES and J. S. BELL 



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First published in English 1960 
Second impression 1963 



Library of Congress Card Number 60-14731 
Printed in Great Britain by J. W. Arrowsmith Ltd., Bristol 



CONTENTS 

Page 

Preface i x 

Notation x 

I. ELECTROSTATICS OF CONDUCTORS 

§1. The electrostatic field of conductors 1 

§2. The energy of the electrostatic field of conductors 3 

§3. Methods of solving problems in electrostatics 9 

§4. A conducting ellipsoid 20 

§5. The forces on a conductor 31 

II. ELECTROSTATICS OF DIELECTRICS 

§6. The electric field in dielectrics 36 

§7. The dielectric permeability 38 

§8. A dielectric ellipsoid 42 

§9. The dielectric permeability of a mixture 45 

§10. Thermodynamic relations for dielectrics in an electric field 47 

§11. The total free energy of a dielectric 52 

§12. Electrostriction of isotropic dielectrics 55 

§13. Dielectric properties of crystals 58 

§14. The sign of the dielectric susceptibility 63 

§15. Electric forces in a fluid dielectric 64 

§16. Electric forces in solids 69 

§17. Piezoelectrics 73 

§18. Thermodynamic inequalities 79 

§19. Ferroelectrics 83 

III. CONSTANT CURRENT 

§20. The current density and the conductivity 92 

§21. The Hall effect 96 

§22. The contact potential 99 

§23. The galvanic cell 101 

§24. Electrocapillarity 103 

§25. Thermoelectric phenomena 104 

§26. Diffusion phenomena 110 

IV. CONSTANT MAGNETIC FIELD 

§27. Constant magnetic field 113 

§28. Magnetic symmetry of crystals 116 

§29. The magnetic field of a constant current 119 

§30. Thermodynamic relations in a magnetic field 126 

§31. The total free energy of a magnetic substance 129 

§32. The energy of a system of currents 131 



vi Contents 

Page 

§33. The self-inductance of linear conductors 136 

§34. Forces in a magnetic field 141 

§35. Gyromagnetic phenomena 144 

V. FERROMAGNETISM 

§36. Ferromagnetics near the Curie point 146 

§37. The magnetic anisotropy energy 149 

§38. Magnetostriction of ferromagnetics 155 

§39. The domain structure of ferromagnetics 158 

§40. The antiferromagnetic Curie point 165 

VI. SUPERCONDUCTIVITY 

§41. The magnetic properties of superconductors 167 

§42. The superconductivity current 169 

§43. The critical field 173 

§44. The intermediate state 178 

VII. QUASI-STATIC ELECTROMAGNETIC FIELD 

§45. Eddy currents 186 

§46. The skin effect 195 

§47. The complex resistance 197 

§48. Capacity in a quasi-steady current circuit 202 

§49. Motion of a conductor in a magnetic field 205 

§50. Excitation of currents by acceleration 210 

VIII. MAGNETIC FLUID DYNAMICS 

§51. The equations of motion for a fluid in a magnetic field 213 

§52. Hydromagnetic waves 218 

§53. Tangential and rotational discontinuities 224 

§54. Shock waves 229 
§55. The spontaneous magnetic field in turbulent motion of a conducting 

fluid 234 

IX. THE ELECTROMAGNETIC WAVE EQUATIONS 

§56. The field equations in a dielectric in the absence of dispersion 239 

§57. The electrodynamics of moving dielectrics 243 

§58. The dispersion of the dielectric permeability 247 

§59. The dielectric permeability at very high frequencies 251 

§60. The dispersion of the magnetic permeability 251 

§61. The field energy in dispersive media 253 

§62. The relation between the real and imaginary parts of e(o>) 256 

§63. A plane wave of a single frequency 263 

§64. Transparent media 266 

X. THE PROPAGATION OF ELECTROMAGNETIC WAVES 

§65. Geometrical optics 269 

§66. Reflection and refraction of electromagnetic waves 272 

§67. The surface impedance of metals 279 



Contents vii 

Page 

§68. The propagation of waves in an inhomogeneous medium 284 

§69. The reciprocity principle 288 

§70. Electromagnetic oscillations in hollow resonators 290 

§71. The propagation of electromagnetic waves in waveguides 293 

§72. The scattering of electromagnetic waves by small particles 299 

§73. The absorption of electromagnetic waves by small particles 303 

§74. Diffraction by a wedge 304 

§75. Diffraction by a plane screen 308 

XI. ELECTROMAGNETIC WAVES IN ANISOTROPIC MEDIA 

§76. The dielectric permeability of crystals 313 

§77. A plane wave in an anisotropic medium 315 

§78. Optical properties of uniaxial crystals 321 

§79. Biaxial crystals 324 

§80. Double refraction in an electric field 329 

§81. Mechanical-optical effects 330 

§82. Magnetic-optical effects 331 

§83. Natural optical activity 337 

XII. THE PASSAGE OF FAST PARTICLES THROUGH MATTER 
§84. Ionisation losses by fast particles in matter: the non-relativistic case 344 

§85. Ionisation losses by fast particles in matter: the relativistic case 349 

§86. Cherenkov radiation 357 

XIII. ELECTROMAGNETIC FLUCTUATIONS 

§87. Current fluctuations in linear circuits 360 

§88. Electromagnetic field fluctuations 361 

§89. Black-body radiation in a transparent medium 367 

§90. Forces of molecular attraction between solid bodies 368 

XIV. SCATTERING OF ELECTROMAGNETIC WAVES 

§91. The general theory of scattering in isotropic media 377 

§92. The principle of detailed balancing applied to scattering 383 

§93. Scattering with small change of frequency 385 

§94. Rayleigh scattering in gases and liquids 387 

§95. Critical opalescence 393 

§96. Scattering in amorphous solids 395 

XV. DIFFRACTION OF X-RAYS IN CRYSTALS 

§97. The general theory of X-ray diffraction 398 

§98. The integral intensity 404 

§99. Diffuse thermal scattering of X-rays 407 

Appendix 410 

Index 413 



PREFACE 

The present volume in the Course of Theoretical Physics deals with the theory 
of electromagnetic fields in matter and with the theory of the macroscopic 
electric and magnetic properties of matter. These theories include a very 
wide range of topics, as may be seen from the Contents. 

In writing this book we have experienced considerable difficulties, partly 
because of the need to make a selection from the extensive existing material, 
and partly because the customary exposition of many topics to be included 
does not possess the necessary physical clarity, and sometimes is actually 
wrong. We realise that our own treatment still has many defects, which we 
hope to correct in future editions. 

We are grateful to Professor V. L. Ginzburg, who read the book in 
manuscript and made some useful comments. I. E. Dzyaloshinski! and 
L. P. Pitaevskii gave great help in reading the proofs of the Russian edition. 
Thanks are due also to Dr Sykes and Dr Bell, who not only carried out 
excellently the arduous task of translating the book, but also made some use- 
ful comments concerning its contents. 

L. D. Landau 
Moscow E. M. Lifshitz 

June, 1959 



NOTATION 
Electric field E 
Electric induction D 
Magnetic field H 
Magnetic induction B 
External electric field © 
External magnetic field # 
Dielectric polarisation P 
Magnetisation M 
Total electric moment of a body ^ 
Total magnetic moment of a body J€ 
Dielectric permeability e 
Magnetic permeability fi 
Current density j 
Conductivity a 
Absolute temperature (in energy units) T 

Thermodynamic quantities : per unit volume for a body 

entropy S S? 

internal energy U tfl 

free energy F & 

thermodynamic potential <X> cp 
Chemical potential £ 

A complex periodic time factor is always taken as e~ iwt . 

The summation convention always applies to three-dimensional (Latin) 
and two-dimensional (Greek) suffixes occurring twice in vector and tensor 
expressions. 



CHAPTER I 

ELECTROSTATICS OF CONDUCTORS 

§1. Tike electrostatic field of conductors 

Like all macroscopic theories, the theory of electromagnetic fields in matter 
deals with physical quantities averaged over elements of volume which are 
"physically infinitesimal", ignoring the microscopic variations of the quan- 
tities which result from the molecular structure of matter. For example, 
instead of the actual "microscopic" value of the electric field e, we discuss 
its averaged value, denoted by E: 

e = E. (1.1) 

The fundamental equations of the electrodynamics of continuous media 
are obtained by averaging the equations for the electromagnetic field in a 
vacuum. This method of obtaining the macroscopic equations from the 
microscopic was first used by H. A. Lorentz. 

The form of the equations of macroscopic electrodynamics and the 
significance of the quantities appearing in them depend essentially on the 
physical nature of the medium, and on the way in which the field varies with 
time. It is therefore reasonable to derive and investigate these equations 
separately for each type of physical object. 

It is well known that all bodies can be divided, as regards their electric 
properties, into two classes, conductors and dielectrics, differing in that any 
electric field causes in a conductor, but not in a dielectric, the motion of 
charges, i.e. an electric current, t 

Let us begin by studying the constant electric fields produced by charged 
conductors, that is, the electrostatics of conductors. First of all, it follows 
from the fundamental property of conductors that, in the electrostatic case, 
the electric field inside a conductor must be zero. For a field E which was 
not zero would cause a current; the propagation of a current in a conductor 
involves a dissipation of energy, and hence cannot occur in a stationary state 
(with no external sources of energy). 

Hence it follows, in turn, that any charges in a conductor must be located 
on its surface. The presence of charges inside a conductor would necessarily 
cause an electric field in it; J they can be distributed on its surface, however, 



t It should be mentioned that the conductor is here assumed to be homogeneous (in 
composition, temperature, etc.)- In an inhomogeneous conductor, as we shall see later, 
there may be fields which cause no motion of charges. 

% This is clearly seen from equation (1.8) below. 

1 



2 Electrostatics of Conductors §1 

in such a way that the fields which they produce in its interior are mutually 
balanced. 

Thus the problem of the electrostatics of conductors amounts to determining 
the electric field in the vacuum outside the conductors and the distribution of 
charges on their surfaces. 

At any point far from the surface of the body, the mean field E in the 
vacuum is almost the same as the actual field e. The two fields differ only 
in the immediate neighbourhood of the body, where the effect of the irregular 
molecular fields is noticeable, and this difference does not affect the averaged 
field equations. The exact microscopic Maxwell's equations in the vacuum are 

dive = 0. (1.2) 

curie = -(l/c)8h/dt, (1.3) 

where h is the microscopic magnetic field. Since the mean magnetic field is 
assumed to be zero, the derivative dhjdt also vanishes on averaging, and we 
find that the constant electric field in the vacuum satisfies the usual equations 

div E = 0, curl E = 0, (1.4) 

i.e. it is a potential field with a potential <f> such that 

E= -grad^, (1.5) 

and <j> satisfies Laplace's equation 

A<f> = 0. (1.6) 

The boundary conditions on the field E at the surface of a conductor 
follow from the equation curl E = 0, which, like the original equation 
(1.3), is valid both outside and inside the body. Let us take the #-axis in the 
direction of the normal to the surface at some point on the conductor. The 
component E z of the field takes very large values in the immediate neigh- 
bourhood of the surface (because there is a finite potential difference over a 
very small distance). This large field pertains to the surface itself and de- 
pends on the physical properties of the surface, but is not involved in our 
electrostatic problem, because it falls off over distances comparable with the 
distances between atoms. It is important to note, however, that, if the 
surface is homogeneous, the derivatives dE z jdx, BE z fdy along the surface 
remain finite, even though E z itself becomes very large. Hence, since 
(curl E) x = dE z \dy—dE y \dz = 0, we find that dE y \dz is finite. This means 
that E y is continuous at the surface, since a discontinuity in E y would mean 
an infinity of the derivative dE y \dz. The same applies to E z , and since 
E = inside the conductor, we reach the conclusion that the tangential 
components of the external field at the surface must be zero: 

E t = 0. (1.7) 

Thus the electrostatic field must be normal to the surface of the conductor 
at every point. Since E = — grad <f>, this means that the field potential must 
be constant on the surface on any particular conductor. In other words, 



§2 The energy of the electrostatic field of conductors 3 

the surface of a homogeneous conductor is an equipotential surface of the 
electrostatic field. 

The component of the field normal to the surface is very simply related to 
the charge density on the surface. The relation is obtained from the general 
electrostatic equation div e = Airp, which on averaging gives 

div E = 4rr/5, (1.8) 

p being the mean charge density. The meaning of the integrated form of this 
equation is well known: the flux of the electric field through a closed surface 
is equal to the total charge inside that surface, multiplied by 4ir. Applying 
this theorem to a volume element lying between two infinitesimally close unit 
areas, one on each side of the surface of the conductor, and using the fact that 
E = on the inner area, we find that E n = Att<j, where a is the surface 
charge density, i.e. the charge per unit area of the surface of the conductor. 
Thus the distribution of charges over the surface of the conductor is given by 

the formula 

W = E n = -d<f>j8n, (1.9) 

the derivative of the potential being taken along the outward normal to the 
surface. The total charge on the conductor is 

«=-!<£ ^d/, (1.10) 

A-n J on 

the integral being taken over the whole surface. 

The potential distribution in the electrostatic field has the following re- 
markable property: the function <f>(x, y, z) can take maximum and minimum 
values only at boundaries of regions where there is a field. This theorem can 
also be formulated thus : a test charge e introduced into the field cannot be 
in stable equilibrium, since there is no point at which its potential energy e$ 
would have a minimum. 

The proof of the theorem is very simple. Let us suppose, for example, 
that the potential has a maximum at some point A not on the boundary of a 
region where there is a field. Then the point A can be surrounded by a small 
closed surface on which the normal derivative d<f>\dn < everywhere. 
Consequently, the integral over this surface j(dcpldn) df < 0. But by La- 
place's equation §(d<f>]dn) df=JA<f> dV = 0, giving a contradiction. 

§2. The energy of the electrostatic field of conductors 

Let us calculate the total energy % of the electrostatic field of charged 
conductors, t 

^ = _Lf£2 dF) (2.1) 

07T J 

t The square E 2 is not the same as the mean square e 2 _oi the actual field near the surface 
of a conductor or inside it (where E = but, of course, e 2 *£ 0). By calculating the integral 
(2.1) we ignore the internal energy of the conductor as such, which is here of no interest, 
and the affinity of the charges for the surface. 



4 Electrostatics of Conductors §2 

where the integral is taken over all space outside the conductors. We trans- 
form this integral as follows : 



<V 



= -— |E.grad<£dF= -— fdiv (<f>E) dV+— [ <f> div E dV. 

07TJ StT J S7T J 



The second integral vanishes by (1.4), and the first can be transformed into 
integrals over the surfaces of the conductors which bound the field and over 
an infinitely remote surface. The latter of these vanishes, because the field 
diminishes sufficiently rapidly at infinity. Denoting by <f> a the constant value 
of the potential on the <zth conductor, we havet 



= ^2f^» d /= 8 ^2^f £ » d /- 



07T ^""' J 877 

a a 

Finally, since the total charges e a on the conductors are given by (1.10) 
we obtain 

^ = 12%^ (2.2) 

which is analogous to the expression for the energy of a system of point 
charges. 

The charges and potentials of the conductors cannot both be arbitrarily 
prescribed; there are certain relations between them. Since the field equa- 
tions in a vacuum are linear and homogeneous, these relations must also 
be linear, i.e. they must be given by equations of the form 

e a = S Cab <f>b, (2.3) 

where the quantities C aa , C a b have the dimensions of length and depend on 
the shape and relative position of the conductors. The quantities C aa are 
called capacity coefficients, and the quantities C ab (a ^ b) are called electro- 
static induction coefficients. In particular, if there is only one conductor, we 
have e = C<f>, where C is the capacity, which in order of magnitude is equal 
to the linear dimension of the body. The converse relations, giving the 
potentials in terms of the charges, are 

<f> a = S C-\ b e b , (2.4) 

where the coefficients C _1 a & form a matrix which is the inverse of the matrix 
Cab- 
Let us calculate the change in the energy of a system of conductors caused 
by an infinitesimal change in their charges or potentials. Varying the original 

f In transforming volume integrals into surface integrals, both here and later, it must be 
borne in mind that E n is the component of the field along the outward normal to the con- 
ductor. This direction is opposite to that of the outward normal to the region of the volume 
integration, namely the space outside the conductors. The sign of the integral is therefore 
changed in the transformation. 



§2 The energy of the electrostatic field of conductors 5 

expression (2.1), we have 8% = (1/4tt) j E-SE dV. This can be further 
transformed by two equivalent methods. Putting E = — grad <f> and using 
the fact that the varied field, like the original field, satisfies equations (1.4) 
(so that div SE = 0), we can write 

8<% = fgrad<£.8EdF= ( div (<f> 8E) dV 

= — 2 $ a j 8En d /> 

that is 

8<% = ^<f> a 8e a , ( 2 - 5 ) 

o 

which gives the change in energy due to a change in the charges. This result 
is obvious; it is the work required to bring infinitesimal charges 8e a to the 
various conductors from infinity, where the field potential is zero. 
On the other hand, we can write 

S<^ = ( E- grad 8<f> dV = ( div (E 8<f>) dV 

4ttJ Att J 

= — 2 fy" f En d /' 

that is 

8% = Xe a 8<f> a , (2.6) 

which expresses the change in energy in terms of the change in the potentials 
of the conductors. 

Formulae (2.5) and (2.6) show that, by differentiating the energy <% with 
respect to the charges, we obtain the potentials of the conductors, and the 
derivatives of <% with respect to the potentials are the charges : 

dW/de a = <£«, d%ld<i> a = e a . (2.7) 

But the potentials and charges are linear functions of each other. Using 
(2.3) we have d 2 <%ld<f> a d<f> b = debjd<j> a = Cba, and by reversing the order of 
differentiation we get C a &. Hence it follows that 

Cab = Cba, (2.8) 

and similarly C~ x a b = C~ x ba- The energy % can be written as a quadratic 
form in either the potentials or the charges : 

% = \ \ Cab Mb = \ \C-^ a b e a e b . (2.9) 

' a,b a,b v 

This quadratic form must be positive definite, like the original expression 
(2.1). From this condition we can derive various inequalities which the 



6 Electrostatics of Conductors §2 

coefficients C a & must satisfy. In particular, all the capacity coefficients are 
positive : 

C aa > (2.10) 

(and also C~\ a > 0).t 

All the electrostatic induction coefficients, on the other hand, are negative : 

Cab <0 (fl^ b). (2.11) 

That this must be so is seen from the following simple arguments. Let us 
suppose that every conductor except the <zth is earthed, i.e. their potentials 
are zero. Then the charge induced by the charged ath conductor on another 
(the 2>th, say) is et, = Cb a <f>a- It is obvious that the sign of the induced charge 
must be opposite to that of the inducing potential, and therefore C a b < 0. 
This can be more rigorously shown from the fact that the potential of the 
electrostatic field cannot reach a maximum or minimum outside the conduc- 
tors. For example, let the potential <£ a of the only conductor not earthed be 
positive. Then the potential is positive in all space, its least value (zero) 
being attained only on the earthed conductors. Hence it follows that the 
normal derivative d$\dn of the potential on the surfaces of these conductors 
is positive, and their charges are therefore negative, by (1.10). Similar 
arguments show that C~\b > 0. 

The energy of the electrostatic field of conductors has a certain extremum 
property, though this property is more formal than physical. To derive it, 
let us suppose that the charge distribution on the conductors undergoes an 
infinitesimal change (the total charge on each conductor remaining unaltered), 
in which the charges may penetrate into the conductors; we ignore the fact 
that such a charge distribution cannot in reality be stationary. We consider 
the change in the integral °ti = (1/8tt) j E 2 dV, which must now be extended 
over all space, including the volumes of the conductors themselves (since 
after the displacement of the charges the field E may not be zero inside the 
conductors). We write 

h% = -—fgrad^-SEdF 

= -— fdiv(08E)dr+~ f^divSEdF. 

4-77 J 47T J 

The first integral vanishes, being equivalent to one over an infinitely remote 
surface. In the second integral, we have by (1.8) divSE = Airhp, so that 
htfl = / <f>Bp dV. This integral vanishes if <f> is the potential of the true electro- 
static field, since then <f> is constant inside each conductor, and the integral 
jSpdV over the volume of each conductor is zero, since its total charge 
remains unaltered. 



t We may also mention that another inequality which must be satisfied if the form (2.9) 
is positive is CaaCbb > Cab 2 - 



§2 The energy of the electrostatic field of conductors 7 

Thus the energy of the actual electrostatic field is a minimumf relative 
to the energies of fields which could be produced by any other distribution 
of the charges on or in the conductors (Thomson's theorem). 

From this theorem it follows, in particular, that the introduction of an 
uncharged conductor into the field of given charges (charged conductors) 
reduces the total energy of the field. To prove this, it is sufficient to compare 
the energy of the actual field resulting from the introduction of the un- 
charged conductor with the energy of the fictitious field in which there are 
no induced charges on that conductor. The former energy, since it has the 
least possible value, is less than the latter energy, which is also the energy 
of the original field (since, in the absence of induced charges, the field would 
penetrate into the conductor, remaining unaltered). This result can also 
be formulated thus : an uncharged conductor remote from a system of given 
charges is attracted towards the system. 

Finally, it can be shown that a conductor (charged or not) brought into 
an electrostatic field cannot be in stable equilibrium under electric forces 
alone. This assertion generalises the theorem for a point charge proved at 
the end of §1, and can be derived by combining the latter theorem with 
Thomson's theorem. We shall not pause to give the derivation in detail. 

Formulae (2.9) are useful for calculating the energy of a system of con- 
ductors at finite distances apart. The energy of an uncharged conductor 
in a uniform external field ©, which may be imagined as due to charges at 
infinity, requires special consideration. According to (2.2), this energy is 
% = ^e<f) y where e is the remote charge which causes the field, and <j> is the 
potential at this charge due to the conductor. °ll does not include the energy 
of the charge e in its own fieldj since we are interested only in the energy of 
the conductor. The charge on the conductor is zero, but the external field 
causes it to acquire a dipole electric moment, which we denote by 0*. The 
potential of the electric dipole field at a large distance r from it is <f> = ^«r/r 3 . 
Hence % = e&'rjlfi. But — er/r 3 is just the field © due to the charge e. 
Thus 

# = -£ 0>©. (2.12) 

Since all the field equations are linear, it is evident that the components 
of the dipole moment &> are linear functions of the components of the field 
©. The coefficients of proportionality between $P and ® have the dimen- 
sions of length cubed, and are therefore proportional to the volume of the 
conductor : 

&i = Vaa&t, (2.13) 

where the coefficients a^ depend only on the shape of the body. The quantities 
Vane form a tensor, which may be called the polarisability tensor of the body. 



f We shall not give here the simple arguments which demonstrate that the extremum is 
a minimum. 



8 Electrostatics of Conductors §2 

This tensor is symmetrical: a^ = olm, a statement which will be proved in 
§11. Accordingly, the energy (2.12) is 

% = -iFottfoCfe. (2.14) 



PROBLEMS 

Problem 1. Express the mutual capacity C of two conductors (with charges ±e) in terms 
of the coefficients Cab. 

Solution. The mutual capacity of two conductors is defined as the coefficient C in the 
relation e = C(<f>2—<f>i), and the energy of the system is given in terms of C by % — %e 2 jC. 
Comparing with (2.9), we obtain 

1/C = C-hi-2C-h2+C-hi 

= (Cll + 2C 12 +C22)/(CllC22-Cl22). 

Problem 2. A point charge e is situated at O, near a system of earthed conductors, and 
induces on them charges e a . If the charge e were absent, and the ath conductor were at 
potential <f>' a , the remainder being earthed, the field potential at O would be <f>'o. Express the 
charges e a in terms of <f>' a and <f>'o- 

Solution. If charges e a on the conductors give them potentials <f> a , and similarly for 
e' a and <j>' a , it follows from (2.3) that 

£ 4>ae a = S <j>aCab<f>'b = 2 <f>'aCa- 
a a,b a 

We apply this relation to two states of the system formed by all the conductors and the 
charge e (regarding the latter as a very small conductor). In one state the charge e is present, 
the charges on the conductors are e a , and their potentials are zero. In the other state the charge 
e is zero, and one of the conductors has a potential <j>' a ^ 0. Then we have e4>'o+e a <f>'a — 0, 
whence e a = —e<f>'ol<l>'a- 

For example, if a charge e is at a distance r from the centre of an earthed conducting sphere 
of radius a(< r), then <f>'o = <f>' a alr, and the charge induced on the sphere is e a = —ea/r. 

As a second example, let us consider a charge e placed between two concentric conducting 
spheres of radii a and b, at a distance r from the centre such that a < r < b. If the outer 
sphere is earthed and the inner one is charged to potential <f>' a> the potential at distance r is 

l/r-1/6 

90 = <f> a— — • 

1/a — l/b 

Hence the charge induced on the inner sphere by the charge e is e a = —ea(b—r)lr(b—a). 
Similarly the charge induced on the outer sphere is eb = —eb(r—a)lr(b—a). 

Problem 3. Two conductors, of capacities Ci and Cz, are placed at a distance r apart 
which is large compared with their dimensions. Determine the coefficients Cab- 

Solution. If conductor 1 has a charge ei, and conductor 2 is uncharged, then in the first 
approximation <f>i = ei/Ci, fa = ei/r ; here we neglect the variation of the field over conductor 
2 and its polarisation. Thus C~hi = 1/Ci, C" 1 ^ = 1/r, and similarly C" 1 2 2 = I/C2. Hence 
we findf 

~ „ /« , CiCaX „ C1C2 „ I CiC 2 \ 

Ca = C,(l+ — ), Q. = - — C 22 - C 2 (l+ -^- a ). 



t The subsequent terms in the expansion are in general of order (in 1/r) one higher than 
those given. If, however, r is taken as the distance between the "centres of charge" of the 
two bodies (for spheres, between the geometrical centres), then the order of the subsequent 
terms is two higher. 



§3 Methods of solving problems in electrostatics 9 

Problem 4. Determine the capacity of a ring (radius b) of thin conducting wire of circular 
cross-section (radius a <^ b). 

Solution. Since the wire is thin, the field at the surface of the ring is almost the same as 
that of charges distributed along the axis of the wire (for a right cylinder, it would be exactly 
the same). Hence the potential of the ring is 

<f>a = — i — 
2irb J r ' 

where r is the distance from a point on the surface of the ring to an element d/ of the axis of 
the wire, the integration being over all such elements. We divide the integral into two parts 
corresponding to r < A and r > A, A being a distance such that a «^ A <^ b. Then for 
r < A the segment of the ring concerned may be regarded as straight, and therefore 

dZ r dl 

^ 2 log(2A/a). 



#7-f 



Vtf 2 +a 2 ) 

A>r -A 

In the range r > A the thickness of the wire may be neglected, i.e. r may be taken as the 
distance between two points on its axis. Then 

dl n f bd<f> 

„, . i, = -2 log tan i<f> , 

2b sin if • 



tf-'J 



where <f> is the angle subtended at the centre of the ring by the chord r, and the lower limit 
of integration is such that 2b sin b<f>o = A, whence ^o = A/6. When the two parts of the 
integral are added, A cancels, and the capacity of the ring is 

c = 



<£« log(86/a) 

§3. Methods of solving problems in electrostatics 

The general methods of solving Laplace's equation for given boundary 
conditions on certain surfaces are studied in mathematical physics, and we 
shall not give a detailed description of them here. We shall merely mention 
some of the more elementary procedures and solve various problems of 
intrinsic interest, t 

(1) The method of images. The simplest example of the use of this method 
is to determine the field due to a point charge e outside a conducting medium 
which occupies a half-space. The principle of the method is to find fictitious 
point charges which, together with the given charge or charges, produce a 
field such that the surface of the conductor is an equipotential surface. In 
the case just mentioned, this is achieved by placing a fictitious charge 
e' = —e at a point which is the image of e in the plane which bounds the 
conducting medium. The potential of the field due to the charge e and its 
image e' is 

* = e ^~~\ (3 * 1} 

t The solutions of many more complex problems are given by W. R. Smythe, Static and 
Dynamic Electricity, 2nd ed., McGraw-Hill, New York, 1950; G. A. Grinberg, Selected 
Problems in the Mathematical Theory of Electric and Magnetic Phenomena (Izbrannye voprosy 
matematicheskoi teorii elektricheskikh i magnitnykh yavlenii), Moscow, 1948. 



10 Electrostatics of Conductors §3 

where r and r' are the distances of a point from the charges e and e'. On 
the bounding plane, r = r' and the potential has the constant value zero, 
so that the necessary boundary condition is satisfied and (3.1) gives the solu- 
tion of the problem. It may be noted that the charge e is attracted to the 
conductor by a force e 2 /(2a) 2 (the image force; a is the distance of the charge 
from the conductor), and the energy of their interaction is — e 2 \Aa. 

The distribution of surface charge induced on the bounding plane by the 
point charge e is given by 

1 \d<l>~\ e a 

It is easy to see that the total charge on the plane is Jad/ = —e, as it should 
be. 

The total charge induced on an originally uncharged insulated conductor 
by other charges is, of course, zero. Hence, if in the present case the conduct- 
ing medium (in reality a large conductor) is insulated, we must suppose 
that, besides the charge —e> a charge +e is also induced, which, however, 
has no finite density, being distributed over the large surface of the conductor. 

Next, let us consider a more difficult problem, that of the field due to a 
point charge e near a spherical conductor. To solve this problem, we use 
the following result, which can easily be proved by direct calculation. The 
potential of the field due to two point charges e and —e', namely <f> = e/r—e'/r', 
vanishes on the surface of a sphere whose centre is on the line joining the 
charges (but not between them). If the radius of the sphere is R and its 
centre is distant / and /' from the two charges, then ///' = {e/e') 2 , R 2 = //'. 

Let us first suppose that the spherical conductor is maintained at a con- 
stant potential <f> = 0, i.e. it is earthed. Then the field outside the sphere due 
to the point charge e at A (Fig. 1), at a distance /from the centre of the sphere, 
is the same as the field due to two charges, namely the given charge e and 
a fictitious charge —e' at A' inside the sphere, at a distance /' from its centre, 
where 

/' = R 2 /l, e' = eRjl (3.3) 

The potential of this field is 

e eR 

* - 7-J? W 

r and r' being as shown in Fig. 1. A non-zero total charge —e' is induced on 
the surface of the sphere. The energy of the interaction between the charge 
and the sphere is 

* = -i "'IV-l') = -%e 2 RI(l 2 -R 2 ), (3.5) 

and the charge is attracted to the sphere by a force F = —d%jdl — 
-e 2 lRj(l 2 -R 2 ) 2 . 



§3 Methods of solving problems in electrostatics 11 

If the total charge on the spherical conductor is kept equal to zero (an 
insulated uncharged sphere), a further fictitious charge must be introduced, 
such that the total charge induced on the surface of the sphere is zero, and 
the potential on that surface is still constant. This is done by placing a 
charge + e' at the centre of the sphere. The potential of the required field 
is then given by the formula 

(3.6) 




OA--1 
OA''-l' 



Fig. 1 
The energy of interaction in this case is 

qt = lee'[ 1 = . (3.7) 

Finally, if the charge e is at A' (Fig. 1) in a spherical cavity in a conducting 
medium, the field inside the cavity must be the same as the field due to the 
charge e at A' and its image at A outside the sphere, regardless of whether 
the conductor is earthed or insulated: 

r Ir 



(2) The method of inversion. There is a simple method whereby in some 
cases a known solution of one electrostatic problem gives the solution of 
another problem. This method is based on the invariance of Laplace's 
equation with respect to a certain transformation of the variables. 

In spherical co-ordinates Laplace's equation has the form 

1 a / 2 d<f>\ 1 
r 2 dr\ drj r 2 

where An denotes the angular part of the Laplacian operator. It is easy to 
see that this equation is unaltered in form if the variable r is replaced by 
a new variable r' such that 

r = R 2 \r' (3.9) 



12 Electrostatics of Conductors §3 

(the inversion transformation) and at the same time the unknown function <f> 
is replaced by $ such that 

<f> = r'<f>'/R. (3.10) 

Here R is some constant having the dimensions of length (the radius of 
inversion). Thus, if the function <f>{t) satisfies Laplace's equation, then so 
does the function 

f(r') = R<f>(R 2 r'lr' 2 )/r'. (3.11) 

Let us assume that we know the electrostatic field due to some system of 
conductors, all at the same potential <j>o, and point charges. The potential 
<£(r) is usually defined so as to vanish at infinity. Here, however, we shall 
define <f>(r) so that it tends to — <£o at infinity. Then <f> = on the conductors. 

We may now ascertain what problem of electrostatics will be solved by 
the transformed function (3.11). First of all, the shapes and relative posi- 
tions of all the conductors of finite size will be changed. The boundary 
condition of constant potential on their surfaces will be automatically satis- 
fied, since <j>' = if <j> — 0. Furthermore, the positions and magnitudes of 
all the point charges will be changed. A charge e at a point ro moves to 
r'o = i? 2 ro/ro 2 and takes a value e' which can be determined as follows. As 
r -> tq the potential ^(r) tends to infinity as ej\8r\, where Sr = r— ro. 
Differentiating the relation r = R 2 r'jr' 2 , we find that the magnitudes of the 
small differences Sr and Sr' =. r'— r' are related by (Sr) 2 = i? 4 (Sr') 2 /r' 4 . 
Hence, as r' ->r' , the function <f>' tends to infinity as e/?/r'o|8r| = er'olR\8r'\, 
corresponding to a charge 

e' = er' JR = eR/r . (3.12) 

Finally, let us examine the behaviour of the function ^'(r') near the origin. 
For r' = we have r -> oo and <£(r) -> —<f>o. Hence, as r' -> 0, the func- 
tion <£' tends to infinity as —Rfajr'. This means that there is a charge 
e = —R<f> at the point r' = 0. 

We shall give, for reference, the way in which certain geometrical figures 
are transformed by inversion. A spherical surface of radius a and centre ro 
is given by the equation (r— ro) 2 = a 2 . On inversion, this becomes 
([i? 2 r'/r' 2 ]— ro) 2 = a 2 , which, on multiplying by r' 2 and rearranging, can be 
written (r'— r'o) 2 = a' 2 , where 

r'o = -R 2 rol(a 2 -r 2 ) } a' = aR 2 l\a 2 -r 2 \. (3.13) 

Thus we have another sphere, of radius a' and centre r'o- If the original 
sphere passes through the origin (a = ro), then a' = oo. In this case the 
sphere is transformed into a plane perpendicular to the vector ro and distant 
r'o— a' ~ R 2 l(a + r ) = R 2 /2a from the origin. 

(3) The method of conformal mapping. A field which depends on only 
two Cartesian co-ordinates (x and y, say) is said to be two-dimensional. 



§3 Methods of solving problems in electrostatics 13 

The theory of functions of a complex variable is a powerful means of solving 
two-dimensional problems of electrostatics. The theoretical basis of the 
method is as follows. 

An electrostatic field in a vacuum satisfies two equations : curl E = 0, 
div E = 0. The first of these makes it possible to introduce the field poten- 
tial, defined by E = — grad <f>. The second equation shows that we can 
also define a vector potential A of the field, such that E = curl A. In the 
two-dimensional case, the vector E lies in the .ry-plane, and depends only 
on x and y. Accordingly, the vector A can be chosen so that it is perpendicu- 
lar to the xy-plane. Then the field components are given in terms of the 
derivatives of <f> and A by 

E x = -d<f>/dx = dA/dy, E y = -8<j>/8y = -dA/dx. (3.14) 

These relations between the derivatives of tf> and A are, mathematically, just 
the well-known Cauchy-Riemann conditions, which express the fact that 
the complex quantity 

w = <f>-iA (3.15) 

is an analytic function of the complex argument z = x + iy. This means that 
the function w(z) has a definite derivative at every point, independent of 
the direction in which the derivative is taken. For example, differentiating 
along the #-axis, we find dwjdz = 8^j8x—i8Aj8x, or 

dw/dz = -E x + iE y . (3.16) 

The function w is called the complex potential. 

The lines of force are defined by the equation dx/E x = dy\E y . Expressing 
E x and Ey as derivatives of A, we can write this as (8A/ 8x)dx +(8A/ 8y)dy 
= dA = 0, whence A{x,y) = constant. Thus the lines on which the 
imaginary part of the function zo(z) is constant are the lines of force. The 
lines on which its real part is constant are the equipotential lines. The 
orthogonality of these families of lines is ensured by the relations (3.14), 
according to which 

86 8 A 86 d A 

dx dx dy dy 

Both the real and the imaginary part of an analytic function zv(z) satisfy 
Laplace's equation. We could therefore equally well take im w as the field 
potential. The lines of force would then be given by re w = constant. 
Instead of (3.15) we should have w = A+i(f>. 

The flux of the electric field through any section of an equipotential line 
is given by the integral § E n dl = — § (d<j>l dri)dl, where dl is an element of 
length of the equipotential line and n the direction of the normal to it. 
According to (3.14) we have d<f>jdn = —8A/81, the choice of sign denoting 
that / is measured to the left when one looks along n. Thus § E n dl 
= § (8Aldl)dl = A2—A1, where A2 and A\ are the values of A at the ends 



14 Electrostatics of Conductors §3 

of the section. In particular, since the flux of the electric field through a 
closed contour is Aire, where e is the total charge enclosed by the contour 
(per unit length of conductors perpendicular to the plane), it follows that 

e = (1/4tt)A^, (3.17) 

where AA is the change in A on passing counterclockwise round the closed 
equipotential line. 

The simplest example of the complex potential is that of the field of a 
charged straight wire passing through the origin and perpendicular to the 
plane. The field is given by E r = 2e/r, E d = 0, where r,6 are polar co- 
ordinates in the xy-plane, and e is the charge per unit length of the wire. 
The corresponding complex potential is 

to = - 2e log z = - 2e log r - lied. (3.18) 

If the charged wire passes through the point (xo, yo) instead of the origin, 
the complex potential is 

to = -2e\og{z-zo\ (3.19) 

where zq = xq + iyo. 

Mathematically, the functional relation w = w(z) constitutes a conformal 
mapping of the plane of the complex variable z on the plane of the complex 
variable w. Let C be the cross-sectional contour of a conductor in the 
ary-plane, and <f>o its potential. It is clear from the above discussion that the 
problem of determining the field due to this conductor amounts to finding a 
function w(z) which maps the contour C in the -sr-plane on the line to = (f>o, 
parallel to the axis of ordinates, in the w-plane. Then re w gives the 
potential of the field. (If the function zv(z) maps the contour C on a line 
parallel to the axis of abscissae, then the potential is im w.) 




Fig. 2 

(4) The wedge problem. We shall give here, for reference, formulae for 
the field due to a point charge e placed between two intersecting conducting 
half-planes. Let the z-axis of a system of cylindrical co-ordinates (r, 6, z) 
be along the apex of the wedge, the angle 6 being measured from one of the 
planes, and let the position of the charge e be (a, y, 0) (Fig. 2). The angle a 
between the planes may be either less or greater than ir\ in the latter case 
we have a charge outside a conducting wedge. 



(3.21) 



§3 Methods of solving problems in electrostatics 15 

The field potential is given byt 

e N sinh(7r£/a) sinh(7r£/a) | 

aV(2<w) J Icosh (Tr^/a) - cos [tt(0 - y)/a] cosh (tt £/a) - cos [tt(0 + y)/a] I 

d£ (3 ' 20) 

x , cosh 7) — (a 2 + r 2 + z 2 )/2ar, rj > 0. 

-v/(cosh £ — cosh 17) 

The potential <f> = on the surface of the conductors, i.e. for 6 = or a. 

In particular, for a = 2tt we have a conducting half-plane in the field of a 
point charge. In this case the integral in (3.20) can be evaluated explicitly, 
giving 

e(l J-cos%(0-y)\ 1 J-co&l{d + Y )\\ 

<j> = — { — cos -1 cos -1 — )}, 

r AR \ cosh £77 J R \ cosh £77 J) 

R2 = a 2 + r 2 + z 2 -2arcos(y-6), 

R'2 = a 2 + r 2 + z 2 -2arcos(y+6). 

In the limit as the point (r, 6, z) tends to the position of the charge e, the 
potential (3.21) becomes 

4> = d>' + elR, where d>' = — — [l+-^^l. (3.22) 

2?ra L sin yj 

The second term is just the Coulomb potential, which becomes infinite as 
R -> 0, while <f>' is the change caused by the conductor in the potential at 
the position of the charge. The energy of the interaction between the charge 
and the conducting half-plane is 

* = W= - — U + -T- 1 . (3-23) 

47T# L sin y J 

PROBLEMS 
Problem 1. Determine the field near an uncharged conducting sphere of radius R placed 
in a uniform external electric field (&. 

Solution. We write the potential in the form ^ = <£o+^i, where ^0 = — ©«r is the 
potential of the external field and <f>i is the required change in potential due to the sphere. 
By symmetry, the function <f>i can depend only on the constant vector (£. The only such solu- 
tion of Laplace's equation which vanishes at infinity is 

^1 = — constant X(£«grad(l/r) = constant X©T/r 3 , 

the origin being taken at the centre of the sphere. On the surface of the sphere <f> must be 
constant, and so the constant in <f>i is R s , whence 

/ R 3 \ I R 3 \ 

*=-ffi.r(l--) = -(£rcos0(l---), 

where is the angle between © and r. The distribution of charge on the surface of the 
sphere is given by 

a (l/47r)[a^/3r] r -R = (3(£/4w) cos 0. 

f This formula was first given by H. M. Macdonald (1895). Its derivation is given by 
him in Electromagnetism, Bell, London, 1934, p. 79. 



16 Electrostatics of Conductors §3 

The total charge e — 0. The dipole moment of the sphere is most easily found by comparing 
4>i with the potential P-r/r 3 of an electric dipole field, whence & = R 3 (&. 

Problem 2. The same as Problem 1, but for an infinite cylinder in a uniform transverse 
field. 

Solution. We use polar co-ordinates in a plane perpendicular to the axis of the cylinder. 
The solution of the two-dimensional Laplace's equation which depends only on a constant 
vector is 

<f>i = constant X (£*grad (log r) = constant X ffi«r/r 2 . 

Adding ^o = — ©«r and putting the constant equal to R 2 , we have 



= -(£r 



<(-£)■ 



The surface charge density is a = ((£/2w) cos d. The dipole moment per unit length of the 
cylinder can be found by comparing ^ with the potential of a two-dimensional dipole field, 
namely 2^-grad (log r) = 2^-r/r 2 , so that ^ = £R 2 (g. 

Problem 3. Determine the field near a wedge-shaped projection on a conductor. 

Solution. We take polar co-ordinates r, d in a plane perpendicular to the apex of the 
wedge, the origin being at the vertex of the angle do of the wedge. The angle d is measured 
from one face of the wedge, the region outside the conductor being ^ d < lit— do. Near 
the apex of the wedge, the potential can be expanded in powers of r, and we shall be interested 
in the first term of the expansion (after the constant term), which contains the lowest power 
of r. The solutions of the two-dimensional Laplace's equation which are proportional to 
r n are r n cos nd and r n sin nd. The solution having the smallest n which satisfies the con- 
dition <f> = constant for 0=0 and = 27r— 0o (i.e. on the surface of the conductor) is 

<f> = constant X r n sin nd, n = ir/(2n— do). 

The value of the constant can be determined only by solving the problem for the whole field. 
The field varies as r n_1 . For do < -n (n < 1), therefore, the field becomes infinite at the apex 
of the wedge. In particular, for a very sharp wedge (do <^ 1 , n = £) E increases as r~* as 
r -> 0. Near a wedge-shaped concavity in a conductor ($o > ir, n > 1) the field tends to 
zero. 

Problem 4. Determine the field near the end of a sharp conical point on the surface of a 
conductor. 

Solution. We take spherical co-ordinates, with the origin at the vertex of the cone and 
the polar axis along the axis of the cone. Let the angle of the cone be 2#o -^ 1 , so that the 
region outside the conductor corresponds to polar angles in the range do < d < it. We seek 
a solution for the variable part of the potential, which is symmetrical about the axis, in the 
form 

* = f/(0), (1) 

with the smallest possible value of n. Laplace's equation 

1 d I a H\ l d I n H\ 

^^r^i+^n^^r^^^ ' 

after substitution of (1), gives 

1 



sin 9 d e ( 8ta9 s) + " ( " +1)/ -°- (2) 

The condition of constant potential on the surface of the cone means that we must have 
/(0o) = 0. 

For small do we seek a solution by assuming that n <^ 1 and f(d) is of the form 
constant X [1+^(0)1, where ifi <^. 1. (For do -> 0, i.e. an infinitely sharp point, we should 
expect that <f> tends to a constant almost everywhere near the cone.) The equation for ifi is 



1 d / . A+ 



sin d dd 



(-3- 



(3) 



§3 Methods of solving problems in electrostatics 17 

The solution having no singularities outside the cone (in particular, at — w) is i/>(0) 
— 2« log sin £0. 

For ~ do <^ 1, ^ is no longer small. Nevertheless, this expression remains valid, since 
the second term in equation (2) may be neglected because is small. To determine the con- 
stant n in the first approximation we must require that the function/ =1+^ vanishes for 
= do. Thusf « = —1/2 log 6o. The field increases to infinity as r~"< 1_B > in the neighbour- 
hood of the vertex, i.e. essentially as 1/r. 

Problem 5. The same as Problem 4, but for a sharp conical depression on the surface of a 
conductor. 

Solution. The region outside the conductor now corresponds to the range < < 0o. 
As in Problem 4, we seek <f> in the form (1), but now n ^> 1. Since <^ 1 for all points in the 
field, equation (2) of Problem 4 becomes 






This is Bessel's equation, and the solution having no singularities in the field is Jo(n0). The 
value of n is determined as the smallest root of the equation Jo(nOo) = 0, whence n = 2*4/0o. 

Problem 6. Determine the energy of the attraction between an electric dipole and a plane 
conducting surface. 

Solution. We take the *-axis perpendicular to the surface of the conductor, and passing 
through the dipole ; let the dipole moment vector 3* lie in the «y-plane. The image of the 
dipole is at the point — * and has a moment &' x = &x, &'v = —&y- The required energy 
of attraction is half the energy of the interaction between the dipole and its image, and is 
% = -(2&> x *+&y 2 )ll6x?. 

Problem 7. Determine the mutual capacity per unit length of two parallel infinite con- 
ducting cylinders of radii a and b, their axes being at a distance c apart.J 




Fig. 3 

Solution. The field due to the two cylinders is the same as that which would be produced 
(in the region outside the cylinders) by two charged wires passing through certain points 
A and A' (Fig. 3). The wires have charges ±«' per unit length, equal to the charges on the 
cylinders, and the points A and A' lie on OO' in such a way that the surfaces of the cylinders 
are equipotential surfaces. For this to be so, the distances OA and O'A' must be such 



f A more rigorous calculation gives the formula n == 1/2 log (2/0o), containing a coefficient 
in the (large) logarithm, which cannot really be obtained by the simple method given here. 

J The corresponding problem for two, spheres cannot be solved in closed form. The 
difference arises because, in the field of two parallel wires bearing equal and opposite charges, 
all the equipotential surfaces are circular cylinders, whereas in the field of two equal and 
opposite point charges the equipotential surfaces are not spheres. 



18 Electrostatics of Conductors §3 

that OA • OA' = a 2 , O'A' • O'A = b\ i.e. di{c-d%) = a 2 , dz(c-di) = 6 2 . Then, for each 
cylinder, the ratio r\r' of the distances from A and A' is constant. On cylinder 1 , r\r' — ajOA' 
= al(c—dz) = di/a, and on cylinder 2, r'/r = dvjb. Accordingly, the potentials of the cylin- 
ders are fa = —2elog {r\r') = —2elog(dila), fa = 2e log (<&/&), ^2—^1 = 2e log (didzlab). 
Hence we find the required mutual capacity C = el(<f>2—<f>i) : 

1/C = 2 log (did z /ab) = 2 cosh" 1 [(c 2 -a 2 -6 2 )/2a6]. 

In particular, for a cylinder of radius a at a distance h (> a) from a conducting plane, we 
put c = b+h and take the limit as b -> 00, obtaining 1/C = 2 cosh -1 (A/a). 

If two hollow cylinders are placed one inside the other (c < b—d), there is no field outside, 
while the field between the cylinders is the same as that due to two wires of charges ±« 
passing through A and A' (Fig. 4). The same method gives 

1/C = 2 cosh" 1 [(a 2 +6 2 -c 2 )/2afe]. 

Problem 8. The boundary of a conductor is an infinite plane with a hemispherical pro- 
jection. Determine the charge distribution on the surface. 




00=c 



Fig. 4 

Solution. In the field determined in Problem 1, whose potential is 

,R 3 \ 



/, R3 \ 
9 = constant X2II —I, 



the plane z = with a projection r = jR is an equipotential surface, on which <f> = 0. Hence 
it can be the surface of a conductor, and the above formula gives the field outside the con- 
ductor. The charge distribution on the plane part of the surface is given by 



47rLad z _o \ r 3 /' 



we have taken the constant in <f> as — Ariroo, so that cto is the charge density far from the pro- 
jection. On the surface of the projection we have 



1 rafi 2 

■ 1 I = A an 



6 1 1 Z 

= 3go— . 

r\r-R R 



Problem 9. Determine the dipole moment of a thin conducting cylindrical rod, of length 
2/ and radius a <^ /, in an electric field © parallel to its axis. 

Solution. Let t(z) be the charge per unit length induced on the surface of the rod, and 
z the co-ordinate along the axis of the rod, measured from its midpoint. The condition of 
constant potential on the surface of the conductor is 



2rr I 

K . - C f r( z')dz' d<f> 

-^rJi — r — =?' 

-i 



2tt J J 
-I 

R* = (*'-*) 2 +4a 2 sin 2 i?, 



§3 



Methods of solving problems in electrostatics 



19 



where <f> is the angle between planes passing through the axis of the cylinder and through two 
points on its surface at a distance R apart. We divide the integral into two parts, putting 
r(z') == t(z) + [t(z')— t(z)]. Since l^> a, we have for points not too near the ends of the rod 

t(z) f r dz'd<f> ^ t(^ r V—z* , , /A , MP-z*) 

2tt J J R ~ 2ir J a 2 sin 2 \$ 



d<f> = t(z) log ■ 



using the result that J log sin <f>d<f> = —w log 2. In the integral which contains the diff- 



erence t(z')—t(z), we can neglect the a 2 term in R, since it no longer causes the integral to 
diverge. Thus 



(&z = t(z) log 



MP-z*) 



+ 



T(Z')-T(Z) 



dz\ 



The quantity t is almost proportional to z, and in this approximation the integral gives 
— 2t(z), the result being 



r(z) = 



&z 



log[4(/ 2 -^ 2 )/a 2 ]-2 

This expression is invalid near the ends of the rod, but in calculating the dipole moment 
that region is unimportant. In the above approximation we have 

-l 

where L = log(2//a)— 1 is large, or (with the same accuracy) 

~ 3 1og(4//a)-7' 

Problem 10. Determine the capacity of a hollow conducting cap of a sphere. 

Solution. We take the origin O at a point on the rim of the cap (Fig. 5), and carry out the 
inversion transformation r = / 2 /r', where / is the diameter of the cap. The cap then becomes 
the half-plane shown by the dashed line in Fig. 5, which is perpendicular to the radius AO 
of the cap and passes through the point B on its rim. The angle y = it— 6, where 20 is the 
angle subtended by the diameter of the cap at the centre of the sphere. 




Fig. 5 

If the charge on the cap is e and its potential is taken as zero, then as r -*■ oo the potential 
^ ->— ^o+e/r. Accordingly, in the transformed problem, as r' -»■ the potential is <f>' ->■ l<f>/r' 
== — I fall* + eft, where the first term corresponds to a charge e' = — l<t>o at the origin. 



20 Electrostatics of Conductors §4 

According to formula (3.22), we have 

Y r' 2tt/\ sinfl/ 

(the potential near a charge e' at a distance / from the edge of a conducting half-plane at zero 
potential). Comparing the two expressions, we have for the required capacity C = e[<f>o 

It 6 \ R 

C = H 1+ "^^ = -0»in 0+0), 
2tt\ sin 6/ it 

where R is the radius of the cap. 

Problem 11. Determine the correction due to edge effects on the value C = Sftird for 
the capacity of a plane condenser (S being the area of the plates, and d <^ \/S the distance 
between them). 



.<p-<^>Q/'d 



(a) -d[ ^=0 




(b) 



Fig. 6 

Solution. Since the plates have free edges, the distribution of charge over them is not 
uniform. To determine the required correction in a first approximation, we consider 
points which are at distances x from the edge such that d <^ x <^ -\/S. For example, taking 
the upper layer (at potential <f> = ifo, Fig. 6a) and neglecting its distance id from the mid- 
plane (the equipotential surface <f> = 0), we have the problem of the field near the boundary 
between two parts of a plane having different potentials (Fig. 6b). The solution is elemen- 
tary f, and the excess charge (relative to the value of a far from the edge) is A a = Enftir 
— <f>o/Sir s x, so that the total excess charge is L / Aa dx = (<j>oL/8ir 2 ) log (\^S/d), where L is 
the perimeter of the plate. In calculating the logarithmically divergent integral, we have 
taken the limits as those of the region d <^.x <^ \/S. Hence we find the capacity} 

S L , VS 
C = 1 log — — . 

4-nd Sit 2 * d 

§4. A conducting ellipsoid 

The problem of the field of a charged conducting ellipsoid and that of an 
ellipsoid in a uniform external field are solved by the use of ellipsoidal co- 
ordinates. These are related to Cartesian co-ordinates by the equation 

X V Z 

+ 1J—-+-* = 1 (a> b> c). (4.1) 



a? + u b 2 + u c 2 + u 



t See §22. In formula (22.2) for the potential we must here put ^ a & == ii>o> ol = tt. 

X A more exact calculation (determining the coefficient in the argument of the logarithm) 
demands considerably more elaborate methods, and the result depends on the shape of the 
plates. If these are circular, of radius R, we obtain Kirchhoff's formula 

R? R / 16nR 



„ R* R/ IbrrR \ 



§4 A conducting ellipsoid 21 

This equation, a cubic in u, has three different real roots |, rj, £, which lie in 
the following ranges: 

£ > -c\ -c 2 ^^^ -b 2 y -b 2 > C> -a 2 . (4.2) 

These three roots are the ellipsoidal co-ordinates of the point x, y, z. Their 
geometrical significance is seen from the fact that the surfaces of constant 
£, rj and £ are respectively ellipsoids and hyperboloids of one and two sheets, 
all confocal with the ellipsoid 

x 2 /a 2 +y 2 /b 2 + z 2 lc 2 = 1. (4.3) 

One surface of each of the three families passes through each point in 
space, and the three surfaces are orthogonal. The formulae for transforma- 
tion from ellipsoidal to Cartesian co-ordinates are given by solving three 
simultaneous equations of the type (4.1), and aret 

(|+a% + a 2 )(£ + a 2 ) 



~VL (b 2 -a 2 )(c 2 -a 2 ) J' 

y VL (c 2 -b 2 \(a 2 -W) y 



~VL (a 2 -C 2 )(b 2 -C 2 ) J* 



(4.4) 



{c 2 -b 2 ){a 2 -b 2 ) 
(i + c^n+fU+f) 
(a 2 -c 2 ){b 2 -c 2 ) 
The element of length in ellipsoidal co-ordinates is 
d/2 = h&ip + WdrP + WdP, 
hi = V[(i-r))^-0V2R s , h = V[(v-CXn-mi2Rv \ (4-5) 

h = V[{t ~ m ~ l)\l^ Ru 2 = (u + a 2 )(u + b 2 )(u + c 2 ), 
u = i, 7], £. 
Accordingly, Laplace's equation in these co-ordinates is 

4 

A<t> = x 

(t-nM-€Kv-Q 

If two of the semiaxes a, b, c become equal, the system of ellipsoidal co- 
ordinates degenerates. Let a = b > c. Then the cubic equation (4.1) 
becomes a quadratic, 

2 2 

9 +-^— = 1, P 2 = x 2 +y\ (4.7) 



a 2 + u c 2 + u 



t Strictly speaking, the ellipsoidal co-ordinates should be taken not as £, 17, £ themselves 
but as V(« 2 + £), V(b 2 + if}, V(c 2 + £)• Then the double signs would not appear in 
(4.4), and the two systems of co-ordinates would be in one-to-one correspondence, as they 
should be. 



22 



Electrostatics of Conductors 



§4 



with two roots whose values lie in the ranges £ ^ — c 2 , —c 2 > rj ^ —a 2 . The 
co-ordinate surfaces of constant £ and r\ become respectively confocal oblate 
spheroids and confocal hyperboloids of revolution of one sheet (Fig. 7). 
As the third co-ordinate we can take the polar angle <f> in the xy-plane 
(# = p cos <f>, y — p sin <j>). For a = b the ellipsoidal co-ordinate £ de- 
generates to a constant, — a 2 . Its relation to the angle <f> is given by the way 
in which it tends to —a 2 as b tends to a, namely 

cos<£ = V[(« 2 + 0/(« 2 -* 2 )] as 6 ^«. ( 4 - 8 ) 




Fig. 7 



This is easily seen from (4.4) or directly from (4.1). The relation between the 
co-ordinates z, p and £, 17 is given, according to (4.4), by 



-*y[ 



(£+c% + c 2 ) 



i*2 /72 



]■ '-yi 



(£+«%+« 2 ) 



t2 — />2 



(4.9) 



The co-ordinates £, 17, <£ are called oblate spheroidal co-ordinates, t 

Similarly, for a > b = c ellipsoidal co-ordinates become prolate spheroidal 
co-ordinates. Two co-ordinates £ and £ are roots of the equation 



x 2 p 2 

a 2 + u b 2 + u 



p2 = y2 _|_ #2^ 



(4.10) 



where £ ^ — & 2 , — b 2 > £ ^ —a 2 . The surfaces of constant £ and £ are 
prolate spheroids and hyperboloids of revolution of two sheets (Fig. 8). 
The co-ordinate 17 degenerates to a constant, — b 2 , for c ->b, and we have 

cos<£ = VW + i)l{b 2 -c 2 )], (4.11) 

where <f> is the polar angle in the yz-plane. The relation between the co- 
ordinates x, p and £, £ is given by 

■(i + a 2 )(C + a 2 )l /r(| + & 2 )(£ + & 2 )| 



_,_ /r(f+« a )(£+flvi /r 



62_«2 



! 1- 



(4.12) 



t We here use the definition of spheroidal co-ordinates such that they are the limit of 
ellipsoidal co-ordinates. Other definitions are used in the literature, but are easily related to ours. 



§4 



A conducting ellipsoid 



In a system of oblate spheroidal co-ordinates the foci of the spheroids and 
hyperboloids lie on a circle of radius <\/{a 2 —c 2 ) in the xy-plane; in Fig. 7 
AA! is a diameter of this circle. Let us draw a plane passing through the 
#-axis and some point P. It intersects the focal circle at two points; let 
their distances from P be r±, r 2 . If the co-ordinates of P are p, z, then 

ri 2 =[p- V(" 2 ~ c 2 )f + * 2 , r 2 2 = [p + V(« 2 ~ c 2 )] 2 + z 2 . 




Fig. 8 



The spheroidal co-ordinates |, r\ are given in terms of r\, r 2 by 
£ = h(r 1 + r 2 ) 2 -a 2 , V = i(r 2 -ri) 2 -a 2 . 



(4.13) 



In a system of prolate spheroidal co-ordinates the foci are the points 
x = ±-\Z(a 2 —b 2 ) on the #-axis (the points A, A' in Fig. 8). If r\ and r% 
are the distances of these foci from P, then 

n 2 = p 2 + [z - *J(a 2 - b 2 )f, r 2 2 = p 2 + [z + V(« 2 ~ b 2 )] 2 , 

and the spheroidal co-ordinates £, £ are given in terms of r±, r 2 by the same 
formulae (4.13), with £ in place of rj. 

Let us now turn to the problem of the field of a charged ellipsoid whose 
surface is given by the equation (4.3). In ellipsoidal co-ordinates this is the 
surface £ = 0. It is therefore clear that, if we seek the field potential as a 
function of £ only, all the ellipsoidal surfaces £ = constant, and in particu- 
lar the surface of the conductor, will be equipotential surfaces. Laplace's 
equation (4.6) then becomes 



whence 



d£< 



m 



«) - •• 



00 



d£ 



24 Electrostatics of Conductors §4 

The upper limit of integration is taken so that the field is zero at infinity. 
The constant A is most simply determined from the condition that at large 
distances r the field must become a Coulomb field and <f> ^ c/r, where e is 
the total charge on the conductor. When r ->- oo, £ ->■ oo, and £ £ r 2 , as 
we see from equation (4.1) with u = £. For large £ we have 1?£ £ £ 3/2 , 
and <£ £ 2A\yJ% = 2.4/r. Hence 2^4 = £, and therefore 

^ = *'Jf (4 * 14) 

g % 

The integral is an elliptic integral of the first kind. The surface of the con- 
ductor corresponds to £ = 0, and so the capacity of the conductor is given 
by 



1 ?dg 

c - *fc (4 - 15) 



The distribution of charge on the surface of the ellipsoid is determined by 
the normal derivative of the potential : 



a= -± \ d l] = _±r±iii 

From equations (4.4) we easily see that for £ = 

x 2 y 2 z 2 r)£ 

— + TT + — = 



1 



47T vW) 



*4 64 ^4 a 2^2 c 2 

Hence 



^nabc 



(x 2 y 2 z 2 \ -* 
^4 + 64 + "?/ 



(4.16) 



For a spheroid the integrals (4.14), (4.15) degenerate and can be expressed 
in terms of elementary functions. For a prolate spheroid (a > b = c) the 
field potential is 

e la 2 — b 2 

= tanh-i / , (4.17) 

r ^{a 2 -b 2 ) V | + fl 2 v ' 

and the capacity is 

V(« 2 -& 2 ) 

C = — -. (4.18) 

cosh _1 (a/6) 

For an oblate spheroid (a = 6 > c) we have 

e /a 2 -c 2 ' V(« 2 -^ 2 ) 

<£ = tan-i / , C = VV t/ ' (4.19) 

Y V"(« 2 -^ 2 ) V £ + c 2 cos-i(c/fl) 



§4 A conducting ellipsoid 25 

In particular, for a circular disc {a = b, c = 0) 

C = laj-n. (4.20) 

Let us now consider the problem of an uncharged conducting ellipsoid 
in a uniform external electric field ©. Without loss of generality we may 
take the field <£ to be along one of the axes of the ellipsoid. In any other case 
this field may be resolved into components along the three axes, and the 
resultant field is a superposition of those arising from each component 
separately. 

The potential of a uniform field <£ along the #-axis (the a-axis of the ellip- 
soid) is, in ellipsoidal co-ordinates, 

<f>o = - ®x = - (£V[(£ + «% + <**){£> + « 2 )/(* 2 - « 2 )(^ 2 - a 2 )]. (4.21) 

We write the field potential outside the ellipsoid as <f> = <f>o +</>', where <j> 
gives the required perturbation of the external field by the ellipsoid, and 
seek <J>' in the form 

*' = <hF(i). (4.22) 

In this function the factors depending on 77 and £ are the same as in ^0 ; this 
enables us to satisfy the boundary condition at £ = for arbitrary 17, £ 
(i.e. on the surface of the ellipsoid). Substituting (4.22) in Laplace's equation 
(4.6), we obtain for F(g) the equation 

d*F dF d 

a? + itvwwn - °- 

One solution of this equation is F = constant, and the other is 



m = ^ J 



g (£ + <> 2 Wi 



(4.23) 



The upper limit of integration is taken so that <f>' -> for g -> 00. The integral 
is an elliptic integral of the second kind. 

We must have <f> =, constant on the surface of the ellipsoid. For this condi- 
tion to be satisfied with £ = and arbitrary 77, £, the constant value of <f> 
must be zero. Determining the coefficient A in F(g) so that F(0) = —1, we 
obtain the following final expression for the field potential : 

*-4-fc^/jfrdy- (4 - 24) 

1 

Let us find the form of the potential <f>' at large distances r from the ellip- 
soid. For large r, the co-ordinate £ is large, and $ £ r 2 , as follows at once 
from equation (4.1). Hence 

r ds ^ r ds 2 
J (s+a*)R s £ J ~sW = ^ 



26 Electrostatics of Conductors §4 

and the potential <f>' = (£ xVj4TTn {x) r z , where F = frrabc is the volume of 
the ellipsoid and n ix \ n ( v\ « fe) are denned by 



n (x) — \abc 



ds , . r ds 

- - 



«(») = i^c 



(* + a 2 )#/ 2 J(* + &2)#/ 

u 

(4.25) 

f ds 
n fe) = \abc . 

o 
The expression for <f>' is, as we should expect, the potential of an electric 
dipole : <£' = x^xj^, where the dipole moment of the ellipsoid is 

& x = <& x VI4irtf*\ (4.26) 

Analogous expressions give the dipole moment when the field G is along the 
y or z axis. 

The positive constants n&\ n^\ w (z) depend only on the shape of the 
ellipsoid, and not on its volume; they are called the depolarisation coefficients A 
If the co-ordinate axes do not necessarily coincide with those of the ellip- 
soid, formula (4.26) must be written in the tensor form 

WVym&t = <&. (4.27) 

The quantities n (x) , n^\ « fe) are the principal values of the symmetrical 
tensor nys of rank two. 

In the general case of arbitrary a, b, c, it follows from the definitions of 
n (z\ n (.y\ nfe) that 

n (x) < n (y) < M fe) if a > b > c . (4.28) 

Further, by adding the integrals for « te >, n^\ « fo) and using as the variable 
of integration u = R s 2 , we find 

r dw 

n (x) + n (y) + n (z) = l aoc ___ 

J U 3 ' 2 

(a be) 1 

whence 

n te> + „<*) + „<«) = 1. (4.29) 

The sum of the three depolarisation coefficients is thus unity; in tensor nota- 
tion, flu = 1. Since these coefficients are positive, none can exceed unity. 
For a sphere (a = b = c) it is evident from symmetry that 

n (x) = n (y) = w fe) = £. (4.30) 

For a cylinder with its axis in the ^-direction (a -> oo), we havet 

»te> = 0, nW = «fe> = i. (4.31) 

f Useful tables of these coefficients have been given by E. C. Stoner (Philosophical 
Magazine [7] 36, 803, 1945). . 

J These values for a sphere and a cylinder agree, of course, with those found in §3, 
Problems 1 and 2. 



§4 



A conducting ellipsoid 



XI 



The elliptic integrals (4.25) can be expressed in terms of elementary func- 
tions if the ellipsoid is a spheroid. For a prolate spheroid {a > b = c) of 
eccentricity e = y^l — £ 2 /a 2 ), 

1-e 2 / 1+e \ 

/log 2e\, nW = n<*> = £(1 -»<*>). 



w te) = 



2*3 



If the spheroid is nearly spherical (e <^ 1) we have approximately 
«to) = i-^ e 2 ) w (y) = w (z) = 1+^2. 

For an oblate spheroid (a = b > c) 



tfe) 



1+e 2 



(e-tan- 1 *;), n<- x) = nW = l{\-n^), 



where e = \Z(a 2 Jc 2 — 1). If e <^ 1, then 



(4.32) 
(4.33) 
(4.34) 
(4.35) 



PROBLEMS 

Problem 1 . Find the field of a charged conducting circular disc of radius a, expressing it 
in cylindrical co-ordinates. Find the distribution of charge on the disc. 

Solution. The charge distribution is obtained by taking the limit of formula (4.16) as 
c -*■ 0, z -*■ 0, with z\c — V(l — r 2 la 2 ) (where r 2 = x z +y 2 ), in accordance with (4.3). This 
gives 

47ra 2 \ a 2 / 

The field potential is given in all space by formula (4.19), where we put c = and express f 
in terms of r and z by means of equation (4.1) with c = 0,u = £, a = b: 



<f> = - tan -1 - 
a Lr 



2a 2 



2 +z 2 -a 2 + V[0- 2 +z 2 -a 2 ) 2 +4a 2 * 2 ] 



]• 



ff / 

Z2L 



'A 



Fig. 9 

Near the edge of the disc, we replace r and z by co-ordinates p and 6 such that z = p sin 0, 
r = a—p cos (Fig. 9; p <^a), obtaining 

in agreement with the general result derived in §3, Problem 3. 



28 Electrostatics of Conductors §4 

Problem 2. Determine the electric quadrupole moment of a charged ellipsoid. 

Solution. The quadrupole moment tensor of a charged conductor is defined as Due 
<= e(3xtxk— r*8uc), where e is the total charge, and the bar denotes an average such as 



XiXjc = - <J) XiXKO d/. 



It is evident that the axes of the ellipsoid are also the principal axes of the tensor Due- Using 
formula (4.16) for a, and for the element of surface of the ellipsoid the expression 



dxdy 
d/= 



dx dy I r* 2 y 2 z 2 l 
= zlc* N ti* 6* e* J' 



we obtain 

— r r 

zdxdy = Jc 2 ; 



= — f 
Andb J 



the integration over x and y covers twice the area of the cross-section of the ellipsoid by the 
xy-plane. Thus 

D xx = J e (2a 2 -& 2 -c 2 ), D yy = ie(26 2 -c 2 -a 2 ), Dzz = J«(2c 2 -a 2 ~& 2 ). 

Problem 3. Determine the distribution of charge on the surface of an uncharged con- 
ducting ellipsoid placed in a uniform external field. 

Solution. According to formula (1.9) we have 



= _jJL|"£f| = _T_L!f| 



by (4.5) the element of length along the normal to the surface of the ellipsoid is Aid£. Sub- 
stituting (4.24) and using the fact that 

"* = ihi ai-U-o = L2a 2 /iJ e -o 

(where v is a unit vector along the normal to the surface), we have a = QzvxfirmW when the 
external field is in the ^-direction. When the direction of the external field is arbitrary this 
becomes 

Air 4-irlnW n^ n<*> J 

Problem 4. The same as Problem 3, but for a plane circular disc of radius a lying parallel 
to the field, t Determine also the dipole moment of the disc. 

Solution. Let us regard the disc as the limit of a spheroid when the semiaxis c tends to 
zero. The depolarisation coefficient along this axis (the 2-axis) tends to 1, and those along 
the x and y axes tend to zero: n< z > = 1 —ncjla, n™ = «<"> = irc/Aa, by (4.34). The com- 
ponent v x of the unit vector along the normal to the surface of the spheroid tends to zero: 

_ x (x*+y 2 z 2 \"-* * c 2 __ xc( _ * 2 +y 2 \~* 

Hence the charge density is 

(£ v x (£p cos <f> 

\lt 70*) ~ 7r 2 V(a 2 -P 2 )' 

where p and <f> axe polar co-ordinates in the plane of the disc. 

The dipole moment of the disc is obtained from formula (4.26), and is & =■ 4a 3 ffi/3w. 
Thus it is proportional to a 3 , and not to the "volume" a 2 c of the disc. 



a = 



t The problem for a disc lying perpendicular to the field is trivial: the field remain 
uniform in all space, and charges a = ± (£/4w are induced on the two sides of the disc. 



§4 A conducting ellipsoid 29 

Problem 5. Determine the field potential outside a conducting spheroid with its axis of 
symmetry parallel to a uniform external field. 

Solution. For a prolate spheroid (a > b = c, with the field ($S in the ^-direction) we find, 
on calculating the integral in formula (4.24), 

tanh-l VK* 2 ~b 2 )K£+a 2 )] - VK« 2 -*> 2 )/(£+a 2 )]| 



<t> = -<£*{l- 



tanh" 1 V(l -& 2 /a 2 ) - V(l -b 2 /a 2 ) J 

The co-ordinate £ is related to * and p = -\/{y 2 +z 2 ) by 

-j£_ + -*_ = ! 
i 2 +* a*+£ 

with < £ < oo in the space outside the ellipsoid. 

For an oblate spheroid (a = b > c) the field © is along the 2-axis. We must therefore re- 
place s+a 2 by $+c 2 and put ^o = —i&z in the integrals in (4.24). Then 

V[(a* -c 2 )/(g+c 2 )] -tan-* V[(a 2 -c 2 )/(£ +c 2 ) 

V(a 2 /c 2 -l)-tan-i V(a 2 /c 2 -l) 

where the co-ordinate £ is related to z and p = V(x 2 +y 2 ) by 



, « f V[(a 2 -c 2 )/(g+c 2 )]-tan-i V[(^-c 2 )/(|+c2) | 

* I V(a 2 /c 2 -l)-tan-i V(a 2 /c 2 -l) /' 



' +^r-l. 



a 2 +£ c 2 +£ 

Problem 6. The same as Problem 5, but with the axis of symmetry perpendicular to the 
external field. 

Solution. For a prolate spheroid (with the field along the #-axis) 

Vtf +a 2 )IU+b 2 )-(a 2 -b 2 )-* tanh" 1 VK« 2 -&W +a 2 )}\ 



<l> = -<E*[l- 



a/6 2 -(a 2 -6 2 )-*tanh- 1 y/{\-b 2 la 2 ) 
For an oblate spheroid (with the field along the %-axis) 

( a 2_ c 2)-» ten -i ^ [ ( a 2_ c 2)/(^ +c a ) ]_ x /(^ +c 2 )/( ^ +a 2 ) 



<£* 1- 



(a 2 -c 2 )-* tan- 1 y/{a 2 \c 2 - 1) -c/a 2 J 

Problem 7. A uniform field Qe in the sr-direction (in the half-space z < 0) is bounded by 
an earthed conducting plane at z = 0, containing a circular aperture. Determine the field 
and charge distribution on the plane. 

Solution. The xy-plane with a circular aperture of radius a and centre at the origin may 
be regarded as the limit of the hyperboloids of revolution of one sheet 

,2 *2 



p" z 



- — = 1, p 2 = x 2 +y 2 , 



F(0 - constant X J^^^r =constantX [-^ - *n-i-2-]; 



a 2 -\ v \ h| 

as |ij| -*■ 0. These hyperboloids are one of the families of co-ordinate surfaces in a system of 
oblate spheroidal co-ordinates with c — 0. The Cartesian co-ordinate z, according to (4.9), 
is given in term6 of £ and 17 by z = \/(£ h|)/a, and V£ must be taken with the positive and 
negative sign in the upper and lower half-space respectively. 

Let us seek a solution in the form <f> = — <&z F(£). For the function F(f) we obtain 

df 
¥(!+*) "™" ~ L~V* " "" ~ V*J 

the constant of integration is put equal to zero in accordance with the condition <f> = for 
z -*■ +oo, i.e. V^ -*" +°o. The inverse tangent of a negative quantity must be taken as 
tan -1 (a/—V0 = w— tan -1 (a/\/£)» and not as —tan -1 (a/V£) since the potential would 
then be discontinuous at the aperture (£ = 0). The constant coefficient is chosen so that, for 
z -*■ — oo (i.e. for V£ -*• — oo and tan _1 (a/\/£) -»■ "■), 4> -*■ — ®*i and so we finally have 

On the conducting plane 17 = and the potential is zero, as it should be. 



30 Electrostatics of Conductors §4 

At large distances r = -y/O^ + p 2 ) from the aperture we have £ = r 2 , and the potential (in 
the upper half-plane) is 

i.e. we have a dipole field, the moment of the dipole being 2P = (£a 3 /377. 

The field decreases as 1/r 3 , and therefore the flux of the field through an infinitely remote 
surface (in the half-space z > 0) is zero. This means that all the lines of force passing through 
the aperture reach the upper side of the conducting plane. 

The distribution of charge on the conducting plane is given by 



r^l = - a 8<f> = + JLr tan -i_^- _ _£_1 



_ l rafi a 8<f> (£ 



where the ^= signs refer to the upper and lower sides of the plane respectively. According 
to the formula 

n 2 Z 2 



a 2 +£ | 

which relates £ to />, z, we have V£ == ±V(p 2 — <* 2 ) °n the plane z = 0. Thus the charge 
distribution on the lower side of the conducting plane is given by the formula 

® / • -i a _l a \ 

a= ~^rA 7T - sm P + v( P 2 -a 2 ))- 

As p -»■ oo we have a = — (£/4ir, as we should expect. On the upper side 

= C / a . _ x a\ 

° 47r2\V(p 2 -a 2 ) Sm p)' 

Problem 8. The same as Problem 7, but for a plane with a slit of width 26. 

Solution. The ay-plane with a slit along the x-axis may be regarded as the limit of the 
hyperbolic cylinders 

y 2 z 2 

b 2 -H ~~M = 

as 1 17 1 -> 0. These hyperbolic cylinders are one of the families of co-ordinate surfaces in a 
system of ellipsoidal co-ordinates with a ->• 00, c -* 0. The Cartesian co-ordinate 

* = V«H)/6. 

As in Problem 7, we seek a solution in the form <f> = —(£zF(£), obtaining for the function F 

f d£ 
F = constant X 



Here the coefficient and the constant of integration are determined by the conditions that 
F = and 1 for z ->- +00 and —00 respectively (i.e. for V£ -»■ + 00 and —00), and the 
final result is 

^l-tva+^o+vi] v\v\, 

lb 

where we now take Vf positive and the two signs =p correspond to the regions z > and 
z < 0. 

At large distances from the slit we have in the upper half -space £ ^ y 2 +z 2 = r 2 , and the 
potential is <f> S i&(£ V(\v\ £) = i&Wzfr 2 , i-e. the field of a two-dimensional dipole of moment 
i&b 2 per unit length of the slit (see the formula in §3, Problem 2). 

The distribution of charge on the conducting plane is given by 

sAv(y z -b 2 ) /" 



§5 The forces on a conductor 3 1 

§5. The forces on a conductor 

In an electric field certain forces act on the surface of a conductor. These 
forces are easily calculated as follows. 

The momentum flux density in an electric field in a vacuum is given by 
the Maxwell stress tensor :t 

— vile = —(i-E 2 ^*; — EiE]c). 

The force on an element df of the surface of the body is just the "flux" of 
momentum through it from outside, and is therefore arad/k = o^n^d/ (the 
sign is changed because the normal vector n is outwards and not inwards). 
The quantity oacnic is thus the force T s per unit area of the surface. Since, at 
the surface of a conductor, the field E has no tangential component, we 
obtain 

F s = n£ 2 /87r, (5.1) 

or, introducing the surface charge density a, 

F s = 2ir(T 2 n = $<rE. 

We therefore conclude that a "negative pressure" acts on the surface of a 
conductor; it is directed along the outward normal to the surface, and its 
magnitude is equal to the energy density in the field. 

The total force F on the conductor is obtained by integrating the force 
(5.1) over the whole surface: 

F = | (tfy&r) df. ( 5 - 2 ) 

Usually, however, it is more convenient to calculate this quantity from the 
general laws of mechanics, by differentiating, the energy °tt. The force, in 
the direction of a co-ordinate q, acting on a conductor is — dW/dq, where 
the derivative signifies the rate of change of energy when the body is trans- 
lated in the ^-direction. The energy must be expressed in terms of the 
charges on the conductors (which give rise to the field), and the differentia- 
tion is performed with the charges constant. Denoting this by the suffix e, 
we write 

F q = -{d%ldq) e . (5.3) 

Similarly, the projection, on any axis, of the total moment of the forces on 
the conductor is 

K=-(d<%ldiP) e , (5.4) 

where if) is the angle of rotation of the body about that axis. 



f See The Classical Theory of Fields, §4-8, Addison- Wesley Press, Cambridge (Mass.), 
1951 ; Pergamon Press, London, 1959. — cr^ is there denoted by T^p. 

In the present case we are applying this formula to a surface which does not precisely 
coincide with that of the body, but is some distance away, in order to exclude the effect of 
the field structure near the surface (see §1). 



32 Electrostatics of Conductors §5 

If, however, the energy is expressed as a function of the potentials of the 
conductors, and not of their charges, the calculation of the forces from the 
energy requires special consideration. The reason is that, to maintain 
constant the potential of a moving conductor, it is necessary to use other 
bodies. For example, the potential of a conductor can be kept constant by 
connecting it to another conductor of very large capacity, a "charge reser- 
voir". On receiving a charge e a , the conductor takes it from the reservoir, 
whose potential <f> a is unchanged on account of its large capacity, although 
its energy is reduced by e a j> a . When the whole system of conductors receives 
charges e a , the energy of the reservoirs connected to them changes by a 
total of —T,e a (f> a . Only the energy of the conductors, and not that of the 
reservoirs, appears in °tt. In this sense we can say that % pertains to a 
system which is not energetically closed. Thus, for a system of conductors 
whose potentials are kept constant, the part of the mechanical energy is 
played not by ^, but by 



#= W-^ecda- (5.5) 

Substituting (2.2), we find that °U and <% differ only in sign: 

% = - <%. (5.6) 

The force F q is obtained by differentiating $ with respect to q for constant 
potentials, i.e. 

F q = -{d%\h\ = W^?V (5.7) 

Thus the forces acting on a conductor can be obtained by differentiating % 
either for constant charges or for constant potentials, the only difference 
being that the derivative must be taken with the minus sign in the first case 
and with the plus sign in the second. 

The same result could be obtained more formally by starting from the 
differential identity 

d ^ = S <f> a de a - F q dq, (5.8) 

in which % is regarded as a function of the charges on the conductors and the 
co-ordinate q. This identity states that 8^jde a = <j> a and d%jdq= —F q . 
Using the variables <f> a instead of e a , we have 

d<# = -Xe a d<f> a -F q dq, (5.9) 

which gives (5.7). 

At the end of §2 we have discussed the energy of a conductor in a uniform 
external electric field. The total force on a conductor in a uniform field is, 
of course, zero. The expression for the energy (2.14) can, however, be 
used to determine the force acting on a conductor in a quasi-uniform field 



§5 The forces on a conductor 33 

<£, i.e. a field which varies only slightly over the dimensions of the conduc- 
tor, In such a field the energy can still be calculated, to a first approxima- 
tion, from formula (2.14), and the force F is the gradient of this energy: 

F = -grad <% = |a ifc Fgrad (<&<&). (5.10) 

The total torque K is in general non-zero even in a uniform external field. 
By the general laws of mechanics K can be determined by considering an 
infinitesimal virtual rotation of the body. The change in energy in such a 
rotation is related to K by h°ll = — K-S<]>, Si|> being the angle of the rota- 
tion. A rotation through an angle S*J> in a uniform field is equivalent to a 
rotation of the field through an angle — SiJ> relative to the body. The change 
in the field is S<£ = — S<p x <£, and the change in energy is 

S^=(a^/a©).S<£= -Sty.&xdWldH. 

But d^jd(& = —&>, as we see from a comparison of formulae (2.13) and 
(2.14). Hence S^ = — ^X©-S<1>, whence 

K= ^x<£, (5.11) 

in accordance with the usual expression given by the theory of fields in a 
vacuum. 

If the total force and torque on a conductor are zero, the conductor remains 
at rest in the field, and effects involving the deformation of the body (called 
electrostriction) become important. The forces (5.1) on the surface of the 
conductor result in changes in its shape and volume. Because the force is 
an extending one, the volume of the body increases. A complete determina- 
tion of the deformation requires a solution of the equations of the theory of 
elasticity, with the given distribution of forces (5.1) on the surface of the body. 
If, however, we are interested only in, the change in volume, the problem 
can be solved very simply. 

To do so, we must bear in mind that, if the deformation is slight (as in 
fact is . true for electrostriction), the effect of the change of shape on the 
change of volume is of the second order of smallness. In the first approxima- 
tion, therefore, the change in volume can be regarded as the result of defor- 
mation without change in shape, i.e. as a volume expansion under the action 
of some effective excess pressure Ap which is uniformly distributed over 
the surface of the body and replaces the exact distribution given by (5.1). 
The relative change in volume is obtained by multiplying Ap by the coeffi- 
cient of uniform expansion of the substance. The pressure Ap is given, 
according to a well-known formula, by the derivative of the electric energy 
% of the body with respect to its volume: Ap = —d°U\dVA 

Let the deforming field be due to the charged conductor itself. Then the 
energy °U = %e 2 jC, and the pressure is Ap = —\e*dC~ x \dV. For a given 



t The quantity thus determined is the pressure exerted on the surface by the body itself; 
the pressure acting on the surface from outside is obtained by changing the sign. 



34 Electrostatics of Conductors §5 

shape, the capacity of the body (having the dimensions of length) is propor- 
tional to the linear dimension, i.e. to V 1/3 . Hence 

tAp = e*l6CV = e<f>/6V. (5.12) 

If an uncharged conductor is situated in a uniform external field ©, its 
energy is given by formula (2.14). The extending pressure is therefore 

A/> = £o«*(gi<g*. (5.13) 



PROBLEMS 
Problem 1. A small conductor of capacity c (equal in order of magnitude to its dimension) 
is at a distance r from the centre of a spherical conductor of large radius a (^> c). The distance 
r—a from the conductor to the surface of the sphere is supposed large compared with c, 
but not large compared with a. The two conductors are joined by a thin wire, so that they are 
at the same potential (f>. Determine the force of their mutual repulsion. 

Solution. Since the conductor c is small, we can suppose that its potential is the sum of 
the potential <f>a/r at a distance r from the centre of the large sphere and the potential e\c 
due to the charge e on the conductor itself. Hence <f> = <f>a/r+e/c, or e = c<f> (1 —a/r). The 
required force of interaction F is the Coulomb repulsion between the charge e on the con- 
ductor and the charge a<i> on the sphere : 



F = ^ 



H)- 



This expression is correct to within terms of higher order in c. Thus the small conductor is 
repelled from the sphere with a force which decreases as it approaches the surface. 

Problem 2. A charged conducting sphere is cut in half. Determine the force of repulsion 
between the hemispheres, t 

Solution. We imagine the hemispheres separated by an infinitely narrow slit, and deter- 
mine the force F on each of them by integrating over the surface the force (E 2 /8v) cos d, 
which is the projection of (5.1) on a direction perpendicular to the plane of separation of the 
hemispheres. In the slit E = 0, and on the outer surface E = e[a?, where a is the radius of 
the sphere and e the total charge on it. The result is F = e 2 /8a 2 . 

Problem 3. The same as Problem 2, but for an uncharged sphere in a uniform external 
field (H perpendicular to the plane of separation. 

Solution. As in Problem 2, except that the field on the surface of the sphere is 
E = 3 (£ cos (§3, Problem 1). The required force is F = 9a 2 (£ 2 /16. 

Problem 4. Determine the change in volume and in shape of a conducting sphere in a 
uniform external electric field. 

Solution. The change in volume AV/V = Ap/K, where K is the modulus of volume 
expansion of the material, and Ap is given by formula (5.13). For a sphere, otjfc = 8«a 
= 3&ijfc/4ir (§3, Problem 1), so that AV/V = 2<&j&-nK. 

As a result of the deformation, the sphere is changed into a prolate spheroid. To determine 
the eccentricity, we may regard the deformation as a uniform pure shear in the volume of 
the body, just as, to determine the change in the total volume, we regarded it as a uniform 
volume expansion. 

The condition of equilibrium for a deformed body may be formulated as requiring that 
the sum of the electrostatic and elastic energies should be a minimum. The former is, by 
(2.12) and (4.26), 

<% = 1_(£2 ~ I(g2 

Srm - 8tt lOir R 



f In Problems 2 and 3 we assume that the hemispheres are at the same potential. 



§5 The farces on a conductor 35 

where R is the original radius of the sphere, a and b the semiaxes of the spheroid, and 
n ^ J— 4 (a—b)/15R is the depolarisation coefficient (see (4.33).) 

Since the deformation is axially symmetrical about the direction of the field (the *-axis), 
only the components u xx and u yy = u zz of the strain tensor are non-zero. Since we are 
considering equilibrium with respect to a change in shape, we can regard the volume as 
unchanged, i.e. uu = 0. Hence the elastic energy may be writtenf 

^el = i«** °HC V = K °XX — Oyy)(u X X — Uyy) V, 

where aue is the elastic stress tensor. We have a xx — °yy = 2p.(u X x —u yy ), where /* is the modu- 
lus of rigidity of the material, and u X x—u y y = (a—b)IR. Hence 

Making the sum <% eB + 'Wei a minimum, we have (a—b)[R — 9<E 2 /407r/i. 

Problem 5. Find the relation between frequency and wavelength for waves propagated 
on a charged plane surface of a liquid conductor (in a gravitational field). Obtain the condi- 
tion for this surface to be stable (Ya. I. Frenkel', 1935). 

Solution. Let the wave be propagated along the x-axis, with the sr-axis vertically upwards. 
The vertical displacement of points on the surface of the liquid is £ = ae*( fca; -«t). When the 
surface is at rest, the field above it is E z = E = 4ttcto, and its potential <f> = — 4iraoz, where 
<ro is the surface charge density. The potential of the field above the oscillating surface can 
be written as <f> = -~4iraoz+<f>i, with 0i = constant X e**^ - "^ e~ kz , <f>i being a small correc- 
tion which satisfies the equation A<f>i = and vanishes for z -> oo. On the surface itself, 
the potential must have a constant value, which we take to be zero, and so <£i = Artaot, for 
* = Q. 

According to (5.1), an additional negative pressure acts on the charged surface of the liquid ; 
this pressure is, as far as terms of the first order in <f>i, E 2 I&it ^ E z 2 I8it ^ 2iroo 2 +[kao<f>i]z~o 
=2irao 2 +4-iroo 2 k£. The constant term 2ttcto 2 is of no importance, since it can be included in 
the constant external pressure. 

The consideration of the hydrodynamical motion in the wave is entirely analogous to the 
theory of capillary waves J, differing only by the presence of the additional pressure mentioned 
above. At the surface of the liquid we have the boundary condition pg£+ p[dQ>l dt]i-o— 
~a.d 2 Hdx 2 —\irao 2 kt, = 0, where a is the surface-tension coefficient, p the density of the 
liquid, and fl> its. velocity potential. <I> and £ are also related by dt/dt = [dQ>ldz]t-o. Sub- 
stituting in these two relations £ = ag*(**-<>>*) and O = Ae^"*'^ and eliminating a and A, 
we find the required relation between k and to ; 

a> 2 = k(gp-47T<ro 2 k+OLk 2 )lp. (1) 

If the surface of the liquid is to be stable, the frequency o> must be real for all values of k 
(since otherwise there would be complex <o with a positive imaginary part, and the factor 
e -ib>t w ould increase indefinitely). The condition for the right-hand side of (1) to be positive 
is (4ir<7o 2 ) 2 — 4gpa < 0, or ao 4 < gpa/47r 2 . This is the condition for stability. 

Problem 6. Find the condition of stability for a charged spherical drop (Rayleigh, 1882). 

Solution. The sum of the electrostatic and surface energies of the drop is ^ «= e 2 /2C + <*S, 
where a is the surface-tension coefficient of the liquid, C the capacity of the drop and S its 
surface area. Instability occurs (with increasing e) with respect to deformation of the sphere 
into a spheroid, and does so when <^ becomes a decreasing function of the eccentricity (for 
a given volume). The spherical shape always corresponds to an extremum of % ; the stability 
condition is therefore [d 2< %ld(a—b) 2 ] a =b > 0, where a and b are the semiaxes of the spheroid, 
and the differentiation is carried out with ab 2 = constant. Using the formula for the surface 
of a spheroid and (4.18) for its capacity, we find after a somewhat lengthy calculation 
e 2 < 16ira s a.. 



t See Theory of Elasticity, §4, Pergamon Press, London, 1959. 
% See Fluid Mechanics, §61, Pergamon Press, London, 1959. 



CHAPTER II 

ELECTROSTATICS OF DIELECTRICS 

§6. The electric field in dielectrics 

We shall now go on to consider a constant electric field in another class of 
substances, namely dielectrics. The fundamental property of dielectrics is 
that a constant current cannot flow in them. Hence the constant electric 
field need not be zero, as in conductors, and we have to derive the equations 
which describe this field. One equation is obtained by averaging equation 
(1.3), and is again 

curl E = 0. (6.1) 

A second equation is obtained by averaging the equation div e = Airp : 

div E = 4w/5. (6.2) 

Let us suppose that no charges are brought into the dielectric from out- 
side, which is the most usual and important case. Then the total charge 
in the volume of the dielectric is zero; even if it is placed in an electric 
field we have j pdV — 0. This integral equation, which must be valid for 
a body of any shape, means that the average charge density can be written 
as the divergence of a certain vector, which is usually denoted by — P: 

p = -divP, (6.3) 

while outside the body P = 0. For, on integrating over the volume bounded 
by a surface which encloses the body but nowhere enters it, we find jpdV 
= — JdivP dV = — <j>P«df = 0. P is called the dielectric polarisation, or 
simply the polarisation, of the body. A dielectric in which P differs from zero 
is said to be polarised. The vector P determines not only the volume charge 
density (6.3), but also the density a of the charges on the surface of the 
polarised dielectric. If we integrate formula (6.3) over an element of volume 
lying between two neighbouring unit areas, one on each side of the di- 
electric surface, we have, since P = on the outer area (cf. the derivation of 
formula (1.9)), 

o = Pn, (6.4) 

where P n is the component of the vector P along the outward normal to the 
surface. 

To see the physical significance of the quantity P itself, let us consider 
the total dipole moment of all the charges within the dielectric; unlike the 

36 



§6 The electric field in dielectrics 37 

total charge, the total dipole moment need not be zero. By definition, it is 
the integral J" tpdV. Substituting p from (6.3) and again integrating over 
a volume which includes the whole body we have 

j rpdV = -j r div FdV = - j> r(df.P) + j (P-grad)r dV. 

The integral over the surface is zero, and in the second term we have 
(P-grad)r = P, so that 

jrpdV = JTdV. (6.5) 

Thus the polarisation vector is the dipole moment (or electric moment) per 
unit volume of the dielectric, t 

Substituting (6.3) in (6.2), we obtain the second equation of the electro- 
static field in the form 

div D = 0, (6.6) 

where we have introduced a quantity D defined by 

D = E + 4ttP, (6.7) 

called the electric induction.% The equation (6.6) has been derived by 
averaging the density of charges in the dielectric. If, however, charges not 
belonging to the dielectric are brought in from outside (we shall call these 
extraneous charges), then their density must be added to the right-hand side 
of equation (6.6) : 

divD = 47rp ex . (6.8) 

On the surface of separation between two different dielectrics, certain 
boundary conditions must be satisfied. One of these follows from the equa- 
tion curl E = 0. If the surface of separation is uniform as regards physical 
properties, || this condition requires the continuity of the tangential compor 
nent of the field: 

Ei* = E 2t ; (6.9) 

cf. the derivation of the condition (1.7). The second condition follows from 
the equation div D = 0, and requires the continuity of the normal compo- 
nent of the induction : 

D ln = D 2n - (6.10) 



t It should be noticed that the relation (6.3) inside the dielectric and the condition P = 
outside do not in themselves determine P uniquely; inside the dielectric we could add to 
P any vector of the form curl f. P can be completely determined only by establishing its 
connection with the dipole moment. 

X Sometimes the electric displacement, a term due to Maxwell, but one which is obsolete. 

|| That is, as regards composition of the adjoining media, temperature, etc. If the 
dielectric is a crystal, the crystallographic direction of the surface must be constant, i.e. the 
surface must be a plane. 



38 Electrostatics of Dielectrics §7 

For a discontinuity in the normal component D n = D z would involve an 
infinity of the derivative dD z \dz, and therefore of divD. 

At a boundary between a dielectric and a conductor, E t = 0, and the condi- 
tion on the normal component is obtained from (6.8): 

E t = 0, D n = ±tto, (6.11) 

where a is the charge density on the surface of the conductor. 

§7. The dielectric permeability 

In order that equations (6.1) and (6.6) should form a complete set of 
equations determining the electrostatic field, they must be supplemented 
by a relation between the induction D and the field E. In the great majority 
of cases this relation may be supposed linear. It corresponds to the first 
terms in an expansion of D in powers of E, and its correctness is due to the 
smallness of the external electric fields in comparison with the internal 
molecular fields. 

The linear relation between D and E is especially simple in the most im- 
portant case, that of an isotropic dielectric. It is evident that, in an isotropic 
dielectric, the vectors D and E must be in the same direction. The linear 
relation between them is therefore a simple proportionality :t 

D = eE. (7.1) 

The coefficient e is the dielectric permeability or dielectric constant of the 
substance and is a function of its thermodynamic state. 

As well as the induction, the polarisation also is proportional to the field: 

P = kEe= ( € -1)E/4t7. (7.2) 

The quantity k is called the polarisation coefficient of the substance, or its 
dielectric susceptibility. Later (§14) we shall show that the dielectric per- 
meability always exceeds unity; the polarisability, accordingly, is always 
positive. The polarisability of a rarefied medium (a gas) may be regarded as 
proportional to its density. 

The boundary conditions (6.9) and (6.10) on the surface separating two 
isotropic dielectrics become 

E«l = E^2, ei-Enl = e2-E"«2. (7.3) 

Thus the normal component of the field is discontinuous, changing in 
inverse proportion to the dielectric permeability of the medium. 



t It should be mentioned, however, that this relation, which assumes that D and E vanish 
simultaneously, is, strictly speaking, valid only in dielectrics which are homogeneous as 
regards physical properties (composition, temperature, etc.). In inhomogeneous bodies D 
may be non-zero even when E = 0, and is determined by the gradients of thermodynamic 
quantities which vary through the body. The corresponding terms, however, are very 
small, and in practice are of no importance. We shall therefore use the relation (7.1) in 
what follows, even for inhomogeneous bodies. 



§7 The dielectric permeability 39 

In a homogeneous dielectric, e = constant, and then it follows from 
div D = that div P = 0. By the definition (6.3) this means that the 
volume charge density in such a body is zero (but the surface density (6.4) 
is in general not zero). On the other hand, in an inhomogeneous dielectric 
we have a non-zero volume charge density 

€-1 1 €-1 1 

p = -divP = -div D = D*grad = E- grade. 

4n-e 4n- e 4n-e 

If we introduce the electric field potential by E = — grad <f>, then equa- 
tion (6.1) is automatically satisfied, and the equation divD= divcE = 
gives 

div (e grad <f>) = 0. (7.4) 

This equation becomes the ordinary Laplace's equation only in a homo- 
geneous dielectric medium. The boundary conditions (7.3) can be re- 
written as the following conditions on the potential: 

(7.5) 
eidfa/dn = €2d<f>2/dn; 

the continuity of the tangential derivatives of the potential is equivalent to 
the continuity of <£ itself. 

In a dielectric medium which is piecewise homogeneous, equation (7.4) 
reduces in each homogeneous region to Laplace's equation A<£ = 0, so 
that the dielectric permeability appears in the solution of the problem only 
through the conditions (7.5). These conditions, however, involve only the 
ratio of the dielectric permeabilities of two adjoining media. In particular, 
the solution of an electrostatic problem for a dielectric body of permeability 
€2, surrounded by a medium of permeability ei, is the same as for a body of 
permeability €2Jei, surrounded by a vacuum. 

Let us consider how the results obtained in Chapter I for the electrostatic 
field of conductors will be modified if these conductors are not in a vacuum 
but in a homogeneous and isotropic dielectric medium. In both cases the 
potential distribution satisfies the equation A<£ = 0, with the boundary 
condition that <£ is constant on the surface of the conductor, and the only 
difference is that, instead of E n = — d<f>/ dn = Ana, we have 

D n = - € d<f>/dn = 4t7(t, (7.6) 

giving the relation between the potential and the surface charge. Hence it 
is clear that the solution of the problem of the field of a charged conductor 
in a vacuum gives the solution of the same problem with a dielectric in place 
of the vacuum if we make the formal substitution <£ -> e<f>, e -> e or j> -> <f> y 
e -> e/e. For given charges on the conductors, the potential and the field 
are reduced by a factor e in comparison with their values in a vacuum. 
This reduction in the field can be explained as the result of a partial "screen- 
ing" of the charge on the conductor by the surface charges on the adjoining 
4 



40 



Electrostatics of Dielectrics 



§7 



polarised dielectric. If, on the other hand, the potentials of the conductors are 
maintained, then the field is unchanged but the charges are increased by a 
factor e. 

Finally, it may be noted that in electrostatics we may formally regard a 
conductor (uncharged) as a body of infinite dielectric permeability, in the 
sense that its effect on an external electric field is the same as that of a di- 
electric (of the same form) as e -> oo. For, since the boundary condition on 
the induction D is finite, D must remain finite in the body even for e ->■ oo. 
This means that E ->■ 0, in accordance with the properties of conductors. 



PROBLEMS 

Problem 1. Determine the field due to a point charge e at a distance h from a plane boun- 
dary separating two different dielectric media. 

Solution. Let O be the position of the charge e in medium 1, and O' its image in the plane 
of separation, situated in medium 2 (Fig. 10). We shall seek the field in medium 1 in the form 
of the field of two point charges, e and a fictitious charge e' at O' (cf. the method of images, 
§3): <f>i = e/eir+e'/eyr', where r and r' are the distances from O and O' respectively. In 
medium 2 we seek the field as that of a fictitious charge e" at O : fa = e"/ezr. On the boun- 
dary plane (r = r') the conditions (7.5) must hold, leading to the equations e—e' = e", 
(e+e')lei = e"lez, whence 



e' = e(ei— e2)/(ei+e2), e" = 2e2e/(ei+e2). 



(1) 




Fig. 10 



For €2 -> oo we have e' — —e, <f>2 = 0, i.e. the result obtained in §3 for the field of a 
point charge near a conducting plane. 

The force acting on the charge e (the "image force") is 



ee' _ t e \ 2 < 
~ (2A) 2 €i ~ \2h) Hi 



(ei-K 2 )' 
F > corresponds to repulsion. 

Problem 2. The same as Problem 1, but for an infinite charged straight wire parallel to 
a plane boundary surface at a distance h. 

Solution. As in Problem 1, except that the field potentials in the two media are 
<f>i = — (2e/ei) log r— (le'/ei) log r', <j>z — — (2e"/e2) logr, where e, e', e" are the charges per 
unit length of the wire and of its images, and r, r' are the distances in a plane perpendicular 
to the wire. The same expressions (1) are obtained for e', e" , and the force on unit length of 
the wire is F = 2ee'[2hei = e 2 (ei— €2)/Aei(ei+c2). 



§7 The dielectric permeability 41 

Problem 3. Determine the field due to an infinite charged straight wire in a medium with 
dielectric constant ei, lying parallel to a cylinder of radius a and dielectric constant €2, at a 
distance b (> a) from its axis-t 

Solution. We seek the field in medium 1 as that produced in a homogeneous dielectric 
(with constant ei) by the actual wire (passing through O in Fig. 11), with charge e per unit 
length, and two fictitious wires with charges e' and —e' per unit length, passing through A 
and O' respectively. The point A is at a distance a 2 Jb from the axis of the cylinder. Then, 
for all points on the circumference, the distances r and r' from O and A are in a constant 
ratio r'jr = a/b, and so it is possible to satisfy the boundary conditions on this circumference. 
In medium 2 we seek the field as that produced in a homogeneous medium (with constant ea) 
by a fictitious charge e" on the wire passing through O. 

00' --b 
AO' =a z /b 




Fig. 11 



The boundary conditions on the surface of separation are conveniently formulated in 
terms of the potential <f> (E = — grad <f>) and the vector potential A (cf. §3), defined by 
D = curl A (in accordance with the equation div D = 0). In a two-dimensional problem, 
A is in the ar-direction (perpendicular to the plane of the figure). The conditions of con- 
tinuity for the tangential components of E and the normal component of D are equivalent 
to <f>i = fa, A\ — Az. 

For the field of a charged wire we have in polar co-ordinates r, 0: $ = — (2e/e) logr+ 
constant, A = 2e0+constant; cf. (3.18). Hence the boundary conditions are 

2 2e" 

— (— e log r—e log r'+e' log a) = log r+constant, 

ei ea 

2[e9+e'0'-e'(6+0')] = 2e"9, 

where the angles are as shown in Fig. 11, and we have used the fact that OO'B and BO' A 
are similar triangles. Hence ez(e-\-e') = cie", e—e'= e", and the expressions for e' and e" 
are again formulae (1) of Problem 1. 
The force acting on unit length of the charged wire is parallel to OO', and is 



2ee'{ 1 1 \ 

F = eE = ( 1 = — 

ei \OA OO') ei( 



2e 2 (€i-e 2 )a 2 



i(ei+e 2 )6(fc2-a2)' 

F > corresponds to repulsion. 

Problem 4. The same as Problem 3, but for the case where the wire is inside a cylinder of 
dielectric permeability e2 (b < a). 



t The corresponding problem of a point charge near a dielectric sphere cannot be solved 
in closed form. 



42 Electrostatics of Dielectrics §8 

Solution. We seek the field in medium 2 as that due to the actual wire, of charge e per 
unit length (O in Fig. 12), and a fictitious wire of charge e' per unit length passing through 
A, which is now outside the cylinder. In medium 1 we seek the field as that of wires with 
charges e" and e—e" passing through O and O' respectively. By the same method as in the 
preceding problem we find e' = — e(ei— e^Kei+ez), e" = 2eiel(ei + e2). For e% > eithe 
wire is repelled from the surface of the cylinder by a force 

lee' 1 2e 2 (e 2 -€i)& 

F = 



e 2 OA e 2 (ei+« 2 )(a2-&2) 




Fig. 12 

Problem 5. Show that the field potential 4>a(xb) at a point xb in an arbitrary inhomogeneous 
dielectric medium, due to a point charge e at Xa, is equal to the potential 4>b(ta) at xa due 
to the same charge at xb. 

Solution. The potentials 4>a{t) and <}>b{t) satisfy the equations 

div(egrad^) = — 47re8(r— xa), div(e grades) = — 4weS(r— xb)- 
Multiplying the first by <f>B and the latter by <f>A and subtracting, we have 

div (4>b e grad <I>a)— div (<}>a e grad </>b) = — 4ne8(x— r^)^(r)+4«8(r- XB)<f>A(r). 
Integration of this equation over all space gives the required relation : 

4>a{tb) = <I>b(xa). 



§8. A dielectric ellipsoid 

The polarisation of a dielectric ellipsoid in a uniform external electric 
field has some unusual properties which render this example particularly 
interesting. 

Let us consider first a simple special case, that of a dielectric sphere in an 
external field ©. We denote its dielectric constant by € tf) , and that of the 
medium surrounding it by e (e) . We take the origin of spherical co-ordinates 
at the centre of the sphere, and the direction of © as the polar axis, and seek 
the field potential outside the sphere in the form <£ (e) = ©*r + ^4©'r/r 3 ; the 
first term is the potential of the external field imposed, and the second, 
which vanishes at infinity, gives the required change in potential due to the 
sphere (cf. §3, Problem 1, solution). Inside the sphere, we seek the field 
potential to the form <£ ( *> = — i?<£«r, the only function which satisfies 
Laplace's equation, remains finite at the centre of the sphere, and depends 
only on the constant vector <& (which is the only parameter of the problem). 

The constants A and B are determined by the boundary conditions on 
the surface of the sphere. It may be seen at once, however, that the field 



§8 A dielectric ellipsoid 43 

in the sphere E tt) = Bf& is uniform and differs only in magnitude from the 
applied field ©. 

The boundary condition of continuity of the potential gives 
E«> = (£(1 — AIR 2 ), where R is the radius of the sphere, and the 
condition of continuity of the normal component of the induction gives 

D«> = efe>(&(l + 2ia//P). 

Eliminating A from these two equations, we obtain 

KD«> + 2e(«>E«>) = €<«>(£ (8.1) 

or, substituting D«> = €«>E (<) , 

E«> = 3e<e>(£/(2 e <a+e<«). (8.2) 

The problem of an infinite dielectric cylinder in an external field per- 
pendicular to its axis is solved in an entirely similar manner (cf. §3, Prob- 
lem 2). The field inside the cylinder, like that inside the sphere in the above 
example, is uniform. It satisfies the relation 

£(D«>+e<e>E«>) = €<*>©, (8.3) 

or 

E«> = 2efeHE/(€W +€<»). (8.4) 

The relations (8.1) and (8.3), in which the dielectric constant e W) of the 
sphere or cylinder does not appear explicitly, are particularly important 
because their validity does not depend on a linear relation between E and 
D within the body; they hold whatever the form of this relation (e.g. for 
anisotropic bodies). The analogous relations 

E» = © (8.5) 

or a cylinder in a longitudinal field and 

D® = € te>© (8.6) 

for a flat plate in a field perpendicular to it are similarly valid; these 
relations are evident at once from the boundary conditions. 

The property of causing a uniform field within itself on being placed in a 
uniform external field is found to pertain to any ellipsoid, whatever the ratio 
of the semiaxes a, b> c. The problem of the polarisation of a dielectric ellip- 
soid is solved by the use of ellipsoidal co-ordinates, in the same way as the 
corresponding problem for a conducting ellipsoid in §4. 

Let the external field be again in the ^-direction. The field potential out- 
side the ellipsoid may again be sought in the form (4.22): <f>' e = <hF(£)> 
with the function F (£) given by (4.23). Such a function cannot, however, 
appear in the field potential <fn inside the sphere, since it does not satisfy the 
condition that the field must be finite everywhere inside the ellipsoid. For let 
us consider the surface £ = — c 2 , which is an ellipse in the #y-plane, lying 
within the ellipsoid. For f -+ —c 2 , the integral (4.23) behaves as ^(g+c 2 ). 



44 Electrostatics of Dielectrics §8 

The field, i.e. the potential gradient, therefore behaves as l/\/(l+c 2 )> and 
becomes infinite at £ = — c 2 . Thus the only solution suitable for the field 
inside the ellipsoid is F(£) = constant, so that <f>t must be sought in the 
form fa = Bfa. We see that the potential fa differs only by a constant 
factor from the potential fa of the uniform field. In other words, the field 
inside the ellipsoid is also uniform. 

We shall not pause to write out the formulae for the field outside the 
ellipsoid, which are of little interest. The uniform field inside the ellipsoid 
can be found without actually writing out the boundary conditions, by using 
some results already known. 

Let us first suppose that the ellipsoid is in a vacuum (c te) = 1). Then there 
must be a linear relation between the vectors E tt) , D ft) and © (which are all 
in the ar-direction), of the form aE x +bD x = (£», where the coefficients a, b 
depend only on the shape of the ellipsoid, and not on its dielectric per- 
meability e (<) . The existence of such a relation follows from the form of 
the boundary conditions, as we saw above in the examples of the sphere and 
the cylinder. 

To determine a and b we notice that, in the trivial particular case € W) = 1, 
we have simply E = D = <£, and so a+b = 1. Another particular case for 
which the solution is known is that of a conducting ellipsoid. In a conductor 
E (i) = 0, and the induction D< f) , though it has no direct physical significance, 
may be regarded formally as being related to the total dipole moment of the 
ellipsoid by D = 4nP = ±rr&>jV. According to (4.26) we then have 
D x = (Sy» (aj) , i.e. b = »<*>, and soa= 1— « te) . Thus we conclude thatt 

(i - «<*>) m x + 1& m x = <s x . (8.7) 

Similar relations, but with other coefficients, hold for the fields in the y and 
z directions. Like the particular formulae (8.1) and (8.3), they are valid 
whatever the relation between E and D inside the ellipsoid. 

The field inside the ellipsoid, when © is in the ^-direction, is found from 
(8.7) by putting £>«>* = €«>£«>*: 

E% = Ck/[1 + (€«>- l)n<*>], (8.8) 

and the total dipole moment of the ellipsoid is 

g; = VP X = (c»-l)FE«y4*r = \abc{^-\)% x f[\ + {^-\)nM]. (8.9) 

If the field © has components along all three axes, then the field inside the 
ellipsoid is still uniform, but in general not parallel to ©. For an arbitrary 
choice of co-ordinate axes we can write the relation (8.7) in the general form 

£»! + ««(/>»*--£»*) = <&. (8-10) 



t This result can also be written El% = ®z — 4mtWP x . The quantity 4mt^P x is some- 
times called the depolarising field. A similar formula holds for a magnetised ellipsoid in a 
uniform external magnetic field (see §27). In this case «<*>, «<»>, n<*> are called demagnetisa- 
tion coefficients. 



§9 The dielectric permeability of a mixture 45 

The transition to the case where the dielectric permeability of the medium 
differs from unity is effected by simply replacing e (,) by € W) /e (e) . Then 
formula (8.7) becomes 

(l-n^)^E% + n^m> x = <m x . (8.11) 

This formula can be applied, in particular, to the field inside an ellipsoidal 
cavity in an infinite dielectric medium. In this case e (<) = 1. 

PROBLEMS t 

Problem 1. Determine the torque on a spheroid in a uniform electric field. 

Solution. According to the general formula (16.13), the torque on an ellipsoid is 
K = & X ©, where 6* is the dipole moment of the ellipsoid. In a spheroid, the vector & is 
in a plane passing through the axis of symmetry and the direction of (J. The torque is per- 
pendicular to this plane, and a calculation of its magnitude from formulae (8.9) gives 

(*-l)2|l-3«|(E 2 Fsin2a 
K = 



87r[ft€+ 1 — «][(1 — n)e+ 1 +n] ' 

where a is the angle between the direction of © and the axis of symmetry of the spheroid, 
and n is the depolarisation coefficient along the axis (so that the depolarisation coefficients 
in the directions perpendicular to the axis are $(1 —it)). The torque is directed so that it 
tends to turn the axis of symmetry of a prolate (n < \) or oblate (w > \) ellipsoid parallel or 
perpendicular to the field respectively. 

For a conducting ellipsoid (e -*■ oo) we have 

K = J 1 " 3 " 1 V<& sin 2a. 
87r?i(l — n) 

Problem 2. A hollow dielectric sphere (of dielectric constant e and internal and external 
radii b and a) is in a uniform external field (£. Determine the field in the cavity. 

Solution. As above in the problem of a continuous sphere, we seek the field potentials 
in the vacuum outside the sphere (region 1) and in the cavity (region 3) in the forms 
<f>i = — (E cos 6 (r—A/r 2 ), <f>z = —B&r cos 0, and that in the dielectric (region 2) as ^2 
= —CCS cos {r—Djr 3 ), where A, B, C, D are constants determined from the conditions of 
continuity of <f> and e d<f>fdr at the boundaries 1—2 and 2—3. Thus the field E 3 = Bd in 
the cavity is uniform, but the field E2 in the sphere is not. A calculation of the constant gives 
the result 

E 3 = 9e©/[(e+2)(2e+l)-2(e-l)2(6/a)3]. 

Problem 3. The same as Problem 2, but for a hollow cylinder in a uniform transverse 
field.} 

Solution. As in Problem 2, with the result 

E 3 = 4ee/[( € +l)2-(e-l)2(6/a)2]. 

§9. The dielectric permeability of a mixture 

If a substance is a finely dispersed mixture (an emulsion, powder mixture, 
etc.), we can consider the electric field averaged over volumes which are 
large compared with the scale of the inhomogeneities. The mixture is a 
homogeneous and isotropic medium with respect to such an average field, 

t In these three Problems the body is assumed to be in a vacuum. 
X In a longitudinal field the solution is clearly E3 = ffi. 



46 Electrostatics of Dielectrics {9 

and so may be characterised by an effective dielectric permeability, which 
we denote by e m ix. If E and D are the field and induction averaged in this 
way, then, by the definition of e m ix, 

T> = e mix E. (9.1) 

If all the particles in the mixture are isotropic, and the differences in their 
dielectric permeabilities are small in comparison with e itself, it is possible 
to calculate emix in a general form which is correct as far as terms of the 
second order in these differences. 

We write the local field as E = E + SE, and the local dielectric permeability 
as e + Se, where 

i = (l/V)jedV (9.2) 

is obtained by averaging over the volume. Then the mean induction is 



D = (e + S e )(E + SE) = e'E + SeSE, (9.3) 

since the mean values of Se and SE are zero by definition. In the zero-order 
approximation e m ix = e"; the first non-zero correction term will, of course, be 
of the second order in Se, as we see from (9.3). 

From the non-averaged equation div D = we have, as far as small terms 
of the first order, 

div[(e + Se)(E + SE)] = edivSE + E-gradSe = 0, 

or, substituting SE = — grad S<£, i /\8<f> = E-grad S e . Taking the gradient, 
we have 

ASE = -(l/e-)(E.grad)gradSe. (9.4) 

The averaging of the product SeSE in (9.3) is done in two stages. We first 
average over the volume of particles of a given kind, i.e. for a given Se. 
The value of SE thus averaged is easily obtained from equation (9.4): on 
account of the isotropy of the mixture as a whole, the operator 8 2 ldxj8xjc 
on the right-hand side of (9.4) becomes, after averaging, JSifcA, so that we 
have ASE = — (l/3e)EA8e, whence SE = -(l/3e")ESe. Multiplying 
by Se and effecting the final averaging over all components of the mixture, 
we obtain SeSE = — (l/3e)E(Se) 2 . Finally, substituting this expression in 
(9.3) and comparing with (9.1), we have the required result: 

emix = e--(l/3e")(Se)2; (9.5) 

This formula can be written in another manner if we put 

/ (Se)2\ 

this is accurate to terms of the second order. Then 

emix* = el (9.6) 



§10 Thermodynamic relations for dielectrics in an electric field 47 

Thus we can say that, in this approximation, the cube root of e is additive. 

PROBLEM 

Determine the dielectric permeability of an emulsion of low concentration but with an 
arbitrary difference between the dielectric permeabilities of the medium (ei) and the disperse 
phase (€2). 

Solution. In the integral 

y f (D-eiE) dV = D-eiE 

the integrand is zero except within particles of the emulsion. It is therefore proportional to 
the volume concentration c of the emulsion, and in calculating it we can assume that the par- 
ticles are in an external field which equals the mean field £. Assuming the particles spherical 
and using formula (8.2), we obtain for the proportionality coefficient between D and E 

€mix = «l+3cei(e2 — ei)/(e2+2ei). 

This formula is correct to terms of the first order in c. When ei and €2 are nearly equal it is 
the same (to the first order in c and the second in *%— €i) as the result given by formula (9.6) 
for small c. 

§10. Thermodynamic relations for dielectrics in an electric field 

The question of the change in thermodynamic properties owing to the 
presence of an electric field does not arise for conductors. Since there is no 
electric field inside a conductor, any change in its thermodynamic properties 
amounts simply to an increase in its total energy by the energy of the field 
which it produces in the surrounding space, t This quantity is quite inde- 
pendent of the thermodynamic state (and, in particular, of the temperature) 
of the body, and so does not affect the entropy, for example. 

On the other hand, an electric field penetrates into a dielectric and so has 
a great effect on its thermodynamic properties. To investigate this effect, 
let us first determine the work done on a thermally insulated dielectric 
when the field in it undergoes an infinitesimal change. 

The electric field in which the dielectric is placed must be imagined as due 
to various external charged conductors, and the change in the field can then 
be regarded as resulting from changes in the charges on these conductors.^ 
Let us suppose for simplicity that there is only one conductor, of charge e 
and potential <f>. The work which must be done to increase its charge by an 
infinitesimal amount he is 

SR = <f>8e; (10.1) 

this is the mechanical work done by the given field on a charge Se brought 
from infinity (where the field potential is zero) to the surface of the conductor, 



t We here neglect the energy of the attachment of the charge to the substance of the 
conductor ; this will be discussed in §22. 

% The final results which we shall obtain involve only the values of the field inside the 
dielectric, and therefore are independent of the origin of the field. For this reason there is 
no need for special discussion of the case where the field is produced, not by charged con- 
ductors, but (for instance) by extraneous charges placed in the dielectric itself or by pyro- 
electric polarisation of it (§13). 



48 Electrostatics of Dielectrics §10 

i.e. through a potential difference of <f>. We shall put 8R in a form which 
is expressed in terms of the field in the space filled with dielectric which 
surrounds the conductor. 

If D n is the component of the electric induction vector in the direction of 
the normal to the surface of the conductor (out of the dielectric and into the 
conductor), then the surface charge density on the conductor is —DuJAtt, 
so that 

Since the potential <f> is constant on the surface of the conductor, we can write 
8R = <f>8e = I^SD-df = f div(<f>8D)dV. 

4-7T J 4"7T J 

The last integral is taken over the whole volume outside the conductor. 
Since the varied field, like the original field, must satisfy the field equations, 
we have div 3D = 0, and so div (<£SD) = ^ div SD + SD»grad <f> = — E»SD. 
Thus the following important formula is obtained: 

8R = J"(E.8D/4ir)dF. (10.2) 

It should be emphasised that the integration in (10.2) is over the whole field, 
including the vacuum if the dielectric does not occupy all space outside the 
conductor. 

The work done on a thermally insulated body is just the change in its 
energy at constant entropy. Hence the expression (10.2) must be included 
in the thermodynamic relation which gives the infinitesimal change in the 
total energy of the body; the latter contains also the energy of the electric 
field. Denoting the total energy by ^, we therefore have 

8<% = T8#>+ — f E.SDdF, (10.3) 

AttJ 

where T is the temperature of the body and S? its entropy, t 
Accordingly we have for the total free energy % 3F = fy—TS? 

8& = -#>8T+— (E-8DdV. (10.4) 

4W 

Similar thermodynamic relations can be obtained for the quantities per- 
taining to unit volume of the body. Let U, S and p be the internal energy, 



t The body in general becomes inhomogeneous in an electric field, and so the volume 
(whose differential is usually included in the expression for 8<#) no longer characterises the 
state of the body. 

X This quantity is meaningful only when the temperature is constant throughout the body. 



§10 Thermodynamic relations for dielectrics in an electric field 49 

entropy and mass of unit volume. It is well known that the ordinary thermo- 
dynamic relation (in the absence of a field) for the internal energy of unit 
volume is dU — TdS+^dp, where £ is the chemical potential of the sub- 
stance.! In the presence of a field in a dielectric, there must be added the 
integrand in (10.3): 

dU= TdS+Zdp + E-dDI4TT. (10.5) 

For the free energy per unit volume of the dielectric, F = U— TS, we 
therefore have 

dF= -Sdr+£dp + E.dD/4Tr. (10.6) 

These relations are the basis of the thermodynamics of dielectrics. 

We see that U and F are the thermodynamic potentials with respect to S, 
p, D and T, p, D respectively. In particular, we can obtain the field by dif- 
ferentiating these potentials with respect to the components of the vector D : 

E = MdU/3D) s , p = 4*(dFldD) T , p . (10.7) 

The free energy is more convenient in this respect, since it is to be differen- 
tiated at constant temperature, whereas the internal energy must be expressed 
in terms of the entropy, which is less easy. 

Together with U and F, it is convenient to introduce thermodynamic 
potentials in which the components of the vector E, instead of D, are the 
independent variables. Such are 

U = E/-E.D/4tt, F = F-E.D/4ir. (10.8) 

On differentiating these we have 



dt/= TdS+tdp-D-dE/4ir, 
dF= -SdT+£dp-D.dE/47r. 



(10.9) 



Hence, in particular, 

D = -47r(dUldE)s, p = -4*(dfldE)T, p . (10.10) 

It should be noticed that the relation between the thermodynamic quanti- 
ties with and without the tilde is exactly that which occurs in §5 for the 
energy of the electrostatic field of conductors in a vacuum. For the integral 
$ E»D dV can be transformed in an exactly similar manner to the one at the 



t See Statistical Physics, §24, Pergamon Press, London, 1958. Instead of the mass 
density we there use the number of particles N per unit volume, which is related to the 
density by p = Nm, where m is the mass of one particle. For this reason the chemical 
potentials as denned here and in Statistical Physics differ by a constant factor (the potential 
here being referred to unit mass, and there to one particle). 

We here denote the chemical potential by £ instead of the more usual letter ft. The use 
of the letter p for the mass density as well as the charge density cannot lead to any mis- 
understanding, because the two quantities never appear together. 



50 Electrostatics of Dielectrics §10 

beginning of §2, with the equation div D = inside the dielectric and the 
boundary condition D n = 4ttg on the surfaces of conductors : 

ij E .DdF=-ljgrad^Ddr 

= ^2/ <f>aDndf = ^- ( ian ) 

Hence we have for the internal energy, for example, 

fE-D 

W = #- I — — dV= Qt-Yfafito (10.12) 

in agreement with the definition (5.5). 

It is useful to derive also the formulae for infinitesimal changes in these 
quantities, expressed in terms of the charges and potentials of the conductors 
(the sources of the field). For example, the variation in the free energy (for 
a given temperature) is 

(&?> = SR = X<f>a8e a . (10.13) 

For the variation of IF we have 

(8&) T = (S^t-SSM, = - Xe a 8<f> a . (10.14) 

We can say that the quantities without the tilde are the thermodynamic 
potentials with respect to the charges on the conductors, while those with 
it are thermodynamic potentials with respect to their potentials. 

It is known from thermodynamics t that the various thermodynamic 
potentials have the property of being minima in a state of thermodynamic 
equilibrium, relative to various changes in the state of the body. In formu- 
lating these conditions of equilibrium in an electric field, it is necessary to 
state whether changes of state with constant charges on the conductors (the 
field sources) or those with constant potentials are being considered. For 
example, in equilibrium 2? and «# are minima with respect to changes in 
state occurring at constant temperature and (respectively) constant charges 
and potentials of the conductors (the same is true for °tt and % at constant 
entropy). 

If any processes (such as chemical reactions) which are not directly related 
to the electric field can occur in the body, the condition of equilibrium with 
respect to these processes is that F is a minimum for given density, tempera- 
ture and induction D, or that P is a minimum for constant density, tempera- 
ture and field E. 

Hitherto we have made no assumptions concerning the dependence of D 
on E, so that all the thermodynamic relations derived above are valid 



t See Statistical Physics, §15. 



§10 Thermodynamic relations for dielectrics in an electric field 51 

whatever the nature of this dependence. Let us now apply them to an 
isotropic dielectric, where a linear relation D = eE holds. In this case 
integration of (10.5) and (10.6) gives 

U= U (S,p) + D^S^ y 

F = F (T, p ) + D2I8tt€, 

where Uq and Fq pertain to the dielectric in the absence of the field. Thus 
in this case the quantity D 2 l8ire = €E 2 J8tt = ED/87T is the change in the 
internal energy (for given entropy and density) or in the free energy (for 
given temperature and density), per unit volume of the dielectric medium, 
resulting from the presence of the field. 

The expressions for the potentials V and F are similarly 

er-tw,)-.*./*,. 

F = F (T,p)-€E2l$7T. K 

We see that the differences U—Uq and U—Uo in this case differ only in 
sign, as they did for an electric field in a vacuum (§5). In a dielectric medium, 
however, this simple result holds good only when there is a linear relation 
between D and E. 

We shall write out also, for future reference, formulae for the entropy 
density S and the chemical potential £, which follow from (10.15): 

/3F\ D 2 / 8e\ 

- * T >» + di>f),' (10 - 17) 

/dF\ E 2 /de \ 

\ dp / t,d 8tt\ dp / t 

These quantities, of course, differ from zero only inside the dielectric. 

The total free energy is obtained by integrating (10.15) over all space. 
By (10.11) we have 

ST-^o = jE.DdF/87r = ^e a <f> a - (10.19) 

This last expression is formally identical with the energy of the electrostatic 
field of conductors in a vacuum. The same result can be obtained directly 
by starting from the variation 8^ (10.13) for an infinitesimal change in the 
charges on the conductors. In the present case, when D and E are linearly 
related, all the field equations and their boundary conditions are also linear. 
Hence the potentials of the conductors must (as for the field in a vacuum) 
be linear functions of their charges, and integration of equation (10.13) gives 
(10.19). 

It should be emphasised that these arguments do not presuppose the di- 
electric to fill all space outside the conductors. If, however, this is so, we 



52 Electrostatics of Dielectrics §11 

can go further and use the results at the end of §7 to draw the following 
conclusion. For given charges on the conductors, the presence of the di- 
electric medium reduces by a factor e both the potentials of the conductors 
and the field energy, as compared with the values for a field in a vacuum. 
If, on the other hand, the potentials of the conductors are maintained con- 
stant, then their charges and the field energy are increased by a factor e. 

PROBLEM 

Determine the height h to which a liquid rises in a vertical plane condenser. 

Solution. For given potentials on the condenser plates, # must be a minimum. <# in- 
cludes the energy JpgA 2 of the liquid under gravity. From this condition we easily obtain 
h = (e-l)E*l8Trpg. 

§11. The total free energy of a dielectric 

The total free energy !F (or the total internal energy *%), as defined in §10, 
includes the energy of the external electric field which polarises the di- 
electric. It is also meaningful to consider the total free energy less the energy 
of the field which would be present in all space if the body were absent. We 
denote this field by <£. Then the total free energy in this sense is 

j(F-<S?/S7r)dV, (11.1) 

where F is the free energy density. Here we shall denote this quantity by 
the letter 3F> which in §10 signified J FdV. It should be emphasised that the 
difference between the two definitions of SF is a quantity independent of the 
thermodynamic state and properties of the dielectric, and hence it has no 
effect on the fundamental differential relations of thermodynamics pertain- 
ing to this quantity, t 

Let us calculate the change in & resulting from an infinitesimal change 
in the field which occurs at constant temperature and does not destroy the 
thermodynamic equilibrium of the medium. Since SF = E«8D/47r, we have 
SJ^ = /(E'SD— (£'S<g)dF/47r. This expression is identically equal to 

8& = J(D-®).aedF/47r + 

+ JE-(SD-8(£)dVI47T- J(D-E).S®dF/477. (11.2) 

In the first integral we write 8© = — grad 8<f> (where <f>o is the potential of 
the field ©) and integrate by parts: 

j grad8<MD-©)dF = <f S<£o(D-(E)-df- Js^ div(D-®)dr. 



t It may be noted that there would be no sense in subtracting E 2 j8v from F, because E 
is the field as modified by the presence of the dielectric, and so the difference F—E 2 j8ir 
could not be regarded as the free energy density of the dielectric as such. 



§11 The total free energy of a dielectric 53 

It is easy to see that both the integrals on the right-hand side are zero. For 
the volume integral this follows at once from the equations div D = and 
div © = which the induction in the dielectric and the field in the vacuum 
must respectively satisfy. The surface integral is taken over the surfaces of 
the conductors which produce the field and over an infinitely distant surface. 
The latter of these is, as usual, zero, and for each of the conductors 
8fo = constant, so that § 8<f>o(D— ©)-df = 8<f>o§(D— <£)-df. The field ©, by 
definition, is produced by the same sources as the field E and induction D 
(i.e. by the same conductors with given total charges e). Hence the two 
integrals § D n df and j (E»d/ are both equal to 4ttc, and their difference is 
zero. 

Similarly, we can see that the second term in (11.2) is also zero, by putting 
E = — grad <f> and using the same transformation. Finally, we have 

8& = - j(T>-E)-8<£dV/47r = - j?>8<£dV. (11.3) 

It should be noticed that the integral in this expression need be taken only 
over the volume of the dielectric medium, since outside it P = 0. 

However, we must emphasise that the integrand P»S© cannot be inter- 
preted as the variation of the free energy density in the same way as was done 
with formulae (10.3), (10.4). First of all, this density must exist outside the 
body, which modifies the field in the surrounding space also. It is clear, 
moreover, that the energy density at any point in the body can depend only 
on the field actually present there, and not on the field which would be 
present if the body were removed. 

If the external field © is uniform, then 

8& = -8®. JPdF = - ^.8©, (11.4) 

where $P is the total electric dipole moment of the body. Hence the thermo- 
dynamic identity for the free energy can be written in this case as 

d^ = -&>&T-0>-m. (11.5) 

The total electric moment of the body can therefore be obtained by dif- 
ferentiating the total free energy : 

& = -{d3F\W) T . (11.6) 

The latter formula can also be obtained directly from the general statistical 
formula 

where ffl is the Hamiltonian of the body as the system of its component 
particles, and A is any parameter characterising the external conditions in 
which the body is placed.! For a body in a uniform external field © ; the 



t See Statistical Physics, §§11, 15. 



54 Electrostatics of Dielectrics §1 1 

Hamiltonian contains a term — ©«^, where $ is the dipole moment opera- 
tor. Taking © as the parameter A, we obtain the required formula. 

If D and E are connected by the linear relation D = eE, we can similarly 
calculate explicitly not only the variation hSF but 3F itself. We have 

3F-3F* = J(E.D-(£ 2 )dF/87r. 

This can be identically transformed into 

Jf-jFo = J(E + ®).(D-(£)dF/87r- J®.(D-E)dF/87r. 

The first term on the right is zero, as we see by putting 

E + ®= — grad(<£ + <£ ) 
and again using the same transformation. Hence we have 

^-jFo(F,T)= -lJ©.Pdr. (11.7) 

In particular, in a uniform external field 

&-3?o{V,T) = -|©.^. (11.8) 

This last equation can also be obtained by direct integration of the rela- 
tion (11.3) if we notice that, since all the field equations are linear when 
D = eE, the electric moment &> must be a linear function of ©. 

The linear relation between the components of &> and © can be written 

&i = Fa!*®*, (11.9) 

as for conductors (§2). For a dielectric, however, the polarisability depends 
not only on the shape but also on the dielectric constant. The symmetry of 
the tensor a**, mentioned in §2, follows at once from the relation (11.6); it 
is sufficient to notice that the second derivative d 2 ^ j d^^i = — d&ijdt&k 
= — Yaw is independent of the order of differentiation. 

Formula (11.7) becomes still simpler in the important case where e is 
close to 1, i.e. the dielectric susceptibility k = (e— l)/4ir is small. In this 
case, in calculating the energy, we can neglect the modification of the field 
due to the presence of the body, putting P = kE ~ k®. Then 

jr_ jr = _!* J(£ 2 dF, (n.io) 

the integral being taken over the volume of the body. In a uniform field, 
the dipole moment &> = Vk(£, and the free energy is 

^r_jr = -\ K V<&. (11.11) 



§12 Electrostriction of isotropic dielectrics 55 

In the general case of an arbitrary relation between D and E, the simple 
formulae (11.7) and (11.8) do not hold. Here the formula 

*-J(*-S dr -J[*-!r-H d,r (1U2) 

may be useful in calculating ^\ its derivation is obvious after the above 
discussion. Here also the integrand in the latter integral is zero outside the 
body, so that the integration is taken only over the volume of the body. 



PROBLEM 

Derive the formula which replaces (11.7) when the body is not in a vacuum but in a 
medium of dielectric permeability e(*>. 

Solution. Using the same transformations as before, we find 
&-&0 = - — [ <g.(D-e« e >E) dF. 

§12. Electrostriction of isotropic dielectrics 

For a solid dielectric in an electric field the concept of pressure cannot be 
defined as for an isotropic body in the absence of a field, because the forces 
acting on a dielectric (which we shall determine in §§15, 16) vary over the 
body, and are anisotropic even if the body itself is isotropic. An exact 
determination of the deformation (electrostriction) of such a body involves 
the solution of a complex problem of the theory of elasticity. 

However, matters are much simpler if we are interested only in the change 
in the total volume of the body. As we saw in §5, the shape of the body 
may then be regarded as unchanged, i.e. the deformation may be regarded 
as a uniform volume compression or expansion. 

We shall neglect the dielectric properties of the external medium (the atmo- 
sphere, for instance) in which the body is situated, i.e. we suppose that e = 1. 
This medium thus serves merely to exert a uniform pressure on the surface 
of the body, which we shall denote by p. If 8F is the total free energy of the 
body, then we have the thermodynamic relation p = —(d^/dV)^ and 
accordingly the expression for the differential dJF contains a term — pdV. 
For example, in a uniform external field, (11.5) becomes 

d^= -SfdT-pdV-&>>d1&. 

We introduce the total thermodynamic potential of the body in accordance 
with the usual thermodynamic relation 

op = &+pV. (12.1) 

The differential of this quantity in a uniform external field is 

Ago = -&>dT+Vdp-0>-d®. (12.2) 



56 Electrostatics of Dielectrics §12 

The change in the thermodynamic quantities in an external electric field is 
usually a relatively small quantity. It is known t that a small change in the 
free energy (for given T and V) is equal to the small change in the thermo- 
dynamic potential (for given T and p). Hence, besides (11.8), we can write 
analogously 

op = ^o-|®-^ (12.3) 

for the thermodynamic potential of a body in a uniform external field. Here 
cpQ is the value for the body in the absence of the field and for given values 
of p and T, while 3F$ in (11.8) is the free energy in the absence of the field 
and for given values of V and T. 

Making explicit the dependence of the dipole moment on V and (£ accord- 
ing to (11.9), we can rewrite (12.3) as 

cp = epo(p, T)-W*nc®&k, (12.4) 

where the correction term must be expressed as a function of temperature 
and pressure by means of the equation of state for the body in the absence 
of the field. In particular, for a substance of small dielectric susceptibility 
this formula becomes simply 

gp = yodj>,T)-\KV<&; (12.5) 

cf. (11.11). 

The required change in volume V— Vq in the external field can now be 
obtained immediately by differentiating cp with respect to pressure for 
constant T and (£. For example, from (12.5) we have 

V-V = -^[d( K V)/dp] T . (12.6) 

This quantity may be either positive or negative (whereas, in electrostric- 
tion of conductors, the volume is always greater in the presence of the field). 
Similarly, we can calculate the amount of heat Q absorbed in a dielectric 
when an external electric field is isothermally applied (the external pressure 
being constant). J Differentiation of cp—cpQ with respect to temperature 
gives the change in the entropy of the body, and by multiplying this by T 
we obtain the required quantity of heat. For example, from (12.5) we obtain 

Q = ±&T[d( K V)ldT] p . (12.7) 

Positive values of Q correspond to absorption of heat. 

PROBLEMS 

Problem 1. Determine the change in volume and the electrocaloric effect for a dielectric 
ellipsoid in a uniform electric field parallel to one of its axes. 



t See Statistical Physics, §15. 

% If the body is thermally insulated, the application of the field results in a change of 
temperature AT = —Q/Cp, where C p is the specific heat at constant pressure. 



§12 Electrostriction of isotropic dielectrics 57 

Solution. From formulae (12.3) and (8.9) we have 

V €-1 „ 

&> = &>o-- — Vfc 2 . 

on ne+l—n 

The relative change in volume is found to be 

V-Vo _ (£ 2 [ €-1 1 1 /3e\ I 

V~ ~ S^lne+1-nK ~ (ne+l—n)A~dp) T J' 

and the electrocaloric effect 



* 8tt lne + 1-n (ne+l-n) z \dT } P \ 



where 1/.K = —<MV)(dV I dp) T is the compressibility of the body, and a = (l/VXdV/dT)? 
the thermal expansion coefficient. 

In particular, for a plane disc in a field perpendicular to it, n = 1, so that 

F-Fo (S 2 r6-1 1 1 fde\ I 
P^~ ~~ 8^hr K ~ l?\dp) tY 

e .«[ii + iffll. 

* 8tt L e e 2 \ar/J 

For a similar disc (or any cylinder) in a longitudinal field, « = 0, and 

V-V (£ 2 f€-l /a e \ I 7V(£ 2 r , /^\1 

Problem 2. Determine the difference between the heat capacity ^ of a plane disc in a 
field perpendicular to it, with a constant potential difference between its faces, and the heat 
capacity ^d at constant induction, the external pressure being maintained constant in 
each case, t 

Solution. According to the results of Problem 1, the entropy of the disc is 

The induction inside the disc is the same as the external field : D = (£. Hence, to calculate 
the heat capacity <&r>, we must differentiate Sf for constant (£. The potential difference 
between the faces of the disc is <f> = El = (£ //e, where / is its thickness. For a uniform com- 
pression or expansion of a body, I is proportional to Vi. Hence, to calculate the heat capacity 
#0, we must differentiate S? for constant (&Vi/e. The required difference is found to be 

Problem 3. Determine the electrocaloric effect in a homogeneous dielectric whose total 
volume is kept constant. 

Solution. Strictly speaking, when an external field is applied the density of the body 
changes (and ceases to be uniform), even if the total volume is kept constant. In calculating 
the change in the total entropy, however, we can ignore this and assume the density p constant 
at every point. J 



f ^ is the heat capacity of a disc between the plates of a plane condenser in circuit 
with a constant e.m.f. In an unconnected condenser with constant charges on the plates, 
the heat capacity of the disc is *& d- 

X The change in density Sp is of the second order with respect to the field (~i? 2 ), and the 
consequent change in the total entropy is of the fourth Order: the term in the change of 
total entropy which is linear in Sp is (dSoldp)f Sp dV, and the integral is zero because the 
total mass of the body is unaltered. 



58 Electrostatics of Dielectrics §13 



According to (10.17) the total entropy of the body is 

1 / 8e 



^^• T >+dw)J E3dr - 



where the integration is over the volume of the body. The amount of heat absorbed is 

*-=(£). J™ 

Problem 4. Determine the difference <$$ — <&d (see Problem 2) when the total volume of 
the disc is kept constant. 

Solution. When the volume, and therefore the thickness, of the disc are constant, dif- 
ferentiation for constant potential difference is equivalent to differentiation for constant field E. 
Using the formula of Problem 3 for the entropy we have 

TVE 2 / de \ 2 

Problem 5. A condenser consists of two conducting surfaces at a distance h apart which is 
small compared with their dimensions ; the space between them is filled with a substance 
of dielectric permeability ei. A sphere of radius a <Z^h and dielectric permeability ez is 
placed in the condenser. Determine the change in capacity. 

Solution. Let the sphere be placed in the condenser in such a way that the potential 
difference <f> between the plates remains unchanged. The free energy for constant poten- 
tials of the conductors is 3?. In the absence of the sphere, & = — £Co<£ 2 , where Co is the 
original capacity of the condenser. Since the sphere is small, we may imagine it to be brought 
into a uniform field (£ = <f>lh, and the change in # is small. The small change in # at con- 
stant potentials is equal to the small change in & at constant charges on the sources of the 
field. Using the formula derived in §11, Problem, and (8.2), we have 

& = -iCo^ 2 -iaV«)(e(')- e ( e ))^ 2 /(2eW + e(*))/i 2 , 

whence the required capacity is 

C = Co+a»eM(e<«.-e<«>)/(2eM + «<«)A«. 



§13. Dielectric properties of crystals 

In 'an anisotropic dielectric medium (a crystal) the linear relation between 
the electric induction and the electric field is less simple, and does not reduce 
to a simple proportionality. 

The most general form of such a relation is 

Dt = D 0i + € ik E k , (13.1) 

where Do is a constant vector, and the quantities et k form a tensor of rank 
two, called the dielectric permeability tensor (or simply the dielectric tensor). 
The inhomogeneous term Do in (13.1) does not, however, appear for all 
crystals. The majority of the types of crystal symmetry do not admit this 
constant vector (see below), and we then have simply 

Di = nicEk- (13.2) 

The tensor e^ is symmetrical: 

Hk = € ki- (13.3) 



§13 Dielectric properties of crystals 59 

In order to prove this, it is sufficient to use the thermodynamic relation 
(10.10) and to observe that the second derivative —4ird 2 FldE k dEi = dDijdEu 
= eoc is independent of the order of differentiation. 
For P itself we have (when (13.2) holds) the expression 

P = F -e ik EiE k IS7T. (13.4) 

The free energy F is 

F = F+EiDilbn- = Fo + e-iaDiDtl&ir. (13.5) 

Like every symmetrical tensor of rank two, the tensor e$ * can be brought 
to diagonal form by a suitable choice of the co-ordinate axes. In general, 
therefore, the tensor e is determined by three independent quantities, namely 
the three principal values e (1) , e (2) , e (3) . All these are necessarily greater than 
unity, just as e > 1 for an isotropic body (see §14). 

The number of different principal values of the tensor e* # may be less than 
three for certain symmetries of the crystal, t 

In crystals of the triclinic, monoclinic and rhombic systems, all three 
principal values are different; such crystals are said to be biaxial.% In 
crystals of the triclinic system, the directions of the principal axes of the 
tensor e^ are not uniquely related to any directions in the crystal. In those 
of the monoclinic system, one of the principal axes must coincide with the 
axis of symmetry of the second order or be perpendicular to the plane of 
symmetry of the crystal. In crystals of the rhombic system, all three princi- 
pal axes of the tensor e^ are crystallographically fixed. 

Next, in crystals of the tetragonal, rhombohedral and hexagonal systems, 
two of the three principal values are equal, so that there are only two inde- 
pendent quantities ; such crystals are said to be uniaxial. One of the principal 
axes coincides with the axis of crystal symmetry of the fourth, third or sixth 
order, but the directions of the other two principal axes can be chosen 
arbitrarily. || 

Finally, in crystals of the cubic system all three principal values of the 
tensor e* # are the same, and the directions of the principal axes are entirely 
arbitrary, ft This means that the tensor €<* is of the form eS^fc, i.e. it is deter- 
mined by a single scalar e. In other words, as regards their dielectric proper- 
ties, crystals of the cubic system are no different from isotropic bodies. 



t The fairly obvious symmetry properties of the tensor em that are given below can be 
very simply obtained by using a result of tensor algebra: to every symmetrical tensor of rank 
two there corresponds a tensor ellipsoid, the lengths of whose semiaxes are proportional to 
the principal values of the tensor. The symmetry of the ellipsoid corresponds to that of the 
crystal. 

X This name refers to the optical properties of the crystals; see §§78, 79. 

|| In this case the tensor ellipsoid degenerates into a spheroid, completely symmetrical 
about the longitudinal axis. It should be emphasised that, as regards the physical properties 
of the crystal which are determined by a symmetrical tensor of rank two, the presence of 
an axis of symmetry of the third or higher order is equivalent to complete isotropy in the 
plane perpendicular to this axis. 

1 1 The tensor ellipsoid here degenerates into a sphere. 



60 Electrostatics of Dielectrics §13 

Let us now examine the dielectric properties of crystals for which the 
constant term Do appears in (13.1). The presence of this term signifies that 
the dielectric is spontaneously polarised even in the absence of an external 
electric field. Such bodies are said to be pyroelectric. The magnitude of this 
spontaneous polarisation is, however, in practice always very small (in com- 
parison with the molecular fields). This is because large values of Do would 
lead to strong fields within the body, which is energetically unfavourable and 
therefore could not correspond to thermodynamic equilibrium. The small- 
ness of Do also ensures the legitimacy of an expansion of D in powers of E, 
of which (13.1) represents the first two terms. 

The thermodynamic quantities for a pyroelectric body are found by 
integrating the relation —fadF/BEt = Di = Doi + e ik E k , whence 

F = Fo-€ ik E i E k l8ir-E i D(Hl+*. (13.6) 

The free energy is 

F = F+EiDi/47T = F + € ik EiE k IS7T 

= Fo + e-h k (Di-Doi)(D k -Do k )IS7T. (13.7) 

It should be noted that the term in F linear in E% does not appear in F.f 

The total free energy of a pyroelectric can be calculated from formula 
(11.12) by substituting (13.7) and (13.1). If there is no external field, (& = 0, 
and we have simply 

F = J[F -(E.Do/87r)]dF. (13.8) 

It is remarkable that the free energy of a pyroelectric in the absence of an 
external field depends, like the field E, not only on the volume of the body 
but also on its shape. 

As has already been pointed out, the phenomenon of pyroelectricity is 
not possible for every crystal symmetry. Since, in any symmetry transforma- 
tion, all the properties of the crystal must remain unchanged, it is clear that 
the only crystals which can be pyroelectric are those in which there is a 
direction which is unchanged (and, in particular, not reversed) in all sym- 
metry transformations, and that this will be the direction of the constant 
vector Do. 

This condition is satisfied only by those symmetry groups which consist 
of a single axis together with planes of symmetry which pass through the 
axis. In particular, crystals having a centre of symmetry certainly cannot be 



t It should also be noted that in these formulae we neglect the piezoelectric effect, i.e. the 
effect of internal stresses on the electric properties of a body; see §17. The formulae given 
here are therefore, strictly speaking, applicable only when the fields are uniform throughout 
the body, and internal stresses do not arise. 



§13 Dielectric properties of crystals 61 

pyroelectric. We may enumerate those out of the 32 crystal classes in which 
pyroelectricity occurs : 

triclinic system: C\ 

monoclinic system: C s , C2 

rhombic system : C^ v 

tetragonal system: C4, C^ v 

rhombohedral system : C3, C$ v 

hexagonal system: C$, Cq v . 
There are, of course, no pyroelectric cubic crystals. In a crystal of class 
Ci the direction of the pyroelectric vector Do is not related to any direction 
fixed in the crystal ; in one of class C s , it must lie in the plane of symmetry. 
In all the remaining classes listed above the direction of Do is that of the 
axis of symmetry. 

It should be mentioned that, under ordinary conditions, pyroelectric 
crystals have zero total electric dipole moment, although their polarisation 
is not zero. The reason is that there is a non-zero field E inside a spon- 
taneously polarised dielectric. Since a body usually has a small but non-zero 
conductivity, the presence of a field gives rise to a current, which flows until 
the free charges formed on the surface of the body annihilate the field inside 
it. The same effect is produced by ions deposited on the surface from the 
air. Experimentally, pyroelectric properties are observed when a body is 
heated and a change in its spontaneous polarisation is detected. 



PROBLEMS 
Problem 1. Determine the field of a pyroelectric sphere in a vacuum. 

Solution. The field inside the sphere is uniform, and the field and induction are related 
by 2E = — D (as follows from (8.1) when © = 0, i.e. when there is no applied external 
field). Substituting in (13.1), we obtain the equation 2Ei+etkEk = — Dm- We take the co- 
ordinate axes to be the principal axes of the tensor ej*. Then this equation gives Ei = 
-Do*/(2+e<*>). The polarisation of the sphere is Pt = (A -Ei)/** = 3Do*/4tt(2 + €<«>). 
The field outside the sphere is that of an electric dipole of moment 3* = J*V. 

Problem 2. Determine the field of a point charge in a homogeneous anisotropic medium. 

Solution. The field of a point charge is given by the equation div D = 47r«8(r) (the 
charge being at the origin). In an anisotropic medium Dt = eucEk = —en d<l>ldxic; taking 
the co-ordinate axes x, y, z along the principal axes of the tensor €*&, we obtain for the poten- 
tial the equation 

eWdmdxZ + cMdmdyZ+eMdmdz 2 = -47re8(*) S(y) 8(z). 

By the introduction of new variables 

x' = x/vV*>, y' = y/V&K z' = z/Ve l * } , (D 

this becomes 

32.A dU dU 4w 
— — H — H — = 8(*') S(y') 8(2'), 



t In Problems 2-5 the anisotropic dielectric is assumed not to be pyroelectric. 



62 Electrostatics of Dielectrics §13 

which formally differs from the equation for the field in a vacuum only in that e is replaced 
by e/V(e (,) « (l "e w )- Hence 



4 = L = f r^l+-z! + .fir; 



V((i*wi) 

In tensor notation, independent of the system of co-ordinates chosen, we have 

<}> = elVdele'hkXiXk), 

where |e| is the determinant of the tensor cm. 

Problem 3. Determine the capacity of a conducting sphere, of radius a, in an anisotropic 
dielectric medium. 

Solution. By the transformation shown in Problem 2, the determination of the field of a 
sphere with charge e in an anisotropic medium reduces to the determination of the field in a 
vacuum due to a charge e' distributed over the surface of the ellipsoid zucx'ix'k = e^x' 2 + 
4- € (v)^ / 2-j- e (z) 2r '2 = a 2 # Using formula (4.14) for the potential due to an ellipsoid, we find 
the required capacity to be given by 

~C = 2V(« ( * ) « (, ' , « ( " ) ) J Lv "^/V "^/V "^>/J d ^' 

Problem 4. Determine the field in a flat anisotropic plate in a uniform external field C 

Solution. From the condition of continuity of the tangential component of the field it 
follows that E = H+An, where (£ is the uniform field outside the plate, n a unit vector 
normal to its surface, and A a constant. The constant is determined from the condition of 
continuity of the normal component of the induction, n-D = n»(fc, or mcvcEje = Tneu&ic J r 
+Aeatmnk = (£««*• Hence A = — {em— §ik)ni<&icl ei m nin m . 

Problem 5. Determine the torque on an anisotropic dielectric sphere, of radius a, in a 
uniform external field © in a vacuum. 

Solution. According to (8.2) we have for the field inside the sphere E x = 3©x/(e( a; )+2), 
and similarly for E y , E z . Here the axes of x, y, z are taken to be the principal axes of the 
tensor ei*. Hence the components of the dipole moment of the sphere are 

4 e<*> — 1 

The components of the torque on the sphere are 

Kz = 0?x(g) 2 = 3a 3 (E, <&y (€<*)-e«»))/(€(*)+2)(e(»)4-2), 

and similarly for K x , K y . 

Problem 6. An infinite anisotropic medium contains a spherical cavity of radius a. Express 
the field in the cavity in terms of the uniform field E^ far from the cavity. 

Solution. The transformation (1) of Problem 2 reduces the equation for the field potential 
in the medium to Laplace's equation for the field in a vacuum. The equation for the field in 
the cavity is transformed into that for the field in a medium with dielectric constants l/e (l \ 
1/eM, l/e< 2 ). Moreover, the sphere is transformed into an ellipsoid with semiaxes a/-\A (ir) ! 
aly/dv), a/V e(e) ' Let nS x \ n<">, n<*> be the depolarisation coefficients of such an ellipsoid 
(given by formulae (4.25)). Applying formula (8.7) to the field of this ellipsoid, we obtain 
the relation 

V ' 8x' e<*> 8x' 8x' ' 

and similarly for the y and z directions. Returning to the original co-ordinates, we have 
d<f>ldx' = Ve {x) d<f>/dx = V^ x) E x , so that the field in the cavity is 

c (z) 
e (*)_ B (*)( e (*)_l) 



§14 The sign of the dielectric susceptibility 63 

§14. The sign of the dielectric susceptibility 

To elucidate the way in which the thermodynamic quantities for a di- 
electric in a field depend on its dielectric constant, let us consider the formal 
problem of the change in the electric component of the total free energy of 
the body when e undergoes an infinitesimal change. 

For an isotropic (not necessarily homogeneous) body we have by (10.19) 
^—^o = J(D 2 /87re)dF. When e changes, so does the induction, and the 
variation in the free energy is therefore 

fD-SD r D 2 rE-SD r E 2 

h&r = dV- 8edV = dV- — StdV. 

J 4tt€ J S-rre 2 J 4tt J 8tt 

The first term in the last member is the same as (10.2), which gives the work 
done in an infinitesimal change in the field sources (i.e. charges on conduc- 
tors). In the present case, however, we are considering a change in the field 
but no change in the sources. This term therefore vanishes, leaving 

Sjr = - jS € (E2/8TT)dV. (14.1) 

From this formula there follows, first of all, an important result: any in- 
crease in the dielectric constant of the medium, even if in only a part of it 
(the sources of the field remaining unchanged), reduces the total free energy. 
In particular, we can say that the free energy is always reduced when un- 
charged conductors are brought into a dielectric medium, since these 
conductors may (in electrostatics) be regarded as bodies whose dielectric 
constant is infinite. This conclusion generalises the theorem (§2) that the 
energy of the electrostatic field in a vacuum is diminished when an uncharged 
conductor is placed in it. 

Formula (14.1) can also be used to prove the statement in §7 that the di- 
electric constant of any body exceeds unity, i.e. the dielectric susceptibility 
(c— l)/47r is positive. To show this, we must first show independently 
that the total change in the free energy of a dielectric when it is placed in an 
electric field is negative, t This can be done by the use of thermodynamic 
perturbation theory, the change in the free energy of the body being regarded 
as the result of a perturbation of its quantum energy levels by the electric 
field. According to this theory we havet 

*-*. = r,--22 E J»- Em «» -2*r( F ™- F ™ )2 - fl4 - 2) 



t The change proportional to the square of the field is meant. It may be recalled that, 
in pyroelectric bodies, the change in the free energy contains also a term linear in the field, 
which is of no interest here. 

X See Statistical Physics, §32, formulae (32.5), (32.6). The formulae given here differ 
from those in Statistical Physics only in form. 



64 Electrostatics of Dielectrics §15 

Here E n {0) are the unperturbed levels, F m «the matrix elements of the perturb- 
ing energy, and the bar denotes a statistical averaging with respect to the 
Gibbs distribution w n = exp {(^Q—En^kT). 

The term V nn in formula (14.2), which is linear in the field, is zero except 
in pyroelectric bodies. The quadratic change in the free energy, which is of 
interest here, is given by the remaining terms. It is evident from the form 
in which they are written here that they are negative. 

If we formally consider the change in the free energy as the result of a 
gradual change in the dielectric constant of the body from 1 to a given value 
€, it follows from formula (14.1) that ^—^o is negative only if e > 1. 
This completes the proof. 

In the same way we can prove the inequalities e W) > 1 for the principal 
values of the tensor e^ in an anisotropic dielectric medium. To do so, it is 
evidently sufficient to consider the energy of a field parallel to each of the 
three principal axes in turn. 

The total free energy is diminished, in particular, when any charge is 
brought up to a dielectric body from infinity (a process which may be 
regarded as an increase of e in a certain volume of the field round the charge). 
In order to conclude from this that any charge is attracted to a dielectric, 
we should, strictly speaking, prove also that F cannot attain a minimum for 
any finite distance between the charge and the body. We shall not pause 
here to prove this statement, especially as the presence of an attractive 
force between a charge and a dielectric may be regarded as a fairly evident 
consequence of the interaction between the charge and the dipole moment of 
the dielectric, which it polarises. 

We can deduce immediately from formula (14.1) the direction of motion 
of a dielectric body in an almost uniform electric field, i.e. one which may 
be regarded as uniform over the dimensions of the body. In this case E 2 
is taken outside the integral, and the difference !F—SF§ is a negative quantity, 
proportional to E 2 . In order to take a position in which its free energy is a 
minimum, the body will therefore move in the direction of E increasing. 



§15. Electric forces in a fluid dielectric 

The problem of calculating the forces (called ponderotnotive forces) which 
act on a dielectric in an arbitrary non-uniform electric field is fairly compli- 
cated and requires separate consideration for fluids (liquids or gases) and 
for solids. We shall take first the simpler case, that of fluid dielectrics. We 
denote by f&V the force on a volume element dV, and call the vector f the 
force density. 

It is well known that the forces acting on any finite volume in a body can 
be reduced to forces applied to the surface of that volume, t This is a conse- 
quence of the law of conservation of momentum. The force acting on the 



t See Theory of Elasticity, §2, Pergamon Press, London, 1959. 



§15 Electric forces in a fluid dielectric 65 

matter in a volume dV is the change in its momentum per unit time. This 
change must be equal to the amount of momentum entering the volume 
through its surface per unit time. If we denote the momentum flux tensor 
by — ewe, then 

(fidV= j>a ik df k , (15.1) 

where the integration on the right is over the surface of the volume V. The 
tensor anc is called the stress tensor. It is evident that ancdfk = criknkdf is 
the ith component of the force on a surface element d/ (n being a unit vector 
along the normal to the surface outwards from the volume under considera- 
tion). 

Similarly, the total torque acting on a given volume also reduces to a 
surface integral, by virtue of the law of conservation of angular momentum. 
This reduction is possible because of the symmetry of the stress tensor 
( ffa . = om), which thus expresses the conservation law mentioned. 

On transforming the surface integral in (15.1) into a volume integral, we 
obtain J/idF= j (daa/ dxic)dV, whence, since the volume of integration is 
arbitrary, 

fi = dancjdxjc. (15- 2 ) 

This is a well-known formula giving the body forces in terms of the stress 
tensor. 

Let us now calculate the stress tensor. Any small region of the surface 
may be regarded as plane, and the properties of the body and the electric 
field near it as uniform. Hence, to simplify the derivation, we can with no 
loss of generality consider a plane-parallel layer of material (of thickness h 
and uniform composition, density and temperature) in an electric field 
which is uniform but whose direction is arbitrary, t This field may be 
imagined to be due to conducting planes, bearing appropriate charge distri- 
butions, applied to the surfaces of the layer. 

Following the general method for determining forces, we subject one of 
the conducting planes (the upper one, say) to a virtual translation over an 
infinitesimal distance £, whose direction is arbitrary and need not be that of 
the normal n. We shall suppose that the potential of the conductor remains 
unchanged at every point, and that the homogeneous deformation of the 
dielectric layer, resulting from the translation, is isothermal. 

A force — own* is exerted by the layer on unit area of the surface. In the 
virtual displacement this force does work — crfjfcWfclf. The work done in an 
isothermal deformation at constant potential is equal to the decrease in J FdV, 
i.e. in hF per unit surface area. Thus 

<*«&»* = K"F) = h8F+F8h. (15.3) 

f We thus ignore any terms in the stress tensor depending on the gradients of tempera- 
tore, field, etc. These terms, however, are vanishingly small in comparison with terms which 
do not contain derivatives, in the same way as any terms containing derivatives which might 
appear in the relation between D and E. 



66 Electrostatics of Dielectrics §15 

The thermodynamic quantities for the fluid depend (for given tempera- 
ture and field) only on its density; deformations which do not change the 
density (i.e. pure shears) do not affect the thermodynamic state. We can 
therefore write for an isothermal variation 8P in a fluid 



„ / dF\ / dP\ 
8P = — .SE+I 8 P 

\ dE/T. \ dp /ET 



DSE / dP\ 
= -__+(_ 8 P . (15.4) 

47T \ Op /E,T 

The change in the density of the layer is related to the change in its thick- 
ness by 8p = —pSh/h. The variation of the field is calculated as follows. 
At a given point in space (with radius vector r) there appears matter which 
was originally at r— u, where u is the particle displacement vector in the 
layer. Since, under the conditions stated (homogeneous deformation, and 
constant potential on the conducting planes), each particle carries its poten- 
tial with it, the change in the potential at a given point in space is 8<f> 
— <f>(r—u)—<f>(r) = — U'grad <f> = u«E, where E is the uniform field in the 
undeformed layer. Since the deformation is homogeneous, however, we 
have 

u = z\\h, (15.5) 

where z is the distance from the lower surface. Hence the variation of the 
field is 

SE = -n(E.§)/A. (15.6) 

Substituting the above expressions in (15.4) and using also the fact that 
&h = £ 2 = £»n, we obtain 

1 dF 

oik&nic = — (n.D)(|.E)-5.np— -+§.ilP 

4-7T Op 

dP. 

Jp~ 

Hence we have finally the following expression for the stress tensor: 

"« = [P-p(dPldp)E,T]8 ik + E i D k l4^. (15.7) 



(EiD k dP \ 

= \— A p-r-Oik + rdijc)^infc. 

\ 4it dp ) 



In isotropic media, which are those here considered, E and D are parallel. 
Hence EiDk = E^Di, and the tensor (15.7) is symmetrical, as it should be. 
If the linear relation D = eE holds, then 

P = Ffo,T)-eE*/&r; (15.8) 

see (10.16). Fq is the free energy per unit volume in the absence of the field. 



§15 Electric forces in a fluid dielectric 67 

According to a well-known thermodynamic relation, the derivative of the free 
energy per unit mass with respect to the specific volume is the pressure : 



• a ( F °Y\ =F _ p ( d J°\ 
.d{ii P )\ P )\ T ° p \b p )t 



p = p ( Pf T) is the pressure which would be found in the medium in the 
absence of a field and for given values of p and T. Hence, substituting (15.8) 
in (15.7), we have 

E 2 r /de\ 1 eEtEjc 
cr ik = -MP»T)S 1k -—^-p^-J J 8 « + -^-- ( 15 - 9 ) 

In a vacuum, this expression becomes the familiar Maxwell stress tensor of 
the electric field :t 

or» = (EiE k -iE28 ik )l47T. 

The forces exerted on the surface of separation by two adjoining media 
must be equal and opposite: oi k n k = — or'^w*, where the quantities with 
and without the prime refer to the two media. The normal vectors n and 
n' are in opposite directions, so that 

o%»& = cr' ik n]c. (15.10) 

At the boundary of two isotropic media the condition of equality of the 
tangential forces is satisfied identically. For, substituting (15.7) in (15.10) 
and taking the tangential component, we obtain E«D» = E'tD' n . This equa- 
tion is satisfied by virtue of the boundary conditions of continuity on E t 
and D n . The condition of equality of the normal forces is, however, a 
non-trivial condition on the pressure difference between the two media. 

For example, let us consider a boundary between a liquid and the atmo- 
sphere (for which we can put € = 1). Denoting by a prime quantities 
pertaining to the atmosphere, and using formula (15.9) for aye, we have 

E 2 /8e\ 6 
-po( P ,T) + -pl-) +-(E n *-m 

= -/>atm + -(^„ 2 -^ 2 ). 

07T 

Using the boundary conditions Et = E' t , D n = eE n = D' n = E' n , we can 
rewrite this equation as 

P E 2 /de\ e-1 
Po(R,T)-p & t m = P — - --— (e£ n 2 + Z^). (15.11) 

07T \Op/T OTT 

This relation is to be taken as determining the density p of the liquid near 
its surface from the electric field in it. 



t See The Classical Theory of Fields, §4—8, Addison- Wesley Press, Cambridge (Mass.), 
1951; Pergamon Press, London, 1959. 



68 Electrostatics of Dielectrics §15 

Let us now determine the body forces acting in a dielectric medium. 
Differentiating (15.9) in accordance with (15.2) gives 

8 r E2 ( 8e \ i E2 8e l r i 8 2 8 i 

dxil Sit \dp/TJ S-rrdxi AttI 2 dxi dxjc J" 

On using the equation div D = 8D k /8x k — 0, the expression in the brackets 
in the last term can be reduced to 

dE k 8E t (dE k 8E t \ 
-eE k -—+D k —- = -D k t- — , 

OXi OXjc \ OXi OX k J 

which is zero, since curl E = 0. Thus we have 

f = -gradM* r) + -grad^2 /0 ^_jj --grade. (15.12) 

If the dielectric contains extraneous charges of density p e x, the force f 
contains a further term E div D/4tt, or, since div D = 47r/o e x, 

PexE; (15.13) 

however, it should not be supposed that this result is obvious (cf. §16, 
Problem 3). 

In a gas, as already mentioned in §7, we can assume the difference e — 1 
to be proportional to the density. Then pde/dp — e— 1, and formula (15.12) 
takes the simpler form 

€-1 

f = -grad/> + -r— grad£2. (15.14) 

Formula (15.12) is valid for media of both uniform and non-uniform 
composition. In the latter case e is a function not only of p and T but also 
of the concentration of the mixture, which varies through the medium. In 
a body of uniform composition, on the other hand, e is a function only of p 
and T, and grad e can be written as 

grade = (deldT) p gradT+(deldp) T gradp. 

Then (15.12) becomes 

f = -gradM* m£grad [*Qj ~(^) V^ T. (15.15) 

If the temperature also is constant through the body, the third term on the 
right is zero, and in the first term grad/>o can be replaced by p grad £o, in 
accordance with the thermodynamic identity for the chemical potential in 
the absence of a field, />d£o = dpo—SodT. Thus 

f = -pgradk (— ) 1. (15.16) 

L 877- \ dp! tj 



§16 Electric forces in solids 69 

The expression in the brackets is just the chemical potential £ in an electric 
field (see (10.18)), and therefore f = — p grade. 

In particular, the condition of mechanical equilibrium f = is, for con- 
stant temperature, 

£ = t,Q-(E 2 l^)(d€Jd P ) T = constant, (15.17) 

in accordance with the thermodynamic condition of equilibrium, t This 
condition can usually be written still more simply. The change in density 
of the medium due to the field is proportional to E 2 . Hence, if the medium 
is of uniform density in the absence of the field, we can put p = constant in 
the last two terms in (15.15) when the field is present; an allowance for the 
change in p is beyond the accuracy of formulae which assume the linear 
relation D = eE. Then, equating to zero f from (15.15), we obtain the 
equilibrium condition at constant temperature in the form 

Po(p, T)-( P E 2 l87r)(deldp) T = constant, (15.18) 

which differs from (15.17) in that £ is replaced by po/p. 

§16. Electric forces in solids 

The dielectric properties of a solid body change not only when its density 
changes (as with liquids) but also under deformations (pure shears) which 
do not affect the density. Let us first consider bodies which are isotropic in 
the absence of the field. In general, the deformed body is no longer iso- 
tropic; in consequence, its dielectric properties also become anisotropic, 
and the scalar dielectric permeability e is replaced by the dielectric tensor 

The state of a slightly deformed body is described by the strain tensor 

1 / dut dujA 
2\dx]c dxi! 

where u(x,y,z) is the displacement vector for points in the body. Since these 
quantities are small, only the first-order terms in uac need be retained in the 
variation of the components e^. Accordingly, we represent the dielectric 
tensor of the deformed body as 

Hk = €<$ik + &iUik + atuifiiic. (16.1) 

Here e is the dielectric permeability of the undeformed body, and the other 
two terms, which contain the scalar constants «i, #2, form the most general 
tensor of rank two which can be constructed linearly from the components 

Uik- 

Let us now see where the derivation given in §15 must be modified. 
Since, in a solid body, P depends on all the components of the strain tensor, 
we must replace (15.4) by 8F = -D.SE/4tt + (dFldu ik )8u ik . For the virtual 



t See Statistical Physics, §25. 



70 Electrostatics of Dielectrics §16 

displacement considered, the vector u is given by formula (15.5), so that the 
strain tensor is u\ k = (tjin k +£ k ni)l2h. Substituting this in oF and using the 
symmetry of the tensor ui k , and therefore of the derivatives dF/duoc, we 
obtain 

8F = -D.SEHw + itotlQdfldun. (16.2) 

It is now evident that we find, instead of (15.7), the following expression for 
the stress tensor :f 

oik = Fo ik + (dFldu ik ) TE +EiD k l4n. (16.3) 

Formula (16.3) is valid whatever the relation between D and E. For a 
body which is neither pyroelectric nor piezoelectric, so that Di = ei k E k , 
F is given by formula (13.4) and the required derivatives are dFjdunc 
= dFQlduiic—(aiEiE k +a2E 2 §ik)l%7T. We then put 6<* = £o$<* everywhere in 
(16.3) and obtain the following formula for the stress tensor: 

a ik = <rfl» tt + (2e - a^EiEjc/Srr - ( €0 + a 2 )E^ ik ^7t . (16.4) 

a (0 \ k is the stress tensor in the absence of an electric field, determined by the 
moduli of rigidity and compression according to the ordinary formulae of 
the theory of elasticity. 

Let us now make similar calculations for anisotropic solids.} The neces- 
sary modification of the above argument is as follows. When the layer 
undergoes a virtual deformation, its crystallographic axes are rotated, and 
their orientation relative to the electric field is therefore changed. On account 
of the anisotropy of the dielectric properties of the crystal, this leads to an 
additional change in F not shown in (16.2). To calculate this change we 
can equally well suppose that the crystal axes rotate through some angle S<|> 
relative to the field E, or that the field rotates through an angle — S<f> relative 
to the axes, and the latter approach is the more convenient. 

Thus the variation of the field (15.6) considered above must be augmented 
by the change in E on rotation through an angle — S<t>: SE= — n(E«^)/A— 
— §4>xE. The angle S<|> is related to the displacement vector u in the 
deformation by S<t> = \ curlu; this equation is easily obtained by noticing 
that, when the body rotates through an angle S<J>, its points are displaced by 
u = S<|>xr. Substituting u from (15.5), we find S<|> = curl z\\2h = nx£/2Z*, 



t The quantity P in this formula, and in all preceding formulae, is the free energy per 
unit volume. In the theory of elasticity, however, a somewhat different definition is usual : 
the thermodynamic quantities are referred to the amount of matter contained in unit volume 
of the undeformed body, which may after deformation occupy some other volume. It is 
easy to go from one definition to the other by expressing the relative volume change in the 
deformation in terms of the tensor mk ; on account of the presence of the derivative with 
respect to uoc in (16.3), this must be done with allowance for second-order terms. As a 
result, the first two terms on the right of (16.3) combine into one of the form dF/duuc, in 
accordance with the usual formula of elasticity theory. 

X We shall see in §17 that the phenomenon of electrostriction in crystals may, for some 
types of symmetry, differ markedly from that in isotropic bodies. Such crystals are said to 
be piezoelectric. Here, however, we discuss only electrostriction in non-piezoelectric bodies. 



§16 Electric forces in solids 71 

and SE= -n(E.^)/A+Ex(nx^)/2A= -[n(E.^) + ?(n.E)]/2A. The first 
term in (16.2) becomes 

--D8E = — : [(n.D)(§.E) + (?.D)(n.E)] = -Uh& . \{EiD k + E k Di). 

Hence we see that the product EiD k in (16.3) must be replaced by the second 
factor in the last expression: 

dF 1 
°ik = F8 ik + —- + —{EiD k + E k D t ). (16.5) 

CUilc 077 

This expression is symmetrical in the suffixes i and k, as it should be. 

The expression (16.1) for the dielectric tensor, involving two scalar con- 
stants, must be replaced in the case of a dielectric crystal by 

Hk = c (0) ik + aiklmUim, (16.6) 

where Oikim is a constant tensor of rank four, symmetrical with respect to 
the pairs of suffixes t, k and /, m (but not with respect to an interchange of 
these pairs). The number of independent non-zero components of this 
tensor depends on the crystal class. 

We shall not pause to write out here the formula for the stress tensor 
(analogous to (16.4)) which is obtained by using (16.6). 

The formulae which we have obtained give the stresses inside a solid di- 
electric. They are not needed, however, if we wish to determine the total 
force F or the total torque K exerted on the body by the external field. Let 
us consider a body immersed in a fluid medium and kept at rest there. 
The total force on it is equal to the integral § ai k n k df, taken over the surface. 
Since the force ai k n k is continuous, it does not matter whether this integral 
is calculated from the values of ai k given by (16.4) or from formula (15.9), 
which relates to the medium surrounding the body. Let us suppose that 
this medium is in mechanical and thermal equilibrium. Then the calcula- 
tion is further simplified if we use the condition of equilibrium (15.18). 
From this condition, part of the stress tensor (15.9) is constant through the 
body, being a uniform compressing or expanding pressure and making no 
contribution to the total force F and torque K acting on the body. These can 
therefore be calculated by writing 0% as 

°ik = (eM(EiE k -±E28 ik ) (16.7) 

simply, where E is the field in the fluid and e its dielectric permeability; this 
expression differs only by a factor e from the Maxwell stress tensor of the 
electric field in a vacuum. Thus 

F = (e/47r)^[E(n.E)-i^ n]d/) (16>8) 

K = (e/477) <([rx E(n- E) -\E*r x n] d/. (16.9) 



72 Electrostatics of Dielectrics §16 

It may also be noted that, since the fluid is in equilibrium, we can take 
these integrals over any closed surface which surrounds the body in question 
(but, of course, does not enclose any of the charged bodies which are sources 
of the field). 

4 The calculation of the total force on a dielectric in an electric field in a 
vacuum can also be approached in another way by expressing this force, 
not in terms of the actual field, but in terms of the field (£ which would be 
produced by the given sources in the absence of the dielectric; this is the 
"external field" in which the body is placed. Here it is assumed that the 
distribution of charges producing the field is unchanged when the body is 
brought in. This condition may not be fulfilled in practice — for example, if 
the charges are distributed over the surface of an extended conductor and the 
dielectric is brought to a finite distance from it. 

In a virtual translation of the body over an infinitesimal distance u, the 
total free energy of the body varies, according to ( 1 1 . 3), by 8$F = — J P • S(£d V, 
where 8© = (£(r + u)— (£(r) = (u«grad)(£ is the change in the field at any 
given point in the body. Since u = constant and curl © = 0, we have 
P.(u«grad)(£= P-grad(u.(g) = U'(P«grad)<£, so that 

8& = — u- J"(P.grad)© dV. 

But 8!F = — u«F, and we therefore have for the required force t 

F = J (P.grad)(£dF. (16.10) 

Similarly, the total torque on the body can be determined. We shall not 
go through the calculation, but merely give the result: 

K = J"Px(&dK+J*rx(P.grad)©dF. (16.11) 

In an almost uniform field, which may be regarded as constant over the 
dimensions of the body, formula (16.10) gives to a first approximation 

F = (J PdF-grad)© = (^.grad)<£, (16.12) 

where £P is the total dipole moment of the polarised dielectric; this result, 
of course, could have been obtained by direct differentiation of SF from 
(11.8). In formula (16.11) we neglect the second term in the first approxi- 
mation and reach the natural conclusion that 

K = ^x<£. (16.13) 



f It should be emphasised, however, that the integrand in (16.10) cannot be interpreted 
as the force density. The reason is that the local forces in the dielectric arise not only from 
the field © but also from the internal fields which, by Newton's third law, contribute nothing 
to the total force, though they modify the distribution of forces over the volume of the body. 



§17 Piezoelectrics 73 

PROBLEMS 

Problem 1 . A dielectric sphere of radius a in a uniform external field © is cut in half by a 
plane perpendicular to the field. Determine the force of attraction between the hemispheres. 

Solution. We imagine the hemispheres separated by an infinitely narrow slit and deter- 
mine the force from formula (16.8) with e = 1, integrating over the surface of a hemisphere ; 
E is the field in the vacuum near the surface. According to (8.2) the field EW inside the sphere 
is uniform and equal to 3©/(2+e), where e is the dielectric constant of the sphere. The field 
in the slit is perpendicular to the surface and is E = D<*> = 3e(fe/(2+e). On the outer surface 
of the sphere we have 

E r = DW r = -^- ® cos 6, Ee = E(% = — (£ sin 6, 

2+e e+2 

where 6 is the angle between the radius vector and the direction of (£. A calculation of the 
integral gives an attractive force f 

F = 9(e-l)2 a z (£ 2 /16(€+2) 2 . 

Problem 2. Determine the change in shape of a dielectric sphere in a uniform external 
electric field. 

Solution. As in §5, Problem 4. In determining the change in shape, we assume the 
volume of the sphere to be unchanged.}: The elastic part of the free energy is given by the 
same expression as in §5, Problem 4. The electric part is given by 

V €<*>— 1 

-i^.®= (£2, 

* 8tt1 +«(e<*> -1) ' 

and the dielectric permeability in the ac-direction is, by (16.1), €<*> = eo+aiUxx = eo+ 
+%ai(u X x— u yy ) = co+§ai(a— b)jR. From the condition that the total free energy is a 
minimum we find 

a-b _ 9& (eo-l)2+5ai 

R ~ 4077/1 (e +2) 2 " 

For e<> -*■ <x> this tends to the value for a conducting sphere. 

Problem 3. Determine the body forces in an isotropic solid dielectric, assumed homoge- 
neous, when extraneous charges are present in it. 

Solution. Assuming eo, ai, az constant and using the equations curl E = 0, div D = 
co div E = 4Tp ex , we have from (16.4) 



/< = 



da ik _ daOhk 1 ., . . 8E 2 
Sxte dxjc 



1 „ v 8E* t ai\ 



§17. Piezoelectrics 

The internal stresses which occur in an isotropic dielectric in an electric 
field are proportional to the square of the field. The effect is similar in 
crystals belonging to some of the crystal classes. For certain types of 
symmetry, however, the electrostriction properties of the crystals are quite 
different. The internal stresses in these piezoelectric bodies resulting from 



t It is by chance that, in the limit e -> oo, this expression tends to the result obtained 
in §5, Problem 3, for a conducting sphere (indeed, the forces are in opposite directions). 
The two cases are evidently not physically equivalent, because there is no field in the slit 
between two Conducting hemispheres at the same potential, whereas in this problem there 
is a field in the slit. 

X The change in volume is determined in §12, Problem 1. 



74 Electrostatics of Dielectrics §17 

an electric field are proportional to the field itself. The converse effect also 
occurs : the deformation of a piezoelectric is accompanied by the appearance 
in it of a field proportional to the deformation. 

Since in a piezoelectric only the principal (linear) effect is of interest, we 
can neglect the terms quadratic in the field in the general formula (16.5). 
Then aac = F8ik+(dFlduijc) T #. In this section we shall use the thermo- 
dynamic quantities referred to the matter in unit volume of the undeformed 
body (see the first footnote to §16). Taking F in this sense, we have simply 

croc = (SFlduiJc) T>E . (17.1) 

Accordingly, the thermodynamic relation for the differential dF is 

dF = -SdT+aikduiic-D. dE/4rr. (17.2) 

The following remark should be made concerning the last term. In the form 
given here, this term (taken from (10.9)) pertains, strictly speaking, to unit 
volume of the deformed body. By ignoring this fact, we commit an error 
which, in the case of a piezoelectric, is of a higher order of smallness 
than the remaining terms in (17.2). 

The independent variables in (17.2) include the components of the tensor 
tint. It is sometimes convenient to use instead the components am. To do so, 
we must introduce the thermodynamic potential, defined as 

0> = F-u ik aijc. (17.3) 

For the differential of this quantity we have 

d$ = -SdT-u ik da ik -D'dEI47T. (17.4) 

It must be emphasised that the use of the thermodynamic potential O in 
electrodynamics in accordance with formulae (17.3) and (17.4) rests on 
the validity of (17.1) and so is possible only for piezoelectric bodies. 

Having thus defined the necessary thermodynamic quantities, let us now 
ascertain the piezoelectric properties of crystals. If o% and E k are taken as 
independent variables, the induction D must be regarded as a function of 
them, and an expansion of this function must include the linear terms in 
them. The linear terms in the expansion of the components of a vector in 
powers of the components of a tensor of rank two can be written, in the 
most general case, as 4myi t ja akh where the constants yijci form a tensor of 
rank three, and the factor Att is introduced for convenience. Since the tensor 
ajd is symmetrical, it is clear that the tensor y ik i may also be supposed to 
have the symmetry property 

7i,kl = 7i,lk- (17-5) 

For clarity we separate the symmetrical suffixes from the remaining one by 
a comma. We call yijci the piezoelectric tensor. If it is known, the piezo- 
electric properties of the crystal are entirely determined. 



§17 Piezoelectrics 75 

Adding the piezoelectric terms to the expression (13.1) for the electric 
induction in the crystal, we have 

Dt = Doi+eikEjc + ^Yi^icrjci. (17-6) 

Corresponding additional terms appear in the thermodynamic quantities. 
The thermodynamic potential of a non-piezoelectric crystal in the absence of a 
field is O = <S> = <S>o—2tMMm0ik<rim, where <X>o pertains to the undeformed 
body, and the second term is the ordinary elastic energy, determined by 
the elastic constant tensor (HkimA For a piezoelectric we have 

<P = ®0 — ilHklm<Tik<nm— HkEiEklSTT — EiDoil^TT — yijiEiajci' (17.7) 

The form of the last three terms is given by the fact that the derivatives of O 
with respect to E\ (for given temperature and internal stresses), found from 
the relation Di — —4^-dOjdEi, must accord with (17.6). 

Knowing <I>, we can obtain from (17.4) a formula giving the strain tensor 
in terms of the stresses aye and the field E: 

tHk = - (^0/3(7ffc)rE = imamoim + yijutEi. (17.8) 

It should be mentioned that to regard the quantities fiycim and cm for a 
piezoelectric as elastic constants and dielectric permeability is to some extent 
conventional. With the definitions used here, they give respectively the 
strains as functions of the elastic stresses for a given field, and the induction 
as a function of the field for given stresses. If, however, the deformation 
occurs with a given value of the induction, or we consider the induction as a 
function of the field for given strains, the elastic constants and the dielectric 
permeability will be represented by other quantities, which can be expressed 
as somewhat complex functions of the components of the tensors /i, e and y. 

The field in a piezoelectric body must be determined together with its 
deformation, leading to a problem in both electrostatics and elasticity 
theory. We must seek a simultaneous solution of the electrostatic equations 

div D = 0, curl E = 0, (17.9) 

with D given by (17.6), and the equations of elastic equilibrium 

da ik /dx fc = 0, (17.10) 

with the appropriate boundary conditions at the surface of the body and use 
of the relation (17.8) between aye and the strains. In general this problem is 
very complex. 



t The tensor iHkim determines the relation between stress and strain: 

1Mk = — d® J don = IHklmOlm- 
In Theory of Elasticity, §10, the converse relation on = Xikimuim is used. It is evident that 
the symmetry properties of the tensor paim are exactly the same as those of Xikim. 
The free energy F contains the elastic energy with the plus sign: 

•Pel = ihklmUikUim. 

The thermodynamic potential is obtained from F by subtracting oikUik, and so 
^el = -Pel — ooeUUe = — ^XikimUikUim = — ilMklmOikOi m . 



76 Electrostatics of Dielectrics §17 

The problem is much simplified for a body of ellipsoidal form with a free 
surface (i.e. one subject to no external mechanical forces). In this case 
(§8), the field inside the body is uniform; the deformation is therefore 
homogeneous, and the elastic stresses o% = 0. t 

Finally, let us consider which types of crystal symmetry allow the existence 
of piezoelectricity; in other words, what are the restrictions imposed on the 
components of the tensor yi ja by the symmetry conditions. In general, this 
tensor (which is symmetrical in the suffixes k and /) has 18 independent non- 
zero components, but in reality the number of independent components is 
usually much smaller. 

In all symmetry transformations of a given crystal, the components of the 
tensor yija must remain unaltered in value. Hence it follows at once that 
no piezoelectric body can have a centre of symmetry or, in particular, be 
isotropic. For, on reflection in the centre (i.e. change of sign of all three co- 
ordinates), the components of a tensor of rank three change sign. 

Of the 32 crystal classes, only 20 allow piezoelectricity. These comprise 
the ten enumerated in §13 as allowing pyroelectricity (all pyroelectrics are 
also piezoelectrics) and the ten following classes : 

rhombic system: D2 
tetragonal system : D4, D^a, $4 
rhombohedral system: D3 
hexagonal system: D 6 , C 3 h, D 3 n 
cubic system: T, T&. 

The non-zero components of the piezoelectric tensor for each class are 
given in the following Problems. 

PROBLEMS 
Problem 1. Determine the non-zero components of the tensor yj,*j for non-pyroelectric 
crystal classes which allow piezoelectricity. 

Solution. The class Dz has three mutually perpendicular axes of symmetry of the second 
order, which we take as the axes of x, y and z. Rotations through 180° about these axes 
change the sign of two out of the three co-ordinates. Since the components yt.jti are trans^ 
formed as the products XiXiexi, the only non-zero components are those with three different 
suffixes: yx.yz, yz.xy, Yy.zx. (The other non-zero components are equal to these, since 
y i>kl = y J( ifc.) Accordingly, the piezoelectric part of the thermodynamic potential ist 

Opie = —2{yx,yzEx<ryz J ryy,xzEya X z-{-yz,xyEzOxy). (l) 

The class Z>2<j is obtained by adding to the axes of class D2 two planes of symmetry passing 
through one axis (the s-axis, say) and bisecting the angles between the other two. Reflection 
in one of these planes gives the transformation x -*■ y, y -> *, z -*■ z. Hence the components 
yi.jfcj which differ by interchange of x and y must be equal, so that only two out of the three 
coefficients in (1) are now independent: yz.xy, Yx,yz = Yv.xz. 



f To avoid misunderstanding it should be recalled that, if we calculate the components 
of the strain tensor mic by direct differentiation of the actual expression for O with respect 
to oik, the derivatives with respect to components aa with i ¥* k give twice the correspond- 
ing components uit; see Theory 0/ Elasticity, §10, Problem, footnote. 



§17 Piezoelectrics 77 

The class T is obtained from the class D2 by adding four diagonal axes of symmetry of 
the third order, rotations about which effect a cyclic permutation of x, y, z, e.g. x -+ z, 
y -> x, z -> y. Hence all three coefficients in (1) are equal: yx.yz = yz.xy = Yy.zx- The same 
result is obtained for the cubic class Ta. 

The class D4 has one axis of symmetry of the fourth order (the z-axis, say) and four of the 
second order lying in the xy-plane. Here the symmetry elements of the class Dz are supple- 
mented by a rotation through 90° about the ar-axis, i.e. the transformation * -> y, y -*■ —x, 
z -> z. Consequently, one of the coefficients in (1) must be zero iyz.xy — —yz.yx — —yz,xy 
= 0), and the other two are equal, but opposite in sign: yx.yz = —yy.xz- The same result is 
obtained for the class De. 

The class 1S4 includes the transformations x -*■ y, y -*■ —x, z -*■ — z and x -> — x, 
y -> — y, z -> z. The non-zero components are yz.xy, Vx.yz = yy.xz, yz.xx = —Yz.yy, 
yx.zx = —yy.zy. One of these can be made to vanish by a suitable choice of the * and y axes. 

The class D3 has one axis of symmetry of the third order (the sr-axis, say), and three of the 
second order lying in the xy-plane ; let one of these be the *-axis. To find the restrictions 
imposed by the presence of a third-order axis, we make a formal transformation by introducing 
the complex "co-ordinates" £ = x+iy, 17 = x—ty; the co-ordinate z remains unchanged. 
We must also transform the tensor yi.jti to these new co-ordinates, in which the suffixes take 
the values g, 17, z. In a rotation through 120° about the .z-axis these co-ordinates undergo 
the transformation f -*■ fe 2 "" 3 , i] -*■ ije -2 "" 3 , z -*■ z. The only components of the tensor yi,ici 
which remain unchanged and so may be different from zero are yz.rf, Yy.zg, Y£,tn> Y£&> Y-n.im 
and yz.zz- A rotation through 180° about the x-axis gives the transformation x -*■ x, y -*■ —y, 
z -> — z, or £ -*■ 17, 7) -»■ £, z -> — z; yz, v g and Yz.zt change sign and so must be zero, while the 
remaining components listed above are mutually transformed in pairs, giving y VtS g = —ys.zj,, 
y{,& = y^,^. In order to write an expression for <f> p i e , we must form the sum —yt.iciEiojei, 
in which the suffixes take the values £, 17, z: 

#pie = — 2y„«5(£i,<7 z5 — Ezozr,)— 75.55(^5^5+^)0^?). 

Here the components Ei and o-j* in the co-ordinates £, 17, z must also be expressed in terms of 
those in the original co-ordinates x, y, z. This is easily done by using the fact that the com- 
ponents of a tensor are transformed as the products of the corresponding co-ordinates. 
Hence, for example, from I s = x 2 —y 2 +2ixy, we have o^ = o X x— oyy+2io X y. The result is 

^pie = 2a(Ey<Xzx — E x o t y)-{-b\2Eya X y— E x (oxx— o*yy)]> (2) 

where a = 2iy v ,z( and b = 2yg.(( are real constants. The relations between the components 
yi,jw in the co-ordinates *, y, z are, as we see from (2), f 

Yy.zx = — Yx.zy — a, yy,xy = — Vx.xx = Yx.yy ^ b. 

The class Dzn is obtained from the class D3 by adding a plane of symmetry (the «y-plane) 
perpendicular to the axis of the third order. Reflection in this plane changes the sign of z, 
and so y v , Z f = 0, so that only the term with the coefficient b remains in (2). 

The class Czn has an axis of the third order and a plane of symmetry perpendicular to it. 
Reflection in this plane changes the sign of z, and so all components yt.ki whose suffixes 
contain z an odd number of times must be zero. Taking into account also the restrictions 
derived above which are imposed by the third-order axis of symmetry, we find that only the 
two components y Vlim and y^.ff are not zero. These quantities must be complex conjugates 
in order that O should be real. Putting 2y v , rrri = a+ib, 2y£,g = a—ib, we find 

Ople= a[2EyO X y— Exioxx — <ryv)]+b[2ExOxy+Ey(oxx — o yy )]. (3) 

Either a orb can be made to vanish by a suitable choice of the x and y axes. 

Problem 2. The same as Problem 1, but for the crystal classes which allow pyroelectricity. 



f In non-orthogonal co-ordinates such as £, 17, z the covariant and contravariant compo- 
nents of tensors must be distinguished. This should have been done in returning to the 
original co-ordinates x, y, z. We avoid this necessity, however, by obtaining the required 
relations between the components yi,*i in the co-ordinates x, y, z directly from the form 
of the scalar combination (2). 



78 Electrostatics of Dielectrics §17 

Solution. Let the #-axis be the axis of symmetry of the second, third, fourth or sixth 
order, or in the class C s be perpendicular to the plane of symmetry. In the classes Cnv the 
xsr-plane is a plane of symmetry. By a suitable choice of axes three more components can be 
made zero in the class d, and one component in the classes C s , Cn- 

We give below for each class all the components yi.ki which are not zero. 
Class C\: all yi.ki. 

C s : all those in which the suffix z appears twice or not at all. 

Cjjwl Yz.xx, Yz.yy, Yz.zz, Yx.xz, Yy.yz. 

Ct.: the same, together with yx.yz, Yy,xz, Yz.xy- 

Civ- Yz.xx = Yz.yy, Yz.zz, Yx.xz = Yy.yz. 

Ca'. the same, together with yx.yz = —yy.xz. 

Csv- Yz.zz, yx.xz = yy.yz, yx.xx = — Yx.yy = — Yv.xy, Yz.xx — Yz.yy. 

Cs: the same, together with yx.yz = —Yy.zx, Yy.xx = —yy.yy = yx.xy. 

Cov'' Yz.zz, yx.xz = Yy.yz, Yz.xx = Yz.yy 

C&: the same, together with yx.yz — —yy.zx. 

Problem 3. Determine Young's modulus (the coefficient of proportionality between the 
extending stress and the relative extension) for a flat slab of a non-pyroelectric piezoelectric 
in the following cases : (a) where the slab is stretched by the plates of a short-circuited 
condenser, (b) where it is stretched by those of an uncharged condenser, (c) where it is 
stretched parallel to its plane with no external field. 

Solution, (a) In this case the field E inside the slab is zero. The only non-zero component 
of the tensor aw is the extending stress a zz (the ar-axis being perpendicular to the slab).f 
From (17.8) we have u Z z = pzzzzOzz, whence Young's modulus is E = l//x ZZ2Z . 

(b) In this case we have in the slab E x = E v = 0, D z = 0. From (17.6) and (17.8) 
we have Dz = e Z zEz+^Yz,zzCFzz = 0, u zz = pzzzzOzz+Yz.zzEz. Eliminating Ez, we obtain 
1/2? = {J.zzzz—4-nyz,zz 2 lezz. 

(c) In this case also, E x = E y — 0, Dz = 0, but the extension is along the *-axis, say. 
Here we have Dz = e zz Ez-\-^yz,xxo X x = 0, u X x = pxxxxOxx+Yz.xxEz. Eliminating Ez, 
we obtain 1/E = iixxxx—^nyz.xx^hzz. 

Problem 4. Obtain an equation for the velocity of sound in a piezoelectric medium. 

Solution. In this problem it is more convenient to use iwc as the independent variables, 
ns tead of one. We write F in the form 

„ 1 1 

F = Fo+^Xikim tine mm — — em EiEh— — EiDoi+Pt.ki Eiujci, 

8ir Artt 

where 

fii.kl = — Afcifc'I' Yi.le'l', 

whence 

oik = dF/diUk = X{klmWm + fil.ikEi. 
The equations of motion from the theory of elasticity are 

doik duim dEi 

Hi — — = Aiklm— VPl.ik- — , 

oxk oXk oxk 

where u is the displacement vector, related to utk by 

1 / 8m 8uk\ 



The equation div D = gives 



1 / dm duk\ 
~ 2\8xk dxj' 



8Ek 8uu _ 

OXi OXi 



and the field can be expressed in terms of the field potential: Ei = —d<j>/dxi, which takes 
into account the equation curl E = 0. 



t It is not assumed to coincide with any particular crystallographic direction. 



§18 Thermodynamic inequalities 79 

In a plane sound wave, u and 4> are proportional to exp[i(k.-r— wt)], and we find from the 
above equations that 

U) 2 m = XiicimkkkiUm—pl.ikkickit, 

eikkikjc<f>+4iTfii,jcikikjcUl = 0. 

Eliminating <f>, we can write the condition of compatibility of the resulting equations for 
m as 

\co 2 8ik — Xmmkikm— ^M.Pl,mikik m )(Pp,g]ckpkq)le r tk r kt\ = 0. 

For any given direction of the wave vector k, this equation determines three phase velocities 
of sound tojk, which are in general different. A characteristic property of a piezoelectric 
medium is the involved relation between the velocity and direction of the wave. 



§18. Thermodynamic inequalities 

According to the formulae of §10, the total free energy can be written as 
the integral 

&=JF(T,p,-D)dV, (18.1) 

taken over all space. We shall suppose that the function D(x, y, z) which 
appears in the integrand satisfies only the equation 

div D = (18.2) 

inside a dielectric and the condition 

D- df = lire (18.3) 

on the surface of a conductor which carries a given charge. These equations 
establish the relation between the field and its sources. Otherwise we regard 
the function D(#, y,z) as arbitrary, and in particular we do not require it to 
satisfy the second field equation curlE = (where E = fadF/dD) or the 
boundary condition <f) = constant on the surface of a conductor. We shall 
show that these equations can then be obtained from the condition that the 
integral (18.1) is a minimum with respect to changes in the function 
D(x,y, z) which satisfy equations (18.2) and (18.3). It should be emphasised 
that the possibility of this derivation is not a priori evident, since the field 
distributions which come into consideration in determining the minimum of 
the integral (18.1) do not necessarily correspond to physically possible states 
(because they do not satisfy all the field equations), whereas, in the thermo- 
dynamic condition that the free energy is a minimum, only the various 
physically possible states are considered. 

The problem of finding the minimum of the integral (18.1) with the 
subsidiary conditions (18.2) and (18.3) is solved by Lagrange's method of 
multipliers. We multiply the variation of the condition (18.2) by some as 
yet undetermined function — <£/4-7r of the co-ordinates, and that of the 



80 Electrostatics of Dielectrics §18 

condition (18.3) by some undetermined constant <£o/4-n-, and then equate to 
zero the sum of variations 

(8FdV (<f>div8DdV+ — (j>SD.df = 0. 

In the first term we write t 

8F = (dF/dD) TfP >8I> = E-SD/477, 

and the second can be integrated by parts: j* <£ divSD dV = j$SD»df— 
— J SD-gr ad <f> dV. The result is 

J(E + grad<£).SDdF+<j# -<£)SD.df = 0. 

Hence we conclude that, throughout the volume, we must have E = — grad <j> 
(and so curl E = 0), and on the surface of a conductor <f> = <f>o = constant. 
These are the correct equations for the field, and the Lagrangian multiplier 
<f> is its potential. 

Similarly it can be shown that the equations for the electric induction are 
obtained from the condition that the integral #= J F (T, />, E)dF is a mini- 
mum, in which the function E(x,y, z) is varied with the subsidiary conditions 
E = —grad <f> and <f> = constant on the surface of a conductor. For 

8# = j(dF/dE)-8EdV=jl>-griidS<f>dVl4iT 

= £ 8<l> D- df/4n— J8cf> div D dV^ = 0. 

The first integral is zero because 8<f> = on the surface, and from the second 
we find the required equation div D = 0, since S<£ is arbitrary in the volume. 
If the body is not in an external electric field (in particular, if there are no 
charged conductors), it may be possible to formulate the condition of thermo- 
dynamic equilibrium as the condition that the total free energy (18.1) has 
an absolute (unconditional) minimum. This amounts to the condition that the 
free energy density F is a minimum as a function of the independent variable 
D: dFjdD = E/47T = 0, i.e. the field must be zero in all space. If it is pos- 
sible to find a distribution of the induction such that div D = 0, this state 
will correspond to thermodynamic equilibrium. J 



t The free energy is the minimum for a given temperature. The variation is with respect 
to two independent quantities D and p. Here we are interested only in the result of varying 
with respect to D. The variation of the integral (18.1) with respect to density (with the 
subsidiary condition of constant mass, i.e. constant J> dV) gives one of the usual conditions 
of thermal equilibrium, namely the constancy of the chemical potential £. 

t Here we are considering bodies in which D need not be zero even if E = (see §19). 
Otherwise we have simply the trivial result E = D = in all space. 



§18 Thermodynamic inequalities 81 

Equating to zero the first variation of the free energy, we find necessary but 
not sufficient conditions for this energy to be a minimum. The calculation 
of the sufficient conditions requires a discussion of the second variation. 
These conditions take the form of certain inequalities (called thermodynamic 
inequalities) and are the conditions which ensure the stability of the state of 
the body.t 

When there is a linear relation between D and E, the situation is much 
simplified, and the thermodynamic inequality of interest here (relating to 
the dielectric properties of the body) becomes evident. The total free energy 
is F Q + J(D 2 /87re)dF. It is clear that this can have a minimum only if 
c > 0, since otherwise the integral could be made to take any large negative 
value by making D 2 large enough. Thus in this case nothing new is learnt, 
since we know already that the dielectric permeability must in fact be not 
only positive but greater than unity (see §14). 

In the general case of an arbitrary relation between D and E, however, it 
is necessary to consider the second variation of the integral (18.1), and to 
vary simultaneously both D and p (leaving only the temperature constant). 
In an isotropic body, F(T, p, D) depends only on the magnitude of the vector 
D, but its three components vary independently. We take the direction of 
the vector D before variation as the s-axis. Then the change in the magni- 
tude of D is given in terms of the changes in its components, as far as the 
second-order terms, by 8D = 8D z + (8D x fl2D + (8D y fl2D. The first and 
second variations of the integral (18.1) are both contained in the expression 

n dF dF 1 d 2 F d*F 1 d*F \ r 

J _SD + — -8p + - — -(8DY + 8D8 P + (5/. 2 dF. 

J \ dD fy P 2 8D* K } dDdp H iJp* F) ) 

Substituting 8D and collecting the second-order terms, we find the second 
variation 



/■ 



±^[ { 8D x f + (8D y ndV+ 



nl d 2 F d*F 1 d*F \ 

♦Jhs^W^W^K (18 - 4) 

These two terms are independent. The first is positive if (\jD)dFjdD > 0. 
But BF/dD = E/4tt, so that the derivative dFjdD is positive or negative 
according as the vectors D and E are in the same or opposite directions. 
Thus these vectors must be in the same direction. 

The conditions for the second term in (18.4) to be positive are 

d^F/dp* > 0, (18.5) 

^^_(^ 2 >0 . (18.6) 



t See Statistical Physics, §21. 



82 Electrostatics of Dielectrics §18 

Since dF/dp = £, dFjdD = Ej\ir, the first of these gives 

(dydp) D , T > 0, (18.7) 

and the second can be rewritten as a Jacobian : 

d(dF/ 3D, dF/dp) _ 1 8(E y Q 

8(D,p) "Sa(A/>) 

Changing from the variables D, p to D, £, we have 

8(E,0 d(E,Q 3(Z),0 (dE\ rib 



> 0. 



\dDlAdpJ D 



d(D,p) d(D,!Z)d(D,p) XdD/^dp) 

by (18.7), this gives 

(BEldD\, T > 0. (18.8) 

Thus we have derived the required thermodynamic inequalities. In the 
absence of a field, the inequality (18.7) becomes the usual condition that the 
isothermal compressibility is positive: (dp/8p)T > O.t The inequality (18.8) 
gives e > 0, since when E -> the induction D -> eE. 

Of the two inequalities (18.5), (18.6) the latter is the stronger; it may be 
violated while the first is not, whereas the reverse is impossible. The equation 



= 



8*F d*F / d*F \ 2 _ d(E, t) 

~dp^~d^~ \ dpdDj ~ d(D, P ) 

corresponds to what' is called the critical state.% This condition is more 
conveniently written in a different form by multiplying it by the non-zero 
factor 8(D, p)\d(E, p): 

8(E, Qld(E, P ) = {dlld P ) Bt T = 0. (18.9) 

The determination of further conditions for the stability of the critical state 
of matter requires a study of the third and fourth variations ; we shall not 
pause to do this, but simply give the results: 

(dH/dp^E.T = 0, (18.10) 

(dK/dp3) E . T > 0, (18.11) 

in analogy with those found in the absence of an electric field. 



t It should be recalled that, in the absence of a field, £ is the thermodynamic potential 
of unit mass and, by the ordinary thermodynamic relations, its differential 



dC = dp/p - (Sip) dT, 

It 
t 

X See Statistical Physics, §80. 



so that (dt/dp)T = (l/p)(S/>/3/>)r. In the above derivation the second of the ordinary 
thermodynamic inequalities (that the specific heat is positive) is ignored. 



§19 Ferroelectrics 83 

PROBLEM 

Determine the displacement of the critical point of a dielectric substance in an electric 
field. 

Solution. Substituting in (18.9) the expression for £ from (10.18), we find (d£o/3p)r — 
— (£ 2 /8w)(3 2 c/3p 2 )r = 0. For the chemical potential when E = we have (3£o/3p)r 
= (l/p)(d/>/3p)r (see the penultimate footnote to this section), where p = p{p, T) is the 
equation of state in the absence of the field. Thus (dpldp)r = (p£ 2 /8ir)(a 2 e/dp 2 ):r. When 
there is no field, the critical point is given by the equation (3p/3p)r = 0, and if it is stable 
we must also have (d 2 p/dp 2 )T = 0. Hence 



\dp/T 



SH> S*p d*p 

— ApH —AT ~ —AT, 

dp 2 r dpdT ~ dpdT 



where AT and Ap are the displacements of the critical temperature and density (assumed to 
be of the same order of smallness, which is confirmed by the result). The temperature 
displacement is therefore 

P E 2 / d 2 e\ 



8ir \ dpt/Tl 



dpdT 

The displacement of the critical pressure is Ap = (dp/dT) fi AT. To determine the displace- 
ment Ap, equation (18.10) must be used in a similar manner. 



§19. Ferroelectrics 

The various crystalline modifications of a given substance may include 
some which are pyroelectric and some which are not. If the change from 
one to the other takes place by means of a second-order phase transition, 
then near the transition point the substance has a number of unusual proper- 
ties which distinguish it from ordinary pyroelectrics ; these are called ferro- 
electric properties. 

In an ordinary pyroelectric crystal, a change in the direction of the spon- 
taneous polarisation involves a considerable reconstruction of the crystal 
lattice. Even if the final result of this reconstruction is energetically favour- 
able, its realisation may still be impossible because it would require the 
surmounting of very high energy barriers. 

In a ferroelectric body, however, the situation is quite different because, 
near a second-order phase transition point (a Curie point), the arrangement 
of the atoms in the crystal lattice of the pyroelectric phase is only compara- 
tively little different from the arrangement in the non-pyroelectric lattice 
(and so the spontaneous polarisation also is small). For this reason the change 
in direction of the spontaneous polarisation here requires only a relatively 
slight reconstruction of the lattice (a slight displacement of the atoms) and 
can occur quite easily. 

The actual nature of the ferroelectric properties of a body depends on its 
crystal symmetry. The direction of the spontaneous polarisation of the pyro- 
electric phase (which we shall call the ferroelectric axis) is determined by the 
structure of the non-pyroelectric phase beyond the Curie point. In some 
cases it is uniquely determined, in the sense that the ferroelectric axis can 
lie in only one, crystallographically determinate, direction; the direction of 



84 Electrostatics of Dielectrics §19 

the spontaneous polarisation is then determined apart from sign, since in the 
non-pyroelectric phase the two opposite directions parallel to the ferro- 
electric axis must be entirely equivalent (otherwise this form of the crystal 
would also be pyroelectric). In other cases, the symmetry of the rion- 
pyroelectric phase may be such as to allow spontaneous polarisation in any 
of several crystallographically equivalent directions, t 

The quantitative theory of ferroelectricity can be developed in terms of 
the general theory of second-order phase transitions ;$ this has been done 
by V. L. Ginzburg (1945). 

The basis of the following considerations will be the thermodynamic 
stability of states. From this point of view the transition is characterised by 
the fact that, on one side of it, a state with D = can be stable, but on the 
other side any such state is unstable, and so there must be a non-zero induc- 
tion even when the field E is zero. For definiteness, we shall suppose below 
that the pyroelectric phase (D ^ 0) corresponds to temperatures T < 
(where is the transition point), but it should be emphasised that this dis- 
position of the phases, though the more usual, is not obligatory, and the 
opposite case is also found in Nature. 

Since our prime interest is in the dielectric properties of the substance, we 
shall first suppose that there are no internal stresses in the body. To deter- 
mine the stability conditions, we can start from the condition that the total 
thermodynamic potential of the body is a minimum (for a given temperature 
and zero stresses). As we have seen in §18, this reduces to the condition that 
the second variation of the thermodynamic potential per unit volume <D 
should be positive. For a state in which the induction is almost zero, the 
second variation of O is simply <E>— <J>o = {\l%Tr)€~\]cDiDic. 

If we take the co-ordinate axes to be the principal axes of the tensor e**, 
then 

The state with D = satisfies the stability conditions (i.e. can correspond to 
a minimum of O) so long as all three coefficients l/e (<) are positive. Hence 
the pyroelectric phase can be formed only when one of these three coeffi- 
cients changes sign. The point at which the second-order phase transition 
takes place is determined by the vanishing of that coefficient. 



t An instance of the first type is sodium potassium tartrate, whose non-pyroelectric 
phase has a rhombic symmetry. The ferroelectric axis appears in it (at the Curie point) 
in a completely definite crystallographic direction (one of the second-order axes), and the 
lattice becomes monoclinic. 

An instance of the second type is barium titanate. Its non-pyroelectric modification has 
a cubic lattice, and any of the three cubic axes may become the ferroelectric axis. After the 
spontaneous polarisation has appeared at the Curie point, these three directions, of course, 
are no longer equivalent. The ferroelectric axis becomes the only fourth-order axis, and the 
lattice becomes tetragonal. 

X See Statistical Physics, Chapter XIV. The discussion here following, however, is not 
based on the usual formulation. 



§19 Ferroehctrics 85 

The ferroelectric axis is then the one for which l/e (<) is zero. Here various 
cases can arise, depending on the symmetry of the non-pyroelectric phase. If 
this symmetry is such that e te) ^ e^ ^ e^, only one of the coefficients 
in (19.1) is zero, and the position of the ferroelectric axis is uniquely defined. 
If e te) = e ( 2/ } = efe> (for which the symmetry must be cubic), all three coeffi- 
cients vanish simultaneously, and the ferroelectric axis may be in one of 
several directions (see below). Finally, if the symmetry is such that e te) 
= eW ^ e fe) } either one or two of the coefficients in (19.1) will vanish at 
the transition point. 

Let us consider first the case where the position of the ferroelectric axis, 
which we take as the #-axis, is uniquely determined. The dielectric properties 
of the crystal in the x and y directions then exhibit no anomalies, and to 
investigate the properties in the s'-direction we need consider only those 
terms in the thermodynamic potential which contain D z . 

The expression (19.1) represents the leading terms in an expansion of <X> 
in powers of D. Since l/e fe) is small near the transition point, it is necessary 
to take into account the next term beyond the quadratic in the expansion in 
powers of D z . There can be no odd powers in this expansion, since they would 
change sign with D z (and so <D would change), whereas here the two direc- 
tions along the #-axis are equivalent. The next term after the quadratic 
therefore involves D z *: 

In order that the state with D z — should be stable at the point T — 0, 
it is clearly necessary that the coefficient B should be positive there, and so 
positive in the neighbourhood of that point. Near the transition point, 
l/e fe) can be expanded in powers of the difference T— 0; the first term in 
the expansion is of the form cc(T— 0), the coefficient a being positive (so 
that 1/efe) > for T > 0). Thus 

€<*> = l/a(T-0), (19.2) 

and the thermodynamic potential is 

a(T-0) B 

<D = <D + -^- L D f+ Dt A. (19.3) 

07T 107T 

These formulae are sufficient for the calculation of all ferroelectric proper- 
ties of present interest near the transition point. First of all, from the formula 
E z = 47rd®ldD z we have 

E z = a(T-0)D z + 5D 2 3. (19.4) 

This is the fundamental relation giving the field as a function of the induction 
in a ferroelectric. 

For T > (in the non-pyroelectric phase), D z is zero for E z = 0. As E z 
increases (for a given value of T— 0), the induction at first increases linearly 



86 Electrostatics of Dielectrics §19 

(D z = E z Jol{T—%)), but for sufficiently large values of E z we have 
T> z = {E z jBy. The proportionality coefficient e fe) in the linear relation is the 
dielectric constant of the non-pyroelectric phase. For T -> it increases 
without limit in inverse proportion to T— 0, but the linear relation then 
ceases to be valid. 

For T < (in the pyroelectric phase), the value D z = cannot corres- 
pond to a stable state. For E z = the induction has a non-zero value, which 
by (19.4) is 

D z = D z0 = ± VW© - T)IB]. (19.5) 

Thus the spontaneous polarisation P z q = D z oj4^r of a ferroelectric decreases 
towards the Curie point as \/(0 — T). 

The "dielectric constant" of the pyroelectric phase may be defined as the 
value of the derivative dD z ldE z for E z = 0. From (19.4) we have 
1 = [— a (0— T) + 3BD z 2 ]dD z ldE z ; substituting (19.5), we have dD z jdE z 
= l/2a(0 — T) for E z = 0. For sufficiently small E z , the relation between D z 
and E z becomes 

D z -D z0 = E z /2x(e-T). (19.6) 

A comparison of (19.2) and (19.6) shows that the "dielectric constant" of 
the pyroelectric phase is half its value in the non-pyroelectric phase at the 
same distance from the Curie point. 

Differentiating O (19.3) with respect to temperature, we can find the 
entropy S= — (d<&[dT)D = *Sb— aZ) 2 2 /87r. Here the fourth-order term can 
be omitted, since the quadratic term is not zero. In the pyroelectric phase 
with E z = we have also D z = 0, so that S = 5b- For the pyroelectric 
phase, substituting D z from (19.5), we find S = So—ol 2 (® — T)I8ttB. Hence 
the specific heat of this phase at the transition point itself is 

C v = TdS/dT = Cpo + o^Q/SttB, (19.7) 

where C v q is the specific heat of the non-pyroelectric phase at this point. 
Thus, if the transition of the ferroelectric from one phase to the other takes 
place with E z — 0, it is accompanied by a sudden change in the specific heat, 
as happens in ordinary second-order phase transitions. Moreover, C p > Cpo, 
i.e. the specific heat increases when pyroelectricity appears. 

Let us further investigate equation (19.4) in the pyroelectric phase (i.e. 
for T < 0). Figure 13 shows the approximate curve of the function D z (E z ) 
given by this equation. We see, first of all, that the part CC of the curve 
(shown dashed in Fig. 13) does not correspond to stable states which can 
occur in Nature: on CC we have dE z \dD z = Atrd^jdD^ < 0, whereas 
the condition that the second variation of the thermodynamic potential 
should be positive requires this derivative to be positive also. The 
ordinates of the points C and C are given by the equation 8E z /3D z = 0, 
and so we conclude that the possible values of \D z \ in the pyroelectric phase 
are bounded below by the condition 

D z 2 > a(0-T)/3£. (19.8) 



§19 



Ferroelectrics 



87 



If we consider states of a ferroelectric with given values of E z , there is still 
an ambiguity in the value of D z , in the range of abscissae between C and C, 
and the question arises of the physical significance of the two values. We 
shall assume the ferroelectric to be a homogeneous flat slab, with the ferro- 
electric axis perpendicular to it, lying between the plates of a condenser, 
which are maintained at given potentials, i.e. which set up a given uniform 
field E = E z . 

For given potentials on the conductors, the condition of stability requires 
that the thermodynamic potential <I> = <J>— E»D/4tt be a minimum. In 
particular, for E = there are two states in which D z has opposite signs (the 
points A and A' in Fig. 13) but O (= O) is the same. These two states, 
therefore, are equally stable, i.e. they are two "phases" which can exist in 
contact. 




Fig. 13 



Hence it is clear that the portions AC and A'C of the curve correspond to 
states which are metastable but not absolutely stable. It is easy to see directly 
that the values of O on AC and A'C are in fact greater than its values of A'B' 
and AB for the same value of E z . The ordinates of A and A' are given by 
formula (19.5). Thus the range of metastability is 



a(e-T)/3B < D z 2 < «(®-T)/B. 



(19.9) 



The existence of these two "phases" with E = is very important, since 
it means that a ferroelectric body can be divided into a number of separate 
regions or domains in which the polarisation is in opposite directions. On 
the surfaces separating these domains, the normal component of D and the 
tangential component of E must be continuous. The latter condition is 
satisfied identically, because E = 0. From the former condition it follows 
that the domain boundaries must be parallel to the #-axis. 

7 



88 Electrostatics of Dielectrics §19 

The actual shapes and sizes of the domains are determined by the condi- 
tion that the total thermodynamic potential of the body should be a mini- 
mum. This subject has not yet been much studied for ferroelectrics. 

If we are not interested in the details of the structure, and consider por- 
tions of the body which are large compared with the domains, we can use 
the induction B averaged over such portions. Its component D z can evi- 
dently take values in the range between the ordinates of A and A' in Fig. 13, 
i.e. 

- VK® - T)/B] <D Z < VW© - T)JB]. (19.10) 

In other words, if D z in Fig. 13 is taken as the induction averaged in this way, 
the vertical segment AA' corresponds to the region of domain structure, and 
the thick curve BAA'B' gives all stable states of the body. 

A ferroelectric must, in particular, have a domain structure if it is not in 
an external electric field. For we have seen in §18 that the conditions of 
thermodynamic equilibrium in the absence of an external field reduce to the 
condition that <I> should be an absolute minimum as a function of D, with 
E = everywhere, t 

Let us consider ferroelectrics which belong (in the non-pyroelectric phase) 
to the cubic system. The cubic symmetry requires that e te) = e ( ^ ) = e te) 
= c, and admits two independent fourth-order invariants formed from the 
components of the vector D, which may, for example, be taken as D x *+D y 4 + 
+D z 4 and D x 2 D y 2 +D x 2 D z 2 +D y W z 2 . Hence the expansion of the thermo- 
dynamic potential is of the form 

<D = ® +oL(T-®)(D x 2 + Dy 2 + D z 2 )ISTT + B(D x *+D y *+D z *)ll(m + 

+ C(D x 2 D y 2 + D X 2 D Z 2 + ZVZ> z 2)/8tt, (19.11) 

where we have again put 1/e = <x.(T—Q), and a, B, C are constants. 

It must be borne in mind, however, that cubic symmetry may admit also 
a third-order invariant DxD y D z ; this happens for the crystal classes T and 
T&, where there is no centre of symmetry. In these cases the state with 
D = certainly cannot satisfy the stability condition (that €> should be a 
minimum), and so no Curie point can exist. Hence the ferroelectric transi- 
tion can occur only in crystals of the classes O, T^, Oa of the cubic system, 
in accordance with the expansion (19.11). 

The sum of the fourth-order terms in (19.11) must be essentially positive. 
Hence we must have 

B > 0, C > -B. (19.12) 

The spontaneous polarisation of a ferroelectric in the absence of an 
external field is determined, as already stated, by the condition that <X> should 



t It should be emphasised that here we are speaking of complete thermodynamic equi- 
librium. This exists in ferroelectrics, but cannot do so in ordinary pyroelectrics, because of 
the difficulty, already mentioned, of reorienting the polarisation and so forming domains 
in them. 



§19 Ferroelectrics 89 

be a minimum as a function of D. In particular, since the second-order 
term in (19.11) is independent of the direction of D, the direction of the 
spontaneous polarisation is determined by the condition that the fourth- 
order terms are a minimum for a given absolute value of D. Two cases are 
possible. If C > B, the minimum of <J> corresponds to D being along any 
one of the axes x, y, z, i.e. along any of the three edges of the cube (the 
crystallographic directions [001], [010], [100]). If, however, C < B, O takes 
its minimum values when D is along any one of the spatial diagonals of the 
cube (the crystallographic directions [111], [111], etc.) i.e. when D x 2 = D y 2 
= D z 2 = %D 2 . In the former case the spontaneously polarised pyroelectric 
phase of the ferroelectric has tetragonal symmetry, and in the latter case it 
has rhombohedral symmetry. 

Let us consider in more detail, for example, the first case (C > B), and 
take as the #-axis the direction of the spontaneous polarisation below the 
Curie point. The magnitude of this polarisation is determined by the mini- 
mum of the expression — <x(@ — T)D 2 /S7t + BD^J\6it, whence 

Do 2 = *(@-T)/B. (19.13) 

The "dielectric permeability" in the ^-direction below the Curie point is, 
of course, different from that in the x and y directions. If the field E is 
small, then D x , D y and D z —Do are also small. Differentiating the expression 
(19.11) gives 

E g = 4nd^ldD z = -x(@-T)D z + BD z 2 ~ 2BD Q \D Z -D ), 
E x = 47rd®/dD x ss [CD 2 - a(0 - T)]D Z , whence 

D 8 -Do-EJ2*&-n (19U) 

D X = BE X ^(Q-T)(C-B). K ' } 

Above the Curie point the dielectric permeability of a cubic ferroelectric is 
the same in all directions : 

€ = l/ a (r-0). (19.15) 

Finally, let us briefly consider the elastic properties of ferroelectrics. 
According to its crystallographic class, the non-pyroelectric phase of a ferro- 
electric may or may not be piezoelectric, t Here particular interest attaches 
to piezoelectric crystals whose symmetry admits a piezoelectric relation 
between the deformation and the polarisation in the direction of the ferro- 
electric axis. These include the classes D2, D%a and S4; in each case the 
induction D z in the direction of the ferroelectric axis appears in the piezo- 
electric part of the thermodynamic potential through a term$— h zxy D z o xy . 



t The non-pyroelectric phase of a ferroelectric is piezoelectric if it belongs to one of 
eight out of the ten classes listed at the end of §17: D2, Da, Z>2<j, 54, D3, De, Csh, Dzn. 

X Since we are using here the potential $>, and not $ as in §17, the piezoelectric tensor 
h,ki is not the same as the tensor yi,ia introduced previously, but their symmetry properties 
are, of course, identical. 



90 Electrostatics of Dielectrics §19 

In the elastic energy of these crystals, the component a xy appears in a term 
—ftxyxyVxy 2 - Thus the thermodynamic potential near the Curie point is 

<D = ® + x(T-T )D 1! ?IS7T + BD z *ll67T-\D z a X y-fx,a X y\ (19.16) 

where for brevity we have put X ZyXy = \ Pxyxy = A*. The terms involving the 
other components are of no interest, since they lead to no anomaly of the 
piezoelectric properties near the Curie point. 

Differentiating <I> with respect to D z and o X y, we find the field E z and the 
deformation u xy :\ 

E z = 47Td®ldD z = a(T- To)D z + BD^-47rXa xy , (19.17) 

U X y = \XD z -\r\LO x y. (19.18) 

In the non-pyroelectnc region when E is small we can neglect the term in 
D 2 3 in (19.17): 

E z = OL(T-To)D z -47T\o x y. 

Substituting D z from (19.18), we find 

A f 2ttA 2 I 

Uxy = Mf-n) Ez+ V + W~n)\ axy ' 

The coefficient of a xy in this formula represents the modulus of elasticity 
for deformations in which the field E z is kept constant, while [jl in formula 
(19.18) is the modulus for constant induction D z . Hence we can write 

jxCB) = / x«>) + 2 7r A2/a(T- T ), (19.19) 

where the superscripts indicate the nature of the deformation. We see that 
the two coefficients behave entirely differently near the Curie point : whereas 
fji {J)) is a finite constant, fi^ E) increases without limit as the Curie point is 
approached. 

In the pyroelectric region, formula (19.18) shows that the spontaneous 
polarisation results in a certain deformation of the body. If there are no 
internal stresses and the field E is zero, the deformation u xy is proportional 
to D z0 , i.e. by (19.5) it is proportional to <\/(@ — T). 

If the symmetry (cubic, for example) of the non-pyroelectric phase of a 
ferroelectric does not admit a piezoelectric effect linear in D, then the first 
non-vanishing terms in an expansion of the thermodynamic potential in 
powers of o-^; and D are quadratic in the components Df, i.e. they are of the 
form 

— ytklmDiDkVlm, ( 1 9.20) 

where yuam is a tensor of rank four, symmetrical with respect to the pairs of 
suffixes i, k and /, m. 



f See the first footnote to §17, Problem 1, concerning differentiation with respect to the 
components uac. 



§19 Ferroelectrics 91 

Doubt might be cast on the legitimacy of using the expression (19.20) in 
the thermodynamic potential, on the grounds that, as stated in §17, this 
potential can be used only when quadratic effects are neglected. However, 
the ferroelectrics form an exception because, near the Curie point, the field 
£ is smallt compared with the induction D. The use of the thermodynamic 
potential involves the neglect of quantities of the order of EDinu (or, what 
is the same thing, EDai^) y whereas the expression (19.20) is of the order 
of D 2 aac. 



t This is seen, for instance, from formula (19.4): the first term on the right-hand side 
contains the small quantity T — 0, and the second term is of the third order in D. 



CHAPTER III 

CONSTANT CURRENT 

§20. The current density and the conductivity 

Let us now consider the steady motion of charges in conductors, i.e. con- 
stant electric currents. We shall denote by j the mean charge flux density or 
electric current density. \ In a constant current, the spatial distribution of j 
is independent of time, and satisfies the equation 

divj = 0, (20.1) 

which states that the mean total charge in any volume of the conductor 
remains constant. 

The electric field in the conductor in which a constant current flows is also 
constant, and therefore satisfies the equation 

curl E = 0, (20.2) 

i.e. it is a potential field. 

Equations (20.1) and (20.2) must be supplemented by an equation relating 
j and E. This equation depends on the properties of the conductor, but in 
the great majority of cases it may be supposed linear (Ohm's law). If the 
conductor is homogeneous and isotropic, the linear relation is a simple 
proportionality: 

j = <xE. (20.3) 

The coefficient a depends on the nature and state of the conductor; it is 
called the electrical conductivity. 

In a homogeneous conductor, a — constant and, substituting (20.3) in 
(20.1), we have divE = 0. In this case the electric field potential satisfies 
Laplace's equation : A<f> = 0. 

At a boundary between two conducting media, the normal component of 
the current density must, of course, be continuous. Moreover, by the 
general condition that the tangential field component is continuous (which 
follows from curlE = 0; cf. (1.7) and (6.9)), the ration/or must be continu- 
ous. Thus the boundary conditions on the current density are 

jnl=jn2, jtt/<71 = jl*2/<72, (20.4) 

or, as conditions on the field, 

<riE n i = o%E n 2y Efi = Et2- (20.5) 

f I n this chapter we ignore the magnetic field due to the current, and therefore the 
reaction of that field on the current. If this effect is to be taken into account, the definition 
of the current density must be refined, which we do in §29. 

92 



§20 The current density and the conductivity 93 

At a boundary between a conductor and a non-conductor we have simply 
/» = 0, or E n = O.t 

An electric field in the presence of a current does mechanical work on the 
current-carrying particles moving in the conductor; the work done per unit 
time and volume is evidently equal to the scalar product j«E. This work is 
dissipated into heat in the conductor. Thus the quantity of heat evolved per 
unit time and volume in a homogeneous conductor is 

yE=aE*=p/o. (20.6) 

This is Joule's law.% 

The evolution of heat results in an increase in the entropy of the body. 
When an amount of heat dQ = j*E dV is evolved, the entropy of the volume 
element dV increases by dQ/T. The rate of change of the total entropy of 
the body is therefore 

dSTjdt = J (j • E/T) dV. (20.7) 

Since the entropy must increase, this derivative must be positive. Putting 
j = ctE, we see that the conductivity a must therefore be positive. 

In an anisotropic body (a single crystal), the directions of the vectors j 
and E are in general different, and the linear relation between them is 

ji = o ik E k , (20.8) 

where the quantities a^ form a tensor of rank two, the conductivity tensor ; 
which is symmetrical (see below). 

The following remark should be made here. The symmetry of the crystal 
would admit also an inhomogeneous term in the linear relation between j 
and E, giving j$ = oikEk+ji (0 \ with j (0) a constant vector. The presence of 
this term would mean that the conductor was "pyroelectric", there being a 
non-zero field in it when j = 0. In reality, however, this is impossible, 
because the entropy must increase: the term j (0) *E in the integrand in (20.7) 
could take either sign, and so dS^/dt could not be invariably positive. 

Just as, for an isotropic medium, dSf\dt > leads to a > 0, so for an 
anisotropic medium this condition means that the principal values of the 
tensor aik must be positive. 

The dependence of the number of independent components of the tensor 
am on the symmetry of the crystal is the same as for any symmetrical tensor 
of rank two (see §13): for biaxial crystals, all three principal values are differ- 
ent, for uniaxial crystals two are equal, and for cubic crystals all three are 



f It should be noticed that the equations curl £ = 0, div(aE) = and the boundary 
conditions (20.5) thereon are formally identical with the equations for the electrostatic field 
in a dielectric, the only difference being that c is replaced by a. This enables us to solve 
problems of the current distribution in an infinite conductor if the solutions of the corres- 
ponding electrostatic problems are known. When the conductor is bounded by a non- 
conductor this analogy does not serve, because in electrostatics there is no medium for 
which e = 0. 

X In Russian "Joule and Lenz's law". 



94 Constant Current §20 

equal, i.e. a cubic crystal behaves as an isotropic body as regards its con- 
ductivity. 

The symmetry of the conductivity tensor 

vik = am (20.9) 

is a consequence of the symmetry of the kinetic coefficients. This general 
principle, due to L. Onsager, may be conveniently formulated, for use here 
and in §§25, 26, as follows.! 

Let x\,X2, ... be some quantities which characterise the state of the 
body at every point. We define also the quantities 

X a = -dSJdxa, (20.10) 

where S is the entropy of unit volume of the body, and the derivative is taken 
at constant energy of the volume. In a state close to equilibrium, the 
quantities x a are close to their equilibrium values, and the X a are small. 
Processes will occur in the body which tend to bring it into equilibrium. The 
rates of change of the quantities x a at each point are usually functions only 
of the values of the x a (or X a ) at that point. Expanding these functions 
in powers of X a and taking only the linear terms, we have 

dx a /dt= -XyabXb. (20.11) 

Then we can assert that the coefficients y a b (the kinetic coefficients) are 
symmetrical with respect to the suffixes a and b: 

Yab — Yba- (20.12) 

In order to make practical use of this principle, it is necessary to choose 
the quantities x a (or their derivatives x a ) in some manner, and then to deter- 
mine the X a . This can usually be done very simply by means of the formula 
for the rate of change of the total entropy of the body 



= -jX Xa ^r dV ' (2(U3) 



d^ f^ „ dx a 

~dt 



where the integration is extended over the whole volume of the body. 

When a current flows in a conductor, dS^jdt is given by (20.7). Compar- 
ing this with (20.13), we see that, if the components of the current density 
vector j are taken as the quantities x a , then the quantities X a will be the 
components of the vector — E/7 1 . A comparison of formulae (20.8) and 
(20.11) shows that the kinetic coefficients in this case are the components of 
the conductivity tensor, multiplied by T. Thus the symmetry of this tensor 
follows immediately from the general relation (20.12). 



f See Statistical Physics, §119, Pergamon Press, London, 1958; Fluid Mechanics, §58, 
Pergamon Press, London, 1959. 



§20 



The current density and the conductivity 



95 



PROBLEMS 

Problem 1. A system of electrodes maintained at constant potentials <f> a is immersed in a 
conducting medium. A current J a flows from each electrode. Determine the total amount 
of Joule heat evolved in the medium per unit time. 

Solution. The required amount of heat Q is given by the integral 

0=/j.EdF= -Jj.grad^dF= -/div(^j)dr, 

taken over the volume of the medium. We transform this into a surface integral, using the 
fact thatjB = at the outer boundary of the medium, while on the surfaces of the electrodes 
4> = constant = <j> a . The result is Q = 2 <f> a Ja. 

Problem 2. Determine the potential distribution in a conducting sphere with a current / 
entering at a point O and leaving at the point O' diametrically opposite to O. 

O 




Solution. Near O and O' (Fig. 14) the potential must be of the forms 4> — J/lnaRi 
and <f> = —JIliraRz respectively, Ri and i?2 being the distances from O and O'. These func- 
tions satisfy Laplace's equation, and the integrals —or J grad ^«df over infinitesimal hemi- 
spheres about O and O' are equal to ±/. We seek the potential at an arbitrary point P in the 
sphere in the form 

2n<ARi R2 

where ^ is a solution of Laplace's equation having no poles in or on the sphere. It is evident 
from symmetry that tfi, like <f>, is a function of the spherical co-ordinates r and only. 

On the surface of the sphere (r = a) we must have d<f>[dr = 0. Differentiating, we find the 
boundary condition on ifi: 



5+4 



dtfi 



1(1 _i) 

2a\Ri Rj 



for r = a. 



If f{r, 6) is any solution of Laplace's equation, then the function 



/ 



f(r, 6) 



dr 



is also a solution, f Comparing this with the above boundary condition, we see that the 



t This is easily seen either by direct calculation or from the fact that any solution f(r, 6) 
of Laplace's equation depending only on r and 9 can be written/ = 'Lc n r n P n (cos 6), where 
the c n are constants and the P n are Legendre polynomials. 



96 Constant Current §21 

condition is met by the solution 

r 

dr 



*-*J(e-s)t 



Substituting i?i,2 = \/(a 2 +r 2 ^2ar cos 0) and effecting the integration, we have finally 

J 1 1 11/ n a+r cos a— r cos 6\\ 

9 = ~ — \Ti 77 + — sinh" 1 : — sinh -1 ; — - — I . 

2iro\Ri Rz 2a\ r sin r sin /) 

Problem 3. Show that the current distribution in a conductor is such that the energy 
dissipated is a minimum. 

Solution. The minimum concerned is that of the intergal J j*E dV = J (j 2 /a) dV, with 
the subsidiary condition div j = (conservation of charge). Varying with respect to j the 
integral J[(j 2 /°') —2<j> div j]dV, where 2<f> is an undetermined Lagrangian multiplier, and equat- 
ing the result to zero, we obtain the equation j = — a grad </> or curl (j/w) = 0, which is the 
same as (20.2) and (20.3). 



§21. The Hall effect 

If a conductor is in an external magnetic field H, the relation between the 
current density and the electric field is again given by j± = ant Ejc, but the 
components of the conductivity tensor ouc are functions of H and, what 
is particularly important, they are no longer symmetrical with respect to the 
suffixes i and k. The symmetry of this tensor was proved in §20 from the 
symmetry of the kinetic coefficients. In a magnetic field, however, this 
principle must be formulated somewhat differently: when the suffixes are 
interchanged, the direction of the magnetic field must be reversed, t Hence 
we now have for the components or^fc(H) the relations 

cr«(H) = ow(-H). (21.1) 

The quantities o-ffr(H) and ct;m(H) are not equal. 

Like any tensor of rank two, 0% can be divided into symmetrical and anti- 
symmetrical parts, which we denote by sue and a\u : 

Vile = Ste + Uijc. (21.2) 

By definition 

*«(H) = s ki (H), a ik (H) = -a M (H), (21.3) 

and from (21.1) it follows that 

**(H) = jm(-H) = ^&(-H), 
a ik (H) = aM(-H) = -a«(-H). 



(21.4) 



Thus the components of the tensor st k are even functions of the magnetic 
field, and those of aa are odd functions. 



f See Statistical Physics, §119. 



§21 The Hall effect 97 

Any antisymmetrical tensor a\ k of rank two corresponds to some axial 
vector, whose components are 

a x = a yz , a y = — a XZy a z = a xy . (21.5) 

In terms of this vector, the components of the product ai k E k can be written 
as those of the vector product Ex a: 

ji = a ik E k = s ik E k + (Ex a)*. (21 .6) 

The Joule heat generated by the passage of the current is given by the 
product j»E. Since the vectors Exa and E are perpendicular, their scalar 
product is zero identically, and so 

j.E = s ik EiE k , (21.7) 

i.e. the Joule heat is determined (for a given field E) only by the symmetrical 
part of the conductivity tensor. 

The external magnetic field may usually be supposed weak, and the compo- 
nents of the conductivity tensor accordingly expanded in powers of that 
field. Since the function a(H) is odd, the expansion of this vector will 
involve only odd powers. The first terms are linear in the field, i.e. they 
are of the form 

at = <x. ik H k . (21.8) 

The vectors a and H are both axial, and the constants a** therefore form an 
ordinary (polar) tensor. The expansion of the even functions ^&(H) will 
involve only even powers. The first term is the conductivity ao,ik in the 
absence of the field, and the next terms are quadratic in the field : 

Sik = <ro,ik + PiklmHiH m . (21.9) 

The tensor fii k i m is symmetrical with respect to i, k and /, m. 

Thus the principal effect of the magnetic field is linear in the field and is 
given by the term E X a; it is called the Hall effect. As we see, it gives rise 
to a current perpendicular to the electric field, whose magnitude is propor- 
tional to the magnetic field. It should be borne in mind, however, that, for 
an arbitrary anisotropic medium, the Hall current is not the only current 
perpendicular to E; the current Si k E k also has a component in such a 
direction. 

The Hall effect may be differently regarded if we use the inverse formulae 
which express E in terms of the current density : Ei = a^ikjic The inverse 
tensor ct -1 ^, like at k itself, can be resolved into a symmetrical part pi k and 
an antisymmetrical part which may be represented by an axial vector b: 

Ei = pikjk+(}xb)i. (21.10) 

The tensor pw and the vector b have the same properties as sac and a. In 
particular, in weak magnetic fields the vector b is linear in the field. In formu- 
la (21.10) the Hall effect is represented by the term j X b, i.e. by an electric 



98 Constant Current §21 

field perpendicular to the current and proportional to the magnetic field and 
to the current j. 

The above relations are much simplified if the conductor is isotropic. 
The vectors a and b must then be parallel to the magnetic field, by sym- 
metry. The only non-zero components of the tensor pug are p X x = Pyy and 
p zz , the field being in the ^-direction. Denoting these two quantities by p 
and p and taking the current to lie in the xs-plane, we have 

E* = P±Jx> Ey = —#*> Ez = Pijz- (21.11) 

Hence we see that, in an isotropic conductor, the Hall field is the only 
electric field which is perpendicular to both the current and the magnetic 
field. 

In weak magnetic fields, the vectors b and H are related (in an isotropic 
body) by 

b = -RH. (21.12) 

simply. The constant R (called HalVs constant) may be either positive or 
negative. The form of the terms quadratic in H in the relation between E 
and j, which enter through the tensor p^, is easily seen from the fact that the 
only vectors linear in j and quadratic in H which can be constructed from j 
and H are (j»H)H and H 2 j. Hence the general form of the relation between 
E and j in an isotropic body, as far as the terms quadratic in H, is 

E = P oj + #Hxj+i8i# 2 j+i82(j-H)H. (21.13) 



PROBLEM 
Express the components of the inverse tensor a -1 ** in terms of those of sue and a. 

Solution. The calculations are most simply effected by taking a system of co-ordinates 
in which the axes are the principal axes of the tensor sin ; the form of the results in an arbitrary 
co-ordinate system can easily be deduced from their form in this particular case. The deter- 
minant J crj is 

SxX O.Z — Oy 

\a\ = —a e Syy a x 

CLy ~~Gx Szz 

= SxxSyySzz + Sxxa X 2 + Syyay 2 + Sztaz 2 . 

In the general case we evidently have 

M = M+Siicaiak. 
From the minors of this determinant we find the components of the inverse tensor: 

C _1 X z = Pxx = (SyySzz + ax 2 )l\o\, 

o _1 X y — Pxy+bi = (axay—azSzz)l\o\, ... . 

The general expressions which give these for the particular system of co-ordinates chosen 
are 

Pile = {* -1 <*M+aia*}/M, bi = —saeajel\a\. 

This completes the solution. 



§22 The contact potential 99 

§22. The contact potential 

In order to remove a charged particle through the surface of a conductor, 
work must be done. The work required for a thermodynamically reversible 
removal of the particle is called the work function. This quantity is always 
positive; this follows immediately from the fact that a point charge is at- 
tracted to any neutral body, and therefore to any conductor (see §14). It 
will be more convenient to refer this work to unit (positive) charge; the sign 
of the work function W thus defined is the same as that of the charge on the 
particle removed. 

The work function depends both on the nature of the conductor (and its 
thermodynamic state, i.e. its temperature and density) and on that of the 
charged particle. For example, the work function for a given metal is differ- 
ent for the removal of a conduction electron and for the removal of an ion 
from the surface. It must also be emphasised that the work function is 
characteristic of the surface of the conductor. It therefore depends, for 
instance, on the treatment of the surface and the "contamination" of it. If 
the conductor is a single crystal, then the work function is different for 
different faces. 

To ascertain the physical nature of the dependence of the work function 
on the properties of the surface, let us establish its relation to the electric 
structure of the surface layer. If p(x) is the charge density not averaged over 
physically infinitesimal segments of the ac-axis (perpendicular to the layer), 
we can write Poisson's equation in that layer as d 2 <£/d* 2 = — 4ny>. Let the 
conductor occupy the region x < 0. Then a first integration gives 



— —4tt \ p dx, 

dx _*L 

and a second integration (by parts) gives 

X X 

<£ — <£(— oo) = — Attx I p dx + 4tt xp dx. 

— oo —oo 

For x -> oo, the integral 

X 

I pdx 

— oo 

tends very rapidly to zero (since the surface of an uncharged conductor is 
electrically neutral). Hence 

00 

<j>( + oo) — <f>(~ oo) = 4-7T xp dx. 



100 Constant Current §22 

The integral on the right is the dipole moment of the charges near the surface 
of the body. These charges form a "double layer", in which charges of 
opposite sign are separated and the dipole moment is non-zero. The struc- 
ture of the double layer, of course, depends on the properties of the surface 
(its crystallographic direction, contamination, etc.). The difference in the 
work function for different surfaces of a given conductor is determined by 
the difference in the dipole moments. 

If two different conductors are placed in contact, an exchange of charged 
particles may occur between them. Charges pass from the body with the 
smaller work function to that with the greater until a potential difference 
between them is set up which prevents further movement of charge. This is 
called a contact potential. 




Fig. 15 

Fig. 15 shows a cross-section of two conductors in contact (a and b) near 
their surfaces AO and OB. Let the potentials of these surfaces be <f> a and 
<f>i, respectively. Then the contact potential is <f> a i, = <f>b~<f>a- The quantita- 
tive relation between this potential and the work functions is given by the 
condition of thermodynamic equilibrium. Let us consider the work which 
must be done on a particle of charge e to remove it from the conductor a 
through the surface AO, transfer it to the surface OB, and finally carry it 
into the conductor b. In a state of thermodynamic equilibrium, this work 
must be zero.t The work done on the particle in the three stages mentioned 
is eW a , e(cf>b—(f>a), and —eWb respectively. Putting the sum of these equal to 
zero, we find the required relation : 

<f> ab =W b -W a . (22.1) 

Thus the contact potential of the neighbouring free surfaces of two conduc- 
tors in contact is equal to the difference in their work functions. 

The existence of the contact potential results in the appearance of an 
electric field in the space outside the conductors. It is easy to determine 



t Of course, in reality a particle can pass from one conductor to another only through 
their surface of contact, and not through the space adjoining them, but the work done is 
independent of the path. 



§23 The galvanic cell 101 

this field near the line of contact of the surfaces. In a small region near this 
line (the point O in Fig. 15), the surfaces may be regarded as plane. The 
field potential outside the conductors satisfies the equation 

1 8 l d<f>\ 1 8*6 
A0 = r— + - = 0, 

r r 8r\ 8rl r* 86* 

where r and 9 are polar co-ordinates with origin at O; on AO and OB the 
potential takes given constant values. We are interested in the solution 
which contains the lowest power of r; this is the leading term in an expansion 
of the potential in powers of the small distance r. The solution concerned is 
<f> = constant x 9. Measuring the angle 9 from AO and arbitrarily taking 
the potential on AO as zero, we have 

<f> = 4> ab djoL, (22.2) 

where a is the angle AOB. Thus the equipotential lines in the plane of the 
diagram are straight lines diverging from O. The lines of force are arcs of 
circles centred at O. The field is 

1 8<j> <f> a b 1 

£=-- t|=-— -; ( 22 - 3 ) 

r 89 a. r 

it decreases inversely as the distance from O. 

As has been said above, "contact" potentials also exist between the various 
faces of a single crystal of metal. Hence an electric field of the kind just 
described must exist near the edges of the crystal, t 

If several metallic conductors (at equal temperatures) are connected 
together, the potential between the extreme conductors is, as we easily de- 
duce from formula (22.1), simply the difference of their work functions, as 
it is for two conductors in direct contact. In particular, if the metal at each 
end is the same, the contact potential between the ends is zero. This is 
evident, however, because if there were a potential difference between two 
like conductors, a current would flow when they were connected, in contra- 
diction to the second law of thermodynamics. 

§23. The galvanic cell 

The statement at the end of §22 ceases to be valid if the circuit includes 
conductors in which the current is carried by different means (e.g. metals 
and solutions of electrolytes). Because the work function of a conductor is 
different for different charged particles (electrons and ions), the total contact 
potential in the circuit is not zero even when the conductors at each end are 
similar. This total potential difference is called the electromotive force or 
e.m.f. in the circuit ; it is just the potential difference between the two like 
conductors before the circuit is closed. When the circuit is closed, a current 
flows in it ; this is the basis of the operation of what are called galvanic cells. 



f In reality, all such fields are usually compensated by the field of ions from the atmo- 
sphere which "adhere" to the surface of the crystal. 



102 Constant Current §23 

The energy which maintains the current in the circuit is supplied by chemical 
transformations occurring in the cell. 

When we go completely round any closed circuit the field potential must, 
of course, return to its original value, i.e. the total change in the potential 
must be zero. Let us consider, for example, a contour on the surface of the 
conductors. When we pass from one conductor to another, the potential has 
a discontinuity <f> a b- The potential drop across any conductor is RJ> where / 
is the total current flowing through it and R is its resistance. Hence the 
total change in the potential round the circuit is E<£ a &— 2/R. Putting this 
equal to zero and using the facts that/ is the same at every point in the 
circuit and H>(j> a b is the electromotive force $ , we find 

JXR = S, (23.1) 

so that the current in a circuit containing a galvanic cell is equal to the e.m.f. 
divided by the total resistance of all the conductors in the circuit (including, 
of course, the internal resistance of the cell itself). 

Although the e.m.f. of a galvanic cell can be expressed as a sum of contact 
potentials, it is very important to note that it is in reality a thermodynamic 
quantity, determined entirely by the states of the conductors and independent 
of the properties of the surfaces separating them. This is clear, because 8 
is just the work per unit charge which must be done on a charged particle 
when it is carried reversibly along the closed circuit. 

To illustrate this, let us consider a galvanic cell consisting of two electrodes 
of metals A and B immersed in solutions of electrolytes AX and BX, X~ 
being any anion. Let £4 and £# be the chemical potentials of the metals A 
and B, and Iax and Cbx those of the electrolytes in solution.! If an elemen- 
tary charge e is carried along the closed circuit, an ion A + passes into solution 
from the electrode A and an ion B + passes out of solution to the electrode 
B, the change in the charges on the electrodes being compensated by the 
passage of an electron from A to B through the external circuit. The result 
is that the electrode A loses one neutral atom, the electrode B gains one, 
and in the electrolyte solution one molecule of BX is replaced by one of AX. 
Since the work done in a reversible process (at constant temperature and 
pressure) is equal to the change in the thermodynamic potential of the sys- 
tem, we have 

e*AB = (Cb-Cbz)-(U-Uz), (23.2) 

which expresses the e.m.f. of the cell in terms of the properties of the material 
of the electrodes and of the electrolyte solution. 

From (23.2) we can also draw the following conclusion. If the solution 
contains three electrolytes AX, BX, CX and three metallic electrodes A, 
B, C, then the e.m.f.s between each pair of them are related by 

&AB + &BC = &AC- (23.3) 



t In this section we use the ordinary chemical potentials, i.e. those defined with respect 
to one particle. 



§24 EUctrocapiUarity 103 

Using the general formulae of thermodynamics, we can relate the e.m.f. 
of a galvanic cell to the heat evolved when a current flows, which of course is 
actually an irreversible phenomenon. Let Q be the amount of heat generated 
(both in the cell itself and in the external circuit) when the unit charge passes 
along the circuit; Q is just the heat of the reaction which occurs in the cell 
when a current flows. By a well-known formula of thermodynamics, t it is 
related to the work <f by 

The definition of the partial derivative with respect to temperature depends 
on the conditions under which the process occurs. For example, if the current 
flows at constant pressure (as usually happens), then the differentiation is 
effected at constant pressure. 



§24. Electrocapillarity 

The presence of charges on the boundary between two conducting media 
affects the surface tension there. This phenomenon is called electrocapillarity. 
In practice, the media concerned are both liquids; usually one is a liquid 
metal (mercury) and the other is a solution of an electrolyte. 

Let 0i, <f>2 be the potentials of the two conductors, and e\, e2 the charges 
at the surface of separation. These charges are equal in magnitude and 
opposite in sign, and thus form a double layer on the surface. 

The differential of the potential gjo of a system of two conductors at given 
temperature and pressure is, taking into account the surface of separation, 

dgb = a.dS-eid<f>i-e2d<f>2, (24.1) 

where the term ccdS is the work done in a reversible change dS in the 
area S of the surface of separation; a is the surface-tension coefficient. t 

The thermodynamic potential cp in (24.1) may be replaced by its "sur- 
face part" ^?s, since the volume part is constant for given temperature and 
pressure, and is therefore of no interest here. Putting e\ = —e2 = e and 
the potential difference <f>i—<f>2 = <f>, we can write (24.1) as 

dgb s = oidS-ed<f>. (24.2) 

Hence 

(d^s/dS)^ = a, (24.3) 

a being expressed as a function of <f>. Integrating, we find that gd s = <x.S. 



t See Statistical Physics, §89. 
X See Statistical Physics, §1 39. 



104 Constant Current §25 

Substitution in (24.2) gives d(a«S) = acLS— ed<f>, or Sda. = — ed(f>, whence 

a = -(d*ld<f>) p ,T, (24.4) 

where a = e/S is the charge per unit area of the surface. The relation (24.4), 
first derived by G. Lippmann and J. W. Gibbs, is the fundamental formula 
in the theory of electrocapillarity. 

In a state of equilibrium, the thermodynamic potential <^> must be a 
minimum for given values of the electric potentials on the conductors. 
Regarding it as a function of the surface charges e, we can write the necessary 
conditions for a minimum as 

dyb s /de = d^/de* > 0, (24.5) 

where the derivatives are taken at constant area S. To calculate these, we 
express gb s in terms of the thermodynamic potential cjo s = §p s (e) : 

&>s = cp s (e)-e\$\-e<$z = cp s {e)-e^>. (24.6) 

The vanishing of the first derivative gives 

= <f> = u, 

de de 

and then the condition for the second derivative to be positive becomes 
d^gos d 2 £fo s d(f> 1 d<f> 



de* de 2 de Sda 

or 



>0, 



da/84, > 0. (24.7) 

This result was to be expected, since the double layer on the surface may be 
regarded as a condenser of capacity de/d4>. 

Differentiating equation (24.4) with respect to <j> and using (24.7), we find 
that 

dV^ 2 < 0. (24.8) 

This means that the point where da./d4> = —a = is a maximum of a as 
a function of <£. 

§25. Thermoelectric phenomena 

The condition that there should be no current in a metal is that there is 
thermodynamic equilibrium with respect to the conduction electrons. This 
means not only that the temperature must be constant throughout the body, 
but also that the sum e<£ + £o should be constant, where Co is the chemical 
potential of the conduction electrons in the metal (for 4> — 0).t If the metal 



t See Statistical Physics, §25. Here we take 5 to be the chemical potential denned in the 
usual manner, viz. per unit particle (electron). 



§25 Thermoelectric phenomena 105 

is not homogeneous, £o is not constant throughout the body even if the 
temperature is constant. Hence the constancy of the electric potential <f> in 
this case does not mean the absence of a current in the metal, although the 
field E = — grad <f> is zero. This makes the ordinary definition of <f> (as the 
average of the true potential) inconvenient, if we wish to take inhomogeneous 
conductors into consideration. 

It is natural to redefine the potential as <f> + Cole, and we shall write this 
henceforward as <f> simply, t In a homogeneous metal, the change amounts to 
the adding of an unimportant constant to the potential. Accordingly, the 
"field" E = — grad <f> (which we shall use henceforward) is the same as the 
true mean field only in a homogeneous metal, and in general the two differ 
by the gradient of some function of the state. 

With this definition, the current and field are both zero in a state of 
thermodynamic equilibrium with respect to the conduction electrons, and 
the relation between them is j = oE (or j\ = at *£*) even if the metal is not 
homogeneous. 

Let us now consider a non-uniformly heated metal, which cannot be in 
thermodynamic equilibrium (with respect to the electrons). Then the field 
E is not zero even if the current is zero. In general, when both the current 
density j and the temperature gradient grad T are not zero, the relation 
between these quantities and the field can be written 

E = j/cr + agradT. (25.1) 

Here a is the ordinary conductivity, and a is another quantity which is an 
electrical characteristic of the metal. Here we suppose for simplicity that 
the substance is isotropic (or of cubic symmetry), and therefore write the 
proportionality coefficients as scalars. The linear relation between E and 
grad T is, of course, merely the first term of an expansion, but it is sufficient 
in view of the smallness of the temperature gradients occurring in practice. 
The same formula (25.1), in the form 

j = a(E-agradT), (25.2) 

shows that a current can flow in a non-uniformly heated metal even if the 
field E is zero. 

As well as the electric current density j, we can consider the energy flux 
density q. First of all, this quantity contains an amount <j>j resulting simply 
from the fact that each charged particle (electron) carries with it an energy 
e<f>. The difference q— <f>j, however, does not depend on the potential, and 
can be generally written as a linear function of the gradients grad <£ = — E 
and grad T, similarly to formula (25.2) for the current density. We shall 
for the present write this as 

q-^j = jSE-ygradr. 

t This definition can also be formulated as follows : the new e<f> is the change in the free 
energy when one electron is isothermally brought into the metal. In other words, <f> = dF/dp, 
where F is the free energy of the metal and p the charge on the conduction electrons per unit 
volume. 



v dV. 



106 Constant Current §25 

The symmetry of the kinetic coefficients gives a relation between the coeffi- 
cient j8 and the coefficient a in (25.2). To derive this, we calculate the rate 
of change of the total entropy of the conductor. The amount of heat evolved 
per unit time and volume is — div q. Hence we can put 

dS? fdivq 

~d7 = J 

Using the equation div j = 0, we have 

divq 1 1 E«j 

~Y~ = y{ div (q-^i) +div ^ = ^- dlv (q-<£0— Y' 

The first term is integrated by parts, giving 

dy fE-i r(q-<f>\)-gradT 

-a-H AV - y i dv - (25 - 3 > 

This formula shows that, if we take as the quantities dx a /dt (see §20) 
the components of the vectors j and q— </>j, then the corresponding quantities 
X a are the components of the vectors — E/T and grad T/T 2 . Accordingly 
in the relations 

E grad T 

E grad T 

q-<f>j = §T—-yT* T2 , 

the coefficients <raT 2 and jST must be equal. Thus jS = axT, so that 
q— <£j = o-aTE— y grad T. Finally, expressing E in terms of j and grad T 
by (25.1), we have the result 

q = (<f> + aT)j - k grad T, (25.4) 

where k = y— To? a is simply the ordinary thermal conductivity, which 
gives the heat flux in the absence of an electric current. 

It should be pointed out that the condition that d«^/d* should be positive 
places no new restriction on the thermoelectric coefficients. Substituting 
(25.1) and (25.4) in (25.3), we obtain 

+ ^V>0, (2,5) 

whence we find only that the coefficients of thermal and electrical conduc- 
tivity must be positive. 

In the above formulae it was tacitly assumed that an inhomogeneity of 
pressure (or density) at constant temperature cannot cause a field (or current) 
to appear in the conductor, and consequently no term in grad/> was in- 
cluded in (25.2) or (25.4). The existence of such terms would, in fact, contra- 
dict the law of the increase of entropy: the integrand in (25.5) would then 



"d7~ J \~iPr 



§25 Thermoelectric phenomena 107 

contain terms in the products j«grad/> and grad T«grad/>, which could 
be of either sign, and so the integral could not be necessarily positive. 

The relations (25.1) and (25.4) indicate various thermoelectric effects. 
Let us consider the amount of heat — div q evolved per unit time and 
volume in the conductor. Taking the divergence of (25.4), we have 

£=-divq 

= div(/cgradT) + E.j+j-grad(aT), 

or, substituting (25.1), 

i 2 

Q = div(*cgrad T) + - — Tj-grada. (25.6) 

a 

The first term on the right pertains to ordinary thermal conduction, and the 
second term, proportional to the square of the current, is the Joule heat. 
The term of interest here is the third, which gives the thermoelectric effects. 
Let us assume the conductor to be homogeneous. Then the change in a 
is due only to the temperature gradient, and grad a = (da/d T) grad T; if, 
as usually happens, the pressure is constant through the body, dccjdT must 
be taken as (da/ #7%. Thus the amount of heat evolved (called the Thomson 
effect) is 

pj.gradT, where p =- Tda/dT. (25.7) 

The coefficient p is called the Thomson coefficient. It should be noticed 
that this effect is proportional to the first power of the current, and not to 
the second power like the Joule heat. It therefore changes sign when the 
current is reversed. The coefficient p may be either positive or negative. 
If p > 0, the Thomson heat is positive (i.e. heat is emitted) when the current 
flows in the direction of increasing temperature, and heat is absorbed when 
it flows in the opposite direction ; if p < the reverse is true. 

Another effect, called the Peltier effect, occurs when a current passes 
through a junction of two different metals. At the surface of contact, the 
temperature, the potential and the normal components of the current density 
and energy flux density are all continuous. Denoting by the suffixes 1 and 2 
the values of quantities for the two metals and equating the normal com- 
ponents of q (25.4) on the two sides, we have, since <f>, T and j x are con- 
tinuous, 

[- icdT/dx]l= -j x T(<x. 2 -<x.i), 

the #-axis being taken along the normal to the surface. If the positive 
direction of this axis is from metal 1 to metal 2, then the expression on the 
left-hand side of this equation is the amount of heat taken from the surface 
per unit time and area by thermal conduction. This heat loss is balanced by 
the evolution at the junction of an amount of heat given by the right-hand 
side of the equation. Thus the amount of heat generated per unit time and 
area is 

jn 12 , where U 12 = - T(* 2 - «i). (25.8) 



108 Constant Current §25 

The quantity II 12 is called the Peltier coefficient. Like the Thomson effect, 
the Peltier effect is proportional to the first power of the current, and changes 
sign when the direction of the current is reversed. The Peltier coefficient 
is additive: II 13 == II 12+ II 23, where the suffixes 1, 2, 3 refer to three different 
metals. 

A comparison of formulae (25.7) and (25.8) shows that the Thomson and 
Peltier coefficients are related by 

d /IIi2\ 

"-« " T «r(-rY (25 - 9) 

Next, let us consider an open circuit containing two junctions, the two 
end conductors being of the same metal (1 in Fig. 16). We suppose that the 
junctions b and c are at different temperatures T\ and T2, while the tempera- 
ture at each end (a and d) is the same. Then there is a potential difference 
called a thermoelectromotive force, which we denote by <fy, between the ends. 



Fig. 16 



To calculate this force, we put in (25.1) j = and integrate the field 
E = a grad T along the circuit (taken to be the #-axis) : 



d 
dT 



f dI f 

g T = a dx = ad7\ 

J dx J 



The integrations from a to b and from c to d are over temperatures from 
T2 to . Ti in metal 1, and that from b to c is over temperatures from T\ to T2 
in metal 2. Thus 



= J(a 2 -ai)dT. (25.10) 



Comparing this with (25.8), we see that the thermo-e.m.f. is related to the 
Peltier coefficient by 



<ot 



c ni2 

= - J ~j=dT (25.11) 



T 



Formulae (25.9) and (25.11) are called Thomson's relations. 

To conclude this section, we shall give the formulae for the current and 
heat flux in an anisotropic conductor. These are derived from the symmetry 



§25 Thermoelectric phenomena 109 

of the kinetic coefficients in exactly the same way as formulae (25.1) and 
(25.4), and the results are 

Ei = o-^jcjjc+cmcdTjdxjc, 

qi-<f>ji = TccMJk- KucdTJdxic. 

Here o~Hk is the tensor inverse to the conductivity tensor ct^, and the tensors 
ak and Kijg are symmetrical. The thermoelectric tensor a**, however, is 
in general not symmetrical. 

PROBLEM 

Find the relations between the coefficients of the various thermogalvanomagnetic effects, 
i.e., those which occur when a current flows in the simultaneous presence of an electric field, 
a magnetic field, and a temperature gradient. 

Solution. The discussion is entirely similar to that given above for thermoelectric effects. 
It is conveniently carried out in tensor form, so as to be applicable to both isotropic and 
anisotropic conductors. We write the electric current density j and the heat flux q as 






(i) 



where all the coefficients are functions of the magnetic field. The symmetry of the kinetic 
coefficients gives 

a,*(H) = a w (-H), <*«*(H) = </*<(-H), 

MH) = a«(-H). 

Expressing E and q— $ in terms of j and grad T from (1), we have 

qi—<h* = P«cjk—KikdT/dxk, 

where the tensors a -1 , a, (3, #c are certain functions of the tensors a, b, c, d, and have the fol- 
lowing symmetry properties resulting from (2) : 

o-MH) = a-i w (-H), 
k«*(H) = /c*«(-H), /3«(H) = 7a*,(-H). K } 

These are the required relations in their most general form. They generalise those found in 
§25 for the case where there is no magnetic field and in §21 for the case where there is no 
temperature gradient. 

For an isotropic conductor in a weak magnetic field we have, as far as the first-order terms 
inH, 

E = j/a+a grad T+i?Hxj+iVHxgrad T, (5) 

q-# = aTj-K grad T+iVTUXj+LHXgrad T. (6) 

Here a and k are the ordinary coefficients of electrical and thermal conductivity, a is the 
thermoelectric coefficient which appears in (25.1), R is the Hall coefficient, and N and L 
are new coefficients. The term iVHxgrad T may be regarded as representing the effect 
of the magnetic field on the thermo-e.m.f . (called the Nernst effect), and the term LH x grad T 
as representing the effect of this field on the thermal conduction (called the Leduc-Righi 
effect). 

At a boundary between media, the normal components of the vectors j and q are continu- 
ous, and therefore so is that of the vector — k grad r+aTj+JVTHxj +LHx grad T. 
The term JVTHxj gives the influence of the magnetic field on the Peltier effect (called the 
Ettingshausen effect). 



110 Constant Current §26 

The amount of heat evolved in the conductor per unit time and volume is Q = — div q. 
Here we must substitute q from (6) and replace — grad <£ = E in accordance with (5). 
If the conductor is homogeneous, then the quantities a, N, L, etc. are functions of tempera- 
ture alone, and so their gradients are proportional to grad T. In the calculation we neglect 
all quantities of the second order in H, and to the same approximation we can take curl (j/a) 
= curl E = 0. We also note that the external field H (arising from sources outside the 
conductor under consideration) is such that curlH = O.f Finally, divj = 0, as for any 
constant current. The result is 

*2 1 A 

Q = — +div(* grad TJ-Tj-grad oc+ — — -(aNT 2 )j xH-grad T. 
a oT di 

The last term gives the change in the Thomson effect resulting from the presence of the 
magnetic field. 



§26. Diffusion phenomena 

The presence of diffusion causes certain phenomena in electrolyte solu- 
tions which do not occur in solid conductors. We shall assume, for simplicity, 
that the temperature is the same everywhere in the solution, and so consider 
only pure diffusion phenomena, uncomplicated by thermoelectric effects. 

Instead of the pressure p and the concentration c, it is more convenient 
to take as independent variables the pressure and the chemical potential £. 
We here define £ as the derivative of the thermodynamic potential of unit 
mass of the solution with respect to its concentration c (at constant p and 
T) ; by the concentration we mean the ratio of the mass of electrolyte in a 
volume element to the total mass of fluid in the same volume. $ It may be 
recalled that the constancy of the chemical potential is (like that of the 
pressure and the temperature) one of the conditions of thermodynamic 
equilibrium. 

The definition of the electric field potential given in §25 has to be some- 
what modified in this case, since the current is now carried by the ions of 
the dissolved electrolyte, and not by the conduction electrons. A suitable 
definition is (cf. the second footnote to §25) <f> = {d^jdp) c , where O is the 
thermodynamic potential and p the sum of the ion charges in unit volume 
of the solution (after differentiating we put p = 0, of course, because the 
solution is electrically neutral). The derivative is taken at constant mass 
concentration, i.e. at a given sum of the masses of ions of both signs in 
unit volume. 



f This neglects the very weak effect on the evolution of heat resulting from the magnetic 
fields of the currents themselves. 

% The chemical potentials are usually defined as £i = 3<B/3mi, £2 = 3G>/3«2, where O is 
the thermodynamic potential of any mass of the solution, and ni, «2 the numbers of particles 
of solute and solvent in that mass of solution. If O is the thermodynamic potential of unit 
mass, then the numbers n\ and nz are related by nwn+ nxm* — 1 (where mi, mz are the 
masses of the two kinds of particle), and the concentration c = mm\. Hence we have 

ao _ ao a«i do dm __ & £2 

dc dni dc dm dc mi mz 
where £ is the chemical potential as here defined. 



§26 Diffusion phenomena 111 

When a gradient of the chemical potential is present, a term proportional 
to it is added to the expression for the current density: 

j = o(E-j8grad9, (26.1) 

in analogy with the added term in (25.2). We shall see below that, for a 
given gradient of the chemical potential (and of the temperature), j must 
be independent of the pressure gradient, and so no term in grad p appears 

in(26.1).t 

As well as the electric current, we have to consider the transport of the 
mass of the electrolyte which takes place at the same time. It must be borne 
in mind that the passage of a current through the solution may be accom- 
panied by a macroscopic motion of the fluid. The mass flux density of the 
electrolyte resulting from this motion is pew, where v is the velocity and p 
the density of the solution. The electrolyte is also transported by molecular 
diffusion. We denote the diffusion flux density by i, so that the total flux 
density is pev+i. The irreversible processes of diffusion cause a further 
increase in entropy ; the rate of change of the total entropy isj 



. JgJdr-j'-g^dr. (26.2) 



Like the electric current density, the diffusion flux may be written as a 
linear combination of E and grad £, or of j and grad £. Using the sym- 
metry of the kinetic coefficients, we can relate one of the coefficients in this 
combination to the coefficient jS in (26.1), in exactly the same way as we did 
for j and q— <£j in §25. The result is 

{dl\dc) Vt T 

The coefficient of grad £ is here expressed in terms of the ordinary diffusion 
coefficient.|| 

The inadmissibility in (26.1) and (26.3) of terms proportional to the 
pressure gradient follows, as in §25, from the law of the increase of entropy: 
such terms would make the derivative of the total entropy (26.2) a quantity 
of variable sign. 

Formulae (26.1) and (26.3) give all the diffusion phenomena in electrolytes, 
but we shall not pause here to examine them more closely. 



f It should be emphasised, however, that, for a given concentration gradient j does 
depend on the pressure gradient: 

grad £ = ( 3£/ 8c) p ,t grade + (d^dp) c , T gradp. 

X The derivation of the second term is given in Fluid Mechanics, §57. 

|| For j = and constant pressure and temperature we have i = — pD grad c. 



112 Constant Current §26 

PROBLEM 

Two parallel plates of a metal A are immersed in a solution of an electrolyte AX. Find the 
current density as a function of the potential difference applied between the plates. 

Solution. When the current passes, metal is dissolved from one plate and deposited on 
the other. The solvent (water) remains at rest, and a mass flux of metal of density t 
pv =jmle occurs in the solution, where j is the electric current density, and m and e are the 
mass and charge of an ion A + . This flux is also given by pv — i + pvc, where t is as shown 
in (26.3) ; assuming the pressure constant throughout the liquid, J we have 

pz ^ = [*-7 (1 -4'' (1) 

where * is the co-ordinate in the direction of a line joining the electrodes. Since j = constant 
in the solution, this gives 

7 _ f 8 P Ddc m 

}l ~ J j3-m(l-c)/ e ' K) 

c i 

where a, cz are the concentrations at the surfaces of the plates, and / is the distance between 
them. 

The potential difference 8 between the plates is most simply found from the total amount 
of energy Q dissipated per unit time and unit area of the plates, which must equal jS. By 
(26.1), (26.2) we have 

d^ r (P Hldc\ z \ 

and therefore, using (1), 



pDdc , f*a{ 



J o(j8-i»(l-c) e) J BcV e '\ 



(3) 



Formulae (2) and (3) implicitly solve the problem. 

If the current j is small, the concentration difference cz—ci is also small. Replacing the 
integrals by ca — ci times the integrands, we find the effective specific resistance of the solution : 



/ 1 1 8l\ 

-T7 = - + 



a pD del e J 

The first term in (3) gives the potential drop (/ (j/o) dx) due to the passage of the current. 
The second term is the e.m.f. due to the concentration gradient in the solution (in a certain 
sense analogous to the thermo-e.m.f.). This latter expression is independent of the con- 
ditions of the particular one-dimensional problem considered, and is the general expression 
for the e.m.f. of a "concentration cell". 



t It may be recalled that the hydrodynamic velocity v in a solution is defined so that p 
is the momentum of unit volume of the liquid; see Fluid Mechanics, §57. Hence the fact 
that in this case only the dissolved metal is moving (relative to the electrodes) does not 
affect the calculation of pv. 

X The change in pressure due to the motion of the liquid gives only terms of a higher 
order of smallness. 



CHAPTER IV 

CONSTANT MAGNETIC FIELD 

§27. Constant magnetic field 

A constant magnetic field in matter satisfies two of Maxwell's equations, 
obtained by averaging the microscopic equations 

lde 4tt 
divh = 0, curlh = — — +— P \. (27.1) 

c ot c 

The mean magnetic field is usually called the magnetic induction and denoted 
byB: 

n = B. (27.2) 

Hence the result of averaging the first equation (27.1) is 

divB = 0. (27.3) 

In the second equation, the time derivative gives zero on averaging, since the 
mean field is supposed constant, and so we have 

curlB = (4*r/<0/»v.' (27.4) 

The mean value of the microscopic current density is in general not zero 
in either conductors or dielectrics. The only difference between these two 
classes is that in dielectrics we always have 



J^F.df=0, (27.5) 



where the integral is taken over the area of any cross-section of the body ; in 
conductors, this integral need not be zero. Let us suppose to begin with that 
there is no net current in the body if it is a conductor, i.e. that (27.5) holds. 
The vanishing of the integral in (27.5) for every cross-section of the body 
means that the vector pv can be written as the curl of another vector, 
usually denoted by cM : 

pv = ccurlM, (27.6) 

where M is zero outside the body; compare the similar discussion in §6. 
For, integrating over a surface bounded by a curve which encloses the body 
and nowhere enters it, we have Jpv«^lf = c JcurlM'df = c|M»dl = 0. 

113 



114 Constant Magnetic Field §27 

The vector M is called the magnetisation of the body. Substituting it in 
(27.4), we find 

curlH = 0, (27.7) 

where the vector H and the magnetic induction B are related by 

B = H + 4ttM, (27.8) 

which is analogous to the relation between the electric field E and induction 
D. Although H is, by analogy with E, usually called the magnetic field, it 
must be remembered that the true mean field is really B and not H. 

To see the physical significance of the quantity M, let us consider the 
total magnetic moment due to all the charged particles moving in the body. 
By the definition of the magnetic moment, t this is 

jrxfivdVJ2c = $Jr xcurlMdF. 

Since pv = outside the body, the integral can be taken over any volume 
which includes the body. We transform the integral as follows : 

JrxcurlMdF = -jr x(Mxdf)- J(Mxgrad) xrdF. 

The integral over the surface outside the body is zero. In the second term 
we have (Mxgrad)xr = — M divr+M = — 2M. Thus we obtain 

(rxpfdV = jMdF. (27.9) 

We see that the magnetisation vector is the magnetic moment per unit 
volume.J 

The equations (27.3) and (27.7) must be supplemented by a relation be- 
tween H and B in order to complete the system of equations. For example, 
in non-ferromagnetic bodies in fairly weak magnetic fields, B and H are 
linearly related. In isotropic bodies, this linear relation becomes a simple 
proportionality : 

B = fjH. (27.10) 

The coefficient /x is called the magnetic permeability. We also have M = %H, 
where the coefficient 

x = G*-l)/4* (27.11) 

is called the magnetic susceptibility. 



1 

2c 



t See The Classical Theory of Fields, §5-9, Addison-Wesley Press, Cambridge (Mass.), 
1951; Pergamon Press, London, 1959. 

X The quantity M is completely determined only when this relation is established. The 
relation (27.6) inside the body, and M = outside it, do not uniquely define M: the gradient 
of any scalar could be added to M inside the body without affecting (27.6) (cf. the similar 
remark in the first footnote to §6). 



§27 Constant magnetic field 115 

Unlike the dielectric constant e, which always exceeds unity, the magnetic 
permeability may be either greater or less than unity. (It is, however, always 
positive, as we shall prove in §30. The reason for the differing behaviour of 
jLt and € is discussed in §31.) The magnetic susceptibility x mav corres- 
pondingly be either positive or negative. 

Another, quantitative, difference is that the magnetic susceptibility of the 
great majority of bodies is very small in comparison with the dielectric 
susceptibility. This difference arises because the magnetisation of a (non- 
ferromagnetic) body is a relativistic effect, of order v 2 /c 2 , where v is the 
velocity of the electrons in the atoms, t 

From the equations div B = 0, curl H =0 it follows (cf. §6) that at a 
boundary between two different media we must have 

B ln = B 2n , Hit = H*. (27.12) 

This system of equations and boundary conditions is formally identical with 
those for the electrostatic field in a dielectric in the absence of free charges, 
differing only in that E and D are replaced by H and B respectively. Since 
curlH = 0, we can put H = — grad^; the equations for the potential ifi 
are the same as those for the electrostatic potential. Thus the solutions of 
the various problems of electrostatics discussed in Chapter II can be im- 
mediately applied to problems with a constant magnetic field. In particular, 
the formulae derived in §8 for a dielectric ellipsoid in a uniform electric 
field hold also, with appropriate substitutions, for a magnetic ellipsoid in a 
uniform magnetic field. 

The tangential component of the magnetic induction, unlike its normal 
component, is discontinuous at a surface separating two media. The magni- 
tude of the discontinuity can be related to the current density on the surface. 
To do this, we integrate both sides of equation (27.4) over a small interval 
A/ crossing the surface along the normal. We then let A/ tend to zero ; the 
integral Jpv d/ may tend to some finite limit. The quantity 

g = J>d/ (27.13) 

may be called the surface current density; it gives the charge passing per unit 
time across unit length of a line in the surface. We take the direction of g 
at a given point on the surface as the jy-axis, and the direction of the normal 
from medium 1 to medium 2 as the #-axis. Then the integration of equation 
(27.4) gives 

r/dB x BB g \ 4ir = 4tt 

J \ 8z dx J c c 

Since B x is continuous, the derivative 8B x /dz is finite, and so its integral 
tends to zero with A/. The integral of BB z jdx gives the difference in the 



t The ratio vjc appears with H in the Hamiltonian of the interaction of the body with 
the magnetic field, and again in the magnetic moments of the atoms or molecules. 



116 Constant Magnetic Field §28 

values of B z on the two sides of the surface. Thus B^ z —B\ z = —fagfc. 
This can be written in vector form : 

4rrg/c = n x(B 2 - Bi) = 4?rn x(M 2 -Mi), (27.14) 

where n is a unit vector along the normal into region 2; the last member of 
(27.14) is obtained by using the continuity of the tangential component of H. 

§28. Magnetic symmetry of crystals 

There is a profound difference between the electric properties of crystals 
and their magnetic properties, which results from a difference in the be- 
haviour of charges and currents with respect to a change in the sign of the 
time. 

The invariance of the equations of motion with respect to this change 
means that the formal substitution t -> —t, on being applied to any state of 
thermodynamic equilibrium of a body, must give some possible equilibrium 
state. There are then two possibilities : either the state obtained by changing 
the sign of t is the same as the original state, or it is not. 

In this section we denote by p(x, y y z) and j(x, y, z) the true (micro- 
scopic) charge and current densities at any given point in the crystal, averaged 
only over time, and not over "physically infinitesimal" volumes as in the 
macroscopic theory. These are the functions which determine the electric 
and magnetic structure of the crystal respectively. 

When t is replaced by —t, j changes sign. If the state of the body remains 
unchanged, it follows that j = — j, i.e. j = 0. Thus there is a reason why 
bodies can exist in which the function }(x, y, z) is identically zero. In such 
bodies, not only the current density but also the (time) average magnetic 
field and magnetic moment vanish at every point (we are speaking, of course, 
of states in the absence of an external magnetic field). Such bodies may 
be said to have no "magnetic structure", and indeed the great majority of 
bodies fall into this category. 

The charge density />, on the other hand, is unchanged when t -> —t. 
There is therefore no reason why this function should be identically zero. In 
other words, there are no crystals without "electric structure", and herein 
lies the essential difference, mentioned at the beginning of this section, 
between the electric and the magnetic properties of crystals. 

Let us now consider crystals for which the change from t to — t results 
in a change of state, so that j # 0. We shall say that such bodies have a 
magnetic structure. First of all, we note that, although j is not zero, there 
can be no total current in an equilibrium state of the body, i.e. the integral 
j j dV taken over an elementary cell must always be zero.t Otherwise the 



t It should be emphasised that the cell spoken of here is the true elementary cell, whose 
definition involves the magnetic structure of the crystal, and which may be different from 
the purely crystallographic cell, which relates only to the symmetry of the charge distribu- 
tion in the lattice. 



§28 Magnetic symmetry of crystals 117 

current would produce a macroscopic magnetic field, and the crystal would 
have a magnetic energy per unit volume increasing rapidly with its dimen- 
sions. Since such a state is energetically unfavourable, it could not corres- 
pond to thermodynamic equilibrium. 

The currents j may, however, produce a non-zero macroscopic magnetic 
moment, i.e. the integral JrxjdF, again taken over an elementary cell, 
need not be zero. Accordingly, the bodies for which j ^ may be divided 
into two types: those in which the macroscopic magnetic moment is not 
zero, called ferromagnetics, and those in which it is zero, called antiferro- 
magnetics. 

The symmetry of the current distribution j can be conveniently regarded 
as the symmetry of the arrangement and orientation of the magnetic moments 
of the individual atoms in the crystal. If j = 0, all these moments are 
changing their orientation in the course of time in an entirely random 
manner, so that the mean value of each moment is zero. In a ferromagnetic, 
the atomic moments are oriented mainly in one direction, causing a non-zero 
total moment in each elementary cell. In an antiferromagnetic, the mean 
atomic moments are not zero, i.e. they are not randomly oriented, but they 
are so arranged as to balance one another in each cell. 

What are the possible symmetry groups of the current distribution 
( x, y, z)} This symmetry contains, first of all, the usual rotations, reflections 
and translations, and so the possible symmetry groups of j always include the 
usual 230 crystallographic space symmetry groups. These, however, are by 
no means all. As has already been mentioned, the substitution t -> —t 
changes the sign of the vector j. For this reason a new symmetry element 
comes in, namely that resulting from the reversal of all currents ; we shall 
denote this transformation by R. If the current distribution itself has the 
symmetry R, it follows that j = — j, i.e. j = 0, and the body has no mag- 
netic structure. A non-zero function }(x, y, z) may, however, be symmetrical 
with respect to various combinations of R with the other symmetry elements 
(rotations, reflections and translations). Thus the problem of determining 
the possible types of symmetry of the current distribution (the magnetic 
space groups) amounts to the enumeration of all possible groups containing 
both the transformations of the ordinary space groups and the combinations 
of these with R. 

If the symmetry of the current distribution is given, the crystallographic 
symmetry of the particle distribution, which is also the symmetry of the 
function p(x, y, z), is determined. It is the symmetry of the space group 
which is obtained from the symmetry group of j by formally regarding the 
transformation R as the identity (as it is with respect to the function p). 

If only the macroscopic properties of the body are of interest, however, it 
is not necessary to know the complete symmetry group of the function 
j(#, y, z). These properties depend only on the direction in the crystal, and 
the translational symmetry of the lattice does not affect them. As regards 
crystallographic structure, the "symmetry of directions" is specified by the 



118 Constant Magnetic Field §28 

32 crystal classes. These are the symmetry groups consisting of rotations 
and reflections only, and are obtained from the space groups by regarding 
every translation as the identity, and the screw axes and glide planes as 
simple axes and planes of symmetry. As regards the magnetic properties, 
the macroscopic symmetry can be classified by groups (consisting of rotations, 
reflections and combinations of these with R) which may be called the mag- 
netic crystal classes. They are related to the magnetic space groups in the 
same way as the ordinary crystal classes to the ordinary space groups. They 
include, firstly, the usual 32 classes, and those classes augmented by the 
element R. These augmented classes are, in particular, the macroscopic 
symmetry groups for all bodies having no magnetic structure, but they occur 
also in bodies with magnetic structure. This happens if the magnetic space 
symmetry group of such bodies includes R only in combination with trans- 
lations, and not alone. 

There are also 58 classes in which R enters only in combination with 
rotations or reflections. Each of these becomes one of the ordinary crystal 
classes if R is replaced by the identity, t 

It should be noted that the occurrence of magnetic structure (ferro- 
magnetic or antiferromagnetic) always involves comparatively weak inter- 
actions, t Hence the crystal structure of a magnetic body is only a slight 
modification of that in the non-magnetic phase, which usually changes into 
the magnetic phase when the temperature is reduced. In this respect ferro- 
magnetics, in particular, differ from ordinary pyroelectrics, but are analogous 
to ferroelectrics. 

If the magnetic crystal class of a body is specified, its macroscopic mag- 
netic properties are qualitatively determined. The most important of these 
is the presence or absence of a macroscopic magnetic moment, i.e. of spon- 
taneous magnetisation in the absence of an external field. The magnetic 
moment M is a vector, behaving as an axial vector (the vector product of 
two polar vectors) under rotation and reflection, and changing sign under 
the operation R. The crystal will possess spontaneous magnetisation if it 
has one or more directions such that a vector M in that direction and having 



f These classes are isomorphous with those discovered by A. V. Shubnikov for the 
symmetry groups of polyhedra with faces of two colours (called by him groups of mixed 
polarity). The element R corresponds to the operation of changing the colour of each face. 
See A. V. Shubnikov, Symmetry and antisymmetry of finite figures (Simmetriya i anti- 
simmetriya konechnykh figur), Moscow, 1951. A direct derivation as symmetry groups for 
the magnetic properties of bodies is given by B. A. Tavger and V. M. Zaitsev, Zhurnal 
eksperimental'noi i teoreticheskoi fiziki 30, 564, 1956; Soviet Physics JETP 3, 430, 1956. 

The total number of magnetic space groups is 1651 ; they are derived (as Shubnikov 
groups) by N. V. Belov, N. N. Neronova and T. S. Smirnova, Trudy Instituta Kristallo- 
grafii 11, 33, 1955 ; A. M. Zamorzaev, Kristallografiya 2, 15, 1957; Soviet Physics: Crystallo- 
graphy 2, 10, 1958. 

% The exchange interaction between the magnetic moments of atoms usually results in 
the saturation of the valency bonds and the formation of non-magnetic structures. A mag- 
netic structure results only from the relatively weak exchange interactions between deep- 
lying d and / electrons of atoms of elements in the intermediate groups of Mendeleev's 
system. 



§29 The magnetic field of a constant current 119 

the above-mentioned properties is invariant under all transformations belong- 
ing to the magnetic crystal class concerned. 

We must again emphasise the difference between these (macroscopic) 
properties and the corresponding ones in electrostatics. The latter are 
qualitatively determined by the ordinary crystal class. In particular, a body 
is pyroelectric if its crystal class admits the existence of a polar vector P 
(the polarisation). It would, however, be entirely wrong to base conclusions 
about the existence or otherwise of a macroscopic magnetic moment on the 
behaviour of the axial vector M with respect to the transformations of the 
non-magnetic crystal class of the body concerned. 

As an illustration, let us consider a tetragonal lattice of identical atoms, 
with magnetic moments parallel to the tetragonal axis.t The magnetic 
crystal class comprises the fourth-order axis C4 (z) , two second-order axes 
combined with jR {C^R and C2 (y) R), the plane of symmetry ofc (z) perpendicu- 
lar to the z-axis, and two vertical planes of symmetry combined with 
jR (o-v^R and cr^R). This group admits the existence of a vector M along 
the tetragonal axis. The crystallographic symmetry class is obtained by 
replacing R by unity, i.e. it is the class D$h. This class does not admit the 
existence of an axial vector M, since the components M x , M y , M z would 
change sign on reflection in the planes o- (z) , a te) , oW respectively. 

The properties of bodies with a spontaneous non-zero macroscopic 
magnetic moment (ferromagnetics) will be discussed in detail in Chapter V. 
In all other crystals, in fairly weak fields, the relation between B and H is 
linear: 

Bi = MkHjc, (28.1) 

with no inhomogeneous term. The magnetic permeability tensor fx^ is sym- 
metrical. This follows from thermodynamic relations which will be derived 
in §30, in exactly the same way as the symmetry of the tensor eijc (§13). 

We may also mention two further phenomena possible in principle. One 
is piezomagnetism, resulting from the existence of a linear relation between 
the magnetic field and the deformation of a body (analogously to piezo- 
electricity; see §17). The other results from a linear relation between the 
magnetic and electric fields in a substance, which would cause, for example, 
a magnetisation proportional to the electric field. % Both these phenomena can 
occur for certain magnetic crystal symmetry classes. || 

§29. The magnetic field of a constant current 

If a conductor carries a non-zero total current, the mean current density 
in it can be written as pv = c curl M+j. The first term, resulting from the 

f Such, for example, is the lattice of iron in its ferromagnetic phase. Crystallographically, 
it is a cubic lattice slightly distorted along one of the fourth-order axes. 

% Effects of this type but quadratic in the field must in principle exist even in isotropic 
bodies, but are negligible. 

|| Examples of these are given by I. E. DzyaloshinskiI, Zhurnal iksperimentaVnol i teore- 
ticheskoi fiziki 33, 807, 1957; 37, 881, 1959 (Soviet Physics JETP 6 (33), 621, 1958; 
10 (37), 628, 1960). 



120 Constant Magnetic Field §29 

magnetisation of the medium, makes no contribution to the total current, 
so that the net charge transfer through a cross-section of the body is given 
by the integral Jj-df of the second term. The quantity j is called the conduc- 
tion current density A The statements made in §20 apply to this current; in 
particular, the energy dissipated per unit time and volume is E«j. 

The distribution of the current j over the volume of the conductor is given 
by the equations of §20, which do not involve the magnetic field due to j 
itself, if we neglect the effect of this field on the conductivity of the body. 
Hence the magnetic field of the currents must be determined for a given 
current distribution. The equations satisfied by this field differ from those 
in §27 by the presence of a term 4ttJ/c on the right-hand side of (27.7): 

div B = 0, (29.1) 

curlH = 4vjjc. (29.2) 

The conduction current density j, which is proportional to the electric field, 
does not become infinite, and in particular is finite on a surface separating 
two media. Hence the term on the right of (29.2) does not affect the boun- 
dary condition that the tangential component of H is continuous. 

To solve equations (29.1), (29.2), it is convenient to use the vector potential 
A, defined by 

B = curl A, (29.3) 

so that equation (29.1) is satisfied identically. Equation (29.3) does not 
uniquely define the vector potential, to which the gradient of any scalar may 
be added without affecting (29.3). For this reason we can impose on A a 
further condition, which we take to be 

divA = 0. (29.4) 

The equation for A is obtained by substituting (29.3) in (29.2). If the linear 
relation B = juH holds we have 

curl /-curl a) = 4irj/c. (29.5) 

In this form the equation is valid for any medium, homogeneous or not. 
In a homogeneous medium, p = constant, and since 

curl curl A = grad div A— A A = — A A 

we find from (29.5) 

A A = -4ir/ij/c. (29.6) 

If we have two or more adjoining media of different magnetic permeability 
fi, the general equation (29.5) has the form (29.6) in each homogeneous 



t The quantity c curl M is sometimes called the molecular current density. This name, 
however, is not in complete accordance with the actual physical picture of motion of charges 
in a conductor. For example, in a metal the conduction electrons, as well as those moving 
in the atoms, contribute to the magnetization Jl. 



§29 The magnetic field of a constant current 121 

medium, while at the interfaces the tangential component of the vector 
(l//i.) curl A must be continuous. Moreover, the tangential component of 
A itself must be continuous, since a discontinuity would mean that the 
induction B was infinite at the boundary. 

The field equations are simpler in the two-dimensional problem of finding 
the magnetic field in a medium infinite and homogeneous in one direction 
(which we take as the ^-direction), the currents which produce the field 
being everywhere in that direction, with the current density j z =j depend- 
ing only on si and y. We make the plausible assumption (to be confirmed 
by the result) that the vector potential of such a field is also in the ^-direction: 
A z — A(x, y). The condition (29.4) is then satisfied identically; the magnetic 
field is everywhere parallel to the ry-plane. We denote by k a unit vector 
in the ^-direction; then 

curl A = curl /4k = grad^4 xk, 
curll-curlAl = curl ( xk) = -kdiv- . 

Hence equation (29.5) becomes 

grad.4 4tt 
div-? = fay), (29.7) 

fJ, c 

i.e. we in fact obtain one equation for the one scalar quantity A(x, y). For 
a piecewise homogeneous medium, (29.7) becomes 

AA = -47r{jLJ(x,y)lc, (29.8) 

with the boundary condition that A and (l//u,) dA/dn are continuous at an 
interface, t 

The magnetic field is easily found if the current distribution is sym- 
metrical about the #-axis: j z =j(r) (where r is the distance from that axis). 
In this case the lines of magnetic force are evidently the circles r — constant. 
The magnitude of the field is found at once from the formula 

(fH-dl = — fj.df, (29.9) 

which is the integral form of (29.2). Thus 

H(r) = 2J(r)/cr, (29.10) 

where J(r) is the total current within the radius r. 



t It should be noticed that the two-dimensional problem with a constant magnetic field 
is equivalent to the two-dimensional electrostatic problem of determining the electric field 
due to extraneous charges of density />«(*, y) in a dielectric medium. The equation to be 
solved in the latter problem is div(cgrad^) = — 4irp e *, where j> is the field potential; 
this differs from (29.7) only in that A,j/c and p are replaced by <f>, p ex and 1/e respectively. 
The boundary conditions on A and <f> are the same. A difference occurs, however, on passing 
to E and B from <f> and A respectively. The vectors E = — grad <f> and B = curl A are 
the same in magnitude but in perpendicular directions at any given point. 



122 Constant Magnetic Field §29 

The reduction of the vector equation (29.5) to a single scalar equation is 
possible also if the current distribution is axially symmetrical and has in 
cylindrical co-ordinates r, <j), z the form j r =j z = 0, j$ = j(r,z). We 
seek the vector potential in the form A r = A z = 0, A$ = A(r, z). The 
components of the magnetic induction B = curl A are B r = — 8A\8z> 
B z — (llr)d(rA)jdr, B$ = 0, and the ^-component of equation (29.2) gives 

8 /l 8A\ 8 1 1 d \ 4tt 

rHr + tHt^ = — j{r > z) - (29 - n) 

czXjjL cz J orX/jLT or J c 

The equations for the magnetic field of the currents can be solved in a 
general form in the important case where the magnetic properties of the 
medium may be neglected, i.e. where we can put ju. = 1. The vector poten- 
tial then satisfies in all space the equation AA = — 4vjjc with no conditions 
at the interfaces between different media (including the surface of the 
conductor on which the current flows). The solution of this equation which 
vanishes at infinity ist 



c) R 



dV, (29.12) 



where R is the distance from the volume element dV to the point at which 
A is to be calculated. In taking the curl of this equation, we must remember 
that the integrand j/R is to be differentiated with respect to the co-ordinates 
of this point, of which j is independent, so that 

curl QIR) = grad (1/12) X j = -R X j/#3, 

where the radius vector R is from dV to the point under consideration. 
Thus 

ixR 

dV. (29.13) 



B = H = lf 



If the conductor on which the current flows is sufficiently thin (a thin 
wire), and if we are interested only in the field in the surrounding space, the 
thickness of the wire may be neglected. In what follows we shall often dis- 
cuss such linear currents. The integration over the volume of the conductor 
is then replaced by an integration along its length: the formulae for linear 
currents are obtained from those for volume currents by making the substi- 
tution }dV ->Jdl, where / is tne tota ^ current in the conductor. For 
example, from formulae (29.12) and (29.13) we have 

J fdl /rdlxR /nni ^ 

A -ih H = l cf-^- (29 - l4) 

The latter formula is Biot and Savart's law. 



t See The Classical Theory of Fields, §5-8. 



§29 The magnetic field of a constant current 123 

This simple formula for the magnetic field of a linear current does not 
depend on the assumption that fi = 1. Since we neglect the thickness of the 
conductor, no boundary conditions at its surface need be applied, and the 
magnetic properties of the conducting material are of no importance (it may 
even be ferromagnetic). The solution of equation (29.6) for the field in the 
medium surrounding the conductor is therefore 

A -7h? b = t)-r^> (2915) 

whatever the magnetic susceptibility of that medium. Thus the presence 
of the medium simply changes the magnetic induction by a factor /u. The 
field H = B/yM is unchanged. 

The problem of determining the magnetic field of linear currents can also 
be solved as a problem of potential theory. Since we neglect the volume of 
conductors, we are in fact determining the field in a region containing no 
currents except along certain line singularities. In the absence of currents, 
a constant magnetic field has a scalar potential, which in a homogeneous 
medium satisfies Laplace's equation. There is, however, an important 
difference between the magnetic field potential and the electrostatic poten- 
tial : the latter is always a one- valued function, because curl E = in all 
space (including charged regions) and so the change in the potential in going 
round any closed contour (i.e. the circulation of E round that contour) is 
zero. The circulation of the magnetic field round a contour enclosing a 
linear current is not zero, but ^nj\c. Hence the potential changes by this 
amount on each passage round a contour enclosing a linear current, i.e. it is 
a many-valued function. 

If the currents lie in a finite region of space (and fx = 1 everywhere), the 
vector potential of the magnetic field at a great distance from the conductors 
is 

A = J£xR/ J R 3 , (29.16) 

where 

Jt = jrxjdVI2c (29.17) 

is the total magnetic moment of the system, t 
For a linear current, this becomes 

M =jjrxdl/2c, 

and can be transformed into an integral over a surface bounded by the line 
of the current. The product df = \t X dl is equal in magnitude to the area 

f See The Classical Theory of Fields, §5-9. In the derivation there given, we use explicitly 
the idea of a current as the result of the motion of individual charged particles. Such a 
derivation is, of course, quite general, but formula (29.16) can also be obtained by macro- 
scopic arguments (see Problem 4). 



124 Constant Magnetic Field §29 

of the triangular surface element formed by the vectors r and dl. The 
vector Jdf is independent of the particular surface (bounded by the current) 
over which it is taken. Thus the magnetic moment of a closed linear current 
is 

M = jjdf/c. (29.18) 

In particular, for a plane closed linear current the magnetic moment is 
simply JS/c, where S is the area of the plane enclosed by the current. 

To conclude this section, we may briefly discuss the energy flux in a 
conductor. The energy dissipated as Joule heat in the conductor is derived 
from the energy of the electromagnetic field. In a steady state, the "equation 
of continuity" which expresses the law of conservation of energy is 

-divS = jE, (29.19) 

where S is the energy flux density, given in a conductor by 

S = cExH/4rr, (29.20) 

which is formally the same as the expression for the Poynting vector for the 
field in a vacuum. This is easily verified directly by calculating div S from 
the equations curlE = and (29.2), when we obtain (29.19). 

Formula (29.20) also follows independently from the obvious condition 
that the normal component of S must be continuous at the surface of a 
conductor, if we use the continuity of E* and H« and the validity of (29.20) 
in the vacuum outside the body. 

PROBLEMS t 
Problem 1. Determine the scalar potential of the magnetic field of a closed linear current. 

Solution. We transform the line integral into one over a surface bounded by the line, 
obtaining 



J r i 

B = curl A = — - (df'grad) grad — 
c J R 



(where we have used the fact that A(1/JR) = 0). Since B = —grad <f>, we have for the 
scalar potential 

^-Jdf.grad-=--j— . 

The integral is, geometrically, the solid angle ii subtended by the closed contour at the point 
considered. The above-mentioned many-valuedness of the potential is seen from the fact 
that, as this point describes a closed path round the wire, the angle Q changes suddenly 
from 2n to — lit. 

Problem 2. Find the magnetic field of a linear current flowing in a circle of radius a. 

Solution. We take the origin of cylindrical co-ordinates r, <f>, z at the centre of the circle, 
with the angle <f> measured from the plane which passes through the ar-axis and the point at 



In Problems 1-4, fi ■■!. 



§29 



The magnetic field of a constant current 



125 



which the field is calculated. The vector potential has only one component, A* — A{r, z), 
and by formula (29.14) we have 



A * = ~J 



J £ cos «f> dl 



R 



c J Via 



a cos <f> d<f> 



■\/(a 2 +r 2 +z 2 — 2ar cos <f>) 



Putting 6 = K^— w )» we find 



A^^rJ^Kl-mK-E], 



where k 2 — 4arl[(a+r) 2 +z 2 ], and K and E are complete elliptic integrals of the first and 
second kinds : 






dd. 



The components of the induction are 

dA* J 2z 

a r = — 



crV[(a+r)*+z*] L (a-r)*+z* J' 

1 a J 2 r a 2 —r 2 —z 2 1 

r or c v[(a+r) a +« 2 ]L (a— r) 2 +z 2 1 

Here we have used the easily verified formulae 

eK E K dE _ E-K 

dk ~ kil-k 2 ) k' dk~ k ' 

On the axis (r = 0) we have B T = 0, B t = 2ira 2 J/c(a 2 +z 2 ) 3 ' 2 , as can also be found by a 
straightforward calculation. 

Problem 3. Determine the magnetic field in a cylindrical hole in a cylindrical conductor 
of infinite length carrying a current uniformly distributed over its cross-section (Fig. 17). 




x,x' 



Fig. 17 



126 Constant Magnetic Field §30 

Solution. If there were no hole, the field in the cylinder would be given by H'x = 
—litjylc, H' y = 2-njxjc. The dimensions and axes are as shown in Fig. 17. If a current of 
density —j were to flow in the inner cylinder, it would produce a field H" x = 2njy'Jc, H" v 
= —l-njx'/c. The required field in the hole is obtained by superposing these two fields. 
Since x—x' = OO' = h, and y — y', we have H x = 0, H v = lirjhlc = 2A//(6 2 — a z )c, i.e. a 
uniform field in the y-direction. 

Problem 4. Derive from (29.12) the formula (29.16) for the vector potential of the field 
far from the currents. 

Solution. We write R = Ro— r, where Ro and r are the radius vectors from the origin 
(situated somewhere among the currents) to the point considered and to the volume element 
dV respectively. Expanding the integrand in powers of r and using the fact that J" j dV == 0, 
we have Ai = (RklcR 3 ) J" xkjt dV. The suffix to R is omitted. Integrating by parts the 
identity J XiXk div j dV = gives J (jtXk+jkXi) dV — 0. Hence we can write 

Ai = (Rkl2cR s )I(xkji-Xij k ) dV, 
which is (29.16). 

Problem 5. Determine the magnetic field produced by a linear current in a magnetically 
anisotropic medium (A. S. Viglin). 

Solution. In the anisotropic medium surrounding the conductor we have 

div B = mtBHkldxt = 0, (1) 

where /*(* is the magnetic permeability tensor of the medium. Instead of introducing the 
vector potential by B = curl A, we use another vector C defined by 

Hi = eikifUcmSCildXm, (2) 

where etki is the antisymmetrical unit tensor. Then equation (1) is again satisfied identically. 
We can also impose on the vector C thus defined the condition 

divCs BCi/Bxi = 0. (3) 

Substituting (2) in curl H = 4-irj/c, we obtain em dHi/dxk = —pkpd z Ci[dXkdx p = 4irjtlc 
(using the condition (3) and the fact that etkieimn = ^im^kn—^in^m). The equation thus 
obtained for C is the same in form as that for the electric field potential resulting from 
charges in an anisotropic medium (§13, Problem 2). The solution is 

jdF 



-;/■ 



V(MrtiW 

where |f*| is the determinant of the tensor ink, and R the radius vector from the point con- 
sidered to dV. For a linear current we have 

/ X dl 



cVM J Vi^ikRiRk) 

§30. Thermodynamic relations in a magnetic field 

The thermodynamic relations for a magnetic substance in a magnetic 
field are, as we shall see, very similar to the corresponding relations for a 
dielectric in an electric field. Their derivation, however, is quite different 
from that given in §10. This difference is ultimately due to the fact that a 
magnetic field, unlike an electric field, does no work on charges moving in it 
(since the force acting on a charge is perpendicular to its velocity). Hence, 
to calculate the change in the energy of the medium when a magnetic field is 
applied, we must examine the electric fields induced by the change in the 
magnetic field and determine the work done by these fields on the currents 
which produce the magnetic field. 



§30 Thermodynamic relations in a magnetic field 127 

Thus the equation which relates electric and variable magnetic fields must 
be used. This equation is 

1 dB 

curlE= ; (30.1) 

c ot 

it follows immediately on averaging the microscopic equation (1.3). 

During a time 8t, the field E does work 8t $ }*EdV on the currents j. 
This quantity with the opposite sign is the work 8R "done on the field" by 
the external e.m.f. which maintains the currents. Substituting 

j = c curl H/47T, 
we have 

8R=-8t~ fE.curlHdF 

4ttJ 
= 8t—(diy(ExH)dV-8t—(H'CurlEdV. 

The first integral, on being transformed to an integral over an infinitely 
distant surface, is seen to be zero. In the second integral we substitute 
curlE from (30.1) and put SB = 8t dB[dt for the change in the magnetic 
induction, obtaining finally 

8R = J*H.SBdF/47r. (30.2) 

This formula appears entirely analogous to the expression (10.2) for the 
work done in an infinitesimal change in the electric field. It must be pointed 
out, however, that the physical analogy between the two formulae is actually 
not complete, since H, unlike E, is not the mean value of the microscopic 
field. 

Having derived formula (30.2), we can write down all the thermodynamic 
relations for a magnetic substance in a magnetic field by analogy with those 
given in §10 for a dielectric in an electric field, simply replacing E and D 
by H and B respectively. We shall give some of these formulae here for 
purposes of reference. The differentials of the total free energy and the 
total internal energy are 

8& = -^Sr+jH.8BdF/4ir, 

(30.3) 
8% = T8y + jU'8BdV/^jT y 

and those of the corresponding quantities per unit volume are 
dF= -Sdr+£dp+H.dB/4ir, 
dU= TdS+£dp + H-dBl47r. ' ' 



128 Constant Magnetic Field §30 

We need also the thermodynamic potentials 

V = C/-H-B/4tt, F = F-H.B/47T, (30.5) 

for which 

dF = - SdT+£d/>-B.dH/47r, 

dfr= rdS+£d/>-B.dH/477. ( ' ' 

If the linear relation B = piH. holds, we can write the expressions for all 
these quantities in the form 

U = U (S, P ) + #78^, F = F (T, p) + &/**?, 

V = U (S,p)-p,H2IS7r, P = F (T,p)-tJiH2/STT. l 

The work 8R (or, what is the same thing, the change 8F at constant 
temperature) can be written in a different form, in terms of the current 
density and the vector potential of the magnetic field. For this purpose we 
put SB = curl SA and 

(SJ*> = _ fH-curlSAdP 

= fdiv(Hx8A)dF+— fsA.curlHdF. 

AttJ AttJ 

The first integral is again zero, and the second gives 

(h&) T = Jj.SAdF/c. (30.8) 

A similar transformation gives 

(&^) r = -J*A.8jdF/c. (30.9) 

It is useful to note that in macroscopic electrodynamics the currents 
(sources of the magnetic field) are mathematically analogues of the poten- 
tials, not of the charges (the sources of the electric field). This is seen by 
comparing formulae (30.8) and (30.9) with the corresponding results for an 
electric field: 

(8&) T =j<f>8 P dV, (&^) T = - fptydV (30.10) 

(see (10.13), (10.14)). We observe that the charges and potentials appear in 
these formulae in the opposite order to the currents and potentials in formu- 
lae (30.8), (30.9). 

On account of the complete formal correspondence between the thermo- 
dynamic relations (expressed in terms of field and induction) for electric 
and magnetic fields, the thermodynamic inequalities derived in §18 can also 



§31 The total free energy of a magnetic substance 129 

be applied to magnetic fields. In particular, we have seen that it follows 
from these inequalities that e > 0. In the electric case this result was of no 
interest, because it was weaker than the inequality e > 1 which follows on 
other grounds. In the magnetic case, however, the corresponding inequality 
[j, > is very important, as it is the only restriction on the values which can 
be taken by the magnetic permeability. 



§31. The total free energy of a magnetic substance 

In §11 expressions have been derived for the total free energy $F of a 
dielectric in an electric field. One of the thermodynamic properties of this 
quantity is that the change in it gives the work done by the electric field on 
the body when the charges producing the field remain constant. In a mag- 
netic field a similar part is played by the free energy &> since for given 
currents producing the field the change in & is the work done on the body. 

The following derivation is entirely analogous to that given in §11. The 
"total" quantity & is defined as 



SF 



-J(' + S dr " (3U) 



where <?) is the magnetic field which would be produced by the given cur- 
rents in the absence of the magnetisable medium. The plus sign appears in 
the parenthesis (instead of the minus sign as in (11.1)) because the value of 
& for a magnetic field in a vacuum is - J(§ 2 /87r) dV (see (30.7)). The 
integration in (31.1) is taken over all space, including the volume occupied 
by the conductors in which flow the currents producing the field, t 

Let us calculate the change in & (for a given temperature and no depar- 
ture from thermodynamic equilibrium in the medium) corresponding to an 
infinitesimal change in the field. Since 8F = -B«SH/4tt, we have 

S#= -J(B-SH-|).8^)dr/47r 

= -J(H-^).S^dr/47r-|B.(SH-8^)dr/47r-J(B-H).^dF/47r. 

(31.2) 

Introducing the vector potential % of the field $), we can write in the 
first term 

(H-£).S£ = (H-£).curlS$l 

= div[8H.(H-£)] + $2l.curl(H-$). 



t In §11 we took the integration in (11.1) over all space except the volume occupied by 
the charged conductors producing the field. This was possible because there is no electric 
field in a conductor, charged or not. There is a magnetic field, however, inside the conduc- 
tors which carry the currents, and they cannot be excluded in calculating the total free energy. 



130 Constant Magnetic Field §31 

By definition, the fields H and $ are produced by the same currents j, the 
distribution of which over the volume of the conductors is (see §29) inde- 
pendent of the field which they produce, i.e. is independent of the presence 
or absence of magnetic substances in the surrounding medium. Hence 
curlH and curl£ are both equal to 47rj/c, and so curl(H— £) = 0. 
The integral of div[S$l«(H — .£))] is transformed into an integral over an 
infinitely distant surface, and so vanishes. 

Similarly, we see that the second term on the right of (31.2) is zero; thus 

S#= -J(B-H).S^dr/477 

= -JM>S$dV. (31.3) 

The expression which we have obtained for S«^ is exactly similar to (11.3) 
for the electrostatic problem. In particular, in a uniform magnetic field ^ 
we have for d^ an expression analogous to (11.5): 

&& = -SP&T-J(-&%, (31.4) 

where M is the total magnetic moment of the body. 

Without repeating the subsequent calculations, we shall write down the 
following formulae by analogy with those in §11. If the linear relation 
B = juH holds, we have 

#-Sr (V,T) = -J*$.MdF. (31.5) 

In particular, if the external field is homogeneous, then 

&-&<&y, T) = -%%'JZ. (31.6) 

In the general case of an arbitrary relation between B and H, & can be 
calculated from the formula 



#= \{ P+ ~t~~ iM '*) dV 
U H-B \ 

= Jr~"~&r~* M '*) dr - (3L7) 



In §11 we gave also the simpler formulae obtained when the dielectric 
susceptibility is small. The analogous case for the magnetic problem is 
especially important because, as mentioned above, the magnetic suscepti- 
bility of the majority of bodies is indeed small. In this case 

f-,P = -l x j&dV. (31.8) 



§32 The energy of a system of currents 131 

We can also derive results for the magnetic field analogous to those 
obtained in §14. These concern the change in the thermodynamic quantities 
resulting from an infinitesimal change in the magnetic permeability fi, the 
field sources being assumed unchanged. It is clear from the foregoing that 
we must consider the change in «^, and not that in 3F as in §14. We shall not 
repeat the derivation, which is similar to that of (14.1), but merely give the 
result : 

S#= -J8(jLH2dV/87T. (31.9) 

In §14 we used this formula to deduce that the dielectric susceptibility of 
any substance is positive. In the magnetic case we cannot draw this conclu- 
sion, and the magnetic susceptibility may be of either sign. The reason for 
this marked difference is that the Hamiltonian of a system of charges moving 
in a magnetic field contains not only terms linear in the field (as in the 
electric case) but also quadratic terms. Hence, in determining the change 
in the free energy of the body in the magnetic field by means of perturbation 
theory as in (14.2), we have a contribution in the first approximation as well 
as the second. In such a case no general conclusion can be drawn concerning 
the sign of the variation. It is positive for paramagnetic bodies and negative 
for diamagnetic ones. 

In §14 we also drew conclusions concerning the direction of motion of 
bodies in an electric field. Similar conclusions follow from (31.9), but, since 
fi may be either greater or less than 1, there is no universal result. For 
example, in an almost uniform field paramagnetic bodies {p, > 1) move in 
the direction of H increasing, and diamagnetic bodies (ji < 1) in the opposite 
direction. 

§32. The energy of a system of currents 

Let us consider a system of conductors with currents flowing in them 
and assume that neither the conductors nor the medium surrounding them 
are ferromagnetic, so that B = fiK everywhere. According to §30, the total 
free energy of the system is given in terms of the magnetic field of the 
currents by 

& = JH-BdVj&n. (32.1) 

Here we omit the quantity «^o> which is a constant (at a given temperature) 
and is not related to the currents. The integration in (32.1) is taken over all 
space, both inside and outside the conductors. 

The same energy can also be expressed in terms of the currents by means 
of the integral 

& = JA-jdVI2c; (32.2) 



132 Constant Magnetic Field §32 

cf. the derivation of (30.8) from (30.2). Here the integration extends only 
over the conductors, because j = outside them. 

Since the field equations are linear, the magnetic field can be written as 
the sum of the fields resulting from each current alone with no current in 
the other conductors: H = SH«. Then the total free energy (32.1) is 

& = S^ aa + Z&ab, (32.3) 

a a>b v ' 

where 

&aa = jH a .B a dF/87r, & ab = jHa.BfrdF/477. (32.4) 

We have put & a b = ^ baj since IVB 6 = juHa'Hb = Hft.B , where [i is the 
magnetic permeability at any point. The quantity & aa may be called the 
free self-energy of the current in the ath conductor, and && the interaction 
energy of the ath. and bth. conductors. It should be borne in mind, however, 
that these names are strictly correct only if the magnetic properties of both 
the conductors and the medium are neglected. Otherwise the field, and 
therefore the energy, of each current depend on the position and magnetic 
permeability of the other conductors. 

The quantities (32.4) can also be expressed in terms of the currents j 
in each conductor, in accordance with formula (32.2) : 

&am = j)a-AadV a l2c, & ab = j} a .A b dV a /c = jj b 'AadV b /c. (32.5) 

The integral in ^ aa is here taken only over the volume of the ath conductor; 
iFab can be written as either of the two expressions, in which the integration 
is over the volume of the ath and Mi conductor respectively. 

When the distribution of the current density over the volume of the 
conductor is given, «^ ao depends only on the total current J a passing through 
a cross-section. Both the current density j and the field which it produces 
will be proportional to J a . Hence the integral BF^ is proportional to J a 2 , 
and we write it 

^aa = Laaja 2 l2c*, (32.6) 

where L aa is called the self-inductance of the conductor. Similarly, the 
interaction energy of two currents is proportional to the product J a J b : 

&ab = Labjajb/c*. (32.7) 

The quantity Lab is called the mutual inductance of the conductors. Thus 
the total free energy of a system of currents is 

* - ^2 W^+^W*/* - ^22 W«ft- (32.8) 
a a>b a b 



§32 The energy of a system of currents 133 

The condition that this quadratic form should be positive definite places 
certain restrictions on the values of the coefficients. In particular L ao > 
for all a, and LaaLbb > Lab 2 - 

The calculation of the energy of currents in the general case of arbitrary 
three-dimensional conductors requires a complete solution of the field 
equations, and is a difficult problem. It becomes simpler if the magnetic 
permeability of both the conductors and the surrounding medium can be 
taken as unity. It should be noted that the energy of the currents is then no 
longer dependent on the thermodynamic state (in particular, on the tem- 
perature) of the bodies, and hence the free energy in the above formulae may 
be referred to simply as the energy. 

For fi = 1 the vector field potential due to the currents j is given by 
formula (29.12). Hence the self-energy of the ath conductor is 



,_LffJ* 

2c* J J R 



^=™)n &ViV '' (32 - 9) 

where both integrations are taken over the volume of the conductor con- 
sidered, and R is the distance between dF"and dV. Similarly, the mutual 
energy of two conductors is 

& ab = -J J^dF a dr & , (32.10) 

where dV a and dVb are volume elements in the two conductors. 

The mutual energy of two linear currents is particularly easy to calculate. 
In formula (32.10) we change from volume currents to linear ones by replac- 
ing jadJ^o and jbdVb by J a dla and Jbdh respectively, and we find that the 
mutual inductance is L & = <j><j>dlo*dl&/.R. In this approximation, there- 
fore, Lab depends only on the shape, size and relative position of the two 
currents, and not on the distribution of current over the cross-section of each 
wire. It must be emphasised that this simple formula can be obtained for 
linear currents without imposing the condition that fi — 1. In the approxi- 
mation where the thickness of the wires is neglected, their magnetic pro- 
perties have no effect on the field which they produce, and therefore no 
effect on their mutual energy. If the magnetic permeability jj, of the medium 
surrounding the wires is different from unity, the vector potential is, by 
(29.15), simply multiplied by fi, and therefore so is the magnetic induction. 
The mutual inductance is therefore multiplied by the same factor, so that 



L ab = nffdla-dh/R. (32.11) 

The self-inductance of linear conductors is much more difficult to calculate ; 
we shall discuss it in §33. 

The total energy of a system of linear currents can be written in still 



134 Constant Magnetic Field §32 

another form. To do this, we return to the integral (32.2), which for linear 
currents becomes 



- iZ>4 



A-dla, (32.12) 



where A is the vector potential of the total field at the element dl« of the ath 
conductor. The main error in going from (32.2) to (32.12) arises from 
neglecting the change in the field (including the field of the current con- 
sidered) over the cross-section of the wire. Each of the contour integrals in 
(32.12) can be transformed into a surface integral: 



j A-dla = J curl A.d& = jB-dfc, 



i.e. it is the flux of the magnetic induction or magnetic flux through the 
circuit of the ath current. We denote this flux by . Then 

* = ^5>° a - (32,13) 

a 

Similarly, the free energy & of a linear current / in an external magnetic 
field, i.e. the energy without the self-energy of the field sources, can be 
expressed in terms of the magnetic flux. Evidently 

SF = JQIc (32.14) 

where <I> is the flux of the external field through the circuit of the current J. 
If the external field is uniform, and fx = 1 in the external medium, then 
O = #• Jdf. Introducing the magnetic moment of the current in accor- 
dance with (29.18), we have & = J£*Si). 

Knowing the energy of a system of currents as a function of their shape, 
size and relative position, we can determine the forces on the conductors by 
simply differentiating with respect to the appropriate co-ordinates. Here, 
however, the question arises which characteristics of the currents should be 
kept constant in the differentiation. It is most convenient to differentiate 
at constant current. In this case the free energy is represented by ^, and 
so the generalised force F q in the direction of a generalised co-ordinate q is 
F q = —{d^jdq)j t T- The suffixes show that the differentiation is effected 
at constant current and constant temperature. Since we omit the term 
independent of the currents in the free energy, & and & differ only in 
sign, and so 

id&\ _ td&\ 1 ^ T T BLai 



\ 1 ^-^ dLab 



\Sqh \8qh 2c^ JV ° dq 
here and henceforward the suffix T to the derivatives is omitted, for brevity. 



§32 The energy of a system of currents 135 

In particular, the forces exerted on a conductor by its own magnetic field 
are given by the formula 

1 8L 

where L is the self-inductance of the conductor. The nature of these forces 
can be seen as follows. For given current (and temperature), & tends to be 
a minimum. Since & = —LJ^jlc 2 , this means that the forces on the con- 
ductor will tend to increase its self-inductance. The latter, having the 
dimensions of length, must be proportional to the dimension of the conduc- 
tor. Thus the effect of the magnetic field is to increase the size of the 
conductor. 

For a current in an external magnetic field we havet 

& = -& = -Jl-%. (32.17) 

In all the above formulae for the energy it is assumed that there is a 
linear relation between the magnetic field and induction. In the general 
case where this relation is arbitrary, analogous differential relations can be 
set up. The change in the free energy resulting from an infinitesimal change 
in the field (at constant temperature) is, by (30.8), 8^ = jySAdV/c or, 
for a system of linear currents, 

8jr = -2/a<|>8A.dl«. 
C a J 

Proceeding as in the derivation of (32.13) from (32.12), we have 

8SF = -yj a 8<I> a . (32.18) 

c*-i 
a 

Similarly, we find from (30.9) 

&#= --]T<D«8/.. (32.19) 

a 

Thus we can say that, for a system of linear currents, & is the thermo- 
dynamic potential with respect to the magnetic fluxes, and & with respect 
to the currents, the two potentials being related by 

& = &--^Ja®a. (32.20) 

a 



t The factor J which appears in (31.6) is absent in (32.17) because the magnetic moment 
of the current in the latter equation is independent of the field, whereas the magnetic moment 
in (31.6) is itself due to the field. 



136 Constant Magnetic Field §33 

Whatever the magnetic properties of the substance, therefore, the thermo- 
dynamic relations 

J a jc = 8&/dQ> ay ^ a /c = -d&jBJa (32.21) 

hold. If these formulae are applied to the case where the field and induction 
are linearly related, so that !F is given by (32.8), we obtain 

0> a = -^Labjb. (32.22) 

C b 

Thus the inductances are the coefficients of proportionality between the 
magnetic fluxes and the currents which produce the magnetic field. The 
product Labjbjc is the magnetic flux through the circuit of the current J a 
due to the current Jb(b # a), and Laajajc is that due to the current J a itself. 

§33. The self-inductance of linear conductors 

In calculating the self-inductance of a linear conductor its thickness 
cannot be entirely neglected as it was in calculating the mutual inductance of 
two conductors. If it were, we should obtain from (32.9) the self -inductance 
L = §§dhdl'lR, where both integrals are taken along the same circuit, 
and this integral is logarithmically divergent because of the contribution 
from small R. 

The exact value of the self-inductance of a conductor depends on the 
distribution of current in it, which may vary with the manner of excitation 
of the current, i.e. with the manner of application of the electromotive force. 
For a linear conductor, however, the self-inductance does not, to a fairly 
high accuracy, depend on the distribution of current over the cross-section, t 

Let us write the self-inductance as L = L e + Li, where L e and Li result 
from the magnetic field energy outside and inside the conductor respec- 
tively. For a linear conductor, the "external" part L e makes the main 
contribution to the self-inductance. This is because most of the magnetic 
energy of a closed linear circuit resides in the field at distances from the 
wire large compared with its thickness. For the energy per unit length of 
an infinite straight wire is 

(fi e l87T)jH*-2iTr4r = (^/frr) \{2J \cry-2-nr dr = (^7 2 /c 2 ) Jdr/r, 

where r is the distance from the axis of the wire and fx e the magnetic perme- 
ability of the external medium. This integral diverges logarithmically for 
large r. For a closed linear circuit, of course, this divergence disappears, 
because the integral is "cut off" at distances of the order of the dimension 



t More precisely, it is independent of the distribution of current provided that the current 
density varies appreciably only over distances comparable with the thickness a of the wire. 
If, however, the distribution is such that the current density varies appreciably over distances 
small compared with a (as happens, for particular reasons, in the skin effect and in super- 
conductors), then the self-inductance does depend on the distribution. 



§33 The self-inductance of linear conductors 137 

of the circuit. We obtain an approximate value for the energy on multi- 
plying this integral by the total length / of the wire, and taking / as the upper 
limit and the radius a of the wire as the lower limit. The result is 
(pej 2 llc 2 ) log(lJa), and hence the self-inductance is 

L = 2fi e l log(//a). (33 1) 

This expression is said to be of logarithmic accuracy: its relative error is of 
the order of l/log(//a), and the ratio Ija is assumed to be so large that its 
logarithm is large, t 

A particular case of a linear conductor is a solenoid, which consists of a 
wire wound in a helix, with the turns very close together. Neglecting the 
thickness of the wire and the distance between the turns, we have simply a 
conducting cylindrical surface with a "surface" conduction current on it. 
The equation curl H = 47rj/c within the conductor is here replaced by the 
boundary condition. 

n x(H 2 -Hi) = 47rg/c, (33.2) 

where g is the surface current density, Hi and H 2 the fields on each side of 
the surface, and n the unit normal vector into medium 2; cf. the derivation 
of (27.14). 

If the solenoid is of infinite length, the magnetic field which it produces 
can be found very simply. The surface currents flow in circles, and their 
density £ = nj, where/ is the current in the wire and n the number of turns 
per unit length of the solenoid. The field outside the cylinder is zero; the 
field inside is uniform and along the axis of the cylinder, and is H = Avnjjc. 
For this field evidently satisfies the equations div H = 0, curl H = in 
all space outside the conducting surface, and also the boundary condition 
(33.2) at that surface. 

Accordingly, the field energy per unit length of the cylinder is 

fieHhr&IZTr = lnhiWii e p\c\ 

where b is the radius of the cylinder and fi e pertains to the material within 
the solenoid. Neglecting the end effects, we can apply this formula also to 
a solenoid whose length h is finite, but large compared with b. Then the 
self-inductance is 

L = WnWhpe = 27T/ji e nbl, (33.3) 

where / = lirbnh is the total length of the wire. The greater self-inductance 
of a solenoid as compared with that of a straight wire of equal length (cf. 
(33.1)) is, of course, due to the mutual induction between adjoining turns. 

t The assertion made above that the self- inductance is independent of the current 
distribution actually applies not only to the approximation (33.1) but also to the next 
approximation, in which terms not containing the large logarithm are included (or, what 
is the same thing, the argument of the logarithm includes a coefficient) ; see the Problems at 
the end of this section. 



138 Constant Magnetic Field §33 

PROBLEMS t 
Problem 1. Determine the self-inductance of a closed circuit of thin wire of circular cross- 
section. 

Solution. The magnetic field in the wire can be taken to be the same as that inside an 
infinite straight cylinder: H = 2Jr/ca 2 , where r is the distance from the axis of the wire and 
a its radius. Hence we find the internal part of the self-inductance : 

2c2 *** ' WAV-Vm, (1) 






where / is the length of the wire. 

To calculate L e , we notice that the field outside a thin wire is independent of the distri- 
bution of current over its cross-section. In particular, the energy ^ e of the external magnetic 
field is unchanged if we assume that the current flows only on the surface of the wire. The 
field inside the wire is then zero, and 3F e may be calculated as the total energy from formula 
(32.2). On account of the assumed surface distribution of the current, the integral in this 
formula becomes a line integral along the axis of the wire, and so the external part of the 
self-inductance is 

2c 2 T f 

*-jr-i;J«l~-'«. 

where the value of A in the integrand is taken at the surface of the wire. In obtaining this 
formula we also use the fact that, in the approximation used here, the field is constant over 
the perimeter of a cross-section. 

Having reduced the problem to that of calculating A for r = a, we now make a different 
assumption concerning the current distribution, namely that the whole current J flows 
along the axis of the wire. The field on the surface of the wire is, in the approximation 
considered, unchanged by this assumption (nor would it be changed for a straight wire of 
circular cross-section). Then, by formula (29.14), we have 



w~-fr#L 



where R is the distance from the element dl of the axis to a given point on the surface of the 
wire. We divide the integral into two parts, one for which R > A and the other for which 
R < A, where A is a distance small compared with the dimension of the circuit but large 
compared with the radius a of the wire.J In the integral where R > A, a may be neglected 
and R taken simply as the distance between two points on the circuit. The vector integral 
where R < A may be assumed to be along the tangent at the point considered. Denoting by 
t the unit vector in that direction, we have 

f f£| g( f dl , 2finh-HA/q) 

L J Rir-a J V(« 2 +/ 2 ) 

R<& -A. 

£ 2t log(2A/a). 
This expression can be written as the integral 

J dVR, 

A>«>*<* 

where R is again the distance between points on the circuit. Adding the two integrals for 
R > A and R < A, we obtain 

J f dl 

R>ia 

for which the arbitrary parameter A has disappeared, as it should. 



t In Problems 1-6 we put (i e = 1. 

X A similar procedure was used to calculate the capacity of a thin ring in §2, Problem 4. 



§33 The self-inductance of linear conductors 139 

The final result is therefore 



-*J : 



The integration here extends over all pairs of points on the circuit whose distance apart 
exceeds \a. 

Problem 2. Determine the self-inductance of a thin wire ring (of radius 6) of circular cross- 
section (of radius a). 

Solution. The integrand in (2), Problem 1, depends only on the central angle 4> subtended 
by the chord R, and R = 26 sin \<f>, while dl'dl' = dldl' cos <f>. Hence 
it 
' cos <f>'2nb'b d<f> 

— — : — - — = 4trb[— log tan i<f>o—2 cos £&>]• 
zo sin i9 

The lower limit of integration is determined from 26 sin £p\> = ia, whence po £ «/26. 
Substituting this value and adding Li = irbpt, we have to the required accuracy 

L = 4w6tlog(86/a)-2+i/ti]. 
In particular, for /*« = 1 we obtain 

L = 47T6[log(86/a)-(7/4)]. 

Problem 3. Determine the extension of a ring of wire (with m = 1) under the action of 
the magnetic field of a current flowing in it. 

Solution. The internal stresses parallel and perpendicular to the axis are, by (32.16), 
given by 

2 J 2 8L p 8L 

ita i a % = , 2irabo. = . 

1 2c 2 d(2irb)' x 2c* da 



Substituting L from Problem 2, we have 

j* r, 8 * 3 i J 2 

a,== -^^L log 7"4j' ai ""5P- 

Hence the required relative extension of the ring ist 

A6 1 , J 2 / 86 3 \ 

— -^..-a-O - 55«(««7-4+H- 

where E is Young's modulus and a Poisson's ratio for the wire. 

Problem 4. Determine the self-inductance per unit length of a system of two parallel 
straight wires (with fit = 1) having circular cross-sections of radii a and 6, with their axes 
a distance h apart, and carrying equal currents J in opposite directions (Fig. 18). 




Fig. 18 



f See Theory of Elasticity, §5, Pergamon Press, London, 1959. 



140 



Constant Magnetic Field 



§33 



Solution. The vector potential of the magnetic field of each current is parallel to the 
axes of the wires, and so the two vector potentials can be added algebraically. For the mag- 
netic field of wire 1, with a uniformly distributed current +J, we have in cylindrical co- 
ordinates 



-#-£) 



for r< a, 



A t = -(c-1-2 log^) for r> a, 



where C is an arbitrary constant; A z is continuous at the surface of the wire. The formulae 
for wire 2 are obtained by substituting b for a and changing the sign of/. Integration over 
the cross-section of wire 1 in formula (32.2) gives 



2c 2 ira 2 



&JM-H-»-t)}* 

S? J J I 1 - ? +Iog m r d * dri = 2^(2 +2 log ih 



2c 2 7ra 2 



The integration over the cross-section of wire 2 gives the same thing with a in place of b. 
The required self-inductance per unit length of the double wire is therefore 

L = l+21og(A 2 /a6). 

Problem 5. Determine the self-inductance of a toroidal solenoid. 

2 




Fig. 19 

Solution. We regard the solenoid as a toroidal conducting surface carrying surface 
currents of density g = NJ/2irr, where N is the total number of turns and J the current; 
the co-ordinates and dimensions are as shown in Fig. 19. The magnetic field outside the 
solenoid is zero, and inside the solenoid Ht r = Hiz — 0, Hi<j, — INJfcr, where r, z, <f> are 
cylindrical co-ordinates ; for this solution satisfies the equations div H = 0, curl H = 
and the boundary condition (33.2). t The energy of the magnetic field in the solenoid is 

f (Hi"/8ir) dV = (N z J z lc*)§z dr/r, 

where the integration is taken along the perimeter of the cross-section, and is easily effected 
by putting z = a sin 0, r — b+a cos 6. The self-inductance is found to be 

L = 4*W"[&— -•(*• -«")]. 

Problem 6. Determine the end-effect correction of order l\h to the expression (33.3) 
with He = 1) for the self-inductance of a cylindrical solenoid. 



t It is valid also for an annular solenoid of any cross-section. 



§34 Forces in a magnetic field 141 

Solution. The self-inductance is calculated as a double integral over the surface of the 
solenoid : 



1 -M/ S ir** 



J 2 
where g is the surface current density (g = nj). In cylindrical co-ordinates 

A h to 

cos 9 dp dsi dsra 





F (A-Ocos^d^dS 



V[C*a-*i) 2 +4* 2 sin 2 i<f q 





= 8w62«2 r r 



V(S 2 +46 2 sin 2 ^)' 



where <f> is the angle between the diametral planes through d/i and d/a, and £ = zz—zi. 
Effecting the integration with respect to £, we have for h ^> b 

L S 8tt62« 2 f [a log -t-t-tt -A+26 sin ipl cos <f> d& 
J V o sin +0 J 

o 

and finally 

L = 4i7 2 6 2 « 2 [A-8J/3w]. 

Problem 7. Determine the factor by which the self-inductance of a plane circuit changes 
when it is placed on the surface of a half-space of magnetic permeability /*,. The internal 
part of the self-inductance is neglected. 

Solution. It is evident from symmetry that, in the absence of the half-space, the magnetic 
field of the current is symmetrical about the plane of the circuit, and the lines of magnetic 
force cross that plane normally. Let this field be Ho. We can satisfy the field equations and 
the boundary conditions on the surface of the half-space by putting H = 2/n«Ho/(^«+l) in 
the vacuum and B = fi e H = 2/x«Ho/(/x e +l) in the medium: B n and H t are then continuous 
at the boundary, and the circulation of H along any line of force is equal to that of Ho. Hence 
we easily see that, when the medium is present, the total energy of the field, and therefore the 
self-inductance of the circuit, are multiplied by 2^«/(^«+l). 



§34. Forces in a magnetic field 

To determine the forces on matter in a magnetic field hardly any further 
calculations are necessary, on account of the complete analogy with electro- 
statics. The analogy is due mainly to the fact that the expressions for the 
thermodynamic quantities in a magnetic field differ from those for an electric 
field only in that E and D are replaced by H and B respectively. In calcu- 
lating the stress tensor in §15 we used the fact that the electric field satisfies 
the equation curl E = 0, and is therefore a potential field. The magnetic 
field satisfies the equation 

curlH = 477J/C, (34.1) 

which reduces to curl H = only in the absence of conduction currents. 
In calculating the stress tensor, however, we must always put j = 0. Since 
j involves the derivatives of the magnetic field, an allowance for the currents 
in calculating the stresses would amount to adding to the stress tensor aw 



142 Constant Magnetic Field §34 

the very small corrections due to the non-uniformity of the field; cf. the 
second footnote to §15. 

Thus all the formulae obtained in §§15 and 16 for the stress tensor can 
be applied immediately for a magnetic field. For example, in a fluid medium 
with B = [iH. we have 

crtk = -po(p,T)Sik- — \fi-pl— J \$ik+ . (34.2) 

From this the volume forces are calculated by the formula ft = da^ldx^. If 
the medium is a conductor carrying a current, the calculation differs from 
that in §15 in that the equation curl H = is replaced by (34.1). 

Differentiating (34.2) and using also the equation div B = div (/i*H) = 0, 
we find 

1 r /du\ I W 

f = -gradp +— gradp^—J J ___ gra d^- 

- — grad H 2 + — (H • grad)H. 

07T 47T 

By a well-known formula of vector analysis, 

(Hgrad)H = |grad#2_ H xcurlH 
= igrad^2 + 4 7 rj x H/c. 

Thus 

f = -grad/> + -!-grad [# 2 />(|^)J -|^grad,*4~j xH < 34 ' 3 ) 

The last term does not appear in the corresponding formula (15.12). It 
would, however, be incorrect to suppose that the presence of this term means 
that a force can be isolated in f which is due to the conduction current. 
The reason is that, by (34.1), the current j is inseparable from non-uniformity 
of the field, and another term in (34.3) also involves the space derivatives 
of the field. When the magnetic permeability of the medium is appreciably 
different from unity, all the terms in (34.3) are in general of the same order 
of magnitude. 

If, however, as usually happens, fi is close to 1, the last term in (34.3) 
gives the main contribution to the force when a conduction current is present, 
and the remaining terms form only a small correction. In calculating the 
forces we can put /i = 1, obtaining simply 

f=jxH/^. (34.4) 

The term — grad/>o is of no interest henceforward, and we omit it. For 
fi = 1 the properties of the substance have no effect on the magnetic pheno- 
mena, and the expression (34.4) for the force is equally valid for fluid and for 



§34 Forces in a magnetic field 143 

solid conductors. The total force exerted by a magnetic field on a conduc- 
tor carrying a current is given by the integral 

F = J xHdV/c. (34.5) 

Formula (34.4) can, of course, be very easily obtained from the familiar 
expression for the Lorentz force. The macroscopic force on a body at rest 
in a magnetic field is just the averaged Lorentz force exerted on the charged 
particles in the body by the microscopic field h: f = pvxh/c. For p = 1 
the field h is equal to the mean field H, and the mean value of pv is the 
conduction current density. 

When a conductor moves, the forces (34.4) do mechanical work on it. At 
first sight it might appear that this contradicts the result that the Lorentz 
forces do no work on moving charges. In reality, of course, there is no 
contradiction, since the work done by the Lorentz forces in a moving con- 
ductor includes not only the mechanical work but also the work done by the 
electromotive forces induced in the conductor during its motion. These 
two quantities of work are equal and opposite; see the second footnote 
to §49. 

In the expression (34.4) H is the true value of the magnetic field due both 
to external sources and to the currents themselves on which the force (34.4) 
acts. In calculating the total force from (34.5), however, we can take H 
to be simply the external field 4) in which the conductor carrying a current 
is placed. The field of the conductor itself cannot, by the law of conservation 
of momentum, contribute to the total force acting on the conductor. 

The calculation of the forces is particularly simple for a linear conductor. 
Its magnetic properties are of no significance, and, if fi = 1 in the surround- 
ing medium, the total force on the conductor is given by the line integral 

F=/<fdlx£/c. (34.6) 

This expression can be written as an integral over a surface bounded by the 
current circuit. Using Stokes' theorem, we replace dl by the operator 
dfx grad, obtaining 4>dlx# = /(dfx grad) x#. Now 

(dfxgrad)x£ = - dfdiv# + grad(df.#) 

= - df div£ + df x curl £ + (df- grad) #. 

But div ijy = 0, and in the space outside the currents curl ^ = also. 
Thus 

F=jj(df.grad)|>/ C . (34.7) 

In particular, in an almost uniform external field .£) can be taken outside 



144 Constant Magnetic Field §35 

the integral, together with the operator grad. With the magnetic moment 
of the current given by (29.18), we then have the obvious result 

F = (^.grad)£. (34.8) 

Since M in this formula is constant, we can also write 

F = grad(^.£), (34.9) 

in agreement with the expression (32.17) for the energy of the current. The 
torque acting on a current in an almost uniform field is easily seen to be given 
by the usual expression 

K = M x£. (34.10) 



PROBLEM 

Determine the force on a straight wire carrying a current J and parallel to an infinite 
circular cylinder with magnetic permeability /*, radius a and axis at a distance b from the 
wire. 

Solution. On account of the relation, mentioned in the second footnote to §29, between 
two-dimensional problems of electrostatics and magnetostatics, the field of the current is 
obtained from the result in §7, Problem 3, by changing the notation. The field in the space 
round the cylinder is the same as that produced in a vacuum by the current J and currents 
+7' and -J' through A and O' (Fig. 11, §7) respectively, where J' = 0*-l)7/0i+l). 
The field within the cylinder is the same as that due to a current J" = 27/0*+ 1) through O. 
The force per unit length of the conductor is 

2777 1 1 \ 

27 2 a 2 (/*-D 
6(& 2 -a 2 )(/i+l)c 2 ' 

Similarly we find (see §7, Problem 4) that a linear conductor passing through a cylindrical 
hole in a magnetic medium is attracted to the nearest surface of the hole by a force 

F = 2J26(/x-l)/(a 2 -6 2 )(/i+l)c 2 . 



§35. Gyromagnetic phenomena 

The possibilities of magnetising (non-ferromagnetic) bodies without 
applying an external magnetic field are severely limited by the requirement 
of invariance with respect to a change in the sign of the time. The electric 
polarisation of many bodies can be achieved without an external electric field 
by, for example, deforming them if they are piezoelectrics. Piezomagnetism, 
however, if it occurs at all, is a very rare phenomenon (see the end of §28), 
and certainly cannot occur in bodies having no magnetic structure. 

Magnetisation without an external magnetic field generally involves setting 
the body in motion. A uniform translation, of course, is of no use, by 
Galileo's relativity principle. A uniform rotation, however, causes a mag- 
netisation which is linearly dependent on the angular velocity Cl (the Barnett 
effect) ; this relation between the vectors M and SI is possible because both 



§35 Gyromagnetic phenomena 145 

change sign when the sign of the time is reversed. Since both are axial 
vectors, the relation can hold even in an isotropic body, where it reduces 
to a simple proportionality between M and St. 

There must also be an inverse effect: a freely suspended body, on being 
magnetised, begins to rotate (the Einstein-de Haas effect). There is a simple 
thermodynamic relation between the two effects; it can be derived as follows. 

As we know,t the thermodynamic potential with respect to the angular 
velocity (for given temperature and volume of the body) is the free energy 
&' of the body in a system of co-ordinates rotating with it. The angular 
momentum L of the body is 

L= -d&'/dSl. (35.1) 

The gyromagnetic phenomena are described by adding to the free energy 
a further term which is the first term, in an expansion in powers of £2 and of 
the magnetisation M at each point in the body, which contains both SI and 
M. This term is linear in both, i.e. it is 

^'gyro = - jX ik CliM k dV = -XijcQ.i^Tc, (35.2) 

where A^ is a constant tensor, in general unsymmetrical. 

According to (35.1) and (35.2) the angular momentum acquired by the 
body as a result of magnetisation is related to its total magnetic moment by 
Lgyroj = \k^k- It is usual to replace A** by the inverse tensor, denned 
as go; = (2mcle)X- 1 iki where e and m are the electron charge and mass. The 
dimensionless quantities gtk are called gyromagnetic coefficients. Then 

#i = (e/2mc)gi k LgyTo,k' (35.3) 

The expression (35.2) also shows that, as regards its magnetic effect, the 
rotation of the body is equivalent to an external field §< = A^Q^ or 

& = (2mc/e)g-hA. (35.4) 

We thus have the possibility, in principle, of calculating the magnetisation 
caused by the rotation. For example, if the magnetic susceptibility xtk °f 
the body is small, the magnetic moment which it acquires is independent of 
its shape and is 

•^i = X<*$fc = {Imcje^ikg'hk&i- 

Formulae (35.3) and (35.4) represent respectively the Einstein-de Haas and 
Barnett effects. We see that both effects are determined by the same tensor 
gik> 



t See Statistical Physics, §26, Pergamon Press, London, 1958. 



CHAPTER V 

FERROMAGNETISM 

§36. Ferromagnetics near the Curie point 

There is a close analogy between the magnetic properties of ferromagnetics 
and the electric properties of ferroelectrics. Both exhibit spontaneous 
polarisation, magnetic or electric, in macroscopic volumes. In each case, 
this polarisation vanishes at a temperature corresponding to a second-order 
phase transition (the Curie point). 

There are also, however, important differences between ferromagnetic 
and ferroelectric phenomena, arising from the difference in the microscopic 
interaction forces which bring about the spontaneous polarisation. In ferro- 
electrics, the interaction between the molecules in the crystal lattice is essen- 
tially anisotropic, and consequently the spontaneous polarisation vector is 
fairly closely related to certain directions in the crystal. In ferromagnetics, 
on the other hand, the spontaneous magnetisation is due mainly to the 
exchange interaction of the atoms, which is quite independent of the direc- 
tion of the total magnetic moment relative to the lattice, t It is true that, 
together with the exchange interaction, there is also a direct magnetic inter- 
action between the magnetic moments of the atoms. This interaction, how- 
ever, is an effect of order v 2 jc 2 (v being the electron velocities), since the 
magnetic moments themselves contain a factor v/c. The effects of this order 
include also the interaction of the magnetic moments of the atoms with the 
electric field of the crystal lattice. All these interactions, which may be called 
relativistic by virtue of the factor 1/c 2 in them, are weak in comparison with 
the exchange interaction, so that they can result only in a comparatively slight 
dependence of the energy of the crystal on the direction of magnetisation.}: 

Consequently, the magnetisation of a ferromagnetic is a quantity which, 
in the first approximation (i.e. on the basis of the exchange interaction), is 
conserved. This fact endows with greater physical significance the thermo- 
dynamic theory given below, in which the magnetisation tyl is regarded as 



t The exchange interaction is a quantum effect resulting from the symmetry of the wave 
functions of the system with respect to interchanges of the particles. The interchange 
symmetry of the wave functions, and therefore the exchange interaction, depend only on 
the total spin of the system, and not on the direction of the spin ; see Quantum Mechanics, 
§60, Pergamon Press, London, 1958. The importance of the exchange interaction in ferro- 
magnetics was first pointed out by Ya. I. Frenkel', Ya. G. Dorfman and W. Heisenbero 
(1928). 

% The order of magnitude of the ratio of the relativistic and exchange interactions is 
given by the ratio U &niS0 IN@, where £/ a mso is the magnetic anisotropy energy per unit volume 
(see §37), N the number of atoms per unit volume, and © the temperature of the Curie 
point. This ratio is usually between 10 -4 and 10~ 5 . 

146 



§36 Ferromagnetics near the Curie point 147 

an independent variable, the actual value of which (as a function of tempera- 
ture, field, etc.) is afterwards determined by the appropriate conditions of 
thermal equilibrium. 

We denote by <I>o the thermodynamic potential per unit volume of the 
substance when H = 0, regarded as a function of the independent variable 
M (and of the other thermodynamic variables). We shall neglect the relati- 
vistic interactions, i.e. take into account only the exchange interaction. Then 
Oo may be a function of the magnitude of M, but not of its direction. 

In order to find the thermodynamic quantities when H is not zero, we 
start from the relation SO/0H = -B/4ir = - (H + 4n-M)/47r. Integrating 
this for a given value of the independent variable M, and using the fact that 
<|> = <1> = Oo for H = 0, we obtain 

$ = Oo(M)-M.H-# 2 /87r. (36.1) 

Hence the potential <X> is 

$ = + H.B/4tt = 0>o + i/ 2 /87r 

= <&o+(B-477M) 2 /87r. (36.2) 

When the magnetic anisotropy of the ferromagnetic is neglected the direc- 
tions of the vectors M and H are, of course, the same and so the vectors in 
formulae (36.1) and (36.2) may be replaced by their magnitudes. 

Near the Curie point, the magnetisation M is small. Using the general 
theory of second-order phase transitions, t we expand <bo(M) as a series in 
powers of the small quantity M. The expansion of an isotropic function in 
powers of the vector quantity M can contain only even powers : 

$ = (Dqo + JaJlfa + \bM* -MH- H 2 /Stt, f 36.3) 

where Ooo, a, b are functions only of temperature (and pressure). 

The Curie point T — is given by the vanishing of the coefficient 
a: a > for T > and a < for T < ®.t Near the Curie point, the 
function a(T) can be expanded in powers of T-0, i.e. we can write 

a = a(r-0), (36.4) 

where a is a positive quantity independent of temperature. The coefficient 
b is positive near the Curie point and may be replaced by its value at that 
point. 

For H = the minimum thermodynamic potential above the Curie point, 
where a > 0, is given by M = 0, i.e. there is no spontaneous polarisation. 
Below the Curie point, the value of M is given by the condition 

aO/dM = [a(T-0) + 6M2]M = 0. 
<E> is a minimum when the expression in brackets is zero, i.e. when 

M= VH®-T)/b]. (36.5) 

t See Statistical Physics, Chapter XIV, Pergamon Press, London, 1958. 
X This relation of the phases occurs in all known ferromagnetics, although it is not thermo- 
dynamically necessary. 



148 Ferromagnetism §36 

Thus, as we approach the Curie point, the spontaneous magnetisation 
decreases as <\/(® — T). 

Like any second-order phase transition, the transition at the Curie point 
(with It = 0) is accompanied by a discontinuity in the specific heat. Neglect- 
ing higher powers of M, we have for the entropy 

S = -8$ldT = Soo-lM^dajdT = Soo-faM*. 

In the non-ferromagnetic phase M = and S = Soo, while in the ferro- 
magnetic phase Mis given by (36.5), so that 

S =Soo + * 2 (T-®)l2b. 

Hence the change in the specific heat C p = TdS/dT is 

AC P = a 2 0/26. (36.6) 

Now let H ^ 0. The condition dft/dM = which determines the 
magnetisation becomes 

x{T-<d)M+bM* = H. (36.7) 

We define the magnetic susceptibility as 

X = (dMidH) H ^. 

From (36.7) 

8M 

— [a(T-0) + 36M2] = 1. 

Above the Curie point M = when H = 0, so that 

X = l/a(T-0), (36.8) 

i.e. we have paramagnetism with susceptibility inversely proportional to 
T— (the Curie-Weiss law). Below the Curie point M is given by formula 
(36.5) when H = 0, and we obtain 

x = l/2a(0-T). (36.9) 

It should be pointed out that this quantity is not the susceptibility in the 
ordinary sense of the word (i.e. the coefficient of proportionality between M 
and H), since M # even when H = O.t 

The susceptibility (36.9) can actually attain values of the order of unity 
only in the immediate neighbourhood of the Curie point. Except in this 
region, which is of little interest, we may suppose that the magnetisation M 
changes only very slightly with the magnetic field and may be regarded as 
a constant for any given temperature. In the following sections we shall 
assume this to be true. 



t Formulae (36.8) and (36.9) are quantitatively correct for x *£ 1 on ty in crystals of the 
cubic system. For uniaxial crystals the anisotropy energy (see §37) should be taken into 
account, which in this case is proportional to M 2 , and for very small M may even exceed 
the term \aM 2 in (36.3). 



§37 The magnetic anisotropy energy 149 

This constitutes a further difference between ferromagnetics and ferro- 
electrics: for the latter, BPjBE is in general not small even near the Curie 
point. The reason again lies in the smallness of the magnetic moments of 
the atoms in comparison with the electric dipole moments of the molecules. 

§37. The magnetic anisotropy energy 

As already mentioned, the anisotropy of the magnetic properties of ferro- 
magnetics is due to the relativistic interactions between their atoms, and these 
interactions are comparatively weak. In the macroscopic theory, the aniso- 
tropy is described by the addition to the thermodynamic potential of the 
magnetic anisotropy energy, which depends on the direction of magnetisation. 

The calculation of the anisotropy energy from the microscopic theory would 
require the use of quantum perturbation theory, the energy of the perturba- 
tion being represented by the terms in the Hamiltonian of the crystal which 
pertain to the relativistic interactions. The general form of the desired expres- 
sions, however, can be deduced without such calculations, from simple argu- 
ments concerning symmetry. 

The Hamiltonian of the relativistic interactions contains terms of the first 
and second powers in the electron spin vector operators; these are respec- 
tively the spin-orbit and spin-spin interactions. When perturbation theory is 
applied, therefore, the anisotropy energy is obtained as a power series in the 
direction cosines of the magnetisation vector. Now the anisotropy energy 
t/aniso, like the potential <X> itself, is invariant with respect to a change in 
the sign of the time, while the magnetisation M changes sign under this 
transformation. Hence it follows that the anisotropy energy must be an 
even function of the direction cosines of the vector M, and so, in the first 
non-zero approximation of perturbation theory, we shall obtain an expression 
of the form 

E/aniso = IfalcMiMu, (37.1) 

where /5» is a dimensionless symmetrical tensor of rank two, whose compo- 
nents are functions of temperature. Near the Curie point, the expression 
(37.1) may also be regarded as the first term in an expansion of the aniso- 
tropy energy in powers of the vector M, which in this range of temperature 
is a small quantity (but we must emphasise that this interpretation of formula 
(37.1) is not valid for other ranges of temperature). Hence it follows that, as 
T -> ®, the quantities jS^ tend to finite non-zero values. 

In uniaxial and biaxial crystals, a symmetrical tensor of rank two has 
respectively two and three independent components. Here, however, it 
must also be borne in mind that there is one quadratic combination, namely 
M x 2 + M y 2 + M z 2 = M 2 , which is independent of the direction of M and so 
cannot appear in the anisotropy energy. Hence the expression (37.1) for 
uniaxial and biaxial crystals contains only one and two independent coeffi- 
cients respectively. 



150 Ferromagnetism §37 

For example, in uniaxial crystals the anisotropy energy can be written 

C/aniso = W 2 + M y *) = ^M 2 ^20, ( 37 . 2 ) 

where is the angle between M and the #-axis, taken to be along the princi- 
pal axis of symmetry of the crystal. If the constant j8 is positive, the aniso- 
tropy energy is least when the magnetisation is in the ^-direction, and the 
#-axis is said to be the direction of easy magnetisation. If, however, j8 < 0, 
the direction of easy magnetisation lies in the xy-plane, in which case it is 
natural to write the anisotropy energy as 

C/aniso = *|j8|M z 2, ( 37 . 3 ) 

which is equivalent to (37.2) but is such that C/aniso = again corresponds to 
the direction of easy magnetisation. t The expression (37.3) is isotropic in 
the ry-plane. Hence the direction of easy magnetisation is here determined 
by terms of higher order (see Problem 1). 

Let us examine the relation between the magnetisation of a uniaxial ferro- 
magnetic and the magnetic field in it, assuming for definiteness that jS > O.t 
It should be recalled that we suppose the magnitude of M to be independent 
of H, so that only rotations of M are considered. It is evident from symmetry 
that the vector M lies in a plane through the #-axis and the direction of H 
(if terms of higher order, anisotropic in the xy-plane, are neglected in the 
anisotropy energy). We take this as the ##-plane. The thermodynamic poten- 
tial, including the anisotropy energy, is [ j 

O = 0) (M)+i J 8M a; 2 -M.H-// 2 /87r 

= 0>o(M) + ±pM2 sin 2 - M(H X sin + H z cos 0) - # 2 /8tt. (37.4) 

The dependence of M on H is given by the equilibrium condition d<t>jdd = 0, 
whence 

j8Afsin0cos0 = H x cos9-H z sm9. (37.5) 

This is an algebraic equation of the fourth degree for £ = sin 0: 

(pM£-H x y{l-e) =HK\ 

in which the coefficients of odd powers of £ are not zero. This equation has 
either two or four real roots (all less than unity). Since all such roots corres- 
pond to extrema of O(0), it is clear that, in the first case, this function has 
one minimum and one maximum and, in the second case, two minima and 
two maxima. In other words, the number of possible directions of the 



t An example of a uniaxial ferromagnetic is hexagonal cobalt, for which )3 > below 
~200 °C and j3 < above that temperature. At room temperature, j3 = 4-2. 

% This case will always be taken when uniaxial crystals are discussed. 

|| In the following discussion we shall use the expression (37.2) for the anisotropy energy. 
It should be pointed out, however, that the expansion of which (37.2) is the first term is 
usually not rapidly convergent in practice. For a satisfactory quantitative description of 
the phenomena, therefore, the next (fourth-order) term should be included; for a hexagonal 
crystal this term is proportional to sin 4 0. 



§37 



The magnetic anisotropy energy 



151 



magnetisation M for a given field H is one and two respectively. Jn the second 
case, one direction (corresponding to the lower minimum of O) is thermo- 
dynamically completely stable, while the other (corresponding to the higher 
minimum) is thermodynamically metastable. 

Either of the two cases can occur, depending on the values of H x and H z . 
When these two parameters vary continuously, one case passes into the 
other at the point where one maximum and one minimum coalesce. The 
curve of $(0) then has a point of inflection instead of an extremum, i.e. both 
d^jdd and d 2 <I>/d0 2 .are zero. Writing equation (37.5) in the form 

sin cos 

and differentiating with respect to 0, we have H x jsm 3 d = -# 2 /cos 3 0. 
Eliminating from these two equations gives 

HfV + Hfl* = (j8M)2/3. (37.6) 

In the HxH z -pfane equation (37.6) represents a closed curve of the kind 
shown in Fig. 20. It divides the plane into two parts, in one of which meta- 
stable states can exist, while in the other they cannot. It is evident without 
further investigation that the region where metastable states do not exist is 
that outside the curve, because f or H -> oo only one direction of M can be 
stable, namely that of the field H. 




Fig. 20 



The existence of metastable states means that what is called hysteresis 
can occur; this is an irreversible change of state in a ferromagnetic body 
when the external magnetic field is varied. The curve shown in Fig. 20 is 
therefore the "absolute limit" of hysteresis; this phenomenon cannot occur 
for fields outside the curve, t 



t The whole of the discussion in this chapter concerns only thermodynamic equilibrium 
states in ferromagnetics and therefore reversible processes in them. In particular, we entirely 
ignore the mechanism of hysteresis phenomena; these may arise from defects in the crystal, 
internal stresses, a polycrystalline state, and so on. 



ii 



152 Ferromagnetism §37 

States in which the field H is perpendicular to the direction of easy mag- 
netisation (H x = H, H z = 0) require special consideration. The thermo- 
dynamic potential is 

H 2 

<D = <d +tfM*&m*e-HMsmO. (37.7) 

StT 

If H > j8M, O has only one minimum, at 9 = \n, i.e. the magnetisation is 
parallel to the field. If, however, H < j8M, then <I> has a minimum when 

M x = Msin0 = H/p, (37.8) 

to which there correspond two possible positions of the vector M (at angles 
6 and tt — 0), symmetrical about the x-axis. Thus in this case there are two 
equilibrium states, which have the same value of <J> and are therefore equally 
stable. 

This result is very important, since it means that two "phases" can exist 
in contact in which the field H is the same but the magnetisation M (and 
therefore the induction B) is different. Thus a new possibility appears for 
reducing the total thermodynamic potential of the body: its volume may be 
divided into separate regions, in each of which the magnetisation has one 
of its two possible directions. These regions are called regions of spontaneous 
magnetisation or domains. The actual determination of the thermodynamic 
equilibrium structure of a ferromagnetic requires a consideration of the 
shape and size of the body as a whole. We shall return to this problem 
in §39. 

Let us consider a portion of the body which is small compared with the 
total volume but large compared with the domains. The field H x can be 
regarded as constant in this portion; we denote by M and B the values of M 
and B averaged over its volume. As well as H Xi the transverse component 
M x = H x jfi of the magnetisation is constant. The longitudinal component 
M z , however, has opposite signs in different domains, so that its mean value 
certainly cannot exceed \M Z \. Since H z = everywhere, the mean induc- 
tion is therefore 



■ = M 1 + ?)' * * ^Ji^-f)- (37 - 9) 



These formulae give the range of values of the mean induction correspond- 
ing to the domain structure of a uniaxial ferromagnetic. 

Let us now consider ferromagnetic crystals of the cubic system. Their 
properties are markedly different from those of uniaxial and biaxial crystals. 
The reason is that the only quadratic combination which is invariant with 
respect to the cubic symmetry transformations and which can be formed 
from the components of the vector M is the sum M x 2 + M y 2 + M z 2 , which is 
independent of the direction of M. Hence the first non-vanishing term in 
the expansion of the anisotropy energy for a cubic crystal is the fourth-order 



§37 The magnetic anisotropy energy 153 

term, not the second-order one. For this reason, the magnetic anisotropy 
effects in cubic crystals are in general less strong than in uniaxial and biaxial 
crystals. 

Cubic symmetry admits the following two quartic invariants formed from 
the components of the vector M: M x 2 M y 2 + M X 2 M Z 2 + M y 2 M z 2 and 
KMaP + My^ + Mz*). These invariants, however, are not independent, 
because their sum is the quantity \{M X 2 + M y 2 + M z 2 ) 2 , which is independent 
of the direction of M. Hence the anisotropy energy of a cubic ferromagnetic 
includes (in the approximation consideredt) only one constant, and may be 
written 

C/aniso = P(M x 2 M y 2 + M x 2 M z 2 + M y 2 M z 2 ), (37.10) 

or, equivalently, 

t/aniso = -IRMJ + MJ + MJ). (37.11) 

If j3 > (as, for example, in iron), the anisotropy energy has equal minimum 
values for three positions of the vector M, namely parallel to the edges of the 
cube (the x, y and z axes or, in crystallographic notation, [100], [010], [001]). 
Thus, in this case, the crystal has three equivalent axes of easy magnetisa- 
tion. 

If, on the other hand, £ < (as, for example, in nickel), then the aniso- 
tropy energy has minima when M x 2 = M y 2 = M z 2 = \M 2 , i.e. when the 
vector M is parallel to one of the four spatial diagonals of the cube [111], 
[Til], etc. These are then the directions of easy magnetisation. 

It should be noticed that, strictly speaking, a ferromagnetic cubic crystal, 
when spontaneously magnetised along one of the directions of easy mag- 
netisation, ceases to possess cubic symmetry, and so there is a displacement 
of the atoms, i.e. a distortion of the crystal lattice. Such a crystal, when 
magnetised parallel to an edge of the cube, becomes slightly tetragonal, 
while one magnetised parallel to a spatial diagonal becomes rhombohedral. 
In this respect cubic crystals differ from uniaxial crystals with the direction 
of easy magnetisation along the principal axis of symmetry, where a mag- 
netisation in this direction evidently does not change the symmetry of the 
crystal. 

The relation between M and H for a cubic crystal can in principle be 
investigated in the same way as was done above for a uniaxial crystal. How- 
ever, we shall not pause to discuss this, because the equations are more 
complex, and explicit analytical formulae cannot be obtained. 

PROBLEMS 
Problem 1. Find the terms of the next order (after the second) in the expansion of Lr anl80 
for a uniaxial crystal, which give rise to anisotropy in the *y-plane. 



f The next approximation involves terms of the sixth order. In calculating the number 
of independent sextic invariants, we must exclude both M 6 , which is independent of the 
direction of M, and expressions which differ from the quartic invariants by a factor M 2 . 
This leaves only one invariant, which may be taken as Mz 2 M y 2 Mz z . 



154 Ferromagnetism §37 

Solution. The problem reduces to finding the independent combinations of lowest (even) 
order, formed from the components of the vector M, which are invariant with respect to 
the symmetry transformations of the crystal concerned and which contain Mx and My 
otherwise than in the form M x 2 +M y 2 . For tetragonal symmetry there is one such combina- 
tion, which may be taken as M x 2 M y 2 \ the combination i(M x 4 +M y A ) gives together with 
M x 2 M y 2 the sum l(M x 2 +M y 2 ) 2 , and is therefore not independent. 

For hexagonal symmetry, the anisotropy in the xy-plane appears only in the sixth-order 
terms ; the invariant combination of this order may be taken as 

—[(M X + iMy)«-(M X -iMy)G] = 2M X My(M X *-—M X Wy 2 + MyA. 

6i \ 3 / 

Rhombohedral symmetry admits this sixth-order term and also one independent quartic 
combination, for example 

iMz[(Mx+iMy) 3 +(Mx-iMy)*] = MxMz(M x 2 -3My 2 ). 

The determination of the direction of easy magnetisation in the xy-plane requires a considera- 
tion of both these terms, since M z is small. 

Problem 2. A uniaxial ferromagnetic crystal is in the shape of a spheroid, the axis of 
easy magnetisation being the axis of revolution, and is placed in an external magnetic field $f. 
Determine the range of values of $) for which the body has a domain structure. 

Solution. According to the general properties of an ellipsoid in a uniform external field 
(§8), the induction B and field S ( = H) averaged over the domain structure are related to .ji) by 

nS z +(l -n)Hz = §> z , i(l -n)B x +Ul +n)H x = &r, 

where n is the demagnetisation coefficient in the direction of the axis of revolution (taken 
as the 2-axis). Putting H z = and using formulae (37.9), we obtain 



H x — 



1+27t(1-m)/j8' 
Elimination of H x gives the required inequality 



a.-fe«-V("-^- 



s-° + „ „y ,. < w 



(47m) 2 [j8+2tt(1-«)]2 
for the range in which there is a domain structure. 

Problem 3. Determine the magnetisation averaged over the crystallites (which have 
uniaxial symmetry) for a polycrystalline body in a strong magnetic field {H ^> AttM). 

Solution. In a particular crystallite, let 6 and ^ be the angles between the direction of easy 
magnetisation and, respectively, M and H. It is evident that, in a strong field, M and H 
will be in almost the same direction, i.e. the angle •& = 0—ift is small. Putting in (37.4) 
M*H = MH cos (d—tfi) and equating to zero the derivative d^jdd, we have & ^ sin •& 
= — (PM/H) sin d cos 9. The average magnetisation is clearly parallel to H, and is 

M = M c~oTl = M{\ -IP) = m[\ - ^—— sin 2 cos 2 0). 

The bar denotes averaging over the crystallites. Assuming that all directions of the axis of 
easy magnetisation of the crystallites are equally probable, we have 

P 2 M 2 \ 



M 



"(*-^)- 



Thus the mean magnetisation approaches saturation in the manner M—M "-' 1 /H 2 . 

Problem 4. The same as Problem 3, but for the case where the crystallites have cubic 
symmetry. 

Solution. The conditions for a minimum of the expression 

-MMx 4 + My* + Mz 4 )-(HxMx+HyMy + HzMz), 

with the subsidiary condition M x 2 +My 2 +M z 2 = constant, are 2frM* 3 +.H'x = XM x ,2$My 3 + 



§38 Magnetostriction of ferromagnetics 155 

+H y = XM V , 2$Mz s +H e = AM:, where A is an undetermined Lagrangian multiplier. For 
large H, we therefore have 

M x S* -H x + — 2pH**+ ... , etc.; 
A A* 

adding the squares of the three equations, we obtain M 2 = H 2 /X 2 , i.e. A ^ H/M. The angle 
& between M and H is found from 

•& 2 ^ sin 2 £ = 



M 2 H 2 
4|3 2 M« 



H 10 



J^H x 2 H y 2 (H x 2 -H y 2 )\ 



where the summation is over cyclic permutations of the suffixes x, y, z. Averaging this 
expression over the orientations of the crystallites is equivalent to averaging over directions of 
the vector H. The latter averaging is effected by integrating over the angles which specify 
the direction of H, and the result isf 

/ 8j8 2 M 6 \ 



§38. Magnetostriction of ferromagnetics 

A change in the magnetisation of a ferromagnetic in a magnetic field 
causes a deformation in it; this phenomenon is called magnetostriction, and 
may be due to either exchange interactions or relativistic interactions in the 
body. Since the exchange energy depends only on the magnitude of the 
magnetisation, its value can change only when this magnitude changes in 
the magnetic field. Although the latter change is, in general, very small, the 
exchange energy is large compared with the anisotropy energy. Hence the 
magnetostriction effects from each type of interaction may be of comparable 
magnitude. 

This happens, for instance, in uniaxial crystals. Marked deformations 
resulting from a change in the direction of M occur in fields H ~ $M\ the 
change in the magnitude M is considerable when H ^ 4ttM. These two 
values of H are almost the same, and so it is in general necessary to take 
account of both effects in discussing the magnetostriction of uniaxial ferro- 
magnetics. We shall not pause here to derive the formulae, which are fairly 
complex. 

In cubic crystals the situation is different, because the anisotropy energy 
is of the fourth order and therefore relatively small. A considerable mag- 
netostriction, due to the change in the direction of M, occurs even in com- 
paratively weak fields, where the change in the magnitude M may be entirely 
neglected. Let us consider these effects. 



f For a cubic crystal there is also a range of fields in which MH is large compared with 
the anisotropy energy but small compared with 4ttM 2 . The formula derived here is then 
invalid, because in deriving it we have neglected the fields in the body resulting from the 
differing directions of magnetisation in different crystallites. A more exact investigation for 
this case gives the same \\H 2 law but with a different coefficient. We shall not discuss this 
problem in more detail, because the results, for some reason not yet understood, disagree 
with those of experiment. 



156 Ferromagnetism §38 

The change in the relativistic interaction energy in the deformed body is 
described by the inclusion in the thermodynamic potential <E> of magneto- 
elastic terms depending on the components of the elasticity stress tensor a^ 
and the direction of the vector M (N. S. Akulov, 1928). The first such terms 
which do not vanish are linear in aye, and quadratic in the direction cosines 
of M because of the symmetry with respect to a change in the sign of the 
time. In general, therefore, the magnetoelastic energy is given by an expres- 
sion of the form 

C/ m _ei = —\iklm0ikMiM m , (38.1) 

where h iklm is a tensor of rank four, symmetrical with respect to the pairs of 
suffixes i,k and l,m (but not with respect to interchange of the two pairs). 
Near the Curie point, where the expansion in powers of the direction cosines 
of the vector M is equivalent to one in powers of its components, the quanti- 
ties \icim tend to constants. 

In calculating the number of independent components of the tensor X-tkim 
it must be borne in mind that the terms in (38.1) which involve the compo- 
nents of M in the form M x 2 + M y 2 + M z 2 are independent of the direction of 
M, and so may be omitted from the magnetoelastic energy, t Thus we find 
that, in a cubic crystal, the magnetoelastic energy contains two independent 
coefficients ; we shall write it as 

C/m-el = ~ Xl((TxxM x 2 + OyyMy 2 + cr zz M z 2 ) - 

- 2\{o X yM x My + O xz M x M z + Gy Z MyM z ). (38.2) 

The strain tensor is obtained by differentiating 4> with respect to the 
various components a«: m» = — d^jdatk, where $ includes also the ordinary 
elastic energy (with reversed sign; see the first footnote to §17). For a 
cubic crystal, the latter energy involves three independent elastic coefficients, 
and is of the form 

Uel = \ki{(s xx 2 + a-yy 2 + cr zz 2 ) + k^{a xx o yy + a xx a zz + a yy a zz ) + 

+ k 3 (a X y 2 + XZ 2 + Oy Z 2 \ (38.3) 

The strain tensor is% 

U XX = klO xx + k 2 (<Tyy + V ZZ ) + X\M X 2 , 

u X y = hza X y-\-\M x M y ^ 

and similarly for the other components. 

These formulae give all the magnetostriction effects in the range of fields 
considered. In particular, if there are no internal stresses the change in the 
deformation resulting from a change in the direction of magnetisation is 



t There is consequently some arbitrariness in the choice of the Xikim, which simply 
reflects the arbitrariness involved in choosing the direction of M for which (applied mech- 
anical forces being absent) we regard the crystal as undeformed. 

t In differentiating o the second footnote to §17 should be recalled. 



§38 Magnetostriction of ferromagnetics 157 

given by u xx = M** 2 , u xy = \MxM y , etc. It should be recalled that the 
magnitude of the deformation itself is to some extent arbitrary, because the 
direction of M for which the deformation is supposed zero is arbitrarily 
chosen. 

Let us now consider magnetostriction in fields so strong (H p 4ttM) that 
the anisotropy energy is unimportant and there is no domain structure, so 
that the directions of M and H may be assumed to coincide. 

Since the anisotropy energy is neglected, the particular symmetry of the 
crystal is of no importance, and the formulae given below are valid for any 
ferromagnetic. 

Let the body be placed in a uniform external magnetic field |>. Its total 
thermodynamic potential go ist 

gb = -M.% = -MV§, (38.5) 

where Jt = MV is the total magnetic moment of a body uniformly mag- 
netised in the direction of the field; we omit the term goo which is unrelated 
to the magnetic field. The strain tensor averaged over the volume of the 
body is u ik = -{\\V)dgb\doi k , whence 

* ik = * W (38.6) 

Thus the deformation is determined by the dependence of the magnetisation 
on the internal stresses. 

For cubic symmetry, any symmetrical tensor of rank two characterising 
the properties of the crystal reduces to a scalar multiple of S«. This is true, 
in particular, of the tensor c\MV)lda ik , so that the magnetostriction deforma- 
tion amounts in this case to a uniform compression or extension. 

If we are interested only in the change W in the total volume of the body, 
we can obtain it by simply differentiating go with respect to the pressure : 

W = dgbjdp = -%d{MV)ldp, (38.7) 

where p is to be regarded as a uniform pressure applied to the surface of the 
body. 

PROBLEM 

Determine the change in volume in magnetostriction of a ferromagnetic ellipsoid in an 
external field $ ~ 4wM parallel to one of its axes. The ferromagnetic is assumed to be a 
cubic crystal.^ 

Solution. When the anisotropy energy is neglected, the range for which the domain 
structure exists is given by B < AttM when H = 0. The bar denotes averaging over the 



t Here the definition of &> is that given in §12, which is applicable except when the 
deformation of the body is appreciably inhomogeneous. 

% In a uniaxial ferromagnetic with § ~ 4wJVf the anisotropy energy would have to be 
taken into account, but this is not necessary in a cubic crystal. 



158 Ferromagnetism §39 

volume of the body; cf. §37. In an ellipsoid nB+(\ —n)H = § ; putting H = 0, we find that 
the domain structure exists when |) < 4imM. Since nB = AmiM = £), the mean magnetisa- 
tion is M = §/47m. Hence the thermodynamic potential is 

&> = -F fMd£= -&VISim. (1) 

o 

If § > 47mM, the ellipsoid is magnetised entirely in the direction of the field, and M = M . 
Then 

& = - M$>V + lirMWn. (2) 

The expressions (1) and (2) are the same for £) = 4irMn. 

The required change in volume is obtained by differentiating gp with respect to pressure: 

8V = - -^ for £ < 4tthM, 

87m 3p 

SF = -$-^ +27m-^ ^ for § > 4-rmM. 

dp dp 

For § > 47mAf we obtain (38.7). 



§39. The domain structure of ferromagnetics 

As already mentioned in §37, there is a wide range of states in which a 
ferromagnetic must have what is called a domain structure, i.e. it must consist 
of various regions in which the directions of magnetisation are different, t 
This is true, in particular, of a ferromagnetic body which is not in an external 
magnetic field. 

Some conclusions concerning the shape of the surfaces separating the 
domains may be obtained directly from the boundary conditions on the 
magnetic field. Since the field H is the same in adjoining domains, the 
condition of continuity of the normal induction B n reduces to the con- 
tinuity of M n . In uniaxial crystals, the sign of M z is different in different 
domains, but M x and M y are the same. Under these conditions the con- 
tinuity of M n means that the surface of separation must be parallel to the 
#-axis, i.e. to the direction of easy magnetisation. 

Let us first examine the properties of the bounding surfaces as such, 
leaving aside the actual shape of the domains. These "surfaces" are in reality 
fairly narrow transition layers in which the direction of the magnetisation 
varies continuously between its directions in the two adjoining domains. 
The "width" of such a layer and the manner in which M varies within it are 
given by the conditions of thermodynamic equilibrium. The additional 
energy due to the non-uniformity of the magnetisation must be taken into 
account. The largest contribution to this "non-uniformity energy" is given 
by the exchange interaction. Macroscopically, this energy can be expressed 



t The concept of domains was first put forward by P. Weiss (1907). The thermodynamic 
theory of domains was given by L. D. Landau and E. M. Lifshitz (1935). 



§39 The domain structure of ferromagnetics 159 

in terms of the space derivatives of M, which can be done in a general form 
if the gradient of the direction of M is supposed relatively small, i.e. if the 
change in the direction of the magnetic moments occurs over distances 
large compared with the distances between the atoms. In the present case, 
this condition is evidently fulfilled, because a considerable difference in the 
directions of the magnetic moments of adjoining atoms would lead to a very 
large increase in the exchange energy, and is therefore thermodynamically 
unfavourable. 

We denote the "non-uniformity energy" by C/non-u- The greatest terms in 
its expansion in powers of the various derivatives of the components of M 
are those quadratic in the first derivatives; there can be no linear terms, on 
account of the symmetry with respect to a change in the sign of the time. 
Next, because it originates from exchange forces, C/non-u cannot depend on 
the absolute direction of M at a given point in the crystal. The most general 
expression satisfying these conditions is 

, m i m i mn 

Unon-u = i^ik— ~ — ' \^- L ) 

OXi CX]c 

where aa is a symmetrical tensor. This quadratic form (in the derivatives) 
must, furthermore, be positive definite. In a uniaxial crystal, the tensor a* fc 
has two independent components, and the non-uniformity energy is of the 
form 

c/non-u^ y +yj+H^)' (39 - 2) 



ai and a 2 being positive. In a cubic crystal we have oci = <X2. 

The following remark should be made concerning (39.1). A thermo- 
dynamic meaning attaches not to Unon-u itself, but only to its integral over 
the volume of the body. It is therefore not necessary to include in Unon-u 
the terms containing products of the components of M and their second 
derivatives with respect to the co-ordinates, even though such terms are 
formally of the same order of magnitude as those in (39.1). The reason is 
that, on being integrated over the volume, they become products of first 
derivatives, i.e. they are included in (39.1).t 

As an example, let us consider the boundary between domains in a uniaxial 
crystal, assuming that the vector M is parallel or antiparallel to the direction 
of easy magnetisation (the .sr-axis). This is true, for example, in the absence 
of an external magnetic field. 

The structure of the transition layer is determined by the condition that 
its total free energy should be a minimum.J Here the exchange energy 



t The symmetry of the crystal may admit terms containing products of the first deriva- 
tives dMildxi and the components Mi. Such terms, on integration over the volume, would 
give expressions depending only on the properties of the surface of the body. 

X Here it is more correct to speak of the total free energy, and not of the total thermo- 
dynamic potential, because the deformation in the layer may be by no means homogeneous. 



160 Ferromagnetism §39 

tends to increase the thickness of the layer (i.e. to make the direction of M 
vary less rapidly). The anisotropy energy has the opposite effect, because 
any deviation of M from the direction of easy magnetisation increases this 
energy. 

We take the #-axis perpendicular to the plane of the layer ; the direction of 
M depends only on x. The rotation of the vector M across the layer must 
take place in the yz-pte.ne, i.e. M x = everywhere. This is seen as follows. 
The non-uniformity and anisotropy energies are independent of the plane 
in which the rotation of the magnetisation takes place. The presence of a 
non-zero component M x would necessarily result in a magnetic field which 
was thermodynamically unfavourable, because of the additional magnetic 
energy. For M x = in the domains; if M x # in the transition layer, then 
divM = dM x \dx ^ 0; since divB = div H + Att div M =0, this implies 
that div H # and therefore that H # 0. 

Let be the angle between M and the #-axis. Then the components of 
M are M x = 0, M y = Msin0, M z = Mcos0. The sum of the non- 
uniformity and anisotropy energies is given by the integral 

oo oo 

}[i«iW+ M z' 2 ) + ii^V] d* = W 2 f(*iO' 2 + P sin 2 0) d* f (39.3) 

—oo —oo 

where the prime denotes differentiation with respect to x. The remaining 
terms in the free energy are independent of the structure of the layer, and 
so can be omitted here. To determine the function 6(x) which makes this 
integral a minimum, we write down the corresponding Euler's equation 
ai0" -j8 sin cos = 0, of which the first integral is 0' 2 -(j8/oci) sin 2 
= constant. Assuming the thickness of the transition layer small compared 
with that of the domains themselves, we can write the boundary conditions 
on this equation as 

= for x = — oo, = tx for x = + oo, 

(39 4) 
0' = for x = ±oo (i.e. for = or ir). v " ' 

These state that adjoining domains are magnetised in opposite directions. 
Then the constant is zero and, integrating the equation 0' 2 = (]8/ai) sin 2 0, 
we obtain 

cos0 = -tanhfcvWai)], (39.5) 

which gives the manner of variation of the direction of magnetisation in the 
transition layer. The "thickness" of this layer is 8 ~ V( a i//0* 
Substituting (39.5) in (39.3), we have 



oc 



dx 



cosh 2 [x-\/(j3/(x.i)] 



§39 



The domain structure of ferromagnetics 



161 



or, effecting the integration, 



2MV(aij8). 



(39.6) 



If we regard the boundary between domains as a geometrical surface, then 
(39.6) is the "surface tension" which must be ascribed to this surface in 
order to take account of the energy needed to create the boundary. 

The shape and size of the domains in thermodynamic equilibrium are 
given by the condition that the total thermodynamic potential should be a 
minimum. They depend considerably on the actual shape and size of the 
body. In the simplest case, that of a ferromagnetic in the form of a flat plate, 
the domains may in principle form either parallel layers, or "filaments" 
across the body. In what follows we shall, for definiteness, speak of layers.! 

The formation of an entire new boundary between domains results in an 
increase in the total "surface tension" energy. This energy consequently 
tends to reduce the number of domains, i.e. to increase their thickness. The 
excess energy near the outer surface of the body has the opposite effect. In 
the body the magnetic field H = 0, and the anisotropy energy is also zero, 
because the vector M is in a direction of easy magnetisation. Near the 
surface, however, this is not so. 

In the limiting case where the coefficient j8 in the anisotropy energy is 
large, the layers must emerge at the surface with no change in the direction 
of M (Fig. 21a, where for definiteness we suppose that the surface is per- 
pendicular to the direction of easy magnetisation). Near the surface there 
is a magnetic field which penetrates into the surrounding space, and into 
the body, to distances of the order of the layer thickness a. 



(o) 



t 




Fig. 21 

In the opposite case of small j8, a more favourable disposition is that where 
there is no magnetic field, and M deviates from the direction of easy mag- 
netisation. For H = we must have everywhere div B = 4tt div M = 0, 
and M n must be continuous at all domain boundaries and at the free surface. 



f The layered structure of domains seems to be the more usual. 



162 Ferromagnetism §39 

This is achieved by the setting up of "domains of closure" of triangular 
cross-section (Fig. 21b), in which the magnetisation is parallel to the surface 
of the body. The total volume of these regions, and therefore the aniso- 
tropy energy in them, are proportional to the layer thickness a. 

Thus in all cases the emergence of the domains at the surface of the body 
results in an excess energy which is the greater, the greater the thickness of 
the domains. This effect therefore tends to reduce the domain thickness. 

The actual thickness of the domains is determined by the equilibrium of 
the two oppositely acting effects. Let us suppose, for example, that the 
domains (in a plate) have the form of plane layers of constant thickness. 
The number of domains is proportional to \ja, while the surface-tension 
energy at the surfaces separating them is proportional to their total area, 
i.e. to L]a, where L is the total dimension of the body in the direction of the 
domains, i.e. the thickness of the plate. The energy of the emergence of the 
domains at the surface of the plate is proportional to a. The sum of these 
two energies has, as a function of a, a minimum when a has a value propor- 
tional to -\/L. 

Thus the thickness of the domains increases with the dimension of the 
body, but the quantitative law a ~ -y/L of this increase is based on the 
assumption that the domains are of constant thickness, and clearly cannot 
be valid for all values of L. The reason is that the thickness of the domains 
at the surface of the body cannot exceed some limiting value ajc which 
depends on the properties of the ferromagnetic substance but not on the 
shape and size of the body. The value of a^ is determined by the point at 
which, as a increases, the "splitting" of the domain near the surface to a 
depth ~ a becomes thermodynamically favourable. Such a point must 
necessarily be reached, since the energy of the emergence of one domain 
increases as a 2 , whereas the excess surface-tension energy resulting from the 
splitting of the domain increases only as a. 

Thus we conclude that, as the size of the body, and therefore the domain 
thickness, increase, a progressive "branching" of the domains occurs as 
they approach the surface of the body (E. M. Lifshitz, 1948). We shall 
not pause to investigate further the various possibilities, which have not yet 
been clearly ascertained, t 

As the dimensions of the body decrease, the formation of any domains at 
all ultimately becomes thermodynamically unfavourable, so that sufficiently 
small ferromagnetic particles are uniformly magnetised single domains. The 
criterion giving their dimension L is obtained by comparing the magnetic 
energy of a uniformly magnetised particle with the non-uniformity energy 
which would result if there were considerable non-uniformity in the distri- 
bution of the magnetisation over its volume. The former energy is of the 



f See S. V. Vonsovskii and Ya. S. Shur, Ferromagnetism (Ferromagnetizm), Moscow, 
1948; C. Kittel, Reviews of Modern Physics 21, 541, 1949; E. M. Lifshitz, Zhurnal tksperi- 
mental'noi i teoreticheskoi fiziki 15, 97, 1945. 



§39 The domain structure of ferromagnetics 163 

order M 2 V, and the latter of the order of oM 2 F/IA The condition for a single 
domain to be formed is therefore 

L 2 ^ a. (39.7) 

The thermodynamic potential of a uniformly magnetised particle in the 
absence of an external field is found by putting # = in the formula 



^ = J[^ + ^-^' M ] dV 



(cf. 31.7)) and substituting for $ the sum of the expression (36.1) and the 
anisotropy energy C/ a niso: 

& = FC/amso-iM.jHdF. (39.8) 

The unimportant constant gi»o is omitted. Since M and H are linearly 
related, the second term is quadratic in the components of M: 

go = VUtniBo + WauMtM** ( 39 ' 9 ) 

where the symmetrical tensor a ik depends only on the shape of the particle. 
If the particle is ellipsoidal, for example, H is constant in it, and (when 
£ = 0) is related to M by 

H t + n ik (B k -H k ) = Hi + 47rn ik M k = 

(cf. (8.10)). In this case, therefore, the components aw are given in terms 
of those of the demagnetisation coefficient tensor by a% k = 4irnui. 

The direction of magnetisation of the particle in an external magnetic 
field £ is given by the condition that go, which now includes a term - V& • M, 
should be a minimum. For a cubic crystal we can put simply 

0b = \Va ik MiM k - r$.M, (39.10) 

neglecting the anisotropy energy. In uniaxial (and in biaxial) ferromagnetics, 
however, the anisotropy energy is a quantity of the same order as the remain- 
ing terms. Writing this energy in the form (37.1), we have 

& = W(^ + MMiM k - VfrM. (39.11) 

In this form the problem is mathematically identical with the one in §37 
concerning the dependence of the local magnetisation M on the local field 
H, the only difference being that H is replaced by |>, and p ik by a ik or 

PROBLEMS 

Problem 1. Determine the "surface-tension" coefficient at a boundary between domains 
in a cubic crystal, if the surface of separation makes an angle x with the plane (100) (the 
ys-plane) and the domains are magnetised in the direction [001] of easy magnetisation (the 
2-axis). 

Solution. As well as the crystallographic co-ordinates x, y, z we use co-ordinates *', 
v', z'\ the z and z' axes coincide, and the yz and y'z' planes are at an angle x- The necessity 



164 Ferromagnetism §39 

of having no considerable magnetic field tends (as in a uniaxial crystal) to keep the vector 
M in the y'z'-plane in the transition layer. The presence of magnetic anisotropy in the x'y'- 
plane, however, causes M to depart slightly from the y'z'-plzne. Since the anisotropy energy 
in a cubic crystal is small, the component M x ' is also small and can be neglected with suffi- 
cient accuracy. Then M x ' = 0, My' = M sin 8, Mz = M cos 8, where 8 is the angle between 
M and the z-axis, or M x = M sin 8 sin x, My = M sin 8 cos x, M z = M cos 8. 

For the sum of the non-uniformity and anisotropy energies in the transition layer we find 

j {izM 2 ^-) +#W 4 (sin 2 cos 2 0+J sin 4 sin 2 2x)} d*'. (1) 

-eo 

Minimising this integral with the same boundary conditions (39.4) gives 

2cot0 
sinh *V(2j3M2/a) = - , n . (2) 

sin 2x 

Substituting in (1) and effecting the integration, we obtain the required surface tension: 



MV(2ai8){ 



1+ 2V(4 i =S^2^ COSh " 1 i^)- (3) 



Formula (3) is valid for any angle x- Equation (2) for the structure of the transition layer 
becomes invalid, however, when x = or &*• I n tn i s case a finite width of the transition 
layer is obtained only when the magnetostriction in it is taken into account. 

Problem 2. Determine the energy of the magnetic field near the surface of a ferromagnetic 
at which plane-parallel domains perpendicular to the surface emerge without change in the 
direction of magnetisation (Fig. 21a). 

Solution. The problem of determining the magnetic field near such a surface is equiva- 
lent to the electrostatic problem of the field due to a plane divided into strips charged alter- 
nately positively and negatively with surface charge density or = ±M. 

Let the surface of the body be the plane z = 0, and let the jc-axis be perpendicular to the 
plane of the domains. The "surface charge density" o{x) is a periodic function with period 
2a (a being the width of the domains), and its value in a typical period is a = —M for 
— a < x < 0, a = +M for < x < a. Its expansion in Fourier series is 

E(2n-\-l)irx 
c n sin , c n = 4M/(2n+l)7r. 
a 
n-0 

The field potential satisfies Laplace's equation 



we seek ^ as a series 



— -H - = 0; 

8x z 8z 2 



#c, z )=Y,b n sin (2n+1)7r * eTdn+Dw/. 



where the two signs in the exponent relate to the half-spaces z > and z < 0. The coeffi- 
cients b n are given by the boundary condition 

— [8<f>/ dz] z -o++[d(f>l dz] t ~o- = 4ito, 

whence b n — 2ac n l(2n+l). 



§40 The antiferromagnetic Curie point 165 

The required field energy can be calculated as the integral J / 0$ d/ over the "charged 
surface". The energy per unit area is 

a 00 
[<r^1«»o dx = - >J c n bn 

8aM 2 A 1 
- 2 2.(2« + l)3- 

The value of the sum is 1 -052, and so the energy is 0-852 aM 2 . 

§40. The antiferromagnetic Curie point 

The passage of a body from an antiferromagnetic to a paramagnetic state 
usually takes place by a second-order phase transition, as in ferromagnetics. 
Since there is no spontaneous magnetisation in either the antiferromagnetic 
or the paramagnetic state, the change in the macroscopic magnetic properties 
of the substance as it passes through the antiferromagnetic Curie point affects 
only its magnetic susceptibility. In accordance with the general properties 
of second-order transitions, the components of the tensor jutk are continuous 
at the transition point, but their first derivatives with respect to the tempera- 
ture are discontinuous. 

The general theory of second-order phase transitions must be used for a 
quantitative investigation of antiferromagnetic transitions. The parameters 
in terms of which the thermodynamic potential must be expanded near the 
transition point are here some linear combinations (depending on the par- 
ticular magnetic symmetry of the lattice) of the mean magnetic moments 
mi, m 2 , ... at the various points of the elementary cell. As the transition 
point is approached, all these parameters tend, as usual, to zero as V(® ~ T )- 

The thermodynamic potential of an antiferromagnetic may involve terms 
of various types, resulting both from the exchange interaction of the magnetic 
moments and from their relativistic interaction; cf. the beginning of §36. 
The former type contain combinations of the moments mi, m2, ... depend- 
ing only on their relative orientation, and not on their orientation with 
respect to the crystal lattice. The relativistic terms, however, depend on the 
directions of the moments in the crystal, i.e. they lead to magnetic anisotropy 
of the crystal. 

As in ferromagnetics, the strong exchange interaction is the principal 
effect which causes an ordered arrangement of magnetic moments in an anti- 
ferromagnetic. The relativistic interactions cause the crystallographic 
anisotropy of its magnetic properties. 

There are certain types of magnetic symmetry which admit ferro- 
magnetism, yet have exchange interactions which do not alone result in ferro- 
magnetism. In such cases the spontaneous magnetisation is due only to the 
relativistic interactions, and is therefore very small. Such a body is anti- 
ferromagnetic, but the disposition of the magnetic moments deviates some- 
what from that of a pure antiferromagnetic, in such a way that the magnetic 



166 Ferromagnetism §40 

moments in a certain direction do not completely balance (I. E. Dzyalo- 
shinskii, 1957t). The ratio of the "ferromagnetic moment" to the value 
which it would have for a pure ferromagnetic is of the same order as the ratio 
of the relativistic and exchange energy densities (~ 10~ 3 to 10 -5 ). 

In sufficiently strong magnetic fields, the antiferromagnetic structure of 
the crystal must be thermodynamically unstable, and the orientation of all 
moments in the direction of the field becomes energetically favourable. This 
change in orientation usually involves a change (reduction) in the elementary 
cell of the magnetic structure of the lattice, and, like any change in the sym- 
metry properties, can occur only at a definite phase-transition point, which 
in most cases is a second-order- transition point. The "critical field" Her 
which destroys the antiferromagnetism is a function of temperature which 
vanishes for T = 0, being the transition point in the absence of the field. 
Thus the region of existence of the antiferromagnetic phase in the T/f-plane 
is bounded by a certain curve. 



f Zhumal eksperimental'nol i teoreticheskoi fiziki 32, 1547, 1957; Soviet Physics J ETP 5, 
1259, 1957. 



CHAPTER VI 

SUPERCONDUCTIVITY 

§41. The magnetic properties of superconductors 

At temperatures close to absolute zero many metals enter a peculiar state 
whose most striking property, discovered by Kamerlingh Onnes in 1911, 
is what is called superconductivity, i.e. the complete absence of electric 
resistance to a constant current. Superconductivity first occurs at a definite 
temperature for each metal, called the superconductivity transition point. 

This absence of electric resistance, however, is not the most fundamental 
property of a superconductor. The transition involves profound changes 
in the magnetic properties of the metal and, as we shall see, the change in 
its electric properties is a necessary consequence. 

The magnetic properties of a superconducting metal can be described as 
follows. The magnetic field does not penetrate into the superconductor; 
since the mean magnetic field in the medium is, by definition, the magnetic 
induction B, we can say that throughout a superconductor 

B = (41.1) 

(W. Meissner and R. Ochsenfeld, 1933). This property holds whatever 
the conditions under which the transition to the superconducting state 
occurs. For example, if the metal is cooled in a magnetic field, then at the 
transition point the lines of magnetic force cease to enter the body.t 

However, it should be mentioned that the equation B = is not valid in 
a thin surface layer. It is found by experiment that the magnetic field 
penetrates into a superconductor to a depth large compared with the distances 
between the atoms, and usually of the order of 10 -5 cm, but depending on 
the metal concerned and on the temperature. For the same reason, the 
equation B = does not hold at all in thin films of metal or colloidal par- 
ticles whose thickness or dimension is of the order of the "penetration 
depth". 

In what follows, we shall consider only thick superconductors, and neglect 
the penetration of the magnetic field into a thin surface layer. 



t It should be mentioned that only "pure" superconductors, i.e. metallic elements and 
chemical compounds containing them in certain proportions, exhibit the above properties. 
We shall discuss only substances of these kinds. In superconducting alloys, the Meissner 
effect is not complete, and the magnetic field may penetrate into the superconductor, although 
there is still no electric resistance. 

The proof below that the vanishing of B implies zero resistance must therefore be taken 
as showing that this equation is a sufficient, but not necessary, condition for the resistance 
to be zero. 

12 167 



168 Superconductivity §41 

As we know, the normal component of the induction must be continuous 
at any boundary between two media ; this condition follows from the equation 
div B = 0, which is universally valid. Since B = in a superconductor, the 
normal component of the external field must be zero on the surface, i.e. the 
field outside a superconductor must be everywhere tangential to its surface, 
the lines of magnetic force having the surface as their envelope. 

Using this result, we can easily find the forces acting on a superconductor 
in a magnetic field. As in §5 for an ordinary conductor in an electric field, 
we calculate the force per unit surface area as <Jiknjc, where o^ = {HiHjc — 
— %H 2 8ik)j47r is the Maxwell stress tensor for a magnetic field in a vacuum. 
Since in the present case n«Hg =0, where H e is the field just outside the 
body, we find 

F s = -H e 2 nlS7T, (41.2) 

i.e. the surface is subject to a compression, of magnitude equal to the field 
energy density. 
According to equation (27.4) 

CurlB = Arrpvlc, (41.3) 

and from the equation B = it follows that the mean current density is also 
zero everywhere inside the superconductor. That is, no macroscopic volume 
currents can flow in a superconductor. It should be emphasised that in a 
superconductor the conduction current cannot meaningfully be isolated 
from ~pv as it can in an ordinary conductor. For the same reason the mag- 
netisation M, and therefore H, have no physical significance here. 

Thus any electric current which flows in a superconductor must be a 
surface current. The surface current density g is given, according to (27.14), 
by the discontinuity in the tangential component of the induction at the 
boundary of the body. Since B = inside the superconductor, and B = H 
outside it, we have 

g = cnxH e /47T. (41.4) 

The presence of surface currents is not peculiar to superconductors. Similar 
currents can occur in any magnetised body, and their density is 
g= cnx(H e — B)/4-7r. Since the tangential component of H = B//* is con- 
tinuous on the surface of a normal (not superconducting) body, we have 
nxH« = nxB//*, and so the expression for g can be written 

c 1 — u. 

g = -nxB— ^ (41.5) 

477 fl 

A fundamental difference between superconductors and other bodies, 
however, appears when we consider the total current through a cross-section 
of the body. In a non-superconductor the surface currents always balance, 
and the total current is zero. This is seen from the condition (41.5) which re- 
lates the current density g to the magnetic induction inside the body, and so to 



§42 The superconductivity current 169 

the current g at every point on the surface. In superconductors, however, 
the condition (41.5) has no meaning. For the transition from the ordinary 
state (with magnetic permeability jh) to the superconducting state corres- 
ponds formally to the limit B -> and fi -> 0. The right-hand side of (41.5) 
then becomes indeterminatej and there is no condition which restricts the 
possible values of the current. 

Thus we have the important result that the currents flowing on the surface 
of a superconductor may amount to a non-zero total current. Of course, 
this can occur only in a multiply-connected body (a ring, for example), or in 
a simply-connected superconductor forming part of a closed circuit which 
includes also a source of the electromotive force needed to maintain the 
currents in the parts of the circuit which are not superconducting. 

It is very important to note that a steady flow of current on a super- 
conductor is possible even if no electric field is present. This means that no 
dissipation of energy occurs, whose replacement would involve the doing of 
work by an external field. This property of a superconductor may also be 
described by saying that it has no electric resistance, a result which is 
thus a necessary consequence of its magnetic properties. 



§42. The superconductivity current 

Let us consider in more detail some properties of superconductors which 
depend on their shape. 

If a superconductor is a simply-connected body, then no steady distribu- 
tion of surface currents on it can exist in the absence of an external magnetic 
field. This can be seen as follows. The surface currents would produce in 
the surrounding space a constant magnetic field vanishing at infinity. Like 
any constant magnetic field in a vacuum, this field would have a potential <j>, 
and by the boundary conditions on the superconductor the normal deriva- 
tive d<f>/dn would vanish at the surface. We know from potential theory, 
however, that if d(f>Jdn = on the surface of a simply-connected body and at 
infinity, then <f> is a constant in all space outside the body. Thus a mag- 
netic field of this kind cannot exist, and therefore neither can the assumed 
surface currents. 

An external magnetic field, on the other hand, causes currents to flow on 
the surface of a simply-connected superconductor, and these currents can be 
observed through the appearance of a magnetic moment of the whole body. 
This "magnetisation" is easily calculated for an ellipsoidal superconductor, t 

Let £ be the external field, parallel to one of the principal axes of the 
ellipsoid. The relation (l-w)H+«B = |> holds for the magnetic field H 
inside a non-superconducting ellipsoid, n being the demagnetisation coeffi- 
cient for the axis in question (see (8.7)). In a superconductor there is no 

t In the present section we always assume that the magnetic field does not exceed the 
value at which superconductivity ceases (see §43). 



170 Superconductivity §42 

"field" H, as we have already shown, and so the magnetisation M = (B— H)/47r 
also lacks its usual significance. Nevertheless, it is here convenient to 
introduce H and M as formal auxiliary quantities in the calculation of the 
total magnetic moment M = M.V (V being the volume of the ellipsoid), 
which retains its usual meaning. Putting B = in the superconducting 
ellipsoid, we find 

H = $/(!-«)> (42.1) 

and 

M = - FH/4tt = - F#/4tt(1 -«). (42.2) 

In particular, for a long cylinder in a longitudinal field n = 0, so that H = 4) 
and M = — V$y\4mA These values of M are the same as would be found if 
the body had a diamagnetic volume susceptibility of — 1/4-7T. 

The magnetic field H e just outside the ellipsoid is everywhere tangential 
to it, and so its magnitude can be determined at once from the condition 
that the tangential component of H is continuous. Within the ellipsoid 
H = jj)/(l— n); taking the tangential component, we have 

(l-n)H e = £sin0, (42.3) 

where 6 is the angle between the direction of the external field 4) and the 
normal to the surface at the point considered. The greatest value of H e 
occurs on the equator of the ellipsoid, and is £>/(l — n). 

It may be pointed out once more that there is no fundamental difference 
between the currents which cause the "magnetisation" of a superconductor 
and those which produce the total current in it: their physical nature is the 
same. This important fact makes possible, in particular, an immediate 
determination of the gyromagnetic coefficients for any superconductor. 
The momentum density of the electrons which form the "magnetising" 
currents differs from the current density only by a factor mje, e and m being 
the charge and mass of the electron. From the definition of the gyro- 
magnetic coefficients (see (35.3)) it follows at once that for a superconductor 

gik = &ik> 

Let us now consider multiply-connected superconductors. Their proper- 
ties are very different from those of simply-connected ones, mainly because 
it is no longer true that a steady distribution of surface currents is impossible 
in the absence of an external magnetic field. Moreover, the surface currents 
need not balance out, and may result in a non-zero total superconductivity 
current on the body, even if no external e.m.f. is applied. 

Let us consider a doubly-connected body (i.e. a ring), with no external 
magnetic field. We shall show that the state of such a body is entirely deter- 
mined if the total current J on it is given. The problem of determining the 
field of the ring can again be solved as a problem of potential theory, but the 



t These relations for a cylinder follow immediately from the continuity condition on H, 
and are therefore valid for a cylinder of any cross-section, circular or otherwise. 



§42 The superconductivity current 171 

potential <f> is now a many-valued function, which changes by \ttJ\c when we 
go round any closed path interlinked with the ring (cf. §29). In order to 
state the problem in mathematically precise terms, we must draw some open 
surface which spans the ring. Then the problem is to solve Laplace's 
equation with the boundary conditions Bj>jdn = on the surface of the 
ring, <f> = at infinity, and fe-fa = ^wjjc on the chosen surface, where <f>i 
and ^2 are the values of the potential on the two sides of that surface. Such 
a problem is known from potential theory to have a unique solution, which 
does not depend on the form of the chosen surface. From the field near the 
surface of the ring, we can in turn uniquely determine the surface current 
distribution. 

The self-inductance of a superconducting ring is entirely determinate 
together with the current distribution. Here there is a marked difference 
from ordinary conductors, where the current distribution, and therefore the 
precise value of the self-inductance, depend on the manner of excitation of 
the current (§33). t 

In §32 we introduced the concept of the magnetic flux O through a linear 
conductor circuit, and showed that <E> = LJjc, where L is the self-inductance 
of the conducting circuit. For a superconducting ring, the magnetic flux is 
meaningful for any thickness, not necessarily small, of the ring. For, since 
the magnetic field is tangential, the magnetic flux through any part of the . 
surface of the ring is zero ; the magnetic flux through every surface spanning 
the ring is therefore the same. Moreover, the formula 

O = LJJc (42.4) 

remains valid, the self-inductance L being again defined in terms of the total 
energy of the magnetic field of the current. The total energy of the magnetic 
field of the superconductor is given by the integral J (H 2 l8ir)dV, taken over 
all space outside the body. Again spanning the ring by a surface C, we can 
use the field potential and write 

JH 2 dVj8ir= -JH.grad<f>dV/STT 

= J^divHdP787r-^tf»^d//8ir. 

The first term is zero, because div H = 0. The surface integral is taken 
over an infinitely remote surface, the surface of the ring, and the two sides 



t The self-inductance of a superconducting ring of radius b, made of wire whose cross- 
section is a circle of small radius a, is the same as the external part of the inductance of a 
non-superconducting ring, namely L = 4irb[\og (8b /a) — 2] ; see §33, Problem 2. The 
exact solution of the problem of a current in a superconducting circular ring was first given 
by V. A. Fok, Physikalische Zeitschrift der Sowjetunion 1, 215, 1932. 



172 Superconductivity , §42 

of the surface C. The first two of these give zero, so that 

jWdV/Zv = j>H n (cl>2-<f>i)dflS7T 
c 

= (J/2c)JH n df = J<l>l2c, 



where <I> is the magnetic flux through the surface C. Comparing this with 
the definition of the self-inductance, we have /3>/2c = LJ 2 /2c 2 , which gives 
(42.4). • 

If the ring is in an external magnetic field, the total magnetic flux O is 
composed of the flux LJjc and the flux <I> e of the external field. A very impor- 
tant property of a superconducting ring is that, even if the external field and 
the current vary, the magnetic flux through the ring remains constant : 

■ LJ/c+<& e = Oo, a constant. (42.5) 

This follows immediately from the integral form of Maxwell's equation in 
the space outside the body: 

i a 



<j)H.df = - (fiE.dl. 

c 8t J J 



If the integration on the left-hand side is taken over a surface C which spans 
the ring, the contour of integration on the right-hand side is a line on the 
surface of the ring. On the surface of a superconductor, the tangential 
component of E is zero (since E = inside a superconductor and E* is 
continuous on the surface). Hence the left-hand side is zero, and therefore 
dO>/d* = 0. 

The relation (42.5) gives the variation of the current in the ring when the 
external field changes. For example, if the ring is made superconducting in 
an external field of flux Oo> which is then removed, a steady current 
J = cOo/L flows in the ring. 

The constancy of the magnetic flux through a superconducting ring holds 
not only when the external field changes but also when the shape of the 
ring or its position in space is altered, t An intuitive statement of this result 
is that the lines of force can never intersect the surface of the superconductor, 
and so cannot escape from the aperture of the ring. 

The above results can be immediately generalised to the case of multiply- 
connected superconducting bodies, including sets of rings. The state of an 
w-ply connected system in the absence of an external field is completely 



t This statement follows at once from the relation between the induced e.m.f. and the 
change in the magnetic flux through the circuit when it is moved (§49). 



§43 The critical field 173 

determined by the « — 1 total currents J a . The relation (42.5) becomes the 
system of equations 

ZLabJl>+®e,a= <*0*. (42-6) 

6 

These equations hold, not only for any external field, but also for any change 
in shape or relative position. 

PROBLEM 
Determine the magnetic moment of a superconducting disc in an external magnetic field 
perpendicular to its plane, f 

Solution. The problem of a superconductor in a constant magnetic field is identical 
with the electrostatic problem of a dielectric of permeability e = 0. Regarding the disc as 
the limit of a spheroid as c -> (cf. §4, Problem 4), and using (8.9), we find with appropriate 
change of notation (the field £ being along the s-axis) M = —2a s £l3n. 

§43. The critical field 

A cylindrical superconductor in a longitudinal magnetic field has an addi- 
tional magnetic energy -\§*M = ^W/Stt. 'For a non-superconducting 
cylinder, on the other hand, the total energy would be almost unchanged 
when the external field was applied (we shall neglect the slight diamagnetism 
or paramagnetism of a non-superconducting metal, i.e. take fi = 1). Thus it 
is clear that, in sufficiently strong magnetic fields, the superconducting state 
must be thermodynamically less favourable than the normal state, and so 
the superconductivity must be destroyed. 

The value of the longitudinal magnetic field at which the superconductivity 
of a cylindrical body is destroyed depends on the metal concerned and on the 
temperature (and pressure). This value is called the critical field Her, and is 
one of the most important characteristics of a superconductor.! 

When the critical field is reached, the superconductivity is destroyed 
throughout the cylinder, because of the uniformity of the field over the 
surface. In bodies of other shapes, however, the destruction of superconduc- 
tivity is a more complex process, in which the volume occupied by matter 
in the normal state gradually extends as £ increases over some range (§44). 

Thus, at any temperature below the transition point, the metal can exist 
in either the superconducting or the normal state, denoted by the suffixes s 
and n respectively. We denote by ^so(V,T) and ^ n {V,T) the total free 
energies of the superconducting and normal body in the absence of an external 

f We consider this problem principally with a view to using the result elsewhere (see 
§75, Problem 2). For a superconducting disc the magnetic fields must in reality be very 
weak, since its superconductivity is very easily destroyed (see §43). 

% There is a sharp transition between the superconducting and normal states only in 
"pure" superconductors (see the footnote to §41), which are the only ones considered here. 
In alloys, the destruction of superconductivity and the penetration of the magnetic field 
occur gradually over a fairly wide range of fields, so that there is no critical field in the sense 
here defined. 



174 Superconductivity §43 

magnetic field; these quantities depend on the substance concerned and on 
the volume, but not on the shape, of the body. The free energy in the normal 
state does not change when the external field is applied, and so we omit the 
suffix in «^»o. In the superconducting state, however, the magnetic field 
considerably affects the free energy. 

For a superconducting cylinder, with given V and T, the free energy in 
a longitudinal external field § is 

& s = & s0 (V, T) + &VI8n. (43.1) 

From this we can derive all the other thermodynamic quantities. Differen- 
tiating (43.1) with respect to the volume, we find the pressure on the body: 

p = rtV,T)-&l*n t (43.2) 

whereto (V> T) is the pressure (for given Fand T) in the absence of the field. 
The equation (43.2) gives the relation between^, V and T, i.e. it is the equa- 
tion of state for a superconducting cylinder in an external magnetic field. 
We see that the volume V(p, T) in the presence of the magnetic field is the 
same as the volume with no magnetic field but a pressure p+gpj&n: This 
result accords, of course, with formula (41.2) for the force on the surface of a 
superconductor in a magnetic field. 

The thermodynamic potential t of the superconducting cylinder is 
cfo s = SF s +pV = ^soiV, T)+poV, the volume V being expressed in terms 
of p and T by (43.2). Hence we can write 

£P S {p, T) = 0°*(p + ^ T )> (43.3) 

where ^o s o(/>, T) is the thermodynamic potential in the absence of the field. 
Differentiating this equation with respect to T and to p, we obtain analogous 
relations for the entropy and the volume : 

^s{p,T) = ^ sQ (p^T^ (43.4) 

V s (p,T)= Vm^^t}. (43.5) 

We can now write down the condition which determines the critical field. 
The transition of the cylinder from the superconducting to the normal state 
occurs when cp n becomes less than cp s (for given p and T). At the transition 
point we have §p s = cp n t i-e. 

SPsQ (p+^ r) = Wf. n ( 43 - 6 ) 

This is an exact thermodynamic relation.t The change in the thermodynamic 



t Here defined as in §12. 

% We give here calculations more accurate than is usually necessary, so as to exhibit more 
clearly the interrelation between the various thermodynamic quantities. 



§43 The critical field 175 

potential in the magnetic field is usually a small correction to cpso(p, T). 
We can then expand the left-hand side of equation (43.6) in series, taking the 
first two terms: 

SPso(p, T) + ^V s0 (p, T) = gpfa n ( 43 - 7 ) 

where V s o(p, T) = dcpso(p, T)\dp is the volume of the superconducting 
cylinder in the absence of the field. Thus, in this approximation, we can say 
that the thermodynamic potential per unit volume is greater by H C t 2 IStt in 
the normal state than its value in the superconducting state. 

We denote by Tct = Tcr(p) the transition temperature in the absence of 
the magnetic field. Experiment shows that the transition concerned is a 
second-order phase transition. Hence, in particular, Hct(T) must tend 
continuously to zero at T = Tct. We know from the general theory of 
second-order phase transitions t that the change in the thermodynamic 
potential near the transition point is proportional to the square of T- T CT - 
We can therefore deduce from (43.7) that the critical field in this temperature 
range varies as the temperature difference T— Tct'. 

H CT = constant x (T cr - T). (43.8) 

Differentiating both sides of equation (43.6) with respect to temperature 
(for given pressure), remembering that Her is a function of T, and using 
(43.4), (43.5), we have 

*-*-:*]*(£)■ (43 - 9) 

where all the quantities £f n , Sf s , V s are for the point of transition between 
the two states of the body (i.e. for H = Her). Multiplying by T, we obtain 
the heat of the transition : 

„.„,._„. _!«^ ,,,., 

When the transition occurs at T = Tct (i.e. in the absence of the magnetic 
field), the quantity Q vanishes with #cr, in accordance with the fact that 
we have a second-order phase transition. A transition at T < Tct (in a 
magnetic field) involves absorption or evolution of heat, i.e. it is a first-order 
phase transition. Experiment shows that Hct increases monotonically with 
decreasing temperature throughout the range from Tct to zero. Hence the 
derivative dHci/dT is always negative, and we see from (43.10) that Q > 0, 
i.e. heat is absorbed in the (isothermal) transition from the superconducting 
to the normal state. 



f See Statistical Physics. §135, Pergamon Press, London, 1958. 



176 Superconductivity 

As T -> 0, the entropy of the whole body must vanish, by Nernst's 
theorem. Hence it follows from (43.9) that dHcrjdT = for T = 0, i.e. 
the curve of Hct{T) intersects the i/cr-axis at right angles. 

We may differentiate the difference £f n — Sf s (43.9) again with respect to 
temperature, and again use equations (43.4), (43.5). Since also (dS^jdpJT 
= - ( 8 V/8 T) p , the result is 

d^n 8& s _ 3* (Hcx 2 x J Vg d (Hcx 2^ 

— — V S 



8T 8T 3T2\ 8 



I 8tt / 8T 8T\ 8tt / 



8V S Y d ///cr 2 \l 2 

Multiplying both sides of this equation by T, we obtain the difference of the 
specific heats (at constant pressure) of the two phases. The terms involving 
the thermal-expansion coefficient and the compressibility are usually very 
small in comparison with the remaining terms ; neglecting them, we have 

V S T 8 2 H CT V s T/8H CT \z 
. *._*._ _ fl(X _ + _(_JL). (4 3,2) 

This formula could also be obtained by direct differentiation of the approxi- 
mate relation (43.7). In this approximation the difference between V 8 and 
V s o, and between ^ s and ^o* may be neglected. 

For T = Tcr, the first term in (43.12) is zero, and we obtain the following 
formula, which relates the change in specific heat in the second-order phase 
transition (in the absence of an external magnetic field) to the temperature 
dependence of H C t : 

(A. J. Rutgers, 1933). Hence we see, in particular, that in this case 
#* > *^n. As the temperature falls, i.e. when the superconductivity is 
destroyed by the magnetic field, the difference *^ s — ^ n changes sign, because 
the difference Sf n — Sf s is zero for T = and for T = Tcr, and must have 
a maximum in between. 

We can similarly discuss effects related to the change in volume in the 
transition. To do so, we differentiate equation (43.6) with respect to pressure 
(for given temperature), Her being a function of p. This gives 



8 1 H^\ 



or 

V n -Vs = ^-^—^, (43.14) 

4tt 8p 



§43 The critical field Ml 

which determines the change in volume at the transition point, t For 
T = Tcr this difference is zero, like the entropy difference. The transition 
at temperatures T < Tcr, however, is accompanied by a change in volume, 
which may be of either sign, depending on the sign of the derivative 
(0Hcr/0p)i. For T = Tcr there is no change in volume, but the compres- 
sibility is discontinuous; the discontinuity is easily found by differentiating 
equation (43.14). It may be noted that, if we substitute in (43.14) 



\ dp ) T ~ \dT ) P \ dp ) Hl 



(obtained by differentiating the equation Hct(j>, T) = constant), we obtain 
the Clapeyron-Clausius equation 

t^t) = Q , (43.15) 

\dTJ H „ T(V n -V s ) 

where the derivative (dp/dT)H CT defines the change in pressure needed to 
keep the applied external field critical when the temperature changes. 

The physical significance of the critical field H CT is much wider than would 
appear from its definition in terms of the behaviour of a superconducting 
cylinder. The equation H = H C r is a condition of equilibrium which must 
be fulfilled at every point of a surface separating normal and superconduct- 
ing phases in the same body. This is evident from the following simple 
arguments. If a cylinder is in a longitudinal magnetic field H C t, then both 
the boundary conditions on the magnetic field and the conditions of thermo- 
dynamic stability are satisfied for all states in which an interior cylindrical 
part is in the superconducting state and the rest of the body is in the normal 
state, and the field at the boundary between these parts is Hei. Thus the 
surface of separation, on which H = Hot, is in "neutral equilibrium" with 
respect to its location. This is a characteristic property of phase equilibrium. 

In a variable magnetic field, the boundary between the superconducting 
and normal phases changes its position. The kinetics of this process is very 
complex, and its discussion requires a simultaneous solution of the equations 
of electrodynamics and of thermal conduction, taking into account the heat 
evolved in the phase transition. We shall not pause to carry out this investi- 
gation,t but merely give the boundary condition which must be satisfied at 
the moving boundary between the normal and superconducting phases. 

To derive this condition we take a co-ordinate system K' moving with the 
velocity v of the boundary between the phases. By the formulae for trans- 
formation of fields, the electric field E' in the system K' is related to the fields 
E and B in a fixed system K by E' = E+vxB/c; see (49.1). Since the 

t This difference must of course be distinguished from the change in volume (magneto- 
striction) of the superconductor when the field changes from zero to H CT . This can be foun 
from (43.5): V,(p, T) - V s0 (P, T) S (H„*l8nXdV.I dp) T . n _ . , , „ 

t It has been completed by I. M. Lifshitz, Zhurnal experimental not i teoreticheskoi 
fiziki 20, 834, 1950; Doklady Akademii Nauk SSSR 90, 363, 1953. 



178 Superconductivity §44 

boundary is at rest in the system K\ the usual condition of continuity of the 
tangential component of E' holds, i.e. nxE' = nxE-wB/c must be con- 
tinuous, where n is a unit vector normal to the surface, in the direction of 
the velocity v. In the superconducting phase E = B = 0, and in the normal 
phase B = Her at the boundary. We therefore find that a tangential electric 
field appears on the moving boundary, its direction being perpendicular to 
that of the magnetic field and its magnitude being 

E = vH CT /c. (43.16) 

§44. The intermediate state 

If a superconducting body of any shape is in an external magnetic field £) 
which is gradually increased, a stage is finally reached where the field at 
some point on the surface of the body becomes equal to the critical field 
Hot, but § itself is still less than Hct. For example, on the surface of an 
ellipsoid placed in a field £> parallel to one of its axes, the greatest value of 
the field occurs on the equator (see (42.3)), and is equal to Her when 
§ = #cr(l-«). 

When <?) increases further, the body cannot remain entirely in the super- 
conducting state. Nor can it pass entirely into the normal state, because 
then the field would become .§ everywhere. Hence the superconductivity 
must be lost only in part. 





(b) 
Fig. 22 

At first sight one might imagine that this process occurs as follows. As § 
increases, the superconductivity is lost in a gradually increasing part of the 
body, while a gradually decreasing part remains superconducting, and the 
whole body becomes normal when § = Her- It is easy to see, however, 
that such states of the body are thermodynamically unstable. On the surface 
separating the superconducting and normal phases the magnetic field is, as 
we know, tangential to the surface, and its magnitude is Her- That is, the 
lines of force are on the surface. If the boundary is convex to the normal 
phase, the equipotential surfaces of the field, being at right angles to the lines 
of force, will diverge into the normal region, as shown by dashed lines in 
Fig. 22a. The field decreases, however, in the direction in which the equi- 
potential surfaces diverge, so that we should have H < H C i in the shaded 
region, contrary to the supposition that this region is in the normal state. 



§44 The intermediate state 179 

If, on the other hand, the boundary of the superconducting phase is concave, 
then the lines of force on that boundary must have a bend on the free surface 
of the superconducting region, to which the field is tangential (at the point 
O, in Fig. 22b). At a bend in a line of force, however, the field becomes 
infinite, which again contradicts the boundary conditions at the surface of the 
superconductor. 

The above arguments represent essentially another form of the situation 
which leads to the domain structure of ferroelectrics and ferromagnetics. 
Here also the conditions of thermodynamic stability have the result that, if 
the magnetic field reaches the value Her at even one point on the surface, 
the body is divided into numerous parallel alternating thin layers of normal 
and superconducting matter (L. Landau, 1937). This state of the supercon- 
ductor is called the intermediate state. As <?> increases, the total volume of the 
normal layers increases, and when £ = #cr the body becomes entirely normal. 

It should be emphasised that a body of arbitrary shape need not be entirely 
in the intermediate state/There may also be regions in the purely super- 
conducting and purely normal states; these must be separated by the region 
which is in the intermediate state. A simpler case is the ellipsoid already con- 
sidered. In a field parallel to the axis the intermediate state occurs in the range 

H CT (l-n) <$>< H CT , (44.1) 

and the whole volume of the ellipsoid is in this state, t 

The shape and size of the normal and superconducting layers in the inter- 
mediate state are determined by the conditions of thermodynamic equili- 
brium of the body as a whole, in the same manner as the shape of the domains 
in a ferromagnetic (§39). As there, the thickness of the layers is determined 
by two oppositely acting factors. The "surface tension" at the boundaries of 
the normal and superconducting phases tends to reduce the number of 
layers, i.e. to increase their thickness. The "energy of emergence' * of the 
layers at the free surface of the body has the opposite tendency. The layer 
thickness increases with the size of the body, and consequently (for the same 
reasons as in ferromagnetic domains) they must eventually branch near the 
surface.^ 

The intermediate state can also be described in an averaged manner if the 
thickness of the regions under consideration is large compared with the layer 
thickness (R. E. Peierls, and F. London, 1936). In this description it is 

t For a sphere (for example), n = J, and the intermediate state exists in the range 
S-Hcr < £ < H CT . For a cylinder in a transverse field, n = \, and the corresponding range 
is iflcr < § < #cr- For a cylinder in a longitudinal field, n = 0; there is no intermediate 
state, and the superconductivity is totally destroyed at £ = Her- Finally, for a flat plate in 
a transverse field n = 1, and it is in the intermediate state for any field £ < H cr . 

% The thickness of unbranched layers is calculated in Problem 2. A discussion of a model 
with multiple branching has been given by L. Landau, Zhurnal experimental' not i teoreti- 
cheskoi fiziki 13, 377, 1943 ; Journal of Physics 7, 99, 1943. 

The interrelation between the two models is considered by E. M. LlFSHlTZ and Yu. V. 
Sharvin, Doklady Akademii Nauk SSSR 79, 783, 1951. 

In certain conditions, when the external field is near zero or H cr , a "filamentary structure 
may be thermodynamically more favourable than the layered structure ; see E. R. Andrew, 
Proceedings of the Royal Society A194, 98, 1948. 



180 



Superconductivity 



§44 



assumed that there is inside the body a magnetic induction B which varies 
from zero in the purely superconducting state to H CT in the purely normal 
state. If we ascribe a non-zero induction to the matter in the intermediate 
state, we must also ascribe to it a definite magnetic " field " fl. To deter- 
mine the. relation between these quantities, we must consider the true 
structure of the intermediate state. 

The magnetic field in a normal layer at its boundary with a superconduct- 
ing layer is Her, and by virtue of the assumed smallness of the layer thick- 
ness we can suppose that the field has this value everywhere in the normal 
layers. In the superconducting layers B = 0. Hence, averaging the magnetic 
field over a volume large compared with the layer thickness, we find that 
the mean induction B = x n HcT, where x n is the fraction of the volume that 
is in the normal state. Next, we determine the thermodynamic potential per 
unit volume of the body, taking as zero the value for the purely superconduct- 
ing state. In the absence of a magnetic field, unit volume of the normal 
phase has an excess thermodynamic potential Hcr 2 /87r.t When a magnetic 
field is present, a further Hqx 2 ^tt is added as magnetic energy, giving alto- 
gether Hcr 2 /47r. The mean thermodynamic potential per unit volume in the 
intermediate state is therefore 

O = XnHo^/^T = H CI S/47T. (44.2) 

The relation between S and fl is obtained from the general thermo- 
dynamic relation H = 47rd<I>/dB. In the present case we find that fl is 
parallel to B and its magnitude is 

H = # cr , (44.3) 

i.e. it is independent of the induction. 




A 
Fig. 23 



If the relation between B and 3 is shown graphically (Fig. 23), then the 
segment OA of the axis of abscissae corresponds to the superconducting 
state, and the line BC (B = S) to the normal state. The vertical line 
AB (S = Her) corresponds to the intermediate state. 

Let n be a unit vector in the direction of the lines of force of the averaged 
magnetic field. Putting fl = HcvTi and substituting in the equation 



t Here we neglect all magnetostriction effects. Instead of the change in the thermo- 
dynamic potential we could therefore speak of the (equal) change in the free energy. 



§44 The intermediate state 181 

curl fl = (which holds in the absence of a volume current), we find that 
curln = 0. Since n 2 = 1, we have 

gradn 2 = 2(n-grad)n + 2nx curln = 0, 

and therefore (n»grad)n = 0. This means that the direction of the vector n 
is constant. Thus the lines of force of the mean field are straight lines. 

Let us apply these results to an ellipsoid in the intermediate state. For a 
uniform field inside the ellipsoid, the relation {\-n)3+nB = <?) holds, 
whatever the relation between B and H. Putting ff = Her, we haVe 

B = $--—H m . (44-4) 

n n 

Thus the mean induction in the ellipsoid varies linearly with the external 
field, from zero when £ = (1 -n)#cr to Her when £ = Her. 

We may also write down an expression for the total thermodynamic 
potential go of an ellipsoid in the intermediate state. To do so, we start from 
the general formula 

-J[-" 2: s a ]«' 

(cf. (31.7)), which is also valid whatever the relation between B and H. 
Substituting 0>, H and B from (44.2)-(44.4), we obtain 

^ = IU r 2-V r -$)2l, (44.5) 

877 L n J 

V being the volume of the ellipsoid; cjhi is taken to be zero in the purely 
superconducting state of the ellipsoid, in the absence of a magnetic field. 
For a superconducting ellipsoid in an external field £ we have gb s = - \M-^ 
= T£ 2 /8tt(1 -«), in accordance with (31.6) and (42.2). These two equations 
give the same result for <?> = Hcr(l -n), as they should. 

Finally, it should be emphasised that the "averaged" description of the 
intermediate state given here is in reality not very accurate, because of the 
comparatively large thickness of the layers. For the same reason, this descrip- 
tion fails to reproduce certain phenomena related to the properties of the 
layer structure. These include the fact that the transition from the super- 
conducting to the intermediate state actually occurs only when £ slightly 
exceeds (1 -n)H CT . The reason for this "delay" is as follows. The passage 
into the intermediate state occurs when that state becomes thermodynami- 
cally stable, i.e. when gb t < cp s . The layered structure, however, has not 
only the "volume" energy (44.5) allotted to it in the "averaged" description 
but also additional energy resulting from the existence of the boundaries 
between the layers and their change in shape near the surface of the body. This 
results in some displacement of the transition point towards stronger fields.t 

t See the references in the second footnote to this section. 



182 Superconductivity §44 

PROBLEMS 
Problem 1. Determine the specific heat of an ellipsoid in the intermediate state. 

Solution. The entropy and thence the specific heat, are found by differentiating the 
thermodynamic potential (44.5) with respect to temperature. Neglecting the terms contain- 
ing the thermal-expansion coefficient, we obtain 

VT 

the prime denoting differentiation with respect to T; <# a is the specific heat of the body in 
the superconducting state, whose slight dependence on § we here neglect. Hence it follows 
that, as $ varies (at constant temperature), the specific heat changes discontinuously at the 
point £ = (1 —n)H CI from <tf s to 

4-TTtl 

and thereafter varies linearly with §, reaching the value 

VT VT VT 

V, - —(Hcr'Z+HcrHcr") + ~ Her' 2 = V» + —Her' 2 

for <?) = H cr , whence it falls discontinuously to <g n . 

Problem 2. Determine the shape and size of the normal and superconducting layers in a 
flat plate in the intermediate state in an external magnetic field § perpendicular to the plate; 
the layers are assumed unbranched (L. Landau, 1937). 

Solution. The normal and superconducting regions are layers parallel to the field, except 
near the surface of the plate. The lines of magnetic force (shown dashed in Fig. 24) pass only 
through the normal layers, and the boundaries of the superconducting layers are also lines of 
force, since B n = there. Since also H = H CT on the boundary between the normal and 
superconducting phases, the conditions at the boundaries of a superconducting layer are 

on BC H x = 0, 

on BA and CD HJ+Hf = H„*, (1) 

the co-ordinate axes being taken as shown in Fig. 24. Far from the plate, the field H must be 
the same as the external field |), i.e. 

for x -> — oo H x = §, H y = 0. (2) 

We use the scalar and vector potentials : H x = —d</>/8x = dA\dy,H y = —d<f>]dy = —dAjdx 
and the complex potential to = <$>—iA (cf. §3, (3)). 

On a line of force A = constant. We put A — on the line of force which reaches O 
and then branches into OCD and OB A, forming the boundary of one superconducting layer. 
The difference between the values of A at the boundaries of two successive superconducting 
layers is equal to the magnetic flux across the segment a = a t +a n , namely $a. Hence the 
value of A at the boundary of any superconducting layer is an integral multiple of §a. Using 
also the "complex field" -q = H x —iH y = —dw/dz, z = x+iy, we can write the conditions 
(1) as 



on BC re t] = 0, 

o: 

We introduce a new variable 



on BA and CD |i?| = H ct . (3) 



£ = exp( —l-nw/^d) — 1 (4) 

and regard i? as a function of £. £ is real on all boundary lines of force and on their continua- 
tions beyond the plate : £ = exp( — lirfl&a) — 1 . 

Since <f> is determined apart from a constant, its value at any one point can be chosen 
arbitrarily. Let <f> — at O. Then £ = there also. On the limiting line of force considered, 



§44 



The intermediate state 



183 



far from the plate, £ = — 1 (since for * -> - oo we have <j> -*- -%x -> + oo). The value of 
£ at B or C, where the line of force enters the plate, is £o, say. On CD and BA, £ varies from 
Jo to oo. Then the conditions (1) and (3) can be written 

V = S, (5) 



for £ = -1 
for < £ < Co re t\ = 0, 
for £o < £ H= H<„. 

The function tj(£) must, furthermore, be everywhere finite. 



D 
S 



On 



(6) 



Fig. 24 
The conditions (6) are satisfied by the function 

For real negative £ the two roots are real, and are taken with the signs shown. For < £ < Co 
both are imaginary, and we take 



(7) 



-^MVf-Vd- 1 )]- 



with the minus and plus signs on OC and OB respectively. For £ > £o 

£o\_. /&>1 



,=MVHWt]> 



with the minus and plus signs on CD and BA respectively. The value of £o is found from the 
condition (5), and is 

l /l \ a 

(8) 



*-&-)'■ 



where h = §/H cr . 
13 



184 Superconductivity §44 

The shape of the layer, i.e. the shape of the limiting line of force, is obtained by integrating 
the relation dz = — dw/17 over real £: 

dw ah r d£ 



Jaw _ ah f 
T) lit J 



v(Z+V 



Substituting 17(C), taking real and imaginary parts, and choosing appropriately the constants 
of integration, we obtain the following parametric equations of the line CD: 

=fh h -v|-v( &+ i,^v§±i;], 

00 

v ah r /Co d£ ah 

I 

where Y = %a s is the value of y for x -> oo ; see Fig. 24. 

The period a of the layered structure is related to the thicknesses a s and a n of the super- 
conducting and normal layers by a = a s +a n , a§ = a n H cr . The latter equation follows from 
the continuity of the magnetic flux, which passes entirely through normal layers. Hence 
a s = a(l —h), a n = ah. 

The period a is determined by the condition that the total thermodynamic potential of the 
plate is a minimum. The existence of "surface tension" at the boundaries between the normal 
and superconducting phases gives a term goi = 2/A • if C r 2 /8ra in the thermodynamic poten- 
tial per unit area of the surface of the plate. Here I is the thickness of the plate, and the surface- 
tension coefficient is written as A • H C t 2 I8t, where A has the dimensions of length. In calculat- 
ing this part of the energy we can, of course, neglect the curvature of the layers near the 
surface of the plate. 

The "energy of emergence" of the layers at the surface of the plate can be written as the 
sum of two parts. First, the increase in the volume of the normal layers as compared with the 
volume they would occupy if they were everywhere plane-parallel gives an additional energy 

GO 

He 



4 f/v ^ Hcr 
SP% = - ( Y— y) dx— — , 

a J 8ir 



where the factor 4 takes into account the presence of four angles (such as B and C in Fig. 24) 
on the two sides of each of the 1/a superconducting layers. 

Second, the emergence of the layers at the surface of the plate changes the energy of the 
system in the external field, i.e. the energy —\M • ^. The magnetic moment of the plate is 
due to currents on the surfaces of the superconducting layers. When the tangential component 
of the induction changes discontinuously from H to zero, the surface current density is 
g — ±cH/4ir. Hence the magnetic moment per unit length in the sr-direction and per boun- 
dary surface of the superconducting layer is 

f H 

— —y ds, ds = \/(dx 2 +dy 2 ). 
J 4ir 
OCD 

If the layer did not emerge at the surface, there would be no segment OC, and on CD we 
should have y = Y. Hence the excess magnetic moment for each of the four angles is 



J Air J Air 



§44 The intermediate state 185 

Accordingly, the excess energy is 

OO 

H 



OCD 

= ^\h<z j (- y d*+y ds) + J H„ dy]. 



The coordinates x and y, expressed in terms of £, are proportional to a. Hence all the 
integrals in fi»2+0°3 are proportional to a 2 , and this part of the thermodynamic potential is 
therefore proportional to a. The sum &>i+&>2+&>s is therefore of the formf 

#cr 2 r/A 



" sas '4rLT +<,r( * ) J- 



The condition that this is a minimum gives a = -\/\lNfW\- 

It should be noted that, in normal layers near the surface of the plate, the magnetic field 
may be considerably less than H CT , i.e. there is a situation corresponding to that shown in 
Fig. 22a. In this case the unfavourable thermodynamic state is made possible by the surface- 
tension energy, which prevents further reduction in the layer thickness. 



t The integrals in &°% and £Pz cannot be expressed in terms of elementary functions. 
The function /(A) has been tabulated by E. M. Lifshitz and Yu. V. Sharvin, loc. cit. 



CHAPTER VII 

QUASI-STATIC ELECTROMAGNETIC FIELD 

§45. Eddy currents 

So far we have discussed only constant electric and magnetic fields, and have 
used Maxwell's equation 

10B 

curlE= (45.1) 

c 8t 

only as a step in deriving the expression for the energy of a magnetic field 
(§30). 

The nature of the variable electromagnetic fields in matter depends greatly 
on the kind of matter concerned and on the order of magnitude of the fre- 
quency of the field. In the present section we shall consider the phenomena 
which occur in extended conductors placed in a variable external magnetic 
field. We shall assume that the rate of change of the field is not too large, 
and therefore satisfies various conditions which will be derived below. 
Electromagnetic fields and currents which satisfy these conditions are said 
to be quasi-static. 

We shall first of all suppose that the wavelength A ~ cjm which corresponds 
(in the vacuum or dielectric surrounding the conductor) to the field frequency 
to is large compared with the dimension / of the body: a> <^ cjl. Then the 
magnetic field distribution outside the conductor at any instant can be 
described by the equations of a static field : 

div B = 0, curlH = 0, (45.2) 

all effects due to the finite velocity of propagation of electromagnetic dis- 
turbances being neglected. Of course, this neglect is permissible only at 
distances from the body which are small compared with A ; these are the only 
distances which need be considered in determining the field inside the body. 
The complete system of field equations inside the conductor consists of 
(45.1) together witht 

divB = 0, (45.3) 

curlH = 4ttoE/c. (45.4) 



f In an anisotropic body, o-E on the right-hand side of (45.4) must be replaced by the 
vector oucEic. 

186 



§45 Eddy currents 187 

The second of these equations has been derived, strictly speaking, only for 
constant currents and magnetic fields. It is therefore necessary to specify 
conditions under which this equation can reliably be used for variable fields. 
In equation (45.4) the current has been written in terms of the electric field 
in accordance with the relation j = ctE with constant a, which holds for a 
steady state. This relation remains valid if the period of the field is large 
compared with the characteristic times of microscopic conduction. That is, 
the field frequency must be small compared with the reciprocal mean free 
time of the electrons in the conductor. For typical metals at room tempera- 
ture, the limiting frequencies given by this condition lie in the infra-red 
region of the spectrum, t 

There is another condition which restricts the applicability of the equations 
in this case. Being macroscopic equations, they presuppose that the electron 
mean free path is small compared with the distances over which the field 
changes appreciably. We shall return to this condition later. 

In equations (45.1) and (45.4), E is the induced electric field resulting from 
the variation of the magnetic field. When H is known, the field E can be 
immediately determined by equation (45.4). The equation for H is obtained 
by eliminating E from (45.1) and (45.4): 

4tt 8B curlH 

= -curl . (45.5) 

c 2 dt o 

In a homogeneous medium of constant conductivity a and constant mag- 
netic permeability n, the factor \\a can be taken in front of the curl operator, 
and by (45.3) we have div B = /u div H = 0. Hence curl curl H = - AH, 
and we obtain the equation 

AH - -trir- (45 - 6) 

With the equation div H =0 this suffices to determine the magnetic field. 
It may be noted that equation (45.6) is a heat-conduction equation, the 
thermometric conductivity x being represented by c 2 /47r^cr. 

The boundary conditions on the magnetic field at the surface of a con- 
ductor are evident from the form of the equations, and are as before 

B nl = B n2 , H a = H, 2 . (45.7) 



t For poor conductors (e.g. semiconductors), equation (45.4) is valid only if a further 
condition, which may be more stringent, is satisfied. For such bodies it may be possible to 
define both a conductivity and a dielectric constant. Then a teim — (e/c) SEjdt is added 
to the right-hand side of (45.4), and the condition for this term to be small in comparison 
with AnaEjc is a/a>^ e. In good conductors (e.g. metals), on the other hand, o/(o^> 1 
throughout the frequency range in which the conductivity can be regarded as constant (see 
also the sixth footnote to this section). 



188 Quasi-static Electromagnetic Field §45 

The expression on the right-hand side of equation (45.4), being bounded, 
does not affect the second of (45.7). For /t =1 we can put simply t 

Hi = H 2 . (45.8) 

The continuity of Ht implies that of (curl H) n and therefore, by (45.4), 
that of (orE) ra . Outside the conductor, a = 0, and we therefore conclude 
that on the surface E n j = 0, where the suffix / denotes the field inside the 
conductor. Since E n is zero, so is j n = oE n . Thus the system of equations 
under consideration necessarily implies the vanishing of the normal com- 
ponent of the current density on the surface of the conductor. In other 
words, in this approximation a variable magnetic field cannot cause the 
appearance of free charges on the surface of the conductor. 

The boundary condition (45.8) is insufficient for a complete formulation 
of the problem if the conductor is composite and its parts have different con- 
ductivities. At the interfaces between the parts we must use both the 
continuity of H and that of E<; the latter implies the condition 

(curlH)n/oi = (curlH), 2 /cT2 (45.9) 

on the magnetic field. 

Having established the basic equations, let us now examine the nature of 
the variable fields which they describe. Suppose that a conductor is placed 
in an external magnetic field which is suddenly removed. The field in and 
around the conductor does not vanish immediately ; the manner of its decay 
with time is given by equation (45.6). To solve a problem of this kind, we 
use the following procedure. We seek solutions of equation (45.6) which 
have the form H = H m (x, y, z)e~ ymt , where y m is a constant. The equation 
for the function H m (#, y, z) is then 

(c 2 /W)AH m = -y m H m . (45.10) 

For a conductor of given shape, this equation has non-zero solutions (satisfy- 
ing the necessary boundary conditions) only for certain y m , the eigenvalues 
of (45.10), all of which are real and positive. J The corresponding functions 



f For ordinary diamagnetic and paramagnetic bodies, /* is very nearly 1, and the inclusion 
of jit in the following formulae would be a pointless refinement. Values of /u. differing consid- 
erably from 1 occur in ferromagnetic metals, whose magnetic properties (in sufficiently weak 
fields) can be described in terms of a large constant permeability. For quite moderate frequen- 
cies, however, such substances exhibit a dispersion of /u. (i.e. a dependence of fi on the frequency 
ou), together with a decrease of fi almost to 1. We shall therefore put p = 1 in the present 
chapter. 

X This is easily seen as follows. So as to avoid having to take account of the boundary 
conditions at the surface of the body, we start from equation (45.5) and suppose a to vanish 
continuously outside the body. Multiplying both sides of the equation 

- 47ry m H m /c 2 - - curl [(1/a) curl H m ] 

by H m * and integrating over all space, we have 

£y» j \H m \*dV = jH OT *.curl CUrl g Hm dF= j- g IcurlHjW, 

whence it is evident that the y m are real and positive. 



§45 Eddy currents 189 

Hm(x, y, z) form a complete set of orthogonal vector functions. Let the field 
distribution at the initial instant be H (tf, y, z). On expanding this in terms 
of the functions H m : 



we obtain the solution of the problem : 

H(x,y, z,t) = X c m e-ymtH m (x,y, z) (45.1 1) 

m 

gives the manner of decay of the field with time. 

The rate of decay is determined principally by the term in the sum for 
which y m is least ; let this be yi . The "decay time" of the field may be defined 
as t = 1/yx. The order of magnitude of t is evident from equation (45.10). 
Since AH ~ H// 2 , where / is the dimension of the conductor, we have 

T - AlTol^C*. (45.12) 

Another type of problem concerns the behaviour of a conductor in an 
external magnetic field which varies with frequency o>. The magnetic field 
penetrates into the conductor and induces in it a variable electric field, 
which in turn causes currents to appear; these are called eddy currents A 
A general idea of the way in which the field penetrates into the conductor 
can be obtained from the analogy already mentioned between equation 
(45.6) and the equation of thermal conduction. It is known from the theory 
of thermal conduction that a quantity which satisfies such an equation is 
"propagated" through a distance ~ V(x0 in time *• We can therefore 
immediately conclude that the magnetic field penetrates into the conductor 
to a distance 8, given in order of magnitude by 8 ~ ^(c 2 jaai). The same is 
true, of course, of the induced electric field and currents. 

In a variable field of frequency co, all quantities depend on the time through 
a factor e~ i(0t . Equation (45.6) then becomes 

AH= -47rt<rcoH/c 2 . (45.13) 

Let us consider two limiting cases. If the penetration depth S is large com- 
pared with the dimension of the body (low frequencies), we can put the right- 
hand side of (45.13) equal to zero as a first approximation. Then the magnetic 
field distribution at any instant will be the same as it would be in a steady 
state with the same external field. Let this solution be Hst; it is independent 
of the frequency (or rather involves the frequency only in the time factor 
e -i">ty The induced electric field appears only in the next approximation, 
being absent in the steady state. This corresponds to the fact that curl Hst 
= 0, and so the value of Est obtained from (45.4) is zero. To calculate E, 



t In Russian "Foucault currents". 



1 90 Quasi-static Electromagnetic Field §45 

therefore, we must use equation (45.1), according to which 

curlE = icoUst/c. (45.14) 

This equation, together with div E = (which follows from (45.4) when a 
is constant in the body), entirely determines the electric field distribution. 
It is seen to be proportional to the frequency co. 

The opposite limiting case is that where 8 <^ / (high frequencies). The 
condition for the macroscopic field equations to be applicable, mentioned at 
the beginning of this section, requires that 8 should still be large compared 
with the mean free path of the conduction electrons, t 

When 8 <^ I the magnetic field penetrates only into a thin surface layer of 
the conductor. In calculating the field outside the conductor we can neglect 
the thickness of this layer, i.e. assume that the magnetic field does not 
penetrate into the conductor at all. In this sense a conductor in a high- 
frequency magnetic field behaves like a superconductor in a constant field, 
and the field outside it must be calculated by solving the corresponding 
steady-state problem for a superconductor of the same shape. 

The true field distribution in the surface layer of the conductor can be 
investigated in a general manner by regarding small regions of the surface 
as plane. It is necessary to solve equation (45.13) for a conducting medium 
bounded by a plane surface, outside which the field has a given value 
Hoe~ i(ot , say. This vector is obtained as shown above, by solving the prob- 
lem for a semi-infinite medium, and is parallel to the surface of the conductor. 
The boundary condition (45.8) shows that the magnetic field in the conductor 
is also Ho£ - * w * at the surface. 

We take the surface of the conductor as the ary-plane, the conducting 
medium being in z > 0. Since the conditions of the problem are inde- 
pendent of x and y, the required field H depends only on the z co-ordinate 
(and on the time). We therefore have divH = dH z jdz = 0, and since 
H z = at the boundary it must be zero everywhere. By (45.13), the equation 
for H is d 2 H/a* 2 + A 2 H =0, where k = V(4Waw/c 2 ) = (l+OVC 2 ™™)/*- 
The solution of this equation which vanishes far from the surface is e ikz . 
Using the boundary condition at z = 0, we obtain 

H = Hoe-* /s eM s -M (45.15) 

where the penetration depth 8 is 

8= cI^(2tto(o) and k = (l + i)/8. (45.16) 

The electric field is now determined by means of equation (45.4). If n is 
a unit vector in the ^-direction, we have 

E = V(W8^)(1 -OH x n. (45.17) 

Thus E ~ H8/X. 



f This condition is, in fact, the first to be violated in metals as the frequency increases. 
The condition a> <^ 1 /t, where r is the mean free time, may, however, be the more stringent 
for semiconductors of low conductivity. 



§45 Eddy currents 191 

If the field H e-* w ' is "linearly polarised", then H can be made real by a 
suitable choice of the origin of time. We then take the direction of Ho as 
the y-axis. Taking the real part in (45.16) and (45.17), we have 



H = H y =H e-*< s cos 



6-4 

E = E X = H Q y/{uf4no)e-*i'ca&£-wt--^. 



(45.18) 



The eddy current density j = oE has the same distribution as E. 

The presence of eddy currents implies a dissipation of. the field energy, 
which appears as Joule heat. The time average energy Q dissipated in the 
conductor per unit time is Q = jyEdV = J <rE 2 dV. It can also be calculated 
as the mean field energy entering the conductor per unit time : 

Q = <J&.df = (c/4*r)<fExH-df, (45.19) 

the integral being taken over the surface of the conductor, t 

We have already seen that, in the limiting case 8 > /, the amplitude of the 
magnetic field inside the conductor is independent of the frequency, while 
that of the electric field is proportional to u>. The energy dissipation Q at 
low frequencies is therefore proportional to to 2 . When S < /, on the other 
hand, the magnetic and electric fields on the surface of the conductor are 
given by formulae (45.15) and (45.17) with z = 0. The Poynting vector is 
normal to the surface, and its mean value is S = (c/167r)\/(ci>/27ra)|Ho| 2 , 
the variation of H over the surface being given by the solution of the prob- 
lem of the static field outside a superconductor of the same shape (cf. above). 
The energy dissipation is 

»-is7^ W * (45 - 20) 

Thus at high frequencies it is proportional to \/co. 

The energy dissipation can also be expressed in terms of the total mag- 
netic moment Jt acquired by the conductor in the magnetic field. In a 
periodic field, the magnetic moment is likewise a periodic function of time, 
with the same frequency. According to formula (31.4), the rate of variation 
of the free energy is given by -Jt'dfrjdt, where £ is a uniform external 



t If any two quantities a(t) and b(t) are written in complex form (proportional to e~ i<0 t), 
the real parts must of course be taken before calculating their product. If, however, we are 
interested only in the time average value of the product, it may be calcula ted as % re ab*. 
The terms containing e ±2i ^* give zero on averaging, and so l(a+a*)(b+b*) = \(ab* + a*b). 
In particular, S can be calculated as the real part of the "complex Poynting vector": 

S = ie[^.*ExH*]. (45.19a) 



192 Quasi-static Electromagnetic Field §45 

field in which the conductor is placed. This expression does not imme- 
diately give the required energy dissipation, because the energy of the body 
changes not only on account of dissipation but also by the periodic move- 
ment of energy between the body and the surrounding field. If we average 
over time, however, the latter contribution vanishes, and the mean dissipa- 
tion of energy per unit time is 



Q = -Jt>d$>/dt. (45.21) 

If M and .£) are written in complex form, then dfy/dt = — icofy, and Q can 
be calculated as 

Q = -| repaid •$*) = \u> im{J(-b*). (45.22) 

The origin of the factor \ is explained in the last footnote. 

The components of the magnetic moment M are linear functions of the 
external field: 

J*i = Kott&t, (45.23) 

where the dimensionless coefficients a^(o>) depend on the shape of the body 
and on its orientation in the external field, but not on its volume V. In this 
formula we assume that M and |) are written in complex form, so that the 
cuijc are also in general complex. The tensor Voctk may be called the magnetic 
polarisability tensor for the body as a whole. This tensor is symmetrical:! 

a-ik = a-ki- (45.24) 

We can therefore write 

If also we write the complex quantities out as auk +i<*4k"> the energy dissipa- 
tion (45.22) becomes 

Q = *FW're(S<$**). (45.25) 

Thus the energy dissipation is determined by the imaginary part of the 
magnetic polarisability. We have already seen that Q is proportional to ofi 
for low frequencies, and to -y/co for high frequencies. We can therefore 
conclude that the quantities a^" in these two limiting cases are proportional 
to co and to 1/-\/oj respectively. Since they decrease both as co ->0 and 
co -> oo, they must have a maximum in between. 

The magnetic moment of a conductor in a variable magnetic field is due 
mainly to the conduction currents set up in the body; it is not zero even if 
fi = 1, when the moment in a constant field vanishes. The latter can be 
obtained from M{oy) by taking the limit as co -> 0. Hence it follows that 
the real part eta of the polarisability tends to a constant limit as co -> 



t See Statistical Physics, §124, Pergamon Press, London, 1958. 



§45 Eddy currents 193 

(the limit being zero for fi = 1), corresponding to magnetisation in a constant 
field. In the limit w -> oo, when the magnetic field does not penetrate into 
the body, a {fc ' tends to a different constant limit, corresponding to the steady 
magnetisation of a superconductor of the same shape. 

PROBLEMS 

Problem 1. Determine the magnetic polarisability of an isotropic conducting sphere of 
radius a in a uniform periodic external field. 

Solution. The field H* inside the sphere satisfies the equations AHi+£ 2 H{ = 0, 
divHi = 0, where k = (l+i')/S. We write this field in the form H< = curl A, where A 
satisfies the equation AA+#>A = 0; since H is an axial vector, A is a polar vector. By 
symmetry, the only constant vector on which the required solution can depend is the ex- 
ternal field 4). We denote by/ the spherically symmetrical solution, finite for r = 0, of the 
scalar equation Af+k 2 f==0, namely /= (1/r) sin kr. Then the polar vector A, which 
satisfies the vector equation AA+ft 2 A = and depends linearly on the constant axial 
vector £), can be written as A = p curl (/£), where P is a constant. Thus we have 

Hi = p curl curl (/£>) 



= /3(y +* 2 /)£-/s(^ +* 2 /)(n-$)n, 



where n is a unit vector in the direction of r; the second derivative/ " has been eliminated 
by means of the equation A/+A 2 / =0- . 

The field H e outside the sphere satisfies the equations curl H« = 0, div H* = 0. We 
put He = — grad <£+|>; <f> satisfies the equation A<£ = and vanishes at infinity. Since <f> 
depends linearly on the constant vector £, we have <j> = — Va. £• grad (1/r), where 
V = 47ra 3 /3. Thus 

H, = Fagrad[($.grad)(l/r)]+$ 

Va 

It is evident that V<x£ is the magnetic moment of the sphere, so that Va is its magnetic 
polarisability (by symmetry, the tensor aij- reduces to a scalar aSa). 

On the surface of the sphere (r = a), all the components of H must be continuous. Equat- 
ing separately the components parallel and perpendicular to n, we obtain two equations to 
determine a and p. The polarisability per unit volume is found to be 



a = a'+ta" = — — 1 — - + — cot ak , 

8?rL a 2 k 2 ak J 

3[ 3 8 sinh (2a/ 8) -sin (2a/S) 1 

a = ~8^L 1_ 2a cosh(2a/S)-cos(2a/8)J' 

9S 2 r a sinh(2a/8)+sin(2a/S) 



a = — 



i['- 



\6ira 2 l 8 cosh(2a/8)-cos(2a/8) 
In the limit of low frequencies (8^> a), 

a .__J_(?) 4 _ 

105ttW 

20tAs; i 

For high frequencies (8 <^ a), 



An a 4 a 2 co 2 
105 c 4 ' 



*ff<0 

10c 2 ' 



lr 1 _i!l = _i.|'i__J£ 1 

JttL 2aJ 8ttL 2aV(2woa>)J' 



9 8 9c 



\6ira \6ira\f(2iTOOi)) 



194 Quasi-static Electromagnetic Field §45 

The limiting value Vol' = — $a s corresponds to the magnetic moment of a superconducting 
sphere; the corresponding value of a" could be found from formula (45.20), using the 
expression (42.3) for the field at the surface of a superconducting sphere. 

Problem 2. The same as Problem 1, but for a conducting cylinder (of radius a) in a 
uniform periodic magnetic field perpendicular to its axis. 

Solution. This problem is the "two-dimensional analogue" of Problem 1. In what 
follows all vector operations are two-dimensional operations in a plane perpendicular to the 
axis of the cylinder, and r is the radius vector in that plane. The field inside the cylinder 
is of the form 

Hi = curl curl (/£) 

(f , ,.,W J 2f ' 



= p\*- + k *f)$-fi(— +* 2 /)(n.|»n, 



where/ = Jo(kr) is the symmetrical solution of the two-dimensional equation A/+& 2 / = 
which is finite for r = 0. The field outside the cylinder is 

He = -2 Fa grad [(|>-grad) log r] + % 

2 Vol 
= — i -[2(n.$)n-$)+$, 

where V = to 2 . The magnetic moment per unit length of the cylinder is Vctfy (see §3, 
Problem 2). From the condition H* = H e for r = a, as in Problem 1, we obtain 



.__±[ 1 _.lm*l>|. 

2ttl kaJo(ka)i 



kaJo(ka)j 

using the relation Jo'{kr) = —kj\{kr). 

For 8 ^> a, expanding the Bessel functions in powers of ka, we have 



24t7\8/ 
8tAs/ 4< 



6c 4 



4c 2 
For 8 <^.a, we use the asymptotic expressions for the Bessel functions, obtaining 



2tt\ a) 2ir\ a\/(2iraa))/' 

1 S c 



a. = 



2tt a 2ira\/(2irow) 

Problem 3. The same as Problem 2, but for a magnetic field parallel to the axis of the 
cylinder. 

Solution. The magnetic field is everywhere parallel to the axis of the cylinder. Outside 
the cylinder we have H e = jr), and inside it H* =/.£), where/ is the symmetrical solu- 
tion of the two-dimensional equation /\J+k 2 f= which is 1 for r = a and finite for 
r = 0: Hi = $)Jo(kr)IJo(ka). The eddy currents in the cylinder are azimuthal (i.e. the only 
non-zero component isjj,), and are given in terms of the field H z = H by 4ir}/c = —dH/dr. 
The magnetic moment generated per unit length of the cylinder by the conduction currents 
is <Jt = ira 2 a.$ = (l/2c)J)VdF= — iJ(3H/3r)r 2 dr; it is parallel to the axis. Evaluating 
the integral, we have 



1 I", 2 Mka)-] 

a. => 1— • 

47TL 



kaJo(ka)Y 

Thus the longitudinal polarisability of the cylinder is half the transverse polarisability de- 
rived in Problem 2. 

Problem 4. Determine the least decay coefficient for the magnetic field in a conducting 
sphere. 



§46 The skin effect 195 

Solution. The solutions of equations (45.10) for a sphere include functions of various 
symmetries. The most symmetrical solution is that which is defined by an arbitrary constant 
scalar. This solution is inapplicable, however, for the following reason: it would be spheri- 
cally symmetrical (H = HAj)) and would have to be H = constant/r in order to satisfy the 
equation divH = (l/r)3(rH)/3r = 0, which is valid both outside and inside the sphere; 
but this function is not finite at the centre of the sphere. 

The least value of y corresponds to one of the solutions defined by an arbitrary constant 
vector. The form of these solutions is evidently the same as has been found in Problem 1, 
the only difference being that the constant term in the field He must be omitted so as to 
have H = at infinity. The quantity k is now real (=V(4w<ry/c 2 )), and the vector |) is 
the arbitrary constant vector. From the boundary condition Hj = H« at r = a we obtain 
two equations, and on eliminating a and /? we find sin ka = 0. The smallest non-zero root 
of" this equation is ka = w, and so the smallest value of y is irc^/loa 2 . 

§46. The skin effect 

Let us consider the distribution of current density over the cross-section 
of a conductor in which a non-zero and variable total current is flowing. 
From the results of §45 we should expect that, as the frequency increases, 
the current will tend to be concentrated near the surface of the conductor. 
This phenomenon is called the skin effect. 

The exact solution of the problem of the skin effect depends, in general, 
not only on the shape of the conductor but also on the manner of excitation 
of the current in it, i.e. the nature of the variable external magnetic field 
which induces the current. An important particular case, however, is that 
where the current flows in a wire of thickness small compared with its 
length; here the current distribution is independent of the manner of 
excitation. 

In calculating the current distribution over the cross-section of a thin 
wire, the latter may be regarded as straight. The electric field is parallel to 
the axis of the wire, and the magnetic field vector H is in a plane perpendicu- 
lar to the axis. 

Let us consider a wire of circular cross-section. This is a particularly 
simple case, because the form of the field outside the wire is immediately 
obvious. By symmetry, E = constant over the surface of the wire (though 
the value of the constant varies with time). With this boundary condition, 
the only solution of the equations div E = 0, curl E = outside the wire 
is E = constant. Similarly, the magnetic field outside the wire must be the 
same as it would be outside a wire carrying a constant current equal to the 
instantaneous value of the variable current. 

Inside the wire, the electric field satisfies the equation AE = (4tt(t/c 2 )^E/3^ 
which is the same as equation (45.6) for H; it is obtained by eliminating H 
from (45.1) and (45.4), just as (45.6) was obtained by eliminating E. In 
cylindrical co-ordinates, with the #-axis along the axis of the wire, the only 
non-zero component of E is E z , which depends only on r. For a periodic 
field of frequency co we have 

;*br) + * w - ' h '^r-T' (4<u) 



196 Quasi-static Electromagnetic Field §46 

where 8 is the penetration depth (45.16). The solution of this equation 
which remains finite at r = is 

E = E z = constant x/ (^) e "^> (46.2) 

where J is the Bessel function. The current density j = oE is similarly 
distributed. 

The magnetic field H± = H is found from the electric field by equation 
(45.1): ■ ' H 

icoHfjc = (curlE)^ = -dE z Jdr. (46.3) 

Since Jo(u) = -Ji(u), we obtain 

H = 11$= - constant x i^(47Tai/a))J 1 (kr)e- i<,)t , (46.4) 

the constant being the same as in (46.2); it is easily determined from the 
condition that H = lljca on the surface of the wire, a being the radius of the 
wire and / the total current in it. 

In the limiting case of low frequencies (a[8 ^ 1) we can take the first few 
terms of the expansions of the Bessel functions at every point in the cross- 
section : 

E z = constant x 1 — i{rj8f (r/S) 4 ] «-*•*, 

2w [ 1 In 
H+ = constant x r 1 — i{rj8f (r/S)* , 

The amplitude of E, and therefore that of the current density, increase as 
1 + (f/28) 4 with increasing distance r from the axis. 

In the opposite limiting case of high frequencies (a/8 > 1) we can use 
the asymptotic formula 

Jo[uV(%)] ~ «-¥!-««, (46.6) 

which is valid for large values of the argument, over most of the cross- 
section. Retaining only the rapidly varying exponential factor, we have 
E z = constant x e -(a-r>/V<a-rW-tf<w 

}2na (46 ' 7) 

H+ = constant x(l+i) / e -ia-r)/8 e i(a-r)/8-io>t. 

These formulae are, of course, the same as (45.15)-(45.17), which are valid 
near the surface of a conductor of any shape when the skin effect is strong. 
In the general case of a wire whose cross-section is not circular, the exact 
calculation of the skin effect is considerably more involved, since the fields 
inside and outside the wire must be determined simultaneously. Only in the 
limiting case of strong skin effect is the problem again simplified, because 
the field outside the wire may then be determined as the static field outside 
a superconductor of the same shape (§45). 



(46.5) 



>—i<i>t 



§47 The complex resistance 197 

§47. The complex resistance 

If the frequency of the variable current is low, the instantaneous current 
J(t) in a linear circuit is determined by the instantaneous e.m.f. S: 

*(*) = Rftt), (47.1) 

where R is the resistance of the wire to a constant current. 

There is no reason, however, to expect a direct relation between the 
values of $ and / at the same instant for all frequencies. We can say only 
that the value of J(t) must be a linear function of the values of S'it) at all 
previous instants. This relation may be symbolically written as / = Z~ x ^ 
or, conversely, 

* = ZJ, (47.2) 

where Z is some linear operator, t If the functions £(t) and J(t) are expanded 
as Fourier integrals, then for each "monochromatic" component (depending 
on time through a factor *-*"*), the effect of the linear operator Z is simply 
multiplication by a quantity Z which depends on the frequency: 

$ = Z{w)J. (47.3) 

The function Z(co) is in general complex. It is called the complex resistance 
or impedance of the conductor. 

It is evident from a comparison of (47.3) and (47.1) that the ordinary 
resistance R is the zero-order term in an expansion of the function Z{w) 
in powers of w. To find the next term, we must take account both of R and 
of the self-inductance L of the conductor.} 

Let us consider a linear circuit containing a variable e.m.f. £(t). By the 
definition of «f, the work done per unit time by the electric field on the 
charges moving in the wire is $J. This work goes partly into Joule heat and 
partly to change the energy of the magnetic field of the current. By the 
definition of R and L, the Joule heat evolved in the wire per unit time is 
Rp, and the magnetic energy of the current is LJ 2 /2c 2 . The law of conserva- 
tion of energy therefore gives the equation 

d LP 1 dj 



or 



= */+i4 («•♦) 



f We shall not pause here to discuss the general properties of this operator, since they 
are entirely analogous to those of the operator e, which will be examined in detail in §§58 
and 62. 

t Here, and in what follows, R and L denote the values for constant current. 



198 Quasi-static Electromagnetic Field §47 

In order to use the quadratic expressions &J and J 2 we must write 
g and J as real functions. Having derived the linear equation (47.4), how- 
ever, we can take complex monochromatic components: $ = $oe~ i<at , 
J = Joe~ i<ot . Then equation (47.4) gives the algebraic relation 



= (*-?*)j. 



whence 



Z = R-—a>L. (47.5) 



Taking the real part inj= $\Z, we have 

JW = /tnl 2T2,^ Ct *( < ° t -®> ^^ = ° )LIC2R ' (47 ' 6) 

y(R 2 +u> 2 L 2 /c 4; ) 

which determines the amplitude of the current and the phase difference 
between the current and the e.m.f. 

The real part of the expression (47.5) is the resistance R, which deter- 
mines the energy dissipation in the circuit. It is easy to see that, whatever 
the function Z{cS), a similar relation holds between re Z and the energy 
dissipation for a given current. On averaging with respect to time the power 
&J required to maintain the periodic current in the circuit, we obtain the 
part of this power which continually makes good the dissipative losses. The 
energy dissipation in the circuit per unit time is therefore Q = \ re {&J*\ 
where $ and J are expressed in complex form ; see the penultimate footnote 
to §45. Substituting $ = ZJ and denoting the real and imaginary parts of 
Z by Z' and Z" respectively :t 

Z = Z' + iZ", (47.7) 

we obtain Q = %Z'IJ\ 2 or, in terms of the real function J(t), 

Q = S'WJS (47.8) 

which gives the required relation. 

It may be noted that, since Q is necessarily positive, Z' is also positive : 

Z' > 0. '47.9) 

We may calculate Z{oS) for a wire of circular cross-section for any fre- 
quency,J i.e. without neglecting the skin effect. To do so, we again use the 
law of conservation of energy, but in a different form. We divide the power 
£J (where $ and / are real) into two parts, one being the change in the 
magnetic field energy outside the wire, and the other the total energy con- 
sumed inside the wire (both in changing the field and in evolution of 



t Sometimes called the resistance and reactance (in Russian : active and reactive resistances). 
X That is, any which satisfies the quasi-steady condition. 



§47 The complex resistance 199 

heat). The second part can be calculated as the total energy flux entering 
the conductor through its surface per unit time. Thus we have 

J dt\2c*) 4tt c* J dt 

where L e is the external part of the self-inductance of the wire, E and H 
the electric and magnetic fields at its surface, a its radius, and / its length. 
The field H is related to the current / by H = IJjca. Hence, dividing the 
above equation by /, we have 

c* at 
This is a linear equation, and hence we can use complex quantities. Then 

i(x)L e 

<?=ZJ = r J+El, 

c 2 

whence 

ico El ioi 2EI 

z '-7^ + J = -^ L ' + ^H- (47 - 10) 

For general frequencies, E and H are given by (46.2) and (46.4), and we 
have 

Z = *l* +i nJgg, (47.11) 

c 2 J x {ka) 

where R = l/irePo. When the skin effect is weak, we use the expansions 
(46.5) ; taking terms as far as (a/8) 4 and separating the real part, we find 

In the opposite case of a strong skin effect we use the expressions (46.7), 
obtaining 

Z' = Ra/28 = (lIcaWHlTTo), 

a* r 28 I co r le ] 

c 2 L a A c 2 l ay(2iTaa})\ 

It is seen from (47.11a) that we can put Z' = R if (7racoa 2 /c 2 ) 2 ^ 12. We 
also have Z"\Z' = wLjc 2 R = (77acoa 2 /c 2 ) 2 log (Ija), where L is given by 
(33.1). Comparing this with the inequality just given, we see that the range 
of frequencies in which the expression (47.5) can be used to take the self- 
inductance into account depends on the ratio l\a and is fairly narrow. 

In practice, however, the most important case is that in which the self- 
inductance of the circuit is due mainly to coils in it, whose self-inductance 
is large compared with that of an uncoiled wire (see §33). In such circuits 

14 



200 Quasi-static Electromagnetic Field §47 

formula (47.5) (i.e. equation (47.4) with constant R and L) can be used 
over a fairly wide range of frequencies. 

Let us consider a circuit in a variable external magnetic field H e , which 
may be generated in any manner. We denote by E e the electric field which 
would be induced by the variable field H e in the absence of conductors. Both 
H c and E e vary only very slightly over the thickness of a thin wire (unlike 
the field of the currents in the wire). We can therefore discuss the circula- 
tion of E e round the current circuit without specifying the exact position of 
the contour of integration in the wire. This circulation is just the e.m.f. $ 
induced in the circuit by the variable external magnetic field. By the integral 
form of Maxwell's equation we have 

r 1 8 r 1 d<D e 

^E,dl= ---J H-df----^ (47.13) 

where Q> e is the flux of the external field through the circuit. Substituting 
this expression in equation (47.4), we obtain 

1 dj 1 dO e 
RJ+—L^ = i (47.14) 

J c 2 dt c dt v ' 

Taking the self-inductance term to the right-hand side, we have 

1 dd> e LdJ 1 dO 



RJ= - 



c dt c 2 dt c dt 



where <I> = ^ e + LJ/c is the total magnetic flux from the external magnetic 
field and the field of the current. In this form the equation gives Ohm's 
law for the whole circuit, i.e. the equality of RJ to the total e.m.f. in the 
circuit. 

The formulation of equation (47.14) as expressing Ohm's law makes pos- 
sible a generalisation of it to the case where the shape of the circuit also 
varies with time. The self-inductance L is then a function of time, and 
(47.14) becomes 

Id 1 dO>e 

RJ= —z-tALJ)— — ^ ( 47 - 15 ) 

c l dt c dt 

In deriving this from the law of conservation of energy we should have to 
take into account also the work done in deforming the conductor. 

If there are several circuits in proximity, carrying currents J a , then for 
each of them <fr e in equation (47.14) is the sum of the magnetic fluxes due to 
all the other circuits (and to the external field, if any). The magnetic flux 
through the ath. circuit due to the current Ji, is Labjbjc, where L a b is the 
mutual inductance of the two circuits. We therefore have the following 
set of equations for the variable currents in the circuits : 

RaJa + -2Lal>4- = ^ ( 47 - 16 ) 

c iZ —> dt 
b 



§47 The complex resistance 201 

The sum over b includes the self-inductance term (b = a), and <o a is the 

e.m.f. produced in the ath circuit by sources external to the system of 
currents considered. 

For periodic currents of a single frequency, the system of differential 
equations (47.16) becomes a set of algebraic equations: 

XZ ab J b = *«, (47.17) 



where the quantities 



lOi 

Zab = $abRa ~L a b (47.18) 



form the impedance matrix. Like (47.5), the expressions (47.18) represent 
the first terms in an expansion of the functions Z a &(o>) in powers of the 
frequency. 

It should be noted that, in this approximation, the circuits have no mutual 
effect on the real parts of their impedances. Such an effect arises because 
the magnetic field of the variable current in one conductor generates eddy 
currents, and therefore an additional dissipation of energy, in the other 
conductor. For linear conductors this effect is negligible, but it may become 
important if extended conductors are located near them. 

Finally, let us consider how the equations of variable currents in linear 
circuits obtained in this section are related to the general equations of a 
variable magnetic field in arbitrary conductors. We shall take the simple 
example of the current set up in a circuit when a constant e.m.f. Sq is 
removed at time t = 0. From equation (47.4) we havet 



J = Sq\R for t < 0, 
/ = (<?olR)e-c 2 ™i< for t > 0. 



(47.19) 



We see that, after the removal of the e.m.f., the current decays exponentially 
with time, the decrement being 

y = c 2 R/L. (47.20) 

If the problem is exactly formulated, this y is the smallest of the y m obtained 
by solving the exact equation (45.10) for the conductor in question. Among 
the y m for a linear conductor there is one, the smallest, which is less than 
the others by a factor of the order of log (Ija), and this is (47.20). 



f Strictly speaking, these formulae are invalid for very small t, when the high-frequency 
terms in the Fourier expansions of the functions are important and so equation (47.4) cannot 
be used. During this short interval of time, however, the current J cannot change signifi- 
cantly, and so formula (47.19) gives the current at subsequent times with sufficient accuracy. 



202 Quasi-static Electromagnetic Field §48 

§48. Capacity in a quasi-steady current circuit 

A variable current, unlike a constant one, can flow in an open circuit as 
well as in a closed one. Let us consider a linear circuit whose ends are con- 
nected to the plates of a condenser, which are at a small distance apart. 
When a variable current flows in the circuit, the condenser plates will be 
periodically charged and discharged, thereby acting as sources and sinks of 
current in the open circuit. 

Since the distance between the condenser plates is small, the magnetic 
energy of the current can again be taken as LJ 2 /2c 2 , where L is the self- 
inductance of the closed circuit which would be obtained by joining the 
condenser plates by a short piece of wire. In applying the law of conserva- 
tion of energy, however, we must take into account not only the magnetic 
energy but also that of the electric field in the condenser. The latter energy 
is e 2 /2C, where C is the capacity of the condenser and + e(t) the charges on 
its plates. Proceeding as in the derivation of equation (47.4), we obtaint 

d LP d e 2 1 dj e de 

SJ = RP+ -+ = RP+—LJ— + . 

J J dt 2c 2 dt2C J c 2 J dt Cdt 

The current / is equal to the rate of decrease and increase of the charges on 
the two plates : J = dejdt. Dividing both sides of the equation by J and 
expressing J in terms of e, we have 

1 d 2 e de e 

— L + R—+— = S. (48.1) 

c 2 dt 2 dt C v ; 

This is the required equation for a variable current in a circuit with a capacity. 
If $ is a periodic function of time having frequency o>, then equation 
(48.1) reduces to an algebraic relation between $ and the charge e, or between 
$ and the current/ = — icoe. We have, in fact, JZ = <f , where the impedance 
Z is defined by 



(oiL 1 \ 

\-i\ . 

\ c 2 coC 



Z = R-i\ — - — \. (48.2) 



Taking real parts in the relation J = S'/Z, we obtain 

Sc\ COS (cut — (t)) 

M " "Tf — 1l 1 

VI + U 2 ~oZc) 



(cjjL 1 \ 1 

tan(£ = I 1 — 

r \ c 2 ojCJ R 



(48.3) 



which give the current in a circuit to which an external e.m.f. $ = $0 cos cot 
is applied. 



t In the present section we neglect the skin effect. 



§48 Capacity in a quasi-steady current circuit 203 

If S = 0, the current in the circuit consists of "free" electric oscillations. 
The (complex) frequency of these oscillations is given by Z = 0, whence 

Re* l\ c 2 {Rc 2 \ 2 l , AQA . 

---•Wb-yj- (48 - 4) 

We may have either periodic oscillations damped with decrement Rc 2 \2h 
or an aperiodically damped discharge, depending on the sign of the radicand. 
In the limit as R -> we have undamped oscillations whose frequency is 
given by Thomson's formula: a> = cj^(LC). 

Equation (48.1) can be immediately generalised to a system of several 
inductively coupled circuits containing condensers. The current J a in the 
ath circuit is related to the charges ± e a on the corresponding condenser by 
Ja = de a jdt, and equation (48.1) is replaced by the set of equations 

21 _ d 2 e b de a e a „ 

— L ab —— + Ra— + -pr = *•• ( 48 - 5 ) 

, c 2 dt 2 dt C a 

o 

For periodic (monochromatic) currents, these equations give the algebraic 
equations 

S^a&/6= *a, ( 48 - 6 ) 

o 

the matrix elements Z a b being given by the formulae 



7 'ab = &ab{Ra 



+ _L r )-^„,. (48.7) 

oiCJ c 2 

The eigenfrequencies of the current system are given by the condition of 
compatibility of equations (48.6) when ff a = 0, i.e. by the condition for the 
determinant \Z a b\ to vanish: 

\Z ab \ = 0. (48.8) 

If the resistances R are not zero, all the "frequencies" have a non-zero ima- 
ginary part, and the electric oscillations are therefore damped. 

It should be noticed that equations (48.5) are formally identical with the 
mechanical equations of motion of a system with several degrees of freedom 
which executes small damped oscillations. The generalised co-ordinates 
are represented by the charges e a , and the generalised velocities by the 
currents J a = de a jdt. The "Lagrangian" of the system is 



^ = Zw^^-Z^+Z 6 " *» < 48 - 9) 

a, b a a 



204 Quasi-static Electromagnetic Field §48 

The kinetic and potential energies of the mechanical system are represented 
by the magnetic and electric energies of the current system, and the quanti- 
ties $ a correspond to the externally applied forces which cause the forced 
oscillations of the system. The quantities R a appear in the dissipative 
function 

R = X \R a e a \ (48.10) 



Equations (48.5) are the analogues of Lagrange's equations 

d Z& dtf BR 



dt de' a de a de a 



(48.11) 



PROBLEMS 
Problem 1. Determine the eigenfrequencies of electric oscillations in two inductively 
coupled circuits containing self-inductances Li and Lz and capacities Ci and Cz, neglecting 
the resistances R\ and Rz. 

Solution. The required frequencies are determined from the condition 

\Z a b\ = Z11Z22 — Z12 2 = 0, 

where 

/ CO 1 \ / o> 1 \ IOJ 

Zn = — i[ — Li — , Z22 = — i\— L2 — I, Zvi.= -Z.12. 

\C 2 toCl/ \C 2 C0C2/ c 2 

Calculation gives 

L1C1+L2C2+ V[(^iCi-L 2 C 2 ) 2 +4CiC2Li2 2 ] 

C01.2 2 = C 2 



2CiC2(LiL -L12 2 ) 

Both frequencies are purely real, owing to the fact that Ri and R2 have been neglected. As 
L12 -»■ 0, 001 and o>2 tend to cl^(Lid) and c/ VCL2C2). These are the frequencies for the two 
circuits separately. 

Problem 2. The same as Problem 1, but for a circuit consisting of a resistance R, a 
capacity C and an inductance L connected in parallel. 

Solution. The impedances of the three branches are Z\ = R, Z2 = ijoiC, 
Zs = —ioiLjc 2 , and the currents in them are such that _/i +.72+73 = 0, Z1J1 = Z2J2 = .Z3./3. 
Hence we have I/Z1+I/Z2+I/Z3 = 0, whence 

™ = ~ 2RC ± V LLC ~ 4R 2 C 2 Y 

Problem 3. Discuss the propagation of electric oscillations in a circuit consisting of an 
infinite succession of identical meshes containing impedances 

as shown in Fig. 25. Find the range of frequencies which can be propagated in the circuit 
without damping. \ 



f The condition for the quasi-steady theory to be applicable to such a periodic circuit is 
that the dimension of one mesh should be small compared with the "wavelength" cjoi. 



§49 



Motion of a conductor in a magnetic field 



205 



Solution. The current in mesh a is denoted by ta, as shown in Fig. 25. Kirchhoff's 
second law gives for this mesh Zii a +Z2(2i a — ta_i — t'a+i) = 0. This is a linear difference 
equation in the integral variable a, with constant coefficients. We seek the solution in the 
form ta = constant X q*, obtaining for the parameter q the equation 



g2 _( 2+ |) g+1 = . 



(1) 



Let —4 ^ Zi/Zt < 0, corresponding to values of a> 2 lying between c 2 /LiCi and 
c 2 (4/C2+l/Ci)/(4Z,2+I,i). Then equation (1) has two complex conjugate roots with moduli 
q\ = 1. This means that the current does not decrease from one mesh to the next, i.e. the 
electric oscillations are propagated in the circuit without being damped. Putting q = e m , 
where I is the length of one mesh and k is the "wave number" of the oscillations propagated 
in the circuit, we can calculate the velocity of propagation u from the general result 
u = douldk. 




Fig. 25 

If, however, (o is outside the range mentioned, equation (1) has two real roots qi and 52, 
say; since qiqz = 1, one root (qi, say) is less than 1 in absolute magnitude, while q% is greater. 
It is easy to see that the propagation of undamped oscillations in the circuit is then impos- 
sible. To elucidate the reason for this, let us consider a circuit of large but finite length. 
An initial oscillatory impulse is given to one end of the circuit, the other end being closed in 
some manner. This closure corresponds mathematically to a certain boundary condition, 
by means of which we can determine the ratio of the coefficients c\ and c% in the general 
solution ci<7i - ( a fc" a) +C2<72 -(a k _a) , where a* is the "co-ordinate" of the end of the circuit. This 
ratio is of the order of unity. As a*— a increases, the second term in the solution rapidly 
becomes very small compared with the first term, because \qz\ > 1. Thus the solution is 
f a = ciqi~^k~ a '> everywhere except for a small part near the end of the circuit, and |t a | de- 
creases towards the end of the circuit. 

It should be emphasised that this damping does not involve dissipative absorption, because 
there is no resistance in the circuit; it can be imagined as being the result of reflection of 
the oscillatory impulse from each successive mesh of the circuit. 

§49. Motion of a conductor in a magnetic field 

Hitherto we have tacitly assumed that a conductor in an electromagnetic 
field is at rest in the frame of reference K in which E, H, etc. are defined. 
In particular, the relation j = aE between the current and the field is generally 
valid only for conductors at rest. 

To determine the corresponding relation in a moving conductor, we 
change from the frame K to another frame K! in which the conductor, or 
some part of it, is at rest at the instant considered. In this frame we have 
j = ctE', where E' is the electric field in K'. The well-known formula for 
the transformation of fields! gives E' in terms of the fields in K: 

E' = E + vxB/c, (49.1) 

f See The Classical Theory of Fields, §3-10, Addison-Wesley Press, Cambridge (Mass.) 
1951 ; Pergamon Press, London, 1959. The microscopic values of the electric and magnetic 
fields are replaced by their averaged values e = E, fa. = B. 



206 Quasi-static Electromagnetic Field §49 

where v is the velocity of K' relative to K, i.e. in this case the velocity of the 
conductor, which we of course suppose small compared with the velocity of 
light. Thus we find 

j = a(E + vxB/c). (49.2) 

This gives the relation between the current and the field in moving con- 
ductors. The following remark should be made concerning its derivation. 
In going from one frame of reference to the other we have transformed the 
field but left the current j unaltered. The correct transformation of the 
current density gives only terms of a higher order of smallness if v <^ c. 
In formula (49.2) the second term, which appears as a result of the field 
transformation, is in general not small compared with the first term, despite 
the factor vjc. For example, if the electric field is due to electromagnetic induc- 
tion from a variable magnetic field, its order of magnitude contains a factor 
\\c as compared with the magnetic field. 

The energy dissipation in a conductor when a given current flows in it 
cannot, of course, depend on the motion of the conductor. The rate of 
evolution of Joule heat per unit volume in a moving conductor is therefore 
given in terms of the current density by the same expression pja as for a 
conductor at rest. The expression j«E, however, is replaced byt / 2 /cr 
= j.(E + vxB/<r). 

Thus, in a moving conductor, the sum E + vxB/c acts as an "effective" 
electric field producing the conduction current. Hence the e.m.f. acting 
in a closed linear circuit C is given by the integral 

£ = £(E + vxB/c).dl. (49.3) 

c 

This expression can be transformed as follows. According to Maxwell's 
equation, curl E = — (l/c)dB/cfr, and so 

<fE-dl = JcurlE-df = JVdf 

c s s 

or, denoting by <E> the magnetic flux through the surface S, which spans the 
circuit C, 

r l/8<!>\ 
cfE-dl = — 



t It is seen from this formula that the additional heat evolved in time St in a conductor 
moving in a magnetic field is 

8* J j-v X B dV/c = - J uj X B dV/c, 

where u = vSt is the displacement in time St. This expression is equal and opposite to the 
work done on the conductor in time St by the volume forces f = j X B/c. This explains the 
apparent contradiction mentioned in §34. 



§49 Motion of a conductor in a magnetic field 207 

The time derivative with the suffix v = denotes the rate of change of the 
magnetic flux due to the time variation of the magnetic field, the position of 
the contour C remaining unchanged. 

In the second term in (49.3), we put v = du/di, where du is an infinitesimal 
displacement of the circuit element d/. Then 

|vxB-dl = cfduxB-dl/d* = -j>B.df/df, 
c s 

where df = duxdl is an element of area on the "side" surface 5 between 
two infinitely close positions C and C" of the current circuit, which it occupies 
at times t and t + dt (Fig. 26). Since the total magnetic flux through any 
closed surface is zero, the flux through s must evidently equal the difference 
of the fluxes through surfaces spanning C and C". Thus 

<|>vXB«dl = -(£<!>/ d*)B=constant, 
c 

where the time derivative denotes the rate of change oi the magnetic flux due 
to the motion of the conductor in a constant field. 




Fig. 26 



Adding the two terms, we have finally 

& = -(l/c)d®/dt, (49.4) 

where the time derivative now denotes the total rate of change of the magnetic 
flux through the moving circuit. Thus the expression (49.4), which is 
Faraday's law, is valid whatever the reason for the change in the magnetic 
flux, whether variation of the field itself (already discussed in §47, formual 
(47.13)) or motion of the conductor. 

In a constant magnetic field, the change in the flux is due entirely to the 
motion of the circuit. If the circuit moves in such a way that every point 
of it moves along a line of force, then the flux through the circuit does not 
vary. This is an obvious result of the fact that the magnetic flux through 
any closed surface is zero, and the flux through the "side" surface described 
by the moving circuit is in this case identically zero (since B n = on this 
surface). Thus we can say that, to induce an e.m.f., the conductor must 
certainly move so as to cross lines of magnetic force. 



208 Quasi-static Electromagnetic Field §49 

The electromagnetic field in a moving conductor is given by the equations 
curlE = -(l/c)dBldt t 
curlH = 4rrj/c = (4w<j/c)(E + vxB/0, 
div B = 0. 

Expressing E in terms of H by means of the second equation and substituting 
in the first, we obtain 

dB c* /curlH\ 

curl (v x B) = curl . (49.5) 

8t 4tt \ a J 

In a homogeneous conductor with constant conductivity a and constant 
magnetic permeability ft, we have 

dU c* 

curl(vxH) = AH, divH = 0. (49.6) 

dt ^nafx 

These equations generalise those obtained in §45. 

It should be pointed out, however, that, if there is only one conductor 
moving as a whole (without change of shape) in an external magnetic field, 
then the solution of the problem is considerably simplified if we use a 
system of co-ordinates fixed in the conductor. In this system the conductor 
is at rest, and the external field varies with time in a given manner, so that we 
return to the eddy-current problems discussed in §45. This possibility does 
not depend on Galileo's (or on Einstein's) relativity principle, since the new 
system of co-ordinates is in general not inertial. The equivalence of the 
problems results from the above-mentioned fact that the electromagnetic 
induction is independent of the cause of the change in the magnetic flux. 
This equivalence can also be demonstrated mathematically. To do so, we 
expand the expression curl (vxB), using the facts that divB = and (for 
motion of the body as a whole) div v =0 (i.e. the body is "incompressible"). 
Then the left-hand side of equation (49.5) becomes 

dB/d* + (v.grad)B-(B.grad)v. (49.7) 

This sum is just the time derivative of B with respect to axes fixed in a 
rotating body. For the sum of the first two terms is the "substantial" time 
derivative dB/d*, which gives the rate of change of B at a point moving with 
velocity v. The third term takes into account the change in the direction 
of B relative to the body; it is zero for pure translation (v = constant) and 
equals — Six B for rotation (v =fixr, where SI is the angular velocity). 

To conclude this section, let us consider the phenomenon of unipolar 
induction, which occurs when a magnetised conductor rotates. If a stationary 
wire is connected to the rotating magnet by means of two sliding contacts A 
and B (Fig. 27) then a current flows in the wire. It is not difficult to calculate 
the e.m.f. which produces the current; the simplest procedure is to use a 
system of co-ordinates rotating with the magnet. If SI is the angular velocity 



§49 



Motion of a conductor in a magnetic field 



209 



of rotation of the magnet, then in the new system the wire rotates with angular 
velocity -SI, while the magnet is at rest. Thus we have a conductor moving 
in a given constant magnetic field B due to a fixed magnet. We neglect the 
distortion of the field by the wire itself. According to formula (49.3), the 
e.m.f. between the ends of the wire is 



= - fvxBdl = — fBx(rxSi)«dl, 

ACB ACB 



(49.8) 



taken along the wire. This is the required solution. 




Fig. 27 



PROBLEMS 

Problem 1. Determine the magnetic moment of a conducting sphere (with ja = 1) rotating 
uniformly in a uniform constant magnetic field, and the torque on the sphere. 

Solution. Let the external field have components <r>x, 0, § z in a fixed system of co- 
ordinates with the #-axis in the direction of the angular velocity vector SI. In a co-ordinate 
system £, -q, z which rotates with the sphere, the field components are §c = |)x cos ilt, 
§ = -§ x sin at, $>z, or, in complex form, £? = $> x e~ int , £„ = -*$*« im , §>*■ 

Thus variable fields of frequency O act along the £ and r, axes, and the magnetic moment 
which they induce is 

Jl % = V re (<x£f;) = V&d*' cos Qf +<x" sin Clt), 
uTt, = V re («&,) = V§> x (-*' sin fi«+a" cos Clt), 
where Vol is the complex magnetic polarisability of the sphere, which has been determined 
in §45 Problem 1. Along the z-sxis, on the other hand, the magnetic field is constant, and 
therefore causes no magnetic moment (if /t = 1). The components of the magnetic moment 
in the fixed system of co-ordinates are Jl x = V«.'$ x , Jt v = VaTfe, -**= 0. Thus in this 
problem a' and a" give the components of the magnetic moment of the sphere respectively 
parallel and perpendicular to the plane of the vectors Si, and |). 

The torque on the sphere is K = M X |>. Its components relative to the fixed axes are 

K x = Va"& x £t, K y = -Fa'§x$ 2 , K. = -Va"$f. 

Problem 2. Determine the e.m.f. due to unipolar induction between the pole and the 
equator (Fig. 27) of a uniformly magnetised sphere rotating uniformly about the direction 
of magnetisation. 

Solution. When the sphere rotates about its direction of magnetisation, it generates a 
constant field, and, since no currents flow within the sphere, we find from (49.5) that 
curl (vxB) = 0. Hence the integral of vxB along the closed contour OACBO (Fig. 27) 



210 Quasi-static Electromagnetic Field §50 

is zero, and so the integration along ACB in formula (49.8) may be replaced by one along 
the path AOB, which lies inside the sphere. The integral along the segment AO of the axis 
of rotation is zero, since £2 and r are parallel ; the integral along the radius OB gives, since 
B and SI are parallel within the sphere, 



a 



Bo&rdr = B Oa 2 /2c, 



where a is the radius of the sphere and Bo the magnetic induction in it. In a uniformly 
magnetised sphere (in the absence of an external field) the induction is related to the mag- 
netisation by Bo+2H = (cf. (8.1)) and Bo—H = AitM, whence Bo = 8ttM/3. In terms 
of the total magnetic moment of the sphere we have finally & = CL^Ica. 

Problem 3. Determine the total charge which flows along a closed linear circuit when the 
magnetic flux through the circuit changes for any reason from one constant value ($i) to 
another (^2). 

Solution. The required total charge is the integral 

o 

Jdt, 



I 



where J{t) is the induction current in the circuit. Mathematically, this integral is the Fourier 
component of the function J(t) that has the frequency w = 0. It is therefore related to the 
corresponding component of the e.m.f. by 



00 00 

[Sdt = Z(0) jjdt; 



see (47.3). Putting Z(0) = R, where R is the resistance of the circuit to a constant current 
and & = — (1/c) d<D/d£, we have 

00 



§50. Excitation of currents by acceleration 

In discussing the motion of a conductor in §49 we have neglected possible 
effects of the acceleration, if any. The accelerated motion of a metal, how- 
ever, is equivalent to the action of additional inertia forces on the conduction 
electrons. If v is the acceleration of the conductor and m the mass of the 
electron, then the force on an electron is -mv. It affects the electron in the 
same way as an electric field mv/e, where — e is the charge on the electron. 
Thus the "effective" electric field on the conduction electrons in an acce- 
lerated metal is 

E' = E + mv/e. (50.1) 

The current density is accordingly 

j = a E f = <r{E + mv/e). (50.2) 



§50 Excitation of currents by acceleration 211 

Expressing E in terms of E' from (50.1), we substitute in the equation curl E 
= —(\jc)dHldt (as usual, we put ju = 1). Then 

1 8U m 

curlE' = + — curly. (50.3) 

c 8t e 

We write v as a sum v = u + SI x r, where u is the translational velocity and 
SI the angular velocity of rotation of the body. Differentiating with respect 
to time, we find the acceleration to be v =u + fllxv+ Slxr = u + flxu+ 
+ fix(flxr) + Axr. The first two terms are independent of r, and there- 
fore give zero on differentiation with respect to the co-ordinates. The third 
term can be written as SI x (Six r) = — | grad (Slxr) 2 , and its curl is there- 
fore zero. Finally, curl (Slxr) — 2€l. Thus, substituting for v in equation 
(50.3), we have curlE' = -(l/c)aH/d* + 2m6/e or 

1 dW 

curlE' = , (50.4) 

c dt 

where 

H' = rl-lmcSlje. (50.5) 

Since SI is independent of the co-ordinates, the equation curl H = 477-j/c 
is still valid if H is replaced by H' : 

curlH' =.4ir<rE'lc. (50.6) 

Eliminating E' from equations (50.4) and (50.6), we obtain for H' the equation 

AH' = (^a/c^dH'/dt, (50.7) 

which is the same as the equation for H in a conductor at rest. 

Outside the body, the field satisfies the equation AH = (the wavelength 
being supposed large compared with the dimension of the body), and H' 
satisfies the same equation. 

Finally, on the surface of the conductor H', like H, is continuous. The 
only difference is in the condition at infinity, where H tends to zero but H' 
tends to the limit —ImcSlje. 

Thus the problem of determining the variable magnetic field H near a 
non-uniformly rotating body is equivalent to that of determining the field 
H' near a body at rest in a uniform external magnetic field 

£ = -2mcSl/e. (50.8) 

The required field H e outside the conductor is obtained by subtracting |) 
from the solution H' of this latter problem, t 



t Misunderstanding may arise from the appearance of the angular velocity Si itself, and 
not its time derivative, in formula (50.8). We may therefore emphasise that the above dis- 
cussion, and therefore the significance here attached to the quantity (50.8), pertain only to 
non-uniform rotation. In particular, the field (50.8) is unrelated to the gyromagnetic effect 
(which appears even when the rotation is uniform, and is a small quantity here neglected). 



212 Quasi-static Electromagnetic Field §50 

The magnetic field thus produced, like any variable field, induces electric 
currents in the conductor itself. In a simply-connected body, these currents 
appear in the form of a magnetic moment. In a non-uniformly rotating 
ring, the effect appears as an e.m.f. — the Stewart-Tolman effect. 

PROBLEMS 

Problem 1. Determine the magnetic moment of a non-uniformly rotating sphere of 
radius a. The rate of rotation is assumed so small that the penetration depth 8 ^> a. 

Solution. The magnetic moment of the sphere in the field £>(t) (50.8) is M = Va.%, 
where a is an operator whose action on the Fourier components of the function $)(t) is given 
by the formulae of §45, Problem 1. For the components with frequencies a> such that 
S^>a we have M — y<x(a>).$ ~ — Amna 5 oioo£ll\5ce. This formula, when written 
M = (4irtna 5 o/15ce) dSi[dt, does not contain to explicitly, and is therefore valid also for 
the functions Jl and Si themselves, as well as their individual Fourier components (on the 
assumption that the Fourier expansion contains chiefly terms whose frequencies satisfy the 
above condition). 

Problem 2. Determine the total charge which flows along a thin circular ring when it 
ceases a uniform rotation about an axis perpendicular to its plane. 

Solution. In the formula obtained in §49, Problem 3, <£> must be taken as the flux of 
t he field $) (50.8). The total charge transferred when the angular velocity changes from Si 



r ,. , 2mc ^ ,„ maV n 

J eKc 2ire 

—00 

where b is the radius of the ring and V its volume. 

Problem 3. Determine the current in a superconducting circular ring which ceases to 
rotate uniformly. 

Solution. From the condition that the total magnetic flux through the ring is constant 
(see (42.5)), we have 

2mc 2 mc 2 b£l 

J = Qwfc 2 = 



eL 2e[log(8bla)-2] 

See the third footnote to §42 concerning the value of L. 



CHAPTER VIII 

MAGNETIC FLUID DYNAMICS 

§51. The equations of motion for a fluid in a magnetic field 

If a conducting fluid moves in a magnetic field, electric fields are induced in 
it and electric currents flow. The magnetic field exerts forces on these currents 
which may considerably modify the flow. Conversely, the currents them- 
selves modify the magnetic field. Thus we have a complex interaction 
between the magnetic and the fluid-dynamic phenomena, and the flow must 
be examined by combining the field equations with those of fluid dynamics. 
We shall use equations (49.6) as the field equations in a moving conduct- 
ing medium. The magnetic permeability of the media considered in magnetic 
fluid dynamics differs only slightly from unity, and the difference is unimpor- 
tant as regards the phenomena under discussion. We shall therefore take 
fi = 1 throughout the present chapter. The equations are then 

div H = 0, (51.1) 

m/dt = curl (v x H) + (c 2 /47r<r) AH. (51.2) 

By using these equations we assume that certain conditions are fulfilled. 
The period of variation of the field must be large compared with the mean 
free time of the conduction electrons. Then the relation between the current 
and the electric field involves the same conductivity a as for a constant 
current (see §45). t Here we assume that a is constant in the medium, and 
therefore, in particular, that the conductivity is independent of the magnetic 
field. For this to be so, the mean free path of the electrons must be small in 
comparison with the radius of curvature of their orbits in the magnetic field. 
That is, the mean free time must be small compared with the reciprocal of 
the electron Larmor frequency eHjmc. This condition may not hold if the 
medium is rarefied and the magnetic field is strong. 

The equations of fluid dynamics are the equation of continuity 

d P ldt+ div (px) = 0, (51.3) 



t In the second footnote to §45 the further condition ola>p> 1 was mentioned as being 
necessary for poor conductors. In good conductors this condition is always satisfied if the 
other conditions are. In the present case the frequency is represented by VjL, where L and 
V axe characteristic parameters of length and velocity which determine the properties of 
the flow. Thus we assume the condition oLjV ^>1 to hold. 

213 



214 Magnetic Fluid Dynamics §51 

where p is the fluid density, and the Navier— Stokes equation 

dv 1 77 1 f 

— + (v^grad)v = grad/> + -Av + -(£ + \tj) grad diw + -, 

8t p p P p 

where 77 and £ are the two coefficients of viscosity for the fluid, and f is the 
volume density of external (in this case, electromagnetic) forces. By formula 
(34.4) we have f = j x H/c = (curl H) x H/4tt. Thus the equation of motion 
of the fluid is 

— + (vgrad)v 

11 1? 1 

= grad/> H x curlH + - Av + -(£ + ^7) grad diw. (51.4) 

P 4tt P p p 

To these equations we must add the equation of state 

p=p( P ,T), (51.5) 

which relates the pressure, density and temperature of the fluid, and the 
equation of heat transfer. In ordinary fluid dynamics the latter ist 

ids \ dvf 

pT (— + vgrads = a'a-r— + div (k grad T). 

\ot / dxjc 

Here s is the entropy per unit mass of the fluid, and the left-hand side of the 
equation is the quantity of heat generated per unit time and volume in a 
moving fluid particle. The right-hand side is the energy dissipated per unit 
time and volume. The first term is due to viscosity ; a'ik is the viscous stress 
tensor : 



/8v t dv k 2 8vi\ 

= V — + -o ik —I + ibijc- 

\oxk oxi 5 oxi I 



The second term gives the dissipation due to thermal conduction, #c being 
the thermal conductivity. In a conducting fluid, a term giving the Joule heat 
must be added. The rate of evolution of this heat per unit volume is j 2 ja 
= (£ 2 /167r 2 <r)(curl H) 2 . The equation of heat transfer in magnetic fluid 
dynamics is therefore 

(ds \ dvi c 2 

— + vgrads = a' tt — + div(*grad T) + — — (curlH) 2 . (51.6) 
Ot / 0Xk iOTT^a 

Equations (51.1)— (51.6) form a complete system of equations of magnetic 
fluid dynamics, on the assumptions stated at the beginning of this section. 



f See Fluid Mechanics, §49, Pergamon Press, London, 1959. 



§5 1 The equations of motion for a fluid in a magnetic field 215 

Equations (51.4) and (51.6) can also be written in forms which express the 
laws of conservation of momentum and energy respectively. The Navier- 
Stokes equation of ordinary fluid dynamics can be written (using the equation 
of continuity) in the formt 

d{pvi)ldt = - dlliicldxic, (51.7) 

where Hoc is the momentum flux density tensor: U ik = pvi v k+ phk— a \ k . 
Equation (51.4) can be brought to the same form, but Uuc now contains an 
additional term. We have Hx curl H = igrad# 2 -(H-grad)H. Thus 

n ik = pvm + p8 ik - a' ik - (HiH k - W 2 8ik)l^r. (51.8) 

The added term is the Maxwell stress tensor of the magnetic field, as it 
should be. 

The equation of heat transfer can be transformed (using the other equa- 
tions of fluid dynamics) into an equation of conservation of energy. In 
ordinary fluid dynamics we have d{\pv 2 + pe)jdt = — divq, where q is the 
energy flux density: 

q = pv(|© 2 + w) - v«o' - Kgrad T; 

€ and w = €+pjp are respectively the internal energy and heat function per 
unit mass of fluid. When a magnetic field is present in the conducting 
medium, the energy density includes also the magnetic energy # 2 /8tt, and 
the energy flux density includes also the Poynting vector cE x H/4tt. Express- 
ing E in the latter in terms of H we obtain 

q = p V (|^ 2 + w) + — H X (v XH) - 
4n- 



c 



H X curlH - v-o' - /cgrad T, (51.9) 

167T 2 <7 



and the equation of conservation of energy is 
d / H 2 \ 






(51.10) 



It is not difficult to verify by direct calculation that equations (51.6) and 
(51.10) are equivalent. 

The equations are somewhat simplified if the moving fluid can be sup- 
posed incompressible. The equation of continuity (51.3) then reduces to 
div v = 0, while in equation (51.4) the last term is zero. For reference, we 
shall write out here the complete system of equations for an incompressible 
fluid (in equations (51.2) and (51.4) we have transformed the terms 



t See Fluid Mechanics, §15. 
IS 



216 Magnetic Fluid Dynamics §51 

curl(vxH) and HxcurlH respectively by the appropriate formulae of 
vector analysis) : 

divH = 0, divv = 0, (51.11) 

mfit + (vgrad)H = (Hgrad) v + (c 2 /4rra) AH, (51.12) 

Sv 

— + (v-grad)v 

1 / H 2 \ 1 

= --grad U + _-)+__ (H-grad)H + ^Av, (51.13) 

p \ 07T / *riTp 

where v = rj/p is the kinematic viscosity. Equation (51.6) is not needed in 
solving the problem of incompressible flow unless we are interested in the 
temperature distribution and its effect on the flow. 

Let us return to the general equation (51.2). In the limiting case of very 
high conductivity it becomes 

dH/dt = curl(v x H), (51.14) 

an equation which has a very important physical interpretation. We expand 
the right-hand side, using the fact that div H = : 

dH/dt = (Hgrad) v - (vgrad)H - H divv. 
Substituting from the equation of continuity (51.3) 

1 dp v«grad/o 

divv = , 

p 8t p 

we obtain after a simple rearrangement of terms 



id \H /H \ 

W + v,grad J7 = l7 ,grad ) v ' 



The left-hand side is the "substantial" derivative, which gives the rate of 
change of H/p for a given fluid particle as it moves about. Denoting this 
derivative by d/d£, we have 

d /H \ 

-(H/ P )= (—gradj*;. (51.15) 

Let us now consider some "fluid line", i.e. a line which moves with the fluid 
particles composing it. Let SI be an element of length of this line ; we shall 
determine how SI varies with time. If v is the fluid velocity at one end of the 
element SI, then the fluid velocity at the other end is v + (Sl»grad)v. During 
a time interval dt> the length of SI therefore changes by d£(Sl»grad)v, i.e. 
d(Sl)/d* = (Sl»grad)v. We see that the rates of change of the vectors SI 
and H/p are given by identical formulae. Hence it follows that, if these 
vectors are initially in the same direction, they will remain parallel, and their 
lengths will remain in the same ratio. In other words, if two infinitely close 



§5 1 The equations of motion for a fluid in a magnetic field 217 

fluid particles are on the same line of force at any time, then they will always 
be on that line of force, and the value of H\p will be proportional to the dis- 
tance between the particles. 

Passing now from particles at an infinitesimal distance apart to those at 
any distance apart, we conclude that every line of force moves with the 
fluid particles which lie on it. We can picture this by saying that (in the 
limit a ->• oo which we are considering) the lines of magnetic force are 
"frozen" in the fluid and move with it. The quantity H\p varies at every 
point proportionally to the extension of the corresponding "fluid line". If 
the fluid may be supposed incompressible p = constant, and the field H 
varies as the extension of the lines of force. 

These results can be viewed in another way: as any closed fluid contour 
moves about in the course of time, it cuts no line of force, i.e. the "number" 
of lines of force passing through the contour remains unchanged. This 
means (cf. §49) that the flux of the magnetic field through any surface 
spanning the fluid contour does not vary with time. 

To the question of when in fact dissipative processes in the fluid may be 
neglected there is no general answer, since the necessary conditions depend 
greatly on the nature of the motion and are, for instance, completely different 
for steady and for non-steady flows. We shall not investigate the general 
problem here. 

PROBLEM 
Determine the velocity distribution in an incompressible viscous conducting fluid in 
steady flow between two parallel solid planes, when a uniform external magnetic field Ho 
is applied perpendicular to the planes (J. Hartmann, 1937). 

Solution. It is natural to assume that the fluid velocity is everywhere in the same direc- 
tion, which we take as that of the *-axis, and depends only on the co-ordinate z (whose 
direction is perpendicular to the planes). The same is true of the longitudinal field H x 
resulting from the motion. The pressure p, however, depends on * also, because there must 
be a constant pressure gradient in the direction of motion in order to maintain a steady flow. 
The equation div v = is satisfied identically, while from div H = it follows that 
H z = constant = Ho. The ar-component of equation (51.13) gives 

07T 

where P(x) is a function of x only. The pressure gradient in the ^-direction is 

— dp/dx = —dP/dx = constant. 
The ^-components of equations (51.12) and (51.13) give 

dz 4wff dz* 

d 2 » Ho dff* dP 

n 1 = constant = — — . \3) 

dz 2 \it dz dx 

The boundary conditions for viscous flow are v = for z = ±a, where 2a is the distance 
between the solid planes, and the plane z = lies half-way between them. The magnetic 
field must satisfy the conditions H x = for z = ±a, since the magnetic field outside the 
fluid is just Ho, and the tangential magnetic field component is continuous at the boundary. 



218 Magnetic Fluid Dynamics §52 

The solution of equations (2) and (3) which satisfies these conditions is 

cosh (a/ A) —cosh (z/A) 



v = vo- 



cosh(a/A) — 1 



tj 47r // ,(*/a)sinh(a/A)-sinh(s/A) 

H x = -VQr- V(°v) ... , (4) 

c cosh (a/ A) — 1 

where A = (clHo)V(.vl a )- The constant vo is the fluid velocity on the median plane z = 0. 
Its relation to the pressure gradient is given by substituting (4) in (3). The fluid velocity 
averaged over the cross-section is 



If, dPaA/a A\ 

v = — - \ v dz — (com— 1. 

2a J d# 17 \ A a / 



The effect of the magnetic field on the flow is characterised by the ratio 

a/A = (aHolc)V(°lv)' 
For a/A <^ 1 we have 

/ z\ dP a 2 

v = vo\ 1 r), v — , 

\ a 2 / dx 3i7 

in accordance with the results of ordinary fluid dynamics. If aj A ^> 1, then 

dP ac 



w = 7 ;( ) (l_g-(a-|2l)/A) > 



dx HoV( or iy 



When the magnetic field increases, the velocity profile is flattened over the major part of the 
cross-section, and the mean velocity is reduced (for a given pressure gradient). 



§52. Hydromagnetic waves 

Let us consider the propagation of small disturbances in a homogeneous 
conducting medium in a uniform constant magnetic field Ho. We shall assume 
that the viscosity, thermal conductivity and electric resistance (l/o-) of the 
medium are so small that their effects, due to the dissipation of energy, on 
the propagation of perturbations may be neglected in a first approximation.! 
Then the perturbations will be propagated as undamped waves.t 

Omitting all dissipative terms, we can rewrite the fundamental equations 
(51.1)-(51.4) as 

divH = 0, (52.1) 

dH/dt = curl(vxH), (52.2) 

dpldt + 6iv(pv) = 0, (52.3) 

dv grad p 1 

— + (v-grad) v = - + (curlH) X H. (52.4) 

Bt p 47T/3 



f It should be noted that, by putting l/o = 0, we extend the range of frequencies in 
which the equations are applicable: the conditions that a should exhibit no dispersion and 
should be independent of the magnetic field are now irrelevant. 

X The condition for this approximation to be valid is that the wave damping coefficient 
(calculated in the Problem at the end of this section) should be small. 



§52 Hydromagnetic waves 219 

Equation (51.6) reduces to the equation of conservation of entropy (the 
condition for adiabatic flow). If the unperturbed medium is homogeneous, 
then this condition means that s = constant in the perturbed medium also, 
i.e. the flow is isentropic. 
We write 

H = H +h, p = po + p', p=po+p', (52.5) 

where the suffix denotes the constant equilibrium value, and h, p and p' 
are the small variations in the wave. The velocity v, which is zero in equili- 
brium, is a small quantity of the same order. Since the flow is isentropic, 
the changes in pressure and density are related by p' = {dpfdp) s p. But 
{dpjdp) s is the square of the velocity of sound, which we denote by u : 
p' = UQ 2 p ' m Neglecting terms of higher order than the first in equations 
(52.1)-(52.4), we obtain the linear equations 

divh = 0, dh/dt = curl(v X H), \ 

0p7& + /»divv = 0, (52.6) 

dv/dt = - (m 2 /p) grad p'-(Hx curl h)/4ir/>. ' 

Here and in what follows we omit, for brevity, the suffix zero to the equili- 
brium values. For a perturbation periodic in time, the first of these equations 
follows from the second and can be omitted. 

We shall seek solutions of these equations which are proportional to 
exp [t(k-r— co*)], i.e. which describe the propagation of plane waves with 
wave vector k and frequency oo. The system of equations (52.6) then gives 
the algebraic equations 

- o»h = k x (v x H), cop = pk.v, 
-a>\ + (u 2 lp)p'k = - H x (k x hW. 

The first of these shows that the vector h is perpendicular to the wave vector 
k, which we shall take to be along the #-axis, with the plane of k and H as 
the xy-plane. We also introduce the phase velocity of the wave, u = o)/k. 
Eliminating p from the third equation by means of the second equation, and 
rewriting the result in components, we have 

uh z = - v z H x , uv z = - H x h z /4irp, (52.7) 

Uky = V X Hy - VyH X , UVy = - H X hy\^p, 

I U ° 2 \ TJU,^ (52 ' 8) 

Vx\u 1 = Hyhyj^np. 

We have here separated the equations into two groups, the first involving 
only h z and v z and the second only hy, v x and v y . It therefore follows that 
perturbations of the two groups of variables are propagated independently. 
The density, and therefore the pressure, belong to the second group, since 

p = pvxfu. (52.9) 



220 Magnetic Fluid Dynamics §52 

The compatibility condition for the two equations (52.7) is 

u = m = H x /^/(4n P ). (52.10) 

In these waves the component h z of the magnetic field which is perpendicular 
to the directions of propagation and of the constant field H oscillates, and 
with it the velocity v z , which is related to h z by 

v z = -h z /V(^r P ). (52.11) 

The relation between to and k (the dispersion relation, as it is called) given 
by (52.10) involves the direction of the wave vector: 

a> = H.k/V(4ir/>). 

The physical velocity of propagation of the waves is called the group velocity 
and is given by the derivative dco/dk. In the present case we have dajjdk 
= Hl-\/(4iTp), which does not involve the direction of k. The direction of 
propagation of the wave, in the sense of the direction of its group velocity, is 
the direction of H. 

Let us now consider waves described by equations (52.8). Equating to 
zero the determinant of these equations, we obtain 



(m 2 — Mo 2 ) I u 2 — 



HJ\ = u 2 Hy 2 
\Ttpl Airp 

The roots of this quartic equation for u aret 

If // „ H 2 H xUo 



11 1/ H* H xUo \ 



47T/J \/( 7r / ) ) 

± y(" + S-^)i- (5zi2) 

Thus we obtain two more types of wave. In these waves the quantities 
h y , v x and v y (and the density />') oscillate. The vectors h and v are in the 
plane of H and k. 

In the limiting case where H 2 < 47rpuo 2 we have u% £ wo, and it follows 
from equations (52.8) that v y <^ v x . In other words, in the limit waves of this 
type become ordinary sound waves propagated with velocity uq. The weak 
transverse field in the wave is related to v x by h y ^ v x H y /uo. 

In the same limiting case, us is the same as u\ to a first approximation, and 
"Vx = 0, v y ^ —h y j^/(ATrp) as in a wave of the first type, but with the vectors 
v and h parallel to the plane of k and H instead of perpendicular to it. 

We see also that in an incompressible fluid (corresponding formally to the 
limit mo -> oo) only one type of wave occurs, with two independent direc- 
tions of polarisation. The dispersion relation for these waves is given by 



t The roots of the quartic equation x*+ px 2 + q = can be written 
x= ± UV(- p + 2Vq) ± V(~ p- 2Vq)}- 



§52 Hydromagnetic waves 221 

formula (52.10); the vectors v and h are perpendicular to k and are related by 

v = - h/V(4ir/>). < 52 - 13 ) 

Such waves were first discussed by H. Alfven (1942). t 

There is a simple interpretation of the result that, in a longitudinal mag- 
netic field, transverse displacements of the fluid are propagated in the form 
of waves. We have seen at the end of §51 that the lines of magnetic force 
behave like fluid lines when a -> oo. The transverse displacement of the 
fluid particles results in a curvature of these lines, and therefore in their 
stretching and, at some points, in their compression. The forces in a mag- 
netic field (expressed by the Maxwell stress tensor) are such as would 
occur if the lines of magnetic force tended to contract and also to repel one 
another.* Hence a curvature of the lines results in "quasi-elastic" forces 
which tend to straighten them, leading to further oscillations. 

It is interesting that, in an incompressible fluid, the plane hydromagnetic 
wave given by formulae (52.10) and (52.13) is in fact an exact solution of the 
equations, valid for any transverse field h (not necessarily small). (This state- 
ment does not apply, however, to a superposition of several plane waves 
propagated in different directions.) For, let us return to the exact equations 
(52.1)-(52.4). In an incompressible fluid, equation (52.3) becomes div v = 0. 
If we seek a solution in which all quantities depend on only one co-ordinate 
9c and the time t , we find from this equation that v x = constant, and by taking 
another system of co-ordinates moving uniformly in the ^-direction we can 
put v x = 0. From the equation div H = it follows that H x = constant. 
Denoting the transverse components of H by h, we obtain from equations 
(52.2) and (52.4) (with v x = 0) dh/dt = H x d\\dx, dvjdt = (Hxfapflhl dx, 
i.e. the exact equations necessarily reduce to the linear equations for a plane 
wave with the phase velocity (52.10), v and h being related by (52.13). 
The x-component of equation (52.4) is 

i* + _L h 3.o, 

p dx 47T/> dx 

whence 

p + A 2 /8tt = constant, (52.14) 

which gives the manner of variation of pressure in the wave. 

Let us return to formulae (52.8) and (52.12), and consider the opposite 
limiting case, where H 2 > 4tt/)Mo 2 - We then have, in the first approxima- 
tion, u 2 = HI^/(4ttp). Since this expression is independent of k, the group 
velocity is of magnitude ui and its direction is that of k. In this wave the 



t They are sometimes called hydromagnetic waves. In the general case, where the magnetic 
fields are not small, the waves cannot be divided into hydromagnetic waves and ordinary 
soundwaves. .... „ ,-. ox 

X For, let a line of force be along the .sr-axis. Then the longitudinal stress ll z « (51.8) 
contains a negative term — H 2 /8tt, and the transverse stresses n** andllyj, contain a positive 
term H 2 IStt. 



222 



Magnetic Fluid Dynamics 



§5 



vector v is perpendicular to H (Fig. 28), and its magnitude is given in terms 
of h = hy by v = h\^{^rrp). For us we have in this limiting case 
us = uoHx/H. The group velocity is daijdk = uqRJH. The vector v in this 
case is antiparallel to H, and its magnitude is given by v = hH 2 /4TTpuoH y . 
When the relation between H 2 and puo 2 is arbitrary, both «2 and us depend 
on the direction of the wave vector. When the angle between k and H 
increases, «2 increases monotonically and us decreases monotonically. It is 
easy to see that the inequalities 

us <: U\ < «2, "2 > uq, us < uo (52.15) 

always hold. If k is parallel to H (H y = 0, H x = H), u% and us are respec- 
tively equal to the greater and the smaller of uq and u\ — Hj-\/(AiTp). If k 
is perpendicular to H (H x = 0, H y = H), then 

H 2 



"2 



V \ ° 477/>/ 



(52.16) 



while u± and us are zero, i.e. only one type of wave exists. 




Fig. 28 



In this last case it is possible to find exact solutions of the equations of 
magnetic fluid dynamics for a plane wave, without assuming its amplitude 
to be small (S. A. Kaplan and K. P. Stanyukovich, 1954). When H x = 0, 
H y = H, equation (52.1) is satisfied identically, and equations (52.2)-(52.4) 
give 

dHJdt + d{v x H)jdx = 0, (52.17) 

dp/dt + d(v xP )ldx = 0, (52.18) 

1 8H 2 1 dp 

p 8x 



dv x dv x 

h V x 1 

8t 8x Snp 



8x 



(52.19) 



From the first two of these equations it is easily seen that the ratio Hjp = b 
satisfies the equation 8bj8t + v x 8bj8x = or db/dt =0, where the total 
derivative signifies the rate of change in a given fluid particle as it moves 



§52 Hydromagnetic waves 223 

about. Hence, if the fluid is homogeneous at some initial instant, so that b 
is constant, then at all subsequent instants we havet 

H/ P = b = constant. (52.20) 

Substituting in the third equation H - pb, we obtain 

d ^ + J^=- l -% + b lA (52.21) 

8t dx P dxV 87/ / 

Thus the magnetic field has been eliminated from the equations, and the 
problem reduces to the solution of equations (52.18) and (52.21). These 
equations differ from those for one-dimensional motion in ordinary fluid 
dynamics only by a change in the equation of state of the gas: the true 
pressure p = p(p) (for given entropy s) must be replaced by p*(p) = p(p) + 
+ 6 2 /o 2 / 87T - Tnis fact enables us to apply the results of ordinary fluid dyna- 
mics to this case of magnetic fluid flow. In particular, the formulae giving 
the exact solution for one-dimensional travelling waves (Riemann's solution)}: 
can be applied, the velocity of sound being represented by 



-mi-ji^i') 



-a 






in accordance with formula (52.16). 



PROBLEM 
Determine the absorption coefficient (assumed small) for a hydromagnetic wave in an 
incompressible fluid. 

Solution. The absorption coefficient for a wave is defined as y — Q/2q, where Q is 
the (time) average energy dissipated per unit time and volume, and q is the mean energy 
flux density in the wave. The amplitude of the wave decreases as e-v* during its propagation. 
Q is given by the right-hand side of equation (51.6); in an incompressible fluid we have for 
a wave propagated in the ^-direction (and so v x = 0) 

Q = i,(dvldx) z +(c 2 ll67T2o)(8hldx)K 

In the energy flux density (51.9), we can omit the small dissipative terms, leaving 
q x = — Hxh.v/477. Using formulae (52.10) and (52.13), we have the result 

j:Lft + ±\ 

r 2«i3\ P 4W 



t In accordance with the general results (§51) concerning the relation between Hip and 
"fluid" lines of force, if we take into account the invariability of the length of these lines 
with time in the present case. 

% See Fluid Mechanics, §94. 



224 Magnetic Fluid Dynamics §53 

§53. Tangential and rotational discontinuities 

The equations of motion for an "ideal" magnetic fluid (having zero vis- 
cosity, thermal conductivity and electric resistance) admit discontinuous 
flows as in ordinary fluid dynamics. To elucidate the conditions which must 
be satisfied on a surface of discontinuity, let us consider an element of the 
surface and use a system of co-ordinates in which it is at rest.t 

First of all, the mass flux must be continuous at a surface of discontinuity : 
the mass of fluid entering from one side must be equal to the mass leaving 
on the other side. Thus pivi n = p2V2n> where the suffixes 1 and 2 refer to 
the two sides of the discontinuity, and the suffix n denotes the component of 
a vector normal to the surface. In what follows we shall denote the difference 
between the values of any quantity on the two sides of the surface of dis- 
continuity by enclosing it in square brackets. Thus [/>#«] = 0. 

Next, the energy flux must be continuous. Using the expression (51.9) and 
omitting the dissipative terms, we obtain 

[qn] = M^ 2 + «0 + VnH^Tr - i7 n v.H/47r] = 0. 

The momentum flux must also be continuous. This condition means that 
[n^»*] = 0, where 11^ is the momentum flux density tensor, and n is a unit 
vector normal to the surface. Using (51.8), we therefore have 



[p + pv n * + {H t * - H n *)/Sir] = 0, 
[pv n v t - HnHtl^rr] = 0, 



where the suffix t denotes the component tangential to the surface. 

Finally, the normal component of the magnetic field and the tangential 
component of the electric field must be continuous. If the conductivity of 
the medium is infinite, the induced electric field is given by E = — vxH/c, 
and the condition [E*] = leads to [H n vt— H#>»] = 0. 

In what follows it is more convenient to use the specific volume of the 
fluid {V = Up) in place of its density. The mass flux density through the 
discontinuity is denoted by j = pv n = v n \V. 

Since j and H n are continuous, we can write the remaining boundary condi- 
tions in the following form: 

j[w + \]W* + W + VU^4tt] = tf„[H r v,]/477, (53.1) 

lp] +P[V] + [H t 2]/87r = 0, (53.2) 

;[v,] = H n \H t ]l47T, (53.3) 

H n [v t ] = j[VH t ]. (53.4) 

This is the fundamental system of equations of discontinuities in magnetic 
fluid dynamics. 



t This condition fixes only the velocity of the co-ordinate system in the direction norma 
to the surface. Any constant vector may be added to its tangential velocity. 



§53 Tangential and rotational discontinuities 225 

In ordinary fluid dynamics, discontinuities of two entirely different kinds 
are possible: shock waves and tangential discontinuities. t Mathematically, 
the two types occur because some of the boundary conditions can be written 
as the vanishing of a product of two factors, and the two different solutions 
are obtained by equating the factors to zero in turn. This feature is not 
present in magnetic fluid dynamics, and it might therefore be supposed 
that only one type of discontinuity occurs. In reality, however, it is found 
that essentially different types of discontinuity again occur (F. de Hoffmann 
and E. Teller, 1950). 

Let us consider, first of all, discontinuities for which j = 0. This means 
that vm = V2n = 0, i.e. the fluid moves parallel to the surface of discon- 
tinuity. If H n ¥= 0, we see from equations (53.1)-(53.4) that the velocity, 
pressure and magnetic field must be continuous. The density (and therefore 
the entropy, temperature, etc.) may have any discontinuity. Such a surface 
may be called a contact discontinuity, and is simply the boundary between 
two media at rest which have different densities and temperatures. 

If both; and H n are zero, then three of the four equations (53.1)-(53.4) 
are satisfied identically, and therefore this is clearly a special case. We thus 
find a type of discontinuity which may be called a tangential discontinuity, as 
in ordinary fluid dynamics. At such a discontinuity the velocity and the 
magnetic field are tangential and can have any discontinuity in both magni- 
tude and direction: 

j = 0, H n = 0, [v,] # 0, \H t ] # 0. (53.5) 

The density discontinuity also can take any value, but the pressure dis- 
continuity is related to that of H t by equation (53.2): 

[V]*0, [/> + ^]=°- ( 53 - 6 ) 

The discontinuities of the other thermodynamic quantities (entropy, tempera- 
ture, etc.) are related to those of V and p by the equation of state. 

Another type of discontinuity is one in which the fluid density is con- 
tinuous. Since the flux; = v n jV is continuous, the normal velocity compo- 
nent is therefore continuous also : 

y^o, [V] = 0, [v n ] = 0. (53.7) 

On the right-hand side of equation (53.4) we can take V outside the brackets 
and divide this equation by equation (53.3), obtaining 

j = H n [V(^rV) (53.8) 

and 

[v] = ViVI^Wtl (53.9) 



t See Fluid Mechanics, §81. 



226 Magnetic Fluid Dynamics §53 

In equation (53.1) we put w = e+pV; since V is continuous, this equation 
can be rewritten as 

H n being replaced in accordance with (53.8). The second term is zero by 
(53.2) and the third term is zero by (53.9), so that we find [e] = 0, i.e. the 
internal energy also is continuous. Every other thermodynamic quantity is 
determined if e and V are given. Hence all the other thermodynamic quanti- 
ties, including the pressure, are continuous. It then follows from (53.2) 
that H* 2 is continuous, i.e. 

[p] = 0, [Ht\ = 0. (53.10) 

The fact that Ht and H n are both continuous means that the magnitude of H 
itself and its angle to the surface are likewise continuous. 

Formulae (53.7)— (53.10) give all the properties of the discontinuities under 
consideration. The thermodynamic quantities are continuous, but the mag- 
netic field is turned through an angle about the normal, its magnitude 
remaining unchanged. The vector Hj, and therefore (by (53.9)) the tan- 
gential velocity component, are discontinuous, but the normal velocity 
component v n =jV is continuous, and its value is 

*» = HnV(VM = Hn/Vi^p)- ( 5 3.H) 

We shall call these rotational discontinuities. 

It is useful to note that, by a suitable choice of the co-ordinate system, 
we can always ensure that the fluid velocity is parallel to the field on each 
side of a rotational discontinuity. To achieve this, we use a co-ordinate system 
moving with velocity vu - Hta V( VjAir) = v^t - ti2tV( VJAtt) . (Compare the 
first footnote to this section.) In the new system the ratio of each component 
of v to the corresponding component of H on either side of the discontinuity 
is V(VM> i-c 

vi = HiV(J74tt), v 2 = H 2 V(JW. (53.12) 

Thus in this system of co-ordinates the velocity is rotated with the magnetic 
field, its magnitude and angle to the normal remaining unchanged. 

The velocity v n is also minus the velocity of propagation of the discon- 
tinuity relative to the fluid. This is equal to the phase velocity u\ of one of 
the three types of hydromagnetic wave (§52). The occurrence of this equality 
for all rotational discontinuities is to some extent accidental, but when the 
discontinuities of the various quantities are small the equality must hold. 
For such a discontinuity is a weak perturbation, in which the velocity v 
and the magnetic field H receive small increments perpendicular to the plane 
through H and the normal n. This perturbation is of the type whose phase 
velocity is u\. The physical velocity of propagation of the front of a small 
perturbation is the normal component of the group velocity, i.e. its compo- 



§53 Tangential and rotational discontinuities 227 

nent in the direction of the vector k. Since the relation between w and k 
is linear, we have k'dio/dk = co, and so this component is the same as the 
phase velocity oijk — u\. 

Although tangential and rotational discontinuities form two different 
types, there are also discontinuities having the properties of both. These 
discontinuities are such that v and H are tangential in direction and con- 
tinuous in magnitude. 

In ordinary fluid dynamics, tangential discontinuities are always unstable 
with respect to infinitesimal perturbations, and so are rapidly broadened into 
turbulent regions. A magnetic field, however, has a stabilising effect on the 
motion of a conducting fluid, and in this case tangential discontinuities may be 
stable. This result is a natural consequence of the fact that a perturbation 
involving fluid displacements transverse to the field leads to a stretching of 
the lines of magnetic force "frozen" in it, and therefore to the appearance of 
forces which tend to restore the unperturbed flow. An investigation of such 
discontinuities in an incompressible fluid by S. I. Syrovatskii (1953) has 
given the following two inequalities, which must both be satisfied if the 
discontinuity is stable: 

Hf + #2 2 > 2*p*2, 

(Hi x H 2 )2 > 2np{(Hi x v)2 + (H 2 x v)2}, 

where v = V2-V1 is the discontinuity of the velocity; see Problem l.t 

In reality, however, the existence of a small but finite viscosity and electric 
resistance in the fluid means that such tangential discontinuities cannot exist 
indefinitely, even if the conditions (53.13) are fulfilled. Although no turbu- 
lence occurs, the sharp discontinuity is replaced by a gradually widening 
transitional region, in which the velocity and the magnetic field change 
smoothly from one value to another. This is easily seen from the equations 
of motion (51.12) and (51.13) if the dissipative terms are retained. We take 
the ar-axis in the direction of the normal to the discontinuity. Assuming all 
quantities to depend on x (and possibly on the time), we can write the trans- 
verse components of these equations as 

8H i /to = (<*l4**)3*H t ld**, 

dvt/dt = vd 2 vtldx 2 , 

the fluid being supposed incompressible. If we assume steady flow, the 
left-hand sides of equations (53.14) are zero, and the only solution which 
remains finite as * -> ± 00 is H t = constant, v t = constant, which contra- 
dicts the assumption that these quantities undergo a change at the discon- 
tinuity. Thus a tangential discontinuity cannot have a constant width such 
as is found for (e.g.) a weak shock wave. Equations (53.14) are heat-conduction 



t If the densities of the incompressible fluids on the two sides of the discontinuity are 
different, then p in these inequalities must be replaced by 2pip2/(pi + P2). 



228 Magnetic Fluid Dynamics §53 

equations. As we know from the theory of thermal conduction, a discon- 
tinuity in a quantity satisfying such an equation is gradually smoothed out 
into a transitional region, whose width increases as the square root of the time. 
Since the coefficients in the two equations (53.14) are different, the widths 
8 V and ojy of the transitional regions for the velocity and the magnetic field 
are also different: 

*v ~ VW, *h = vW<0- (53.15) 

Rotational discontinuities in an incompressible fluid are stable with respect 
to infinitesimal perturbations, whatever the strength of the magnetic field 
(S. I. Syrovatskii, 1953). Like tangential discontinuities, however, they 
cannot have constant widths, but are gradually smoothed out by the viscosity 
and electric resistance of the fluid (see Problem 2). 

PROBLEMS 
Problem 1. Derive the condition for the stability of a tangential discontinuity in an 
incompressible perfectly conducting non-viscous fluid in a magnetic field (S. I. Syrovatskii). 

Solution, f We write v = vo+v', p = po+p', H = Ho+H', where vo, po and Ho are 
the constant (on each side of the discontinuity) unperturbed values, and v', p' and H' are 
small perturbations. Substituting in equations (51.11)— (51.13), we have for an ideal fluid 

div u' = 0, div v' = 0, (1) 

du'/dt = (u'grad)v'— (vgrad)u' (2) 

8v' 1 

f-(v«grad)v' = grad^'— u x curl u' 

dt p 

1 
= - - grad (p'+pw u') + (u • grad)u', (3) 

P 

where for brevity we have omitted the suffix and put u = H/y^wp). We take the diver- 
gence of equation (3) and use (1), obtaining 

Ate'+pu-u') = 0. (4) 

Let * = be the plane of the discontinuity, to which the vectors v and u are parallel. 
In each of the half-spaces x > and x < we seek all quantities v', u', p' in a form pro- 
portional to exp{i(k'r—<ot)+Kx}, where k is a vector in the yz-plsaie. From equation (4) 
we find that k 2 — k 2 = 0, so that we must put k — k for x < and * = — k for * > 0. 
We eliminate v' x from the ^-components of equations (2) and (3), obtaining^ 

p'+pu>u' = -«V^— {(«-k-v)*-(k.u)*}. (5) 

K'UK 

Let $ = £(y, z, t) be the displacement of the surface of discontinuity in the ^-direction 
due to the perturbation. The conditions (53.5) and (53.6) must hold on the displaced 
surface : 

[p+ P (u+u') 2 ] £ [p'+pu-ul = 0, \ 

uni+u'm £ u'xi— ui«grad £ = 0, > (6) 

Mn2+w'n2 = u' X 2— U2«grad £ = 0; I 



t Cf. Fluid Mechanics, §30. 

X The case where the expression in braces vanishes is of no interest, since a> is then real, 
whereas instability can occur only for complex to. 



§54 Shock waves 229 

the condition of zero mass flux through the surface is satisfied automatically. Putting 
£ = constant Xexp{t(k»r—o>f)} and eliminating £, u x \ and u x z from the three equations (6), 
we obtain an equation giving the possible values of a»: 

(a,-k-vi)2+(<o-k-v a ) 2 = (k-ui) 2 +(k-u a ) 2 . 

This quadratic equation has no complex root if 

2(k.ui) 2 +2(k-ua) 2 -{k-(v 2 -vi)} 2 > 0, 

or 

{2uiiUllc + 2ll2iUZk— {V2i— Vli)(V2k— Vlk))kikk > 0. 

This quadratic form is positive-definite if the trace and determinant of the tensor of rank 
two in the braces are both positive, and hence we obtain the conditions (53.13). 

Problem 2. Find the manner of widening of a rotational discontinuity with time. 

Solution. Assuming all quantities to depend only on the co-ordinate * (and on the time), 
we find from the equations div v = and div H = that v x = constant and H x = constant. 
Let the co-ordinate system be such that the values of v and H on each side of the discon- 
tinuity (outside the transitional layer) are related by (53.12). Then v x = u x , where u has 
the same meaning as in Problem 1. For the transverse components u* and \t we have from 
equations (51.12) and (51.13) 

3u t du t dv t c 2 a^t 

Vu x = ux— h — — , 

8t dx dx Aira dx 2 

dvt 3vt 3ut 3 2 v t 

\-U X = Ms— \-V . 

dt dx dx dx 2 

Since the difference Vt— u* tends to zero for x ->■ ±oo, because of the relations (53.12), 
this difference must be small in the transitional layer in comparison with the sum Vt+u*. 
Adding the equations (1), we can therefore neglect a term in Vt— Ut, obtaining 

a 1/ c 2 \ a 2 

■^ Vt+Ut) ^2(w + fe (Vt+Ut) - 
From this we see that the width of the discontinuity varies in a manner given by 

+M. 



V(( 



4ira 



§54. Shock waves 
Let us now consider the type of discontinuity in which 

; * 0, [F] # 0. (54.1) 

Such discontinuities are called shock waves, as in ordinary fluid dynamics. 
They are characterised by a discontinuity of density and by the fact that the 
gas moves through them (v n \ and v n 2 being non-zero). The normal com- 
ponent of the magnetic field may or may not be zero. 

On comparing equations (53.3) and (53.4) we see that, when H n i= 0, the 
vectors H« 2 -H«i and FaEfo- FiHa are parallel to the same vector v 2 «- v Ut 
and therefore to each other. Hence it follows that H«i and H«2 are parallel, 
i.e. the vectors Hi, H 2 and the normal to the surface are coplanar, unlike 
what happens (in general) in tangential and rotational discontinuities. This 
result holds also when H n = ; in this case, which we shall discuss later, it 
follows from (53.4) that VJlti = FssH^. 



230 Magnetic Fluid Dynamics §54 

The velocity discontinuity v$i — Vt2 lies in the same plane as Hi and H2. 
We can evidently, without loss of generality, assume that the vectors vi and 
V2 themselves lie in this plane, so that the motion in the shock wave is two- 
dimensional. Furthermore, it is easy to see that, if H n ^ 0, a suitable trans- 
formation of the co-ordinates will always ensure that the vectors v and H 
are parallel on each side of the discontinuity. To achieve this, we use a co- 
ordinate system which moves with velocity vt — (v n lH n )H.t = Vt — (jVjH v )'H.t 
(the value of this expression is the same on each side of the discontinuity, 
by (53.4)). In the following formulae, however, the choice of this particular 
co-ordinate system is not implied. 

Let us derive the relation for shock waves in magnetic gas dynamics which 
corresponds to the shock adiabatic (Hugoniot adiabatic) in ordinary gas 
dynamics. Eliminating \yi\ from (53.3) and (53.4) we have 

P[VH t ] = H n *[Ht]l4ir; (54.2) 

here we have replaced H$ by Ht, since H^i and H^ are parallel. In order to 
eliminate [v*] from equation (53.1), we rewrite that equation as 

+ [ra?]/4ir - H n *[jty]l32n*j* = 0. 

The third term is zero by equation (53.3) and so v< does not appear. In the 
last term we substitute p from (54.2) and in the second term from (53.2), 

p = {p2-pi + (flia 8 - Ha*)l&r}l(Vi - V 2 ). (54.3) 

A simple calculation then gives 

€2 - «1 + \{p2 + pl)(V 2 ~Vi) + 

+ (Vi - FiX^fc - H tl fl\(m = 0. (54.4) 

This is the equation of the shock adiabatic in magnetic gas dynamics. It 
differs from the ordinary equation by the presence of the third term. 

We may also write out again equation (53.3), which gives the discontinuity 
of vt in terms of that of Ht : 

v t 2 - v tl = H n (H t2 - H tl )l4TTJ. (54.5) 

Equations (54.2)-(54.5) form a complete system of equations of shock waves. 
As the discontinuities of all quantities tend to zero, the velocity of propaga- 
tion of the shock wave must tend to its value for small perturbations. In 
ordinary gas dynamics this means that the velocity of a weak shock wave 
tends to the velocity of sound. In magnetic gas dynamics, however, there 
are two different velocities 112 and M3 with which weak shock waves can be 
propagated, t 



f The velocity mi is, as mentioned in §53, the velocity of propagation of perturbations 
corresponding to rotational discontinuities. 



§54 Shock waves 231 

Let us consider weak shock waves in more detail, and ascertain in which 
direction the various quantities change in them. Expanding equation (54.4) 
in powers of the discontinuities of pressure and entropy, we obtaint 

l07r\ dp I s 

When the gas passes through the shock wave, its entropy can only increase : 
s 2 > si. By an inequality of thermodynamics, we have (dV/dp) s < 0, and 
the derivative (dWldp 2 ) s is in fact positive for all the substances in question 
here. Hence we see from (54.6) that the inequality s 2 > *i implies that 
p 2 > pi, and therefore V 2 < V\. Thus we have a compression wave, as in 
ordinary gas dynamics. This result, which we have proved for weak shock 
waves, seems to hold for shock waves of any intensity. 

For weak shock waves we can also derive certain results concerning the 
direction of variation of the magnetic field. The changes in the various 
quantities when the state of the gas undergoes a slight perturbation are related 
by formulae (52.8) and (52.9). The changes 8p = p 2 -pi and 8(H t 2 ) 
= H t2 2 -H t i 2 are such that 8(H t 2 ) = Stt(u 2 - u Q 2 )8p. Since u 2 > « and« 3 < «o 
(see (52.15)), and from the above we necessarily have 8p > 0, we see that the 
quantities H t 2 , and therefore H 2 = H t 2 + H n 2 , vary in opposite directions in 
the two kinds of weak shock wave. The magnetic field is increased in a 
shock moving with velocity ~ u 2 , but it is reduced in one moving with 
velocity ~ Ms- 
Let us now consider shock waves in weak magnetic fields, i.e. assume that 
H 2 ^ pu 2 on either side of the discontinuity. No restriction is placed on the 
discontinuity of any quantity; in particular, the discontinuity in the magnetic 
field may be comparable with the magnitude of that field. 

There are again two possibilities. If the discontinuities of density and 
pressure are not small, we can neglect, in a first approximation, the last term 
in equation (54.4) and the magnetic field in formula (54.3). We thereby 
return to the formulae of ordinary fluid dynamics. Thus the relation between 
the discontinuities of the various thermodynamic quantities, and the rate of 
propagation of the shock, will be the same as for ordinary shock waves. The 
change in the magnetic field can be found from the relation (54.2). Since 
the right-hand side of this equation is of the third order of smallness with 
respect to the field, the same must be true of the left-hand side. As a first ap- 
proximation we can put [VH t ] = V 2 H t2 - V\H t \ £ 0, whence H t2 \H t \ 
= Vi/V 2 = p 2 jpi. Since in an ordinary shock wave we always have V\ > V 2 , 
we see that the magnetic field is strengthened in a shock of this type. 



t See Fluid Mechanics, §83. 
16 



232 Magnetic Fluid Dynamics §54 

Equations (54.2)-(54.4) admit also another type of solution. The assump- 
tion that the field is small is also compatible with equation (54.2) for a wave 
in which V\ % V% and^ 2 is the second-order quantity 

P = HffaV, (54.7) 

where V is the common value of Vi and V 2 . It is seen from equation (54.3) 
that if we put V± = V2, we must to the same approximation put 

P2-P1 = - (H t2 2 - Ha*)l&r. (54.8) 

The continuity of the density means that a shock wave of this type can be 
regarded as a discontinuity in an incompressible fluid. The vector H t (and 
therefore y t ) has a discontinuity in magnitude but not in direction, and the 
discontinuity of pressure is given in terms of that of the magnetic field by 
formula (54.8) when the density is continuous. The rate of propagation of 
the discontinuity is v n2 = v nl =jV = H n y/{V\^Tr). This is a natural result, 
and the necessity for the existence of such discontinuities could have been 
foreseen. We saw in §52 that, in an incompressible fluid, there is only one 
velocity of propagation of small perturbations of the magnetic field, namely 
Hl\^(4iTp). Hence the fronts of small perturbations move with velocity 
u\ = H n l<s/(4TTp), whether the change SH in the field is parallel or perpendicu- 
lar to the plane of H and n. The latter case corresponds to weak rotational 
discontinuities (already discussed in §53) and the former, when the dis- 
continuities are small, is the type just considered. 

To ascertain the direction of variation of the magnetic field strength in such 
discontinuities, we return to equation (54.4), which has not yet been used, 
and rewrite it in the form (54.6), in whose derivation the discontinuity in the 
magnetic field was not assumed small compared with that field itself. Substi- 
tuting p 2 ~pi from (54.8), we find that the second term on the right of (54.6) 
is of the fourth order in the field, whereas the first term is of the sixth order 
and may be omitted. It follows at once from the condition s 2 > *i that 
Ht2 < Ha, i.e. that the magnetic field is weakened in such a discontinuity. 

Returning now to shock waves of any intensity in magnetic fields of any 
strength, we may consider two particular cases. Let the magnetic field in 
medium 1 be perpendicular to the shock wave front, i.e. H«i = 0. Then equa- 
tion (54.2) becomes pV 2 H t 2 = Hr?H t 2\^n- Hence it follows that either 
H12 = 0, or;' 2 = H n 2 l47rV2 with no restriction on H t 2- In the former case 
the magnetic field remains perpendicular to the surface of the discontinuity, 
and does not affect the properties of the shock wave, since it does not appear 
in the equations. In the second alternative we have a shock wave in which 
the field changes direction, propagated with velocity v n2 = jV 2 = H n /\/(47rp2) 
relative to the gas behind it. 

Another particular case is a shock wave parallel to the field on either 
side of it (H n = 0).t From (54.5) we then have v t2 = v t i, i.e. the tangential 

t For H n = there is only one type of shock wave, in accordance with the fact that m 
is zero. The shock waves corresponding to us become weak tangential discontinuities at 
rest relative to the fluid. 



§54 Shock waves 233 

velocity component is continuous. By a suitable choice of co-ordinates 
therefore, we can always ensure that v t = on either side of the discontinuity, 
i.e. the gas moves perpendicularly to the discontinuity, and we shall hence- 
forth assume this. From equation (54.2) we have V2H2 = Pi-Hi- This rela- 
tion shows that equations (54.3) and (54.4) can be written 

j2 =(p 2 *-p 1 *)l(V 1 -V 2 ) i 6 2 *-ei* + i(/)2*+/>i*)(^2-Fi) =0, 

which differ from the ordinary equations for shock waves in the absence of a 
magnetic field only by a change in the equation of state: the true equa- 
tion of state p = p(V,s) must be replaced by p* =p*(V,s), where 
p* =p + b 2 ISiTV 2 , and b denotes the constant product HV. Accordingly e* 
must be defined so as to satisfy the thermodynamic relation ( dt* / d V) s = -p* , 
whence e* = e + b 2 /87rV. 

It has been shown in §53 that there are discontinuities which exhibit the 
properties of both tangential and rotational discontinuities. The discon- 
tinuities discussed here are related in this way to shock waves also. The 
transition between shock waves and rotational discontinuities is formed by 
a discontinuity in which there is no change in density and the only change in 
the magnetic field is that H t is reversed. The transition between shock waves 
and tangential discontinuities is formed by discontinuities in which v n = 0, 
H n = 0, and H* has any discontinuity in magnitude but none in direction. 

We may summarise as follows the discontinuities discussed in §§53-4: 

(1) Contact discontinuities: 

j = 0, KJ = 0, [V] * 0, \p] = 0, H n # 0, [H,] = 0. 

(2) Tangential discontinuities : 

j = 0, H ^0, [V] * 0, \p + ^J = 0, H n = 0, [H«] # 0. 

(3) Rotational discontinuities : 

j # 0, [x t ] * 0, [V] = 0, [/>] = 0, H n # 0, 
Ht changes direction but not magnitude. 

(4) Shock waves: 

j ^0, [V] ^ 0, Hi, H 2 and n coplanar. 
The following diagram shows the possible transitional cases: 




234 Magnetic Fluid Dynamics §55 

§55. The spontaneous magnetic field in turbulent motion of a con- 
ducting fluid 

Turbulent motion of a conducting fluid has the remarkable property that 
it may lead to spontaneous magnetic fields which are quite strong. There are 
always small perturbations in a conducting fluid, resulting from causes 
extraneous to the fluid motion itself, t and accompanied by very weak electric 
and magnetic fields. The question is whether these perturbations are, on 
the average, amplified or damped by the turbulent motion in the course of 
time. The following arguments show that either may occur, depending on 
the properties of the fluid itself. % 

The manner of variation with time of magnetic field perturbations, once 
they have arisen, is determined by two physical agencies. The dissipation 
of magnetic energy, which is converted into the Joule heat of the induced 
currents, tends to diminish the field. The magnetic field tends to increase, 
on the other hand, by the purely magnetic effect of the "stretching" of 
the lines of force. We have shown at the end of §51 that, when a fluid of 
sufficiently high conductivity is in motion, the lines of magnetic force move 
as fluid lines, and the magnetic field varies proportionally to the stretching 
at each point on each line of force. In turbulent motion any two neighbour- 
ing particles move apart, on the average, in the course of time. As a result, 
the lines of force are stretched and the magnetic field is strengthened. 

We shall show that in certain conditions these two opposite tendencies 
may balance, and this will provide a criterion distinguishing the cases where 
the magnetic field perturbations increase from those where they are damped. 

While the magnetic field resulting from the motion remains weak its 
reciprocal effect on the motion can be neglected. That is, we may consider 
ordinary fluid turbulence as providing a given "background" on which the 
magnetic perturbations develop. We assume a steady turbulent velocity 
distribution, the word "steady" being used in the sense usual in turbulence 
theory, i.e. referring to the average values of the motion. || 

Mathematically, we neglect the terms quadratic in the field in the equation 
of motion (51.13), returning to the ordinary Navier-Stokes equation: 

d-v/dt + (vgrad)v = - grad(p//>) + vAv 

(the fluid being supposed incompressible). If we use the formula (vgrad)v 
= \ grad v 2 — v x curl v and take the curl of the above equation, we obtain 

dStfdt = curl(v x SI) + v&Sl, (55.1) 

where we have put for brevity SI = | curl v. 



t For example, the magnetomechanical effect in rotating parts of a fluid, or even thermal 
fluctuations. 

X The results in §55 are due to G. K. Batchelor (1950). 

|| The averaging is over times which are of the order of the periods of the corresponding 
turbulent fluctuations, but are, of course, small compared with the total time during which 
the system is observed. 



§55 Spontaneous magnetic field in turbulent motion of conducting fluid 235 

Let us compare this equation with (51.2): 

dU/dt = curl(v x H) + (cS/W) AH, (55.2) 

which (for a given velocity distribution) determines the time variation of the 
magnetic field. We see that SI and H satisfy equations of the same form, which 
become identical if v = c^JAttg. In this case, therefore, there is a solution of 
equation (55.2) for which 

H = constant x SI. (55.3) 

Thus, if 

v = C2/W, (55.4) 

a steady magnetic field (in the same sense of the word "steady") can exist. 
This field, on the average, neither increases nor decreases, whatever the value 
of the constant coefficient in (55.3). We may say that there is neutral equili- 
brium, in which the two factors, mentioned above as determining the magnetic 
field, are exactly balanced. 

Hence, in turn, it is evident that, if the conductivity of the fluid exceeds 
c^j^ttv, the dissipative loss of electromagnetic energy will be insufficient to 
compensate the increase of the magnetic field by the stretching of the lines 
of force. Thus we obtain the inequality 

4-m/a/c 2 > 1 (55.5) 

as the condition for the spontaneous appearance of magnetic fields by the 
growth of small magnetic perturbations, t 

We can say that this is the condition for turbulent motion to be unstable 
with respect to infinitesimal magnetic perturbations. It is noteworthy that 
the criterion can be established quantitatively, and not merely in orders of 
magnitude.}: 

The condition (55.5) as a criterion of the behaviour of the field is valid 
so long as the neglect of the reciprocal effect of the magnetic field on the flow, 
on which the derivation of (55.5) is based, holds good. The field will increase 
until some steady state, in which the reciprocal effect of the field cannot be 
neglected, is set up. Although, strictly speaking, the fluid-mechanical 
properties of the turbulence in this state are not those given a priori, the 
qualitative distribution and the order of magnitude of the resulting magnetic 
field can be determined as if they were. 



t The condition (55.5) is very stringent. For example, in mercury (a^l0 16 sec 1 , 
v = 1 -2 X 10- 3 cm 2 /sec), the quantity on the left of (55.5) is only 1 -5 X 10~ 7 . Since a and v 
increase with the mean free paths of the corresponding carriers of charge and mass, the 
condition (55.5) may be fulfilled, for example, in the Sun's chromosphere and corona, and 
in the ionised interstellar gas. 

% It should be mentioned that the foregoing arguments, however convincing, are not 
entirely conclusive. For example, Ya. B. Zel'dovich has shown (Zhurnal iksperimental'noi 
i teoreticheskoi fiziUZl, 154, 1956; Soviet Physics JETP 4, 460, 1957) that they are invalid 
in a hypothetical case of "two-dimensional" turbulence. 



236 Magnetic Fluid Dynamics §55 

It is easy to see that the magnetic field distribution must be similar to the 
turbulent distribution of SI. For SI may be regarded as the angular velocity 
of the fluid at any given point. Since the lines of magnetic force move with 
the fluid, the vector H rotates with the same angular velocity. Hence, if at 
any two points of a turbulent flow the instantaneous values of SI are uncorre- 
cted, the vectors H at those points will rotate in an uncorrelated manner, 
and their relative direction will vary randomly with time. 

In this connection we may refer to some purely fluid-mechanical properties 
of turbulence, t Turbulent flow may be regarded as a superposition of turbu- 
lent eddies of various sizes, from the largest / (the "external scale" of the 
turbulence) to the smallest Ao (the "internal scale"). The former is equal 
to a characteristic length which gives the size of the region in which the 
turbulent flow occurs. The quantity Ao gives, in order of magnitude, the 
distances at which viscosity, and the energy dissipation which it entails, 
become important; it can be expressed in terms of / and the Reynolds 
number R ~ uljv of the turbulent flow as a whole (u being of the order of 
the change in the mean velocity over a distance /), or in terms of the energy 
e dissipated in unit mass of the fluid in unit time : 

A - (v3/e)i - IIRK (55.6) 

The correlation between the velocities vi and V2 at two points 1 and 2 at 
a distance A apart is determined mainly by the eddies of size A. According 
to Kolmogorov and Obukhov's law, we have, for distances A > Ao, AvtAvje 
~ A 2/3 , where Av = V2 — vi. At distances A < Ao, on the other hand, 
AviAvjc ~ A 2 . From this we can easily find the correlation of angular veloci- 
ties. Since the components of Sli and SI2 are expressed in terms of the 
derivatives of vi and V2, we find, by differentiating AvtAvje once with respect 
to the co-ordinates of point 1 and once with respect to those of point 2, 



SluCkk ~ A" 4 / 3 for A > A , 
0.uQ.2k ~ constant for A <^ Ao. 

These formulae show that an appreciable correlation between the angular 
velocities exists only at distances up to those of the order of Ao, falling off 
rapidly at greater distances. 

From the above discussion, the distribution of the steady spontaneous 
magnetic field must be similar. The distribution is correlated only over 
regions of dimension ~ Ao. At greater distances the relative direction of the 
vectors H is practically random. 

The order of magnitude of the magnetic field can now be easily deter- 
mined by estimating the terms in the complete equation of motion 

dx/dt + (vgrad)v = - grad (/>//>) + v/\v - (1/4tt/j)H x curlH. 



t See Fluid Mechanics, §§31-33. 



§55 Spontaneous magnetic field in turbulent motion of conducting fluid 237 

Since the vector H changes its direction completely over distances ~ Ao, 
the order of magnitude of the last term on the right-hand side is H 2 \A7rpXo. 
Let us now estimate the term (v»grad)v. For eddies of size A it is of the 
order of v^/X, where vx is the change in the velocity over a distance A. 
According to the formulae of turbulence theory we have 

v x ~ w(A//)* for A > A , 

v x ~ m(A//)VR for A < Ao. 

Hence the ratio t^ 2 /A~ A _1/3 for A > Ao and ~ A for A < Ao- Its greatest value 
is therefore reached when A ~ Ao. Thus (v»grad)v ~ v^/Xo. Finally, if 
the two terms are comparable in magnitude, we have 

H 2 „ 4tt P v X0 \ (55.9) 

According to (55.6) and (55.8), vxq ~ «R~ 1/4 ~ (ev) 1/4 . Hence we can also 
write 

H 2 ~ 4irpU 2 /^R „ 4tt P V(^)- (55.10) 

These formulae give the order of magnitude of the spontaneous magnetic 
field. It is of interest to compare the energy of this field with the 
kinetic energy of the turbulent flow. The latter energy resides mainly in the 
largest eddies (of size ~ /), and its order of magnitude is pu 2 . The mag- 
netic energy resides mainly in the "magnetic eddies", which are of small size 
(~ Ao). By (55.9), it is comparable with the kinetic energy of the turbulent 
eddies of this same size, but, by (55.10), it is small in comparison with the 
total kinetic energy. A more exact mathematical formulation of these state- 
ments can be attained by expanding the spatial distribution of velocity and 
magnetic field as Fourier integrals. The kinetic energy then resides mainly 
in the components with small wave numbers (k ~ 1//), while the magnetic 
energy is mainly in those with large wave numbers (k ~ 1/Ao). 

Turbulent flow results in a continuous transfer of energy from large eddies 
to small ones, with almost no viscous dissipation. This "energy flux" is 
dissipated only in the eddies of size Ao. In the absence of a magnetic field, 
the dissipation is due entirely to the viscosity of the fluid, but in the turbu- 
lence here considered the energy in the eddies of size Ao is partly dissipated 
by viscosity, partly converted into the energy of the magnetic field and only 
then dissipated as Joule heat. 

Let us estimate the time required to establish the steady state. For this 
purpose we return to equation (55.2). The two terms on the right-hand 
side are in order of magnitude respectively Hv\JXq = HvxqXo/Xo 2 ~ Hv/Xq 2 

and c 2 H/4it0Xq 2 . Since we know precisely the condition (55.4) for the occur- 
rence of neutral equilibrium, we also know the exact relation between the 
coefficients in these two terms, and can write 

8H / c 2 \H 

dt \ 4W Ao 2 



238 Magnetic Fluid Dynamics §55 

Hence we see that small perturbations increase with time according to the 
exponential function 

Ht -£)£)• (55 - n) 

If 4ttov/c 2 > 1 we have simply exp(v*/A 2 ). The time r during which an 
initial small perturbation ~ Hq develops into the steady field H (55.10) is 
then, in order of magnitude, 

Ao 2 , H 2 /v\* p(ev)i 

-v log w^(;) log w- (55 - 12) 

The random variation of the magnetic field with time in turbulent flow 
means that the (time) average value of H is zero. In other words, we can 
say that, in the case considered here (i.e. when a spontaneous field is possible), 
a non-zero mean field is incompatible with turbulence. The result must be 
that, when a moderate external magnetic field is applied to a fluid in turbu- 
lent motion (in a finite volume), the latter will behave like a superconductor. 
A strong field (H 2 > pu 2 ) must necessarily penetrate into the fluid and will 
suppress the turbulence. 



CHAPTER IX 

THE ELECTROMAGNETIC WAVE EQUATIONS 

§56. The field equations in a dielectric in the absence of dispersion 

In §45 we gave the equations for a variable electromagnetic field in a metal : 
curlH = 4t7(7E/c, curlE= - (I /c) dB/dt, (56.1) 

which hold when the field changes sufficiently slowly: the frequencies of the 
field must be such that the dependence of j on E (and of B on H, if needed) 
is that corresponding to the static case, f 

We shall now examine the corresponding problem for a variable electro- 
magnetic field in a dielectric, and shall formulate equations valid for fre- 
quencies such that the relations between D and E, and B and H, are the same 
as when the fields are constant. If, as usually happens, these relations are 
simple proportionalities, this means that we can put 

D = eE, B = fxH, (56.2) 

with the static values of e and fi. 

These relations are not valid (or, as we say, e and fj, exhibit dispersion) at 
frequencies comparable with the eigenfrequencies of the molecular or 
electronic vibrations which lead to the electric or magnetic polarisation of 
the matter. The order of magnitude of such frequencies depends on the 
substance concerned, and varies widely. It may also be entirely different 
for electric and for magnetic phenomena. J 

The equations 

div B = 0, (56.3) 

curlE= -(1/c) dB/dt (56.4) 

are obtained immediately by replacing e and h in the exact microscopic 
Maxwell's equations by their averaged values E and B, and therefore are 
always valid. The equation 

divD = (56.5) 

is obtained (§6) by averaging the exact microscopic equation div e = 47r/>, 



f The condition I <^ A does not relate to the validity of equations (56.1) as they stand. 
In the problems discussed in Chapter VII this condition was necessary in order to justify 
the neglect of retardation effects in the field outside the conductor. 

% In diamond, for example, the electric polarisation is due to the electrons, and the 
dispersion of e begins only in the ultra-violet. In a polar liquid such as water, the polarisation 
is due to the orientation of molecules with permanent dipole moments, and the dispersion 
of e appears at frequencies a <~ 10 11 , i.e. in the centimetre wavelength range. The dispersion 
of ix in ferromagnetics may begin at even lower frequencies. 

239 



240 The Electromagnetic Wave Equations §56 

using only the fact that the total charge on the body is zero. This result is 
evidently independent of the assumption made in §6 that the field is static, 
and equation (56.5) is therefore valid in variable fields also. 

A further equation is to be obtained by averaging the exact equation 

lde 4tt 

curlh = 1 pv. (56.6) 

c dt c 



A direct averaging gives 



1 dE 4tt_ 

curl B = + — pv. (56.7) 

c dt c 



When the macroscopic field depends on time, the establishment of the rela- 
tion between the mean value jov and the other quantities is fairly difficult. 
It is simpler to effect the averaging in the following more formal way. 

Let us assume for the moment that extraneous charges of volume density 
pex are placed in the dielectric. The motion of these charges causes an 
"extraneous current" j ex , and the conservation of charge is expressed by an 
equation of continuity : 

dpex/dt + divjex = 0. 

Instead of equation (56.5) we have divD = 4ir/oex; see (6.8). Differentiat- 
ing this equation with respect to time and using the equation of continuity, 
we obtain d(divD)/dt = 4irdpex/dt — -4rr divjex, or 



/8D \ 

divl — - + 47rjexl = 0. 



Hence it follows that the vector in parentheses can be written as the curl of 
another vector, which we denote by cH. Thus 

4ir 1 3D 

curlH = — j ex + — — • (56.8) 

c c dt 

Outside the body this must be the same as the exact Maxwell's equation 
for the field in a vacuum, and therefore H is the magnetic field. Inside the 
body, in the static case, the current j ex is related to the magnetic field by the 
equation curl H = 47rj e x/c, where H is the quantity introduced in §27 and 
related in a definite manner to the mean field B. Hence it follows that, in 
the limit of zero frequency, the vector H in equation (56.8) is the static 
quantity H(B), and our present assumption that the field varies "slowly" 
means that the same relation H(B) holds between these variable fields. Thus 
H is a definite quantity, so that we can drop the auxiliary quantity jex and 
obtain the final equation 

curlH = (1/c) dD/dt. (56.9) 



§56 The field equations in a dielectric in the absence of dispersion 241 

This equation replaces in dielectrics the first equation (56.1) for the field 
in metals. It might be supposed that the term in 8E\8t ought to be included 
when this equation is used for variable fields in metals also, giving 

*** e BE 

curlH = —aE + -—. (56.10) 

c c 8t 

In good conductors such as the true metals, however, the introduction of 
this term is pointless. The two terms on the right-hand side of (56.10) are 
essentially the first two terms in an expansion in powers of the field fre- 
quency. Since this frequency is assumed small, the second term must repre- 
sent at most a small correction. In actual fact, in metals the corrections for 
the effect of the spatial non-uniformity of the field become important sooner 
than the frequency correction (see the sixth footnote to §45). 

There are, however, substances, namely poor conductors, for which 
equation (56.10) may be meaningful. For such reasons as the small number 
of conduction electrons in semiconductors, or the small mobility of the ions 
in electrolyte solutions, these substances exhibit anomalously low conduc- 
tivity, and hence the second term on the right of equation (56.10) may be 
comparable with, or even exceed, the first term at frequencies for which a 
and e may still be regarded as constants. In a field of a single frequency to, 
the ratio of the second term to the first is co>/47ra. If this ratio is small, 
the body behaves as an ordinary conductor of conductivity a. At frequencies 
o) > Arrafe, it behaves as a dielectric with dielectric constant e. 

In a homogeneous medium with constant «• and /x, equations (56.3)— (56.5) 
and (56.9) become 

divE = 0, divH=0, (56.11) 

u8H e8E 

curlE = - - — , curlH = . (56.12) 

c Bt c 8t 

Eliminating E in the usual manner, we obtain 

e 8 €fx sm 
curl curl H = curlE = , 

c8t c 2 8t 2 

and, since curl curl H = grad divH— AH = —AH, we reach the 
wave equation 

eu8m 

AH-— = 0. 

C 2 8fi 

A similar equation for E can be obtained by eliminating H. We see that 
the velocity of propagation of electromagnetic waves in a homogeneous 
dielectric is 

c/VM- (56.13) 



242 The Electromagnetic Wave Equations §56 

The electromagnetic energy flux density in a dielectric is given by the 
same formula as in a metal : 

S = cEx H/4tt. (56.14) 

This is easily seen by calculating div S. Using equations (56.4) and (56.9), 
we obtain 

divS = — (H-curlE - E-curlH) 

1 / 3D 8B\ 3U , 4 x 

= E + H = , (56.15) 

Att\ dt BtJ at ' 

in accordance with the expression dU = (E«dD + H«dB)/47r for the dif- 
ferential of the internal energy of a dielectric at given density and entropy. 
The general requirements of relativistic invariance have the result that 
the energy flux density must be the same, apart from a factor c 2 , as the space 
density of the field momentum, f which is therefore 

E x H/4ttc. (56.16) 

This expression must, in particular, be used in determining the forces on a 
dielectric in a variable electromagnetic field. The force f per unit volume 
may be calculated from the stress tensor ow'. ft = Soutjdxit. Here, however, 
it must be remembered that aw is the momentum flux density, which in- 
cludes the momentum of both the matter and the electromagnetic field. If f is 
taken as the force on the medium, the rate of change of the field momentum 
per unit volume must be subtracted : 

**_1(BXH)«_ 

8xi 8t \ttc 

In a constant field the last term is zero, and so this question did not arise 
previously. 

Since the field varies "slowly", the stress tensor may be taken to have the 
same value as in a constant field. For instance, in a fluid dielectric, ow is 
given by the sum of the electric part (15.9) and the magnetic part (34.2). In 
differentiating these expressions with respect to the co-ordinates we must 
use the fact that the equations curl E = 0, curl H = for a constant field 
(in the absence of currents) are replaced by equations (56.12). The result is 

E 2 f /de\ E 2 1 H 2 

f = - grad^o - ^S rade + S rad [p[y) ^J ~ §^ grad ^ + 

r /du\ H 2 l €ii - 1 d 



t This follows from the symmetry of the four-dimensional energy-momentum tensor; 
see The Classical Theory of Fields, §4-7, Addison- Wesley Press, Cambridge (Mass.), 1951; 
Pergamon Press, London, 1959. 



§57 The electrodynamics of moving dielectrics 243 

§57. The electrodynamics of moving dielectrics 

The motion of a medium results in an interaction between the electric and 
magnetic fields. Such phenomena for conductors have been discussed in 
§49; we shall now discuss them for dielectrics. Here we are in practice 
concerned with the phenomena occurring in moving media when external 
electric or magnetic fields are present. It should be emphasised that they 
are in no way related to the appearance of fields as a result of the motion 
itself (§§35, 50). 

Our starting point in §49 was the formulae giving the transformation of 
the field when the frame of reference is changed. There it was sufficient to 
know the general formulae for the transformation of electric and magnetic 
fields in a vacuum, the averaging of which gives immediately the formulae 
for the transformation of E and B. In dielectrics the problem is considerably 
more complex, because the electromagnetic field is described by a greater 
number of quantities. 

In the motion of macroscopic bodies, the velocities involved must in 
practice be small compared with the velocity of light. To obtain the necessary 
approximate transformation formulae, however, it is simplest to use the 
exact relativistic formulae which hold for all velocities. 

In the electrodynamics of the field in a vacuum, the components of the 
electric and magnetic field vectors e and h are actually components of an 
antisymmetrical four- dimensional tensor (or "four-tensor") of rank two.f 
The same is true of E and B, which are the mean values of e and h. Thus 
there is a four-tensor Fac whose components are given byt 



Foe = 



B z 


— By 


-iE x 





Bx 


— lEy 


-B x 





~iE z 


iE y 


iE z 






(57.1) 




-B z 

By 

iE x 

Using this tensor, the first two Maxwell's equations, 

div B = 0, curl E = - (1/c) dB/dt, (57.2) 

can be written in the four-dimensional form 

8F ik dF kl dF u 

~T~ + -J- + T~ = °- < 57 ' 3 ) 

OXi CXf OX]z 

This shows the relativistic invariance of the equations. The applicability of 
equations (57.2) to moving bodies is evident, since they are obtained directly 
from the exact microscopic Maxwell's equations by replacing e and h by 
their averaged values E and B. 



t See The Classical Theory of Fields, §§3-9, 4-1. 

% In the present section (but not in the Problems) the tensor suffixes take the values 1,2, 
3, 4, corresponding to the four-dimensional co-ordinates x\ — x, xz = y, x% — z, Xi = ict. 



244 



The Electromagnetic Wave Equations 



§57 



The second pair of Maxwell's equations 



divD = 0, curl H = (1/c) dDjdt 



(57.4) 



also retain their form in moving media. This is seen from the arguments 
given in §56, in which we used only general properties of bodies (e.g. that 
the total charge is zero), equally valid for moving bodies and bodies at rest. 
However, the relations between D and E, and B and H, need not be the same 
as in bodies at rest. 

Since they are valid for bodies both at rest and in motion, equations 
(57.4) must be unaltered by the Lorentz transformation. For a field in 
a vacuum, the vectors D and H are the same as E and B, and the relativis- 
tic invariance of the second pair of Maxwell's equations appears in the 
fact that they also can be written in four- dimensional form, using the same 
tensor Fac'. dFijcjdxjc = O.f Hence it is clear that, to ensure the relativistic 
invariance of equations (57.4), it is necessary that the components of the 
vectors D and H should be transformed as the components of a four-tensor 
exactly similar to Fi/c, which we denote by Hm : 






H z 


— Hy 


-iD x 


H z 





H x 


-iD y 


Hy 


— H x 





-iD z 


iD x 


iD y 


iD z 






Hik = 



Using this tensor, we can write equations (57.4) in the form 

dHacjdxic = 0. 



(57.5) 



(57.6) 



Having elucidated that the quantities E, D, H, B form four-dimensional 
tensors, we have also ascertained the law of their transformation from one 
frame of reference to another. However, we are interested rather in the 
relations between the quantities in a moving medium, which generalise the 
relations D = cE and B = juH valid in a medium at rest. 

We denote by m the velocity four- vector of the medium ; its components 
are related to the three-dimensional velocity v by 

«iA3 = ««r../«/(i - y - «• = */j(i - y ■ 

From this four-vector and the four-tensors Fuc and Ha we form combina- 
tions which become E and D in a medium at rest. These combinations are 
the four- vectors FacUjc and HocUjc', for v = their time components are 



t See The Classical Theory of Fields, §4—5. 



§57 The electrodynamics of moving dielectrics 245 

zero and their space components are E and D respectively. The four- 
dimensional generalisation of the equation D = eE is therefore evidently f 

HucUjc = eFikUic. (57.7) 

Similarly, we see that the generalisation of B = /uH is the four-dimensional 
equation 

FocUi + F k iUi + Fuujc = n(H ik ui + H k m + H u u k ). (57.8) 

Returning from the four-dimensional to the three-dimensional notation, 
we derive from these two equations the vector relations! 

D + V x Ulc = e(E + v x B/c), 

(57 9) 
B + E X v/c = ju(H + D X v/c). V ' 

These formulae, first derived by H. Minkowski (1908), are exact in the 
sense that no assumption has yet been made concerning the magnitude of 
the velocity. If the ratio v/c is assumed small the equations can be solved 
for D and B as far as terms of the first order to give 

D = eE + (ep - l)v x H/c, (57.10) 

B = M H + (e/x - 1)E X v/c. (57.11) 

These formulae, together with Maxwell's equations (57.2) and (57.4), form 
the basis for the electrodynamics of dielectrics in motion. 

The boundary conditions on Maxwell's equations are also somewhat 
modified. From the equations div D = 0, div B = the continuity of the 
normal components of the inductions follows as before : 

D n i = D n 2, B n \ = B n 2. (57.12) 

The conditions on the tangential components of the fields are most simply 
obtained by changing from the fixed frame of reference K to another, K\ 
which moves with the surface element considered, whose velocity along the 
normal n we denote by v n . The usual conditions, namely that E't and H' t 
are continuous, hold in the frame K'. By the relativistic transformation 
formulae, 1 1 these are equivalent to the continuity of the tangential compo- 
nents of the vectors E + vxB/c and H-vxD/t. Taking the components 
perpendicular to n and using equations (57.12), we obtain the required 
boundary conditions : 

n X (Ea - E0 = v n (B 2 - Bitfc, 

n x (H 2 - Hi) = - v n (D 2 - Ih)lc. l ' j 



t It should be noted that, by writing down relations involving only the local value of the 
velocity, we neglect slight effects due to the possibility of a velocity gradient, such as gyro- 
magnetic effects (§35). 

X If either of the relations D = eE and B = /xH does not hold in the medium at rest, 
the corresponding relation (57.9) is replaced by a different functional relation between the 
vector sums on the two sides of the equation. 

|| See The Classical Theory of Fields, §3-10. 



246 The Electromagnetic Wave Equations §57 

If we substitute here the expressions (57.10) and (57.11), and neglect 
terms of higher order in v/c, we obtain 

n x (E 2 - Ei) = vnfa - pifHtlc, 

n X (H 2 - Hi) = - v n (€ 2 - €i)E*/<r. ( ' ' 

In this approximation the values of H and E on the two sides of the surface 
need not be distinguished on the right-hand sides of equations (57.14). 

If the body moves so that its surface moves tangentially to itself (e.g. a 
solid of revolution rotating about its axis), then v n = 0. Only in this case 
do the boundary conditions (57.13) or (57.14) reduce to the usual condi- 
tions that Et and H^ are continuous. 



PROBLEMS 

Problem 1. A dielectric sphere rotates uniformly in a vacuum in a uniform constant 
magnetic field £. Determine the resulting electric field near the sphere. 

Solution. In calculating the resulting electric field, the magnetic field may be taken to 
be the same as for a sphere at rest, since an allowance for the reciprocal effect of the magnetic 
field variation would give corrections of a higher order of smallness. Within the sphere, the 
magnetic field has the uniform value H«) = 3.|>/(2+/x); cf. (8.2). 

Since the rotation is steady, the resulting electric field is constant and, like any constant 
electric field, has a potential: E = — grad <f>. Outside the sphere, the potential satisfies the 
equation A^ (e) = 0; inside the sphere, it satisfies 

A4> w = 2(e,x-l)£2.HW/ce, (1) 

where Si is the angular velocity. The latter equation is obtained from div D = by sub- 
stituting for D the expression (57.10) with v = S2Xr. The condition that the normal 
component of D is continuous at the surface of the sphere gives 

~ e RrL= + ! ^^- H(i) -( a - n )( H(i) - n )] = -[-^r] • ( 2 ) 

Here a is the radius of the sphere and n a unit radial vector. 

From the symmetry of the sphere, the required electric field is determined by only two 
constant vectors, Si and .£). From the components of these vectors we can form a bilinear 
scalar $)-Si and a bilinear tensor §jOfc+§fc^i— i&tk$)'Si, the sum of whose diagonal terms 
is zero. Accordingly, we seek the field potential outside the sphere in the form 

am — *n ^ 2 l^\ *r> niTllc 
6 dxidxjc\rJ 2 r 3 

where Dm is a constant tensor (with Da — 0), the electric quadrupole moment tensor of 
the sphere. f No term of the form constant/r can appear in ft e \ since such a term would 
give a non-zero total electric flux through a surface surrounding the sphere, whereas the 
sphere is uncharged. The field potential inside the sphere is sought in the form 

4>m = I~Damnt+ ^-fl •»«>(** -a"). (4) 

2a* 3ce 

The first term is the solution of the homogeneous equation /\<j> = 0, and the coefficient is 
chosen so as to give continuity of the potential, and therefore of Et, at the surface of the 
sphere. Substituting (3) and (4) in (2), we obtain 

a* 3(e/*-l) ^ 

Dik = n ,; v J , [&Q*+6*Qi-S8 tt 3.fl]. (5) 

c (3+2e)(2 + /n) 



t See The Classical Theory of Fields, §5-6. 



§58 The dispersion of the dielectric permeability 247 

Thus a quadrupole electric field is formed near the rotating sphere, and the quadrupole 
moment of the sphere is given by formula (5).f In particular, if the axis of rotation (the 
ar-axis) is parallel to the external field, A* has only the diagonal components 

a* 4(c/*— 1) , 

c (3+2e)(2+/i) 

Problem 2. A magnetised sphere rotates uniformly in a vacuum about its axis, which is 
parallel to the direction of magnetisation. Determine the resulting electric field near the 
sphere.J 

Solution. The magnetic field inside the sphere is uniform, and is expressed in 
terms of the constant magnetisation M by the equations B<*'+2H< < ) = (cf. (8.1)) and 
B(*)~H«> = 4ttM, whence B«> = 8*rM/3, H<*> = -4ttM/3. The second of formulae 
(57.9) does not hold in this case, because the formula B = /tH is not valid for a ferro- 
magnetic at rest; from the first of (57.9) we have, inside the sphere, 

D = eE+evxB/c— vxH/c 

= eE+4w(2e+l)vXM/3c. 

The potential of the resulting electric field outside the sphere satisfies the equation 
/\$>(e) = o, and that inside the sphere satisfies A<£ (<) = 8u<2e+l)MQ/3ce. 
The boundary condition that D n is continuous at the surface of the sphere gives 

L dr \ r -a 3c L dr J r =-o 

where 9 is the angle between the normal n and the direction of SI and M (the sr-axis). We 
seek $ (e) and $ (i) in the forms 

Dantmi Dzz 

*(•> = —^ = —(3 cos 2 0-l), 

r 2 4irC2e4-l') 

^(i) = —Dz& cos 2 »-l)+ - n ; M Cl(r*-aZ). 
\aP 9ce 

From the boundary condition we obtain the following expressions for the electric quadrupole 
moment of the rotating sphere : 

4(2e+l) 
D™ = — „ , n , n .a z &^t D X x — Dyy = — iDzz, 
3c(2e+3) 

where *# is the total magnetic moment of the sphere. For a metal sphere we must take 
e -»■ oo, giving 

D zz = —\&J(cP\Zc. 

§58. The dispersion of the dielectric permeability 

Let us now go on to study the important subject of rapidly varying electro- 
magnetic fields, whose frequencies are not restricted to be small in comparison 
with the frequencies which characterise the establishment of the electric and 
magnetic polarisation of the substances concerned. 

An electromagnetic field variable in time must necessarily be variable in 
space also. For a frequency <a, the spatial periodicity is characterised by a 



t Similarly, a quadruple magnetic field occurs near a sphere rotating in a uniform electric 
field. The magnetic quadrupole moment is given by (5) if the sign is changed and c, /x, £ 
are replaced by fi, e, ($* respectively. 

X If the direction of magnetisation is not the same as that of the axis of rotation, the 
problem is considerably changed, since the sphere then emits electromagnetic waves. 

17 



248 The Electromagnetic Wave Equations §58 

wavelength A ~ cjto. As the frequency increases, A eventually becomes 
comparable with the atomic dimensions a. The macroscopic description of 
the field is thereafter invalid. 

The question may arise whether there is any frequency range in which, on 
the one hand, dispersion phenomena are important but, on the other hand, 
the macroscopic formulation still holds good. It is easy to see that such a 
range must exist. The most rapid manner of establishment of the electric or 
magnetic polarisation in matter is the electronic mechanism. Its relaxation 
time is of the order of the atomic time a/v, where v is the velocity of the 
electrons in the atom. Since »<c, even the wavelength A ~ ac/v corres- 
ponding to these times is large compared with a. 

In what follows we shall assume the condition A > a to hold.f It must 
be borne in mind, however, that this condition may not be sufficient: for 
metals at low temperatures there is a range of frequencies in which the 
macroscopic theory is inapplicable, although the inequality cfto > a is 
satisfied (see §67). 

The formal theory given below is equally applicable to metals and to 
dielectrics. At frequencies corresponding to the motion of the electrons 
within the atoms (optical frequencies) and at higher frequencies, there is, 
indeed, not even a quantitative difference in the properties of metals and 
dielectrics. 

It is clear from the discussion in §56 that Maxwell's equations 

divD = 0, divB = 0, (58.1) 

curlE = - (l/*)0B/0*, curlH = (l/c)8DJdt (58.2) 

remain formally the same in arbitrary variable electromagnetic fields. These 
equations are, however, largely useless until the relations between the 
quantities D, B, E and H which appear in them have been established. At 
the high frequencies at present under consideration, these relations bear no 
resemblance to those which are valid in the static case and which we have 
used for variable fields in the absence of dispersion. 

First of all, the principal property of these relations, namely the depen- 
dence of D and B only on the values of E and H at the instant considered, 
no longer holds good. In the general case of an arbitrary variable field, the 
values of D and B at a given instant are not determined only by the values of 
E and H at that instant. On the contrary, they depend in general on the 
values of E(t) and H(t) at every previous instant. This expresses the fact 
that the establishment of the electric or magnetic polarisation of the matter 
cannot keep up with the change in the electromagnetic field. The frequencies 
at which dispersion phenomena first appear may be completely different for 
the electric and the magnetic properties of the substance. 



t The effects (called the natural optical activity) resulting from terms of the next order 
in the small ratio a\ A will be considered in §83. 



§58 The dispersion of the dielectric permeability 249 

In the present section we shall refer to the dependence of D on E; the 
specific features of the dispersion of magnetic properties will be discussed 
in §60. 

The polarisation vector P has been introduced in §6 by means of the 
definition p = - div P, p being the true (microscopic) charge density. This 
equation expresses the electric neutrality of the body as a whole, and together 
with the condition P = outside the body it shows that the total electric 
moment of the body is j P dV. This derivation is evidently valid for variable 
as well as for constant fields. Thus in any variable field, even if dispersion 
is present, the vector P = (D-E)/4tt retains its physical significance: it is 
the electric moment per unit volume. 

In rapidly varying fields, the field strengths involved are in practice always 
fairly small. Hence the relation between D and E can always be taken to 
be linear, f The most general linear relation between D(*) and the values of 
the function E(t) at all previous instants can be written in the integral form 

00 

B(t) = E(t) + J7(r)E(* - r)dr. (58.3) 



It is convenient to separate the term E(f), for reasons which will become 
evident later. In equation (58.3)/(t) is a function of time and of the proper- 
ties of the medium. By analogy with the electrostatic formula D = eE, 
we write the relation (58.3) in the symbolic form D = eE, where £ is a 
linear integral operator whose effect is shown by (58.3). 

Any variable field can be resolved by a Fourier expansion into a series of 
components of a single frequency, in which all quantities depend on time 
through the factor e-* w *. For such fields the relation (58.3) between D and 
E becomes 

D = e(w)E, (58.4) 

where the function e( co) is defined as 

00 

e(co)= 1+ ffW'dT. (58.5) 



Thus, for periodic fields, we can regard the dielectric permeability (the 
coefficient of proportionality between D and E) as a function of the frequency 
as well as of the properties of the medium. The dependence of e on the 
frequency is called its dispersion law. 



f Here we assume that D depends linearly on E alone, and not on H. In a constant field, 
a linear dependence of D on H is excluded by the requirement of invariance with respect 
to a change in the sign of the time. In a variable field, this condition no longer applies, and 
a linear relation between D and H is possible if the substance possesses symmetry of various 
kinds. It is, however, a small effect of the order of a/A, and is indeed the effect mentioned in 
the last footnote. 



250 The Electromagnetic Wave Equations §58 

The function e(o>) is in general complex. We denote its real and imaginary 
parts by e' and e" : 

€(co) = e'(to) + ie"(to). (58.6) 

From the definition (58.5) we see at once that 

e (_ w ) = €*(a>). (58.7) 

Separating the real and imaginary parts, we have 

e'(- a) = e'(co), e"(- ft>) = - e"(co). (58.8) 

Thus e is an even function of the frequency, and e" is an odd function. 

For frequencies which are small compared with those at which the dis- 
persion is large, we can expand e(o>) as a power series in to. The expansion 
of the even function €'(<*>) includes only even powers, and that of the odd 
function e"(co) includes only odd powers. In the limit as to -> 0, the func- 
tion e(co) in dielectrics tends, of course, to the electrostatic dielectric constant, 
which we here denote by eo. In dielectrics, therefore, the expansion of 
e'(o)) begins with the constant term e , while that of e"(co) begins, in general, 
with a term in to. 

The function e(w) at low frequencies can also be discussed for metals, if 
it is defined in such a way that, in the limit o> -> 0, the equation 

curlH = (l/<:)flD/a* 

becomes the equation 

curlH = AttoEIc 

for a constant field in a conductor. Comparing the two equations, we see 
that for co -> we must have SDjdt -> 4ttoE. But, in a periodic field, 
dD/dt = —itoeE, and we thus obtain the following expression for e(a>) in 
the limit of low frequencies : 

e(co) = Airicr/co. (58.9) 

Thus the expansion of the function e(a>) in conductors begins with an 
imaginary term in ljco, which is expressed in terms of the ordinary conduc- 
tivity a for constant currents. f The next term in the expansion of e(to) is 
a real constant, although for metals this constant does not have the same 
electrostatic significance as it does for dielectrics.}: 

Moreover, this term of the expansion may again be devoid of significance 
if the effects of the spatial non-uniformity of the field of the electromagnetic 
wave appear before those of its periodicity in time. 



t The imaginary part of the function e{oi) is sometimes represented in the form (58.9) 
for all frequencies; this amounts to introducing a new function a(o>), which has no physical 
significance apart from its relationship to e"(w). 

% To avoid misunderstanding, we should point out a slight change in notation in com- 
parison with §56. In equation (56.10) for poor conductors, e(a>) is (4mo/a>) + e. 



§60 The dispersion of the magnetic permeability 25 1 

In superconductors there is always considerable non-uniformity, resulting 
from the smallness of the "penetration depth" of the magnetic field. It is 
not yet clear whether the concept of the dielectric permeability e(<o) has any 
meaning for superconductors. 

§59. The dielectric permeability at very high frequencies 

In the limit as to -> oo, the function e(o>) tends to unity. This is evident 
from simple physical considerations: when the field changes sufficiently 
rapidly, the polarisation processes responsible for the difference between the 
field E and the induction D cannot occur at all. 

It is possible to establish the limiting form of the function e(o>) at high 
frequencies, which is valid for all bodies, whether metals or dielectrics. The 
field frequency is assumed large compared with the "frequencies" of the 
motion of all, or at least the majority, of the electrons in the atoms forming 
the body. When this condition holds, we can calculate the polarisation of 
the substance by regarding the electrons as free and neglecting their inter- 
action with one another and with the nuclei of the atoms. 

The velocities v of the motion of the electrons in the atoms are small 
compared with the velocity of light. Hence the distances vja> which they 
traverse during one period of the electromagnetic wave are small compared 
with the wavelength c/to. For this reason we can assume the wave field 
uniform in determining the velocity acquired by an electron in that field. 

The equation of motion is m dv'/dt = eE = eEo<r iw ', where e and m are 
the electron charge and mass, and v' is the additional velocity acquired by 
the electron in the wave field. Hence v' = ieEjmoi. The displacement r of 
the electron due to the field is given by f = v', and therefore r = -eE/ma> 2 . 
The polarisation P of the body is the dipole moment per unit volume. 
Summing over all electrons, we find P = Ser = -e 2 NE/ma> 2 , where N is 
the number of electrons in all the atoms in unit volume of the substance. By 
the definition of the electric induction, we have D = eE = E+4nP. We 
thus have the formula 

e(a>) = 1 - 4rriV<? 2 /mco 2 . (59.1) 

The range of frequencies over which this formula is applicable begins, in 
practice, at the far ultra-violet for light elements and at the X-ray region 
for heavier elements, f 

§60. The dispersion of the magnetic permeability 

Unlike the dielectric polarisability, the magnetic susceptibility ceases to 
have any physical meaning at relatively low frequencies. To take account 
of the deviation of fi(to) from unity would then be an unwarrantable refinement. 



t If e(<o) is to retain the significance which it has in Maxwell's equations, the frequency 
must also satisfy the condition o> < c\a. We shall see later (§97), however, that the expres- 
sion (59.1) can be allotted a certain physical significance even at higher frequencies. 



252 The Electromagnetic Wave Equations §60 

To show this, let us investigate to what extent the physical meaning of the 
quantity M = (B — H)/47r, as being the magnetic moment per unit volume, 
is maintained in a variable field. The magnetic moment of a body is, by 
definition, the integral 

— ( r XptdV. (60.1) 

The mean value of the microscopic current density is related to the mean 
field by equation (56.7): 

4tt_ 1 BE 
curl B = — pv + --—. (60.2) 

c c ot 

Subtracting the equation curl H = (lJc)8D/dt, we obtain 

pv = ccurlM + 8P/8L (60.3) 

The integral (60.1) can, as shown in §27, be put in the form JM dV only if 
p~v = c curl M and M = outside the body. 

Thus the physical meaning of M, and therefore of the magnetic suscep- 
tibility, depends on the possibility of neglecting the term dT?/dt in (60.3). 
Let us see to what extent the conditions can be fulfilled which make this 
neglect permissible. 

For a given frequency, the most favourable conditions for measuring the 
susceptibility are those where the body is as small as possible (to increase 
the space derivatives in curl M) and the electric field is as weak as possible 
(to reduce P). The field of an electromagnetic wave does not satisfy the 
latter condition, because E ~ H. Let us therefore consider a variable field, 
say in a solenoid, with the body under investigation placed on the axis. The 
electric field is due only to induction by the variable magnetic field, and the 
order of magnitude of E inside the body can be obtained by estimating the 
terms in the equation curlE= -(l/c)dB/8t, whence E/l ~ toH/c or 
E ~ (wllc)H, where / is the dimension of the body. Putting c— 1 ~ 1, we 
have dPJdt ~ <oE ~ oj 2 IH/c. For the space derivatives of the magnetic 
moment M = #H we have |ccurlM| ~ cxH/l. If \dP/dt\ is small com- 
pared with \c curl M|, we must have 

J2 <^ X c*lufi. (60.4) 

It is evident that the concept of magnetic susceptibility can be meaningful 
only if this inequality allows dimensions of the body which are (at least) just 
macroscopic, i.e. if it is compatible with the inequality / > a, where a is 
the atomic dimension. This condition is certainly not fulfilled for the 
optical frequency range ; for such frequencies, the magnetic susceptibility is 
always ~ v^jc 2 , where v is the electron velocity in the atom;f but the optical 



f The relaxation times for any paramagnetic or ferromagnetic processes are certainly 
large in comparison with the optical periods. 



§61 The field energy in dispersive media 253 

frequencies themselves are ~ v/a, and therefore the right-hand side of the 
inequality (60.4) is ~ a 2 . 

Thus there is certainly no meaning in using the magnetic susceptibility 
from optical frequencies onward, and in discussing such phenomena we 
must put /* = 1. To distinguish between B and H in this frequency range 
would be an over-refinement. Actually, the same is true for many phen- 
omena even at frequencies well below the optical range. 

§61. The field energy in dispersive media 

The formula 

S = cEx H/4tt (61.1) 

for the energy flux density remains valid in variable electromagnetic fields, 
even if dispersion is present. This is evident from the arguments given at 
the end of §29: on account of the continuity of the tangential components 
of E and H, formula (61.1) follows from the condition that the normal 
component of S is continuous at the boundary of the body and the validity 
of a similar formula in the vacuum outside the body. 

The rate of change of the energy in unit volume of the body is div S. 
Using Maxwell's equations, we can write this expression as 

1 / dD „ 8B \ 

- divS = d E -ir +H -i7) ; < 6L2 > 

see (56.15). In a dielectric medium without dispersion, when € and /x are 
real constants, this quantity can be regarded as the rate of change of the 
electromagnetic energy 

U= ( € E2 + m H2)/8tt, (61.3) 

which has an exact thermodynamic significance: it is the difference between 
the internal energy per unit volume with and without the field, the density 
and entropy remaining unchanged. 

In the presence of dispersion, no such simple interpretation is possible. 
Moreover, in the general case of arbitrary dispersion, the electromagnetic 
energy cannot be rationally defined as a thermodynamic quantity. This is 
because the presence of dispersion in general signifies a dissipation of energy, 
i.e. a dispersive medium is also an absorbing medium. 

To determine this dissipation, let us consider an electromagnetic field of 
a single frequency. By averaging with respect to time the expression (61.2), 
we find the steady rate of change of the energy, and this is the mean quantity 
Q of heat evolved per unit time and volume. 

Since the expression (61.2) is quadratic in the fields, all quantities must 
be written in real form. If, as is convenient for a field of a single frequency, 
we take E and H to be complex, then in (61.2) we must substitute for E and 
SDJdt respectively £(E+E*) and £(— iweE+icoe^E*), and similarly for H 



254 The Electromagnetic Wave Equations §61 

and dB/dt. On averaging with respect to time, the products E«E and E*»E*, 
which contain factors e T2io)t , give zero, leaving 

Q = TirK 6 * - e ) E * E * + 0** - J") H ' H *] = ^( € "I E I 2 + /*'W)- 

J.07T 07T 

This expression can also be written 

Q = o>(e"E2 + ^"!P)/47r, (61.4) 

where E and H are the real fields, and the bar denotes an average with 
respect to time. 

This important formula shows that the absorption (dissipation) of energy 
is determined by the imaginary parts of e and /*. The two terms in (61.4) 
are called the electric and magnetic losses respectively. On account of the 
law of increase of entropy, the sign of these losses is determinate : the dis- 
sipation of energy is accompanied by the evolution of heat, i.e. Q > 0. It 
therefore follows from (61.4) that the imaginary parts of e and //, are always 
positive : 

e" > 0, p" > (61.5) 

for all substances and at all frequencies.! The signs of the real parts of € 
and p for a> ^ are subject to no physical restriction. 

Any non-steady process in an actual body is to some extent thermo- 
dynamically irreversible. The electric and magnetic losses in a variable 
electromagnetic field therefore always occur to some extent, however slight. 
That is, the functions e"(<o) and p"(<o) are not exactly zero for any frequency 
other than zero. We shall see in §62 that this statement is of fundamental 
importance, although it does not exclude the possibility of only very small 
losses in certain frequency ranges. Such ranges, in which e" and /x" are 
very small in comparison with * and p, are called transparency ranges. It 
is possible to neglect the absorption in these ranges and to introduce the 
concept of the internal energy of the body in the electromagnetic field, in 
the same sense as in a constant field. To determine this quantity, it is not 
sufficient to consider a field of only a single frequency, since the strict 
periodicity results in no steady accumulation of electromagnetic energy. Let 
us therefore consider a field whose components have frequencies in a narrow 
range about some mean value a>o. The field strengths can be written 

E = E (t)e-^ot, H = Ho(t)e-i»ot, (61.6) 



t Strictly speaking, this statement applies to bodies which, in the absence of the variable 
field, are in thermodynamic equilibrium; we assume this condition to hold. If the body is 
not in thermal equilibrium, then Q may in principle be negative. The second law of thermo- 
dynamics requires only a net increase in entropy as a result of the effects of the variable 
electromagnetic field and of the absence of thermodynamic equilibrium, the latter effect 
being independent of the presence of the field. A hypothetical example of such a body is 
one in which all the atoms have been excited artificially (i.e. otherwise than by spontaneous 
thermal excitation). 



§61 The field energy in dispersive media 255 

where E (*) and Ho(0 are functions of time which vary only slowly in com- 
parison with the factor *-*»o'. The real parts of these expressions are to be 
substituted on the right-hand side of (61.2), and we then average with 
respect to time over the period 27r/o> , which is small compared with the time 
of variation of the factors Eo and Ho. 

The first term in (61.2), with E written in complex form, is 

i(E+E*)4(D + D*)/4tt, 

and similarly for the second term. The products E-D and E*.D* vanish 
when averaged over time, and can therefore be ignored, leaving 

1 / 3D* ^ 3D \ 

E-— - + E*.— • (61-7) 

16tt\ dt dt I 

We write the derivative 8D/dt as /E, where / is the operator d£/dt, and 
ascertain the effect of this operator on a function of the form (61.6). If Eo 
were a constant, we should have simply/E = /(w)E, where/(o>) = -*W(co). 
We expand the function Eo(f) as a series of Fourier components Eo a e _fa ', 
with constant Eo a . Since Eo(*) varies only slowly, this series will include 
only components with a <^ w . We can therefore put 



/E 0a e-*< w o + a > = /(a + a> )Eoa<H< ft, o + 



<x)t 



da>o 

Summing the Fourier components, we have 

d/(ajo) dEo . # 
fEo{t)e-i"ot = /(o*)E <H<V + i ^^ L ~^ e °' 

Omitting henceforward the suffix to o>, we thus obtain 

3D d(a>e) dE . , ... m 

= - icoe(co)E + _l—i— «-•* (61.8) 

3t do> dt 

Substituting this expression in (61.7) and neglecting the imaginary part of 
e(o>) gives 



1 dHL, »Eo 8Eo*\ 1 d(««) d 



since E-E* = E «Eo*. Adding a similar expression involving the magnetic 
field, we conclude that the steady rate of change of the energy in unit volume 
is given by dU/dt, where 

O _ _L« E .E* + ^*Wl . (61.9) 

167T L da> dco J 



256 The Electromagnetic Wave Equations §62 

In terms of the real fields E and H this expression can be written 

1 rd(a»e) — d(a> / a) — ] 



V 



1 rdfcue)— d(cou)—l 
= — -T-^ 2 + -—^HA. (61.10) 

o7rL dco da> J 



This is the required result: V is the mean value of the electromagnetic 
part of the internal energy per unit volume of a transparent medium. If there 
is no dispersion, e and p are constants, and (61.10) becomes the mean value 
of (61.3), as it should. 

If the external supply of electromagnetic energy to the body is cut off, the 
absorption which is always present (even though very small) ultimately 
converts the energy V entirely into heat. Since, by the law of increase of 
entropy, there must be evolution and not absorption of heat, we must have 
V > 0. It therefore follows, by (61.9), that the inequalities d(we)/dto > 0, 
d((t)fi)/dco > must hold. In reality, these conditions are necessarily ful- 
filled, by virtue of more stringent inequalities always satisfied by the functions 
e(w) and fi(to) in transparency ranges (see §64). f 

Considerable interest attaches to the determination of the (time) average 
stress tensor giving the forces on matter in a variable electromagnetic field. 
This problem is meaningful for both absorbing and non-absorbing media, 
whereas that concerning the internal energy can be proposed only if absorp- 
tion is neglected. The corresponding formulae, however, have not yet been 
derived. 



§62. The relation between the real and imaginary parts of e(a>) 

The function /(t) in (58.3) is finite for all values of t, including zero.t 
For dielectrics it tends to zero as r -*-oo. This simply expresses the fact 
that the value of D(*) at any instant cannot be appreciably affected by the 
values of E(*) at remote instants. The physical agency underlying the 
integral relation (58.3) consists in the processes of the establishment of the 
electric polarisation. Hence the range of values in which the function /(t) 
differs appreciably from zero is of the order of the relaxation time which 
characterises these processes. 

The above statements are true also of metals, the only difference being 
that the function /(t) — Atto, rather than/(r) itself, tends to zero as t -> oo. 
This difference arises because the passage of a steady conduction current, 
though it does not cause any actual change in the physical state of the metal, 
in our equations leads formally to the presence of an induction D such that 



f The sum of the inequalities (64.1) and (64.2) shows, in fact, that the derivative d(we)/do) 
always exceeds unity. 

X It was to ensure this that the term E(£) was separated in (58.3), since otherwise the 
function /(t) would have a delta-function singularity at t = 0. 



§62 The relation between the real and imaginary parts of c(o>) 257 

(llc)8Djdt = ^naEjcoT 

t 00 

D(f) = f 4tt<tE(t) dr = -W j E(t - t) dr. 

-00 

We have defined the function e(oi) by 

CO 

e(a>) = 1 + je^f(r) dr. (62.1) 



It is possible to derive some very general relations concerning this function 
by using the methods of the theory of functions of a complex variable. To 
do so, we regard w as a complex variable (co = to'+ito"), and ascertain 
the properties of the function e(cu) in the upper half of the <o-plane. From 
the definition (62.1) and the above-mentioned properties of the function 
/(t), it follows that e(o>) is a one- valued regular function everywhere in the 
upper half-plane. For, when a>" > 0, the integrand in (62.1) includes the 
exponentially decreasing factor c~ w " T and, since the function /(t) is finite 
throughout the region of integration, the integral converges. The function 
e(w) has no singularity on the real axis (a/' = 0), except possibly at the origin 
(where, for metals, «(o>) has a simple pole).f 

It is useful to notice that the conclusion that e(w) is regular in the upper 
half-plane is, physically, a consequence of the causality principle. The 
integration in (58.3) is, on account of this principle, taken only over times 
previous to t, and the region of integration in formula (62.1) therefore 
extends from to oo rather than from - oo to oo. 

It is evident also from the definition (62.1) that 

€ (_ w *) = € *(o>). (62.2) 

This generalises the relation (58.7) for real to. In particular, for purely 
imaginary w we have e(*V) = «*(««"), i.e. the function e(w) is real on the 
imaginary axis : 

ime = for to = iui" . (62.3) 

It should be emphasised that the property (62.2) merely expresses the fact 
that the operator relation D = eE must give real values of D for real E. If 
the function E(f) is given by the real expression 

E = Eo*-*"* + E V W * , (62.4) 



t In the lower half-plane, the definition (62.1) is invalid, since the integral diverges. 
Hence the function €(oj) can be defined in the lower half-plane only as the analytical continu- 
ation of formula (62.1) from the upper half-plane, and in general has singularities. 

The function e(co) has a physical as well as a mathematical significance in the upper half- 
plane : it gives the relation between D and E for fields whose amplitude increases as e . 
In the lower half-plane, this physical interpretation is not possible, if only because the 
presence of a field which is damped as e-l w "l* implies an infinite field f or t -> — oo. 



258 The Electromagnetic Wave Equations §62 

then, applying the operator £ to each term, we have 

D = €(co)Eoe- io,t + e(- o>*)Eo*«*°**, 

and the condition for this to be real is just (62.2). 

According to the results of §61, the imaginary part of e(a>) is positive for 
positive real co = co', i.e. on the right-hand half of the real axis. Since, by 
(62.2), ime(-w') = — im e(to'), the imaginary part of e(co) is negative on 
the left-hand half of this axis. Thus 

ime > for oo = a/ > 0. (62.5) 

At co = 0, im e changes sign, passing through zero for dielectrics and 
through infinity for metals. This is the only point on the real axis for which 
im e(co) can vanish. 

When <o tends to infinity in any manner in the upper half-plane, e(co) 
tends to unity. This has been shown in §59 for the case where co tends to 
infinity along the real axis. The general result is seen from formula (62. 1) : 
if co -» oo in such a way that co" -> oo, the integral in (62. 1) vanishes because 
of the factor e~ W ' T in the integrand, while if co" remains finite but \co'\ -» oo 
the integral vanishes because of the oscillating factor e ib)r . 

The above properties of the function e(co) are sufficient to prove the 
following theorem: the function e(tu) does not take real values at any finite 
point in the upper half-plane except on the imaginary axis, where it de- 
creases monotonically from eo > 1 (for dielectrics) or from + oo (for metals) 
at co = iO to 1 at to = too. Hence, in particular, it follows that the function 
e(co) has no zeros in the upper half-plane. 

We shall not pause to prove this theorem, because it is identical with a 
general theorem concerning the "generalised susceptibility" (and the prop- 
erties of e(co) enumerated above exhibit a similar analogy), f For the same 
reason, the function e(a>) satisfies the general relations between the real and 
imaginary parts of the generalised susceptibility. We shall repeat here the 
derivation of these relations, in order to emphasise certain differences between 
dielectrics and metals. 

Let us take some real value coq of co, and integrate the expression 
(e— l)/(co— coq) round the contour C shown in Fig. 29. This contour in- 




Fig. 29 



t See Statistical Physics, §122, Pergamon Press, London, 1958. The generalised suscep- 
tibility a(co) used there corresponds to e(to) — 1, which vanishes as « -> oo. 



§62 The relation between the real and imaginary parts of e(to) 259 

eludes the whole of the real axis, indented upwards at the point to = too > 0, 
and also at the point to = if the latter is (as in metals) a pole of the func- 
tion e(to), and is completed by a semicircle of infinite radius. At infinity 
c -> 1, and the function (e — l)/(to- too) therefore tends to zero more rapidly 
than 1/to. The integral 



dco (62.6) 



f 

J (a — coo 
c 



consequently converges; since c(to) is regular in the upper half-plane, and 
the point <o = a> has been excluded from the region of integration, the 
function (e — l)/(co — too) is analytic everywhere inside the contour C, and 
the integral is therefore zero. 

The integral along the semicircle at infinity is also zero. We pass round 
the point too along a semicircle whose radius p -> 0. The direction of 
integration is clockwise, and the contribution to the integral is — ^7r[e(too) — 1]. 
If the function e(to) pertains to a dielectric, the indentation at the origin is 
unnecessary, and we therefore have 

-p + w 00 

lim j — dco + dco - zV[e(coo) - 1] = 0. 

P-+°\ J CO — coo J CO — coo ) 

-oo p + w 

The expression in the braces is the integral from — oo to oo, taken as a 
principal value. Thus we have 

00 1 

P f dco - H>(co ) - 1] = 0. (62.7) 

J CO — coo 

—oo 

Here the variable of integration <a takes only real values. We replace it 
by x, call the given real value to instead of coo, and write the function e(co) 
of the real variable to, as in §58, in the form e(to) = e'(to)+/c"(to). Taking 
the real and imaginary parts of (62.7), we obtain the following two formulae: 



00 

1 r e 

E 'H - l = -p - 

TT J X 



"(*) 



dx, (62.8) 



1 r €'(*) - 1 
6 "(o,) = — P\ — — dx % (62.9) 

TT J X— CO 
-oo 

first derived by H. A. Kramers and R. de L. Kronig (1927). It should be 
emphasised that the only important property of the function c(to) used in 



260 The Electromagnetic Wave Equations §62 

the proof is that it is regular in the upper half-plane, f Hence we can say 
that Kramers and Kronig's formulae, like this property of e(a>), are a direct 
consequence of the causality principle. 

Using the fact that e"(x) is an odd function, we can rewrite (62.8) as 

1 f e"(x) 1 ? e"(x) 

e'(co) - 1 = -P \ — —dx + -P — — d* 

TT J X — CO TT J X + CO 



2 r xe"(x) 
= -P ~Y^ dx - (62.10) 

TT J X 1 — CO 1 


If a metal is concerned, the function e(o>) has a pole at the point to = 0, 
near which e = 47ria/co (58.9). The passage along a semicircle round this 
point gives a further real term —(47tct/coo)7t, which must be added to the 
left-hand side of equation (62.7). Thus formula (62.9) becomes 

1 fV(#) -1 Ait a 

€ "(o>) = - -P\ -Aj! d* + — , (62.11) 

77 J X — CO CO 

-oo 

but (62.8) and (62.10) remain unchanged. A further remark is also necessary 
as regards metals. We have said at the end of §58 that there may be ranges 
of frequency for metals in which the function e(to) becomes physically 
meaningless on account of the spatial non-uniformity of the field. In the 
formulae given here, however, the integration must be taken over all fre- 
quencies. In such cases e(<o) must be taken, in the frequency ranges con- 
cerned, as the function obtained by solving the formal problem of the 
behaviour of the body in a fictitious uniform periodic electric field (and not 
in the necessarily non-uniform field of the electromagnetic wave). 

Formula (62.10) is of particular importance: it makes possible a calcula- 
tion of the function e'(co) if the function e"{<o) is known even approximately 
(for example, empirically) for a given body. It is important to note that, for 
any function e"(a>) satisfying the physically necessary condition e" > 
for to > 0, formula (62.10) gives a function e'(w) consistent with all physical 
requirements, i.e. one which is in principle possible (the sign and magnitude 
of e' are subject to no general physical restrictions). This makes it possible 
to use formula (62.10) even when the function ^"(co) is approximate. 
Formula (62.9), on the other hand, does not give a physically possible 
function €"(co) for an arbitrary choice of the function e'(o>), since the condi- 
tion that e"(a>) > is not necessarily fulfilled. 



t The property e -> 1 as a> -> oo is not important: if the limit e(oo) were other than 
unity, we should simply take e — c(co) in place of e — 1, with corresponding obvious 
changes in formulae (62.8), (62.9). 



§62 The relation between the real and imaginary parts of e(a>) 261 

In dispersion theory the expression for e'(to) is customarily written in 
the form 

e>) _ 1 = _^ P f^Ld*, (62.12) 

o 

where e and m are the charge and mass of the electron, and /(to) da> is 
called the oscillator strength (or "number of dispersion electrons") in the 
frequency range dto. According to (62.10), this quantity is related to e"(a>) 
by 

m 
/H = TT7" e "H- (62 - 13) 

For metals, /(co) tends to a finite limit as to -> 0. 

For sufficiently large w, x 2 can be neglected in comparison with to 2 in 
the integrand in (62.10). Then 

dx. 



c'(co) - 1 = f Xe"(x) 

WOT J 
Q 



For the dielectric constant at high frequencies, on the other hand, formula 
(59.1) holds, and a comparison shows that 



00 

m 



27r 2 e 2 
o 



f co € "(o>)do> = f f (to) dot = iV, (62.14) 



where iV is the total number of electrons per unit volume. 

If € f (w) is regular at <o = 0, we can take the limit to -» in formula 
(62.10), obtaining 

e '(0)-l = - — ^d*. (62.15) 

o 

If the point to = is a singularity of e"( o>) (as in metals), the limit of the 
integral (62.10) as to -> is not what is obtained by simply deleting the term 
in to. To calculate the limit, we must first replace e"(x) in the integrand by 
e"(x)- / hro/x; the value of the integral is unchanged, because 

d# 



P = 0. 

J x 2 — to 2 



262 The Electromagnetic Wave Equations §62 

For a dielectric, formula (62.15) can be rewritten as 

4ire*N 

€0 _ l = o>- 2 , (62.16) 

m 

where the bar denotes averaging with respect to the "number of oscillators": 

N J 



co * = — I — dco. 







The expression (62.16) may be useful in estimating co- 

The following formula f relates the values of e(co) on the upper half of 
the imaginary axis to those of e"{<*>) on the real axis: 

2 F Xe"(x) 

</co)-l=- — ±Ldx. (62.17) 

IT J X 1 + CO 1 



Integrating this relation over all co, we obtain 

00 00 

f[ C (/a>)-l]dco = fe"(cu)dco. (62.18) 



All the above results are applicable, apart from slight changes, to the 
magnetic permeability /*(«>). The differences are due principally to the fact 
that the function /x(a>) ceases to be physically meaningful at relatively low 
frequencies. Hence, for example, Kramers and Kronig's formula must be 
applied to /a(o>) as follows. We consider not an infinite but a finite range of 
co (from to coi), which extends only to frequencies where p is still meaning- 
ful but no longer variable, so that its imaginary part may be taken as zero ; 
let the real quantity ja(coi) be denoted by (i±. Then formula (62.10) must be 
written as 

w i 
2 C xa"(x) 
/.'(«) - R = -P\ -f^d*. (62.19) 

IT J X L — (O z 



Unlike eo, the value /jlq of fj,(0) may be either less than or greater than unity. 
The variation of fj,(w) along the imaginary axis is again a monotonic decrease, 
from j^o to fj,i < fjbo- 



t See Statistical Physics, formula (122.19). 



§63 A plane wave of a single frequency 263 

§63. A plane wave of a single frequency 

Maxwell's equations (58.2) for a wave of a single frequency are 

za>/z(cu)H = ccurlE, /o>e(cu)E = — ccurlH. (63.1) 

These equations as they stand are complete, since equations (58.1) follow 
from (63.1) and so do not require separate consideration. Assuming the 
medium homogeneous, and eliminating H from equations (63.1), we obtain 
the second-order equation 

AE + £/x(w 2 /c 2 )E = 0; (63.2) 

elimination of E gives a similar equation for H. 

Let us consider a plane electromagnetic wave propagated in an infinite 
homogeneous medium. In a plane wave in a vacuum, the space dependence 
of the field is given by a factor e^'', with a real wave vector k. In con- 
sidering wave propagation in matter, however, it is in general necessary 
to take k complex: k = k'+*k", where the vectors k' and k" are real. 

Taking E and H as proportional to ei^' r , and carrying out the differentia- 
tion with respect to the co-ordinates in equations (63.1), we obtain 

o>/xH = ck x E, o>eE = -ckxH. (63.3) 

Eliminating E and H from these two equations, we obtain for the square of 
the wave vector 

£2 = #2 _ £"2 + 2zk'-k" = €fx^lc\ (63.4) 

We see that k can be real only if e and /x are real and positive. Even then, 
however, k may still be complex if k'»k" = 0; we shall meet with such a 
case in discussing total reflection in §66. 

It must be borne in mind that, in the general case of complex k, the term 
"plane wave" is purely conventional. Putting e ik ' r = e^- '*e~W' r , we see 
that the planes perpendicular to the vector k' are planes of constant phase. 
The planes of constant amplitude, however, are those perpendicular to k", 
the direction in which the wave is damped. The surfaces on which the field 
itself is constant are in general not planes at all. Such waves are called 
inhomogeneous plane waves, in contradistinction to ordinary "homogeneous" 
plane waves. 

The general relation between the electric and magnetic field components 
is given by formulae (63.3). In particular, taking the scalar product of these 
formulae with k, we obtain 

k.E = 0, k.H = 0, (63.5) 

and, squaring either and using (63.4), 

E2 = filPje. (63.6) 

It must be remembered, however, that because all three vectors k, E and H 
are complex these formulae do not in general have the same evident signifi- 
cance as when the vectors are real. 



264 The Electromagnetic Wave Equations §63 

We shall not give the cumbersome relations valid in the general case, but 
consider only the most important particular cases. Especially simple results 
are obtained for a wave propagated without damping in a non-absorbing 
(transparent) homogeneous medium. The wave vector is real, and its 
magnitude is 

k = ■\Z(€fi)colc = ncojc, (63.7) 

where n = VC^) * s called the refractive index of the medium. The electric 
and magnetic fields are both in a plane perpendicular to the vector k (a pure 
transverse wave) ; they are mutually perpendicular, and are related by 

H = <v/(*//*)l X E » (63.8) 

where 1 is a unit vector in the direction of k. Hence it follows that eE 2 = fiH 2 , 
but this does not mean (as it would in the absence of dispersion) that the 
electric and magnetic energies in the wave are equal, since these energies 
are given by different expressions (namely, the two terms in formula (61.10)). 
The velocity u with which the wave is propagated in the medium is given 
by the familiar expression for the group velocity :f 

dcu c 

U = ~TT = A/ v . • (63.9) 

ak d(«ct>)/da> 

It is easy to verify that 

u = SJC, (63.10) 

in accordance with its significance as the velocity of transfer of energy in the 
wave packet; here V is the energy density given by formula (61.9), and 

S = f/-E-E* (63.11) 

is the mean value of the Poynting vector. In the absence of dispersion, when 
the refractive index is independent of frequency, the expression (63.9) 
becomes simply c/n; cf. (56.13). 

Next, let us consider a more general case, the propagation of an electro- 
magnetic wave in an absorbing medium, the wave vector having a definite 
direction (i.e. k' and k" being parallel). Then the wave is literally plane, 
since the surfaces of constant field in it are planes perpendicular to the 
direction of propagation (a homogeneous plane wave). 

In this case we can introduce the "length" k of the wave vector, given by 
k = kl (1 being a unit vector in the direction of k' and k"), and from (63.4) 
we have k = y^e/^co/c. The complex quantity V( € / x ) * s usually written in 
the form n+iic, with real n and k, so that 

k = -\/(€ix)wJc = (n + iKJw/c. (63.12) 



t When considerable absorption occurs, the group velocity cannot be used, since in an 
absorbing medium wave packets are not propagated but rapidly "ironed out". 



§63 A plane wave of a single frequency 265 

The quantity n is called the refractive index of the medium, and k the 
absorption coefficient'^ the latter gives the rate of damping of the wave during 
its propagation. It should be emphasised, however, that the damping of 
the wave need not be due to true absorption: dissipation of energy occurs 
only when e and /x. are complex, but k is different from zero if «r and p are 
real and of opposite sign. 

We may express n and k in terms of the real and imaginary parts of the 
dielectric constant (taking fj, = 1). From the equation 

n 2 — k 2 + 2inK = e = e' + U" 

we have n 2 — k 2 = e', 2«k = e". Solving these equations for n and k, we 
havef 

n = VW + V(*' 2 + "**)]}, (63.13) 

In particular, for metals and in the frequency range where formula (58.9) is 
valid, the imaginary part of e is large compared with its real part, and is 
related to the conductivity by e" = faa/to; neglecting e' in comparison with 
e", we find that n and k are equal : 

n = K = ^(lira/to). (63.14) 

The relation between the fields E and H in this homogeneous plane wave 
is again given by formula (63.8), but e and y, are now complex. The formula 
again shows that the two fields and the direction of propagation are mutually 
perpendicular. If /* = 1, we write -y/e = <\/(n 2 +K 2 )exp[i tan -1 (*/«)], 
which shows that the magnetic field is -y/(n 2 +K 2 ) times the electric field in 
magnitude and tan _1 (i<r/«) from it in phase; in particular, when (63.14) 
holds, the phase difference is |-7r. 



PROBLEM 

At a given instant < = Oan electromagnetic perturbation occurs in some region of space. 
The perturbation is not maintained by external agencies, and is therefore damped in time. 
Find the damping decrement. 

Solution. We expand the initial perturbation as a Fourier integral with respect to the 
co-ordinates, and consider a component having a (real) wave vector k. The time dependence 
of this component is given, for sufficiently large t, by a factor e _ * w * with a complex "fre- 
quency" to, which is to be determined; the damping decrement is — im to. 

From the equations — tile = curl E = tkXE, f)/c = curl H = tk x H we have, eliminat- 
ing H, 

fi/c a =kx(kxE). (1) 

We take the direction of k as the x-axis. The "longitudinal" part of the perturbation there- 
fore satisfies D x — 0, whence D x = 0. 



t Since e" > 0, the signs of n and k must be the same, in accordance with the fact that 
the wave is damped in the direction of propagation. The choice of positive signs in (63.13) 
corresponds to a wave propagated in the positive ac-direction. 



266 The Electromagnetic Wave Equations §64 

The relation between D x and E x is of the form 

e 

Ex{t) = t-W x = J F(t-r)D x (r) dr; (2) 

—00 

cf. §58. Since we have D x (r) = for t > 0, it follows that 

o 
Exit) = J Fit-r)Dxir) dr. (3) 

-co 

Hence we see that, for large t , the time dependence of Ex is given essentially by that of the 
function Fit). 

For a field of a single frequency, (2) gives 

1 °° 
= f Fix)e iax dx, 

or, conversely, 

00 

1 f 1 

2n- J €(a>) 

—00 

To estimate this integral for large t , we displace the path of integration into the lower half- 
plane of a, where the integrand decreases rapidly. The singularities of the function l/e(co), 
i.e. the zeros and branch points of e(o>), must be excluded from the contour. The integral 
is then essentially proportional to e"* u o*, where a»o is the singularity nearest the real axis. 
This gives the solution for the longitudinal part of the perturbation. 

For the transverse components, we have from (1) Dy,zlc 2 +k z E y , t = 0. A similar analysis 
gives the result that the required "frequency" wo is in this case the zero or branch point 
of the function a> 2 e(a>)— c 2 A 2 which lies nearest the real axis. 

§64. Transparent media 

Let us apply the general formulae derived in §62 to media which absorb 
only slightly in a given range of frequencies, i.e. assuming that for these 
frequencies the imaginary part of the dielectric permeability may be 
neglected. 

In such a case there is no need to take the principal value in formula 
(62.10), since the point x = o> does not in practice lie in the region of 
integration. The integral can then be differentiated in the usual way with 
respect to the parameter o>, giving 

00 

de 4ct> f Xe '(x) 

dx. 



-vJ 



dcu it J (to 2 — x 2 ) 2 
o 

Since the integrand is positive throughout the region of integration, we 
conclude that 

de(o>)/do> > 0, (64.1) 

i.e.* if absorption is absent the dielectric constant is a monotonically increasing 
function of the frequency. 



§64 Transparent media 267 

Similarly, in the same frequency range we obtain another inequality, 

d 4o> p *V'(*) 

—[o>2( e -!)] = _- K) dx > 0, 

dco it J (x* — ix) z Y 

o 

or 

d e /do> > 2(1 - e)/o>. (64.2) 

If c < 1, this inequality is more stringent than (64.1). 

It may be noted that the inequalities (64. 1) and (64.2) (together with the 
corresponding ones for fi(o>)) ensure that the inequality u < c is satisfied by 
the velocity of propagation of waves. For example, if p = 1 we have 
n = y'e and, replacing e by n 2 in (64.1) and (64.2), 

d(no>)/da> > n, d(nco)/da> > 1/w. (64.3) 

Thus we obtain two inequalities for the velocity u (63.9): u < c/n and 

u < en, whence u < c whether n < 1 or n > 1. These inequalities also 

show that u > 0, i.e. the group velocity is in the same direction as the wave 

vector. This is quite natural, even if not logically necessary. 

Let us suppose that the weak absorption extends over a wide range of 

frequencies, from a>i to a>2 (> a>i), and consider frequencies a> such that 

a>i <^ a* <^ o)2. The region of integration in (62.10) divides into two parts, 

x < a>i and x > a>2. In the former region we can neglect x in comparison 

with a>, and in the latter region co in comparison with x, in the denominator 

of the integrand : 

w i 
2 ¥ dx 2 C 
e(co) = 1 + - €"(*) X€"(x)dx, (64.4) 

It J X 7T6t> 2 J 

i.e. the function e(<o) in this range is of the form a — b/cn 2 , where a and b 
are positive constants. The constant b can be expressed in terms of the 
"number of dispersion electrons" N± responsible for the absorption in the 
range from to coi (cf. (62.14)): 

e(o>) = a — ATrN\e 2 jmco 2 . (64.5) 

From this expression it follows, in particular, that, when the region of 
weak absorption is sufficiently wide, the dielectric permeability in general 
passes through zero. In this connection it should be recalled that a literally 
"transparent" medium is one in which e(a>) is not only real but also positive; 
if e is negative, the wave is damped inside the medium, even though no 
true dissipation of energy occurs. 

For the frequency at which e = the induction D is zero identically, 
and Maxwell's equations admit a variable electric field satisfying the single 
equation curl E = 0, with zero magnetic field. In other words, longitudinal 
electric waves can occur. To determine their velocity of propagation, we 



268 The Electromagnetic Wave Equations §64 

must take into account the dispersion of the dielectric permeability not only 
in frequency, but also with respect to the wave vector. The value of m for 
which e = is also a function of the wave vector. If the medium is iso- 
tropic, the next term after the zero-order term in the expansion of the scalar 
function a>(k) for which e[<o(k)] = is proportional to k 2 : to = a> +|ak 2 . 
Hence the velocity of propagation is u = dcofdk = <xk, and is proportional 
to the wave vector itself. 

PROBLEM 

A plane electromagnetic wave with a sharply defined forward front is incident normally 
on the boundary of a half-space (x > 0) occupied by a transparent medium with /* = 1. 
Determine the structure of the front of the transmitted wave (A. Sommerfeld and L. 
Brillouin, 1914). 

Solution. Let the wave be incident on the boundary of the medium at time t — 0, so 
that at * = the field (E or H) of the incident wave is E = for t < 0, E ~ <r i( V for 
t > 0. Expanding this field as a Fourier integral with respect to time, we reduce the prob- 
lem to that of waves of various frequencies and infinite extent incident on the boundary. 
The amplitude of the Fourier component of frequency w is proportional to 

00 

j* g<(o)-wo)T dr. 


When a wave of frequency o> is incident, the transmitted wave is of the form a(a>) e~ iu>t + ia>nx,c f 
where the amplitude a(w) is a slowly varying function of frequency. Hence the wave field in 
the medium in the present problem is 

oo oo 

E ~ Jdoi a(a))e-* w '+* wna; / c f e*< w -<V T dr. 

—oo 

In the region near the wave front, the important values of a> in this integral are those 
close to o>o. Using a new variable £ = a>— o>o, we replace a(o>) by a(a>o), and expand the 
exponent in powers of f . Omitting unimportant constants and phase factors, we have 



00 00 

E ~ J j exp{*f (t-« + ^ -i^ 8 *^-} di dr, 



where u = «(a>o) is the velocity of propagation (63.9), and «' = [dM/dw] w _ 0)() . Effecting 
the integration over £, we easily bring E to the form 

oo 

E ~ fe* ir > dij, w = (x-ut)lV(2x\u'\), 
to 

the sign in the exponent depending on that of «'. The intensity distribution near the wave 

front is given by 

00 I 2 

/ ~ J e*V 2 dy . 

This expression is of the same form as that which gives the intensity distribution near the 
edge of the shadow in Fresnel diffraction, f For to > the intensity decreases monotoni- 
cally with increasing w, but for w < it oscillates with decreasing amplitude about a constant 
value to which it tends as w -> — oo . J ♦ 



t See The Classical Theory of Fields, §7-8. 

% At large distances preceding the front here considered there are found "precursors" 
propagated with velocity c. These correspond to the high-frequency Fourier components, 
for which e -> 1 . 



CHAPTER X 

THE PROPAGATION OF ELECTROMAGNETIC 

WAVES 

§65. Geometrical optics 

The condition for geometrical optics f to be applicable is that the wave- 
length A should be small in comparison with the characteristic dimension / 
of the problem. The relation between geometrical and wave optics is that, 
for A <^ /, any quantity <f> which describes the wave field (i.e. any component 
of E or H) is given by a formula of the type <f> = aeW where the amplitude 
a is a slowly varying function of the co-ordinates and time, and the phase iff 
is a large quantity which is "almost linear" in the co-ordinates and the 
time; it is called the eikonal, and is of great importance in geometrical 
optics. The time derivative of «/r gives the frequency of the wave: 

difi/dt = - a», (65.1) 

and the space derivatives give the wave vector: 

grad«£ = k, (65.2) 

and consequently the direction of the ray through any point in space. 

For a steady wave of a single frequency, the frequency is a constant and 
the time dependence of the eikonal is given by a term — at. We then intro- 
duce a function fa (also called the eikonal), such that 

j/t = — cot + (oo/c)iffi(x,y,z). (65.3) 

Then 0i is a function of the co-ordinates only, and its gradient is 

gradi/ri = n, (65.4) 

where n is a vector such that 

k = conjc. (65.5) 

The magnitude of n is equal to the refractive index n of the medium. J 
Hence the equation for the eikonal in ray propagation in a medium of refrac- 
tive index n(x, y, z) (a given function of the co-ordinates) is 

*-»-(■£)'♦(■£)■♦(£)•-* <*> 



t See The Classical Theory of Fields, §7-1, Addison- Wesley Press, Cambridge (Mass.), 
1951 ; Pergamon Press, London, 1959. 
X Only transparent media are considered in geometrical optics. 

269 



270 The Propagation of Electromagnetic Waves §65 

The equation of ray propagation in a steady state can also be derived from 
Fermat's principle, according to which the integral /k«dl (or, equivalently, 
the integral ?/ri = Jn»dl = Jnd/) along the path of the ray between two 
given points A and B has a value less than for any other path between A 
and B. Equating to zero the variation of this integral, we have 

B 

S^i = j (Sndl + wSdZ) = 0. 

A 

Let Sr be a displacement of the ray path under the variation. Then 
Sn = Sr»grad«, S d/ = l«dSr, where 1 is a unit vector tangential to the 
ray. Substituting in S^i and integrating by parts in the second term (using 
the fact that Sr = at A and B), we have 

B B 

S^tjl = I Sr«gradwd/ + I «l«dSr 



= J( grad „_^)). 8rd/ = . 



Hence 



d(«l)/d/ = gradw. (65.7) 

Expanding the derivative and putting d«/d/ = l«grad«, we obtain 

dl 1 

— = -[gradw - l(l.gradn)]. (65.8) 

dl n 

This is the equation giving the form of the rays. 

We know from differential geometry that the derivative dl/d/ along the 
ray is equal to N/2?, where N is the unit vector along the principal normal 
and 2? the radius of curvature. Taking the scalar product of both sides of 
(65.8) with N, and using the fact that N and 1 are perpendicular, we have 

1 gradn 

- = N.5 ; (65.9) 

2? n 

the rays are therefore bent in the direction of increasing refractive index. 

The velocity of propagation of rays in geometrical optics is in the direc- 
tion of 1, and is given by the derivative 

u = dco/dk. (65.10) 

This is also called the group velocity, the ratio w/k being called the phase 
velocity. It must be remembered, however, that the latter is not the velocity 
of physical propagation of any quantity. 



§65 Geometrical optics 271 

It is easy to derive also the equation which gives the rate of change of the 
radiation intensity along a ray. The intensity J is the magnitude of the 
(time) average Poynting vector. This vector, like the group velocity, is in 
the direction of 1: § = 71. In a steady state, the mean field energy density 
is constant at any given point in space. The equation of conservation of 
energy is therefore div § = 0, or 

div(71) = 0. (65.11) 

This is the required equation. 

Finally, let us consider how the direction of polarisation of linearly 
polarised radiation varies along a ray (S. M. Rytov, 1938). As we know 
from differential geometry, a curve in space (in this case, the ray) is charac- 
terised at every point by the mutually perpendicular unit vectors along the 
tangent (1), the principal normal (N) and the binormal (b), which form the 
natural trihedral. Since the electromagnetic waves are transverse, the vectors 
E and H are always coplanar with N and b. 

Let the direction of E at some point on the ray be the same as that of N, 
i.e. let E lie in the osculating plane (that of N and 1). The deviation of the 
curve from the osculating plane over a length d/ is of the third order of 
smallness with respect to d/. We can therefore say that, over a length d/ 
of the ray, the vector E remains in the original osculating plane. The ospu- 
lating plane at the other end of d/ is inclined to the original one at an angle 
d<f> = dl/T, where T is the radius of torsion. This is therefore the angle 
turned through by the vector E relative to N in the normal plane. Thus, 
over a distance d/ along the ray, the direction of polarisation rotates in the 
normal plane, its angle to the principal normal varying in accordance with 
the equation 

d<f>/dl = ljT. (65.12) 

In particular, when the torsion is zero, i.e. the ray is a plane curve, the 
direction of the vector E in the normal plane is constant, as is in any case 
evident from symmetry. 

PROBLEM 

Determine the velocity of propagation of light in a medium moving relative to the 
observer. 

Solution. Let o> and k be the frequency and wave vector of the light wave in a fixed 
frame of reference K, and o>', k' the corresponding quantities in a frame K' moving with 
the medium at velocity v relative to K. In the first approximation with respect to vjc (the 
only one we shall consider), the motion perpendicular to k has no effect on the propagation 
of light, and so, without loss of generality, we can assume that v and k are in the same 
direction. 

In the frame K' the medium is at rest, and a/ and k' are therefore related by 

ck' = a)'n(w')- (1) 

According to the relativistic transformation formulae f we have, as far as terms of the first 



f See The Classical Theory of Fields, §6-4. 



272 



The Propagation of Electromagnetic Waves 



§66 



order in vjc t to' = oi—kv, k' = k — cov/c 2 . Substituting these expressions in (1) and ex- 
panding the function n(w'), we obtain to the same accuracy 



ta vu> I ' d(«co)\ 

k = n— + — Hl-M— 1, 

c c 2 \ dw / 



where n = n(u>). The velocity of propagation (the group velocity) is therefore 

vnco duo 



u = uo+v\ 1 —I — 



(2) 



(3) 



where uo = c[d(wa>)/da>] -1 is the velocity of propagation in a medium at rest. The phase 
velocity is 

to c I 1 co dn\ 

— = -+v[l- — + — 
k n \ n? n 



M 



do/' 



The first two terms in (3) can also be obtained by simply applying the relativistic formula 
for the addition of velocities, and the third is a dispersion effect, first discussed by H. A. 
Lorentz. 

§66. Reflection and refraction of electromagnetic waves 

Let us consider the reflection and refraction of a plane electromagnetic 
wave (of a single frequency) at a plane boundary between two homogeneous 
media, f Medium 1, from which the wave is incident, is assumed trans- 
parent, but not (for the present) medium 2. Quantities pertaining to the 
incident and reflected waves will be distinguished by the suffixes and 1 
respectively, and those for the refracted wave by the suffix 2 (Fig. 30). The 
direction of the normal from the boundary plane into medium 2 is taken as 
the sr-axis. 




Fig. 30 



Since there is complete homogeneity in the ry-plane, the dependence of 
the solution of the field equations on x and y must be the same in all space. 
The components k x , k y of the wave vector must therefore be the same for 
all three waves. Consequently, the directions of propagation of the three 
waves lie in one plane, which we take as the a^-plane. 



t We take /tt = 1 in both media. 



§66 Reflection and refraction of electromagnetic waves 273 

From the equations 

fax = fax = fax (66.1) 

we find 

k\ z = — ko z = — (a)jc)\/ei cos 0o, 
k 2z = vT(*>A0 2€ 2 - fax 2 ] = W C W(*2 ~ e i sin2 o ). 



(66.2) 



The vector ko is, by definition, real, and so is ki. The quantity k 2z , however, 
is complex in an absorbing medium, and the sign of the root must be taken 
so that im&2 2 > 0, the refracted wave being damped towards the interior 
of medium 2. 

If both media are transparent, equations (66.1) give the familiar laws of 
reflection and refraction: 

sin 09 I e± n\ 

01 = 00, -7-^= /- = -• ( 66 - 3 ) 

sin 0o V «2 »2 

To determine the amplitudes of the reflected and refracted waves, we must 
use the boundary conditions at the surface of separation (z = 0), and we 
shall consider separately the two cases where the electric field Eo is in the 
plane of incidence and perpendicular to that plane ; from the results we can 
obtain the solution for the general case, where Eo can be resolved into compo- 
nents in these two directions. 

Let us first suppose that Eo is perpendicular to the plane of incidence. It 
is evident from symmetry that the same will be true of the fields Ei and E2 
in the reflected and refracted waves. The vector H is in the xsr-plane. 
The boundary conditions require f the continuity of E y = E and H x \ by 
(63.3) H x = -cfaEy/w. 

The field in medium 1 is the sum of the fields in the incident and reflected 
waves, so that we obtain the two equations E0 + E1 = E2, fa z (Eo — E\) = fa z E 2 . 
The exponential factors in E cancel because k x (and therefore to) is the same 
in all three waves. In what follows, E signifies the complex amplitude of a 
wave. The solution of the above equations gives FresneVs formulae'. 



faz- &2z _ V e i cos 0o - -v/( € 2 - € i sin 2 o ) 
hj\ = Eo = Eo, 

faz + faz V € l cos #0 + V( € 2 - € 1 sm2 #o) 

2&oz „ 2-vAicos0o 

Eq = — — Eq. 



(66.4) 



faz + faz V € i cos ^o + V( € 2 - e i sin 2 6c) 



t The boundary conditions on the normal components of B and D give nothing new in 
the present problem, because the equations div B = 0, div D = are consequences of 
equations (63.1). 



274 The Propagation of Electromagnetic Waves §66 

If both media are transparent, these formulae become, by (66.3), 

sin(0 2 -0 o ) 

hl = • , n TTT*' ' 

sm(0 2 + O ) 

. . n (66.5) 

2 cos do sin #2 

E 2 = Eo. 

sin(0 2 + o ) 

The case where E lies in the plane of incidence can be discussed similarly. 
Here it is more convenient to carry out the calculations for the magnetic 
field, which is perpendicular to the plane of incidence. A further two Fresnel's 
formulae are obtained: 



(66.6) 



€ 2 ko z - ei&2z u * 2 COS 0<> ~~ V( e l e 2 - ei 2 Sin 2 O ) „ 

tl\ = Hq = ; /zO, 

«2&0z + «l&2z e 2 COS ^o + \/( e l e 2 — ei 2 sin 2 0o) 

2e 2 ^0z 2e 2 COS do 

ti 2 = Hq = .no* 

€2^oz + nk2z € 2 cos 6 + V( € i e 2 - n 2 sin 2 6 ) 

If both media are transparent, these formulae may be written 

tan(0 o -0 2 ) 

Ul = 77i ^~T-"0> 

tan(0 o + 2 ) 

sin 20n 

H 2 = H . 

sin (0 O + 2 ) cos (0 O - 6 2 ) 

The reflection coefficient R is defined as the ratio of the (time) average 
energy flux reflected from the surface to the incident flux. Each of these 
fluxes is given by the averaged ^-component of the Poynting vector (63.11) 
for the wave in question, 

A Aicos0 1 |Ei| 2 |Ei|2 



(66.7) 



R = 



-vAicos0 o |Eo| 2 |Eo| s 



For normal incidence (0 = 0) the two modes of polarisation are equiva- 
lent, and the reflection coefficient is given by 

•\Ai — V € 2 



R = v . *. . (66.8) 

V e i + V e 2 

This formula is valid whether the reflecting medium is transparent or not. 
If we put <\/e 2 = n 2 + iK 2 , and if medium 1 is a vacuum (ei = 1), then 

(n 2 — l) 2 + «"2 2 
R = — — . (66.9) 

(«2 + I) 2 + *2 2 

The remaining discussion assumes that both media are transparent. The 
following general remark should be made first of all. The boundary between 
two different media is in reality not a geometrical surface but a thin transition 



§66 Reflection and refraction of electromagnetic waves 27 S 

layer. The validity of the formulae (66.1) does not rest on any assumptions 
concerning the nature of this layer. The derivation of Fresnel's formulae, 
on the other hand, is based on the use of the boundary conditions, and 
assumes that the thickness 8 of the transition layer is small compared with 
the wavelength A. The thickness 8 is usually comparable with the distances 
between the atoms, which are always small compared with A if the macro- 
scopic description of the field is legitimate, and so the condition A > 8 is 
usually fulfilled. In the opposite limiting case the phenomenon of refraction 
is entirely different in character. For 8 > A, geometrical optics is valid (A 
being small compared with the dimensions of the inhomogeneities in the 
medium). In this case, therefore, the propagation of the wave can be re- 
garded as the propagation of rays which undergo refraction in the transition 
layer but are not reflected from it. The reflection coefficient, therefore, 
would be zero. 

Let us return now to Fresnel's formulae. In reflection from a transparent 
medium, the coefficients of proportionality between Ei, E2 and Eo in these 
formulae are real, f This means that the wave phase either remains unchanged 
or changes by it, depending on the sign of the coefficients. In particular, 
the phase of the refracted wave is always the same as that of the incident 
wave. The reflection, on the other hand, may be accompanied by a change 
in phase.J For example, with normal incidence the phase of the wave is 
unchanged if ei > e 2 , but if ei < e 2 the vectors Ei and Eo are in opposite 
directions, i.e. the wave phase changes by it. 

The reflection coefficients for oblique incidence are, by (66.5) and (66.7), 

_ sin^-«o) _ «*■»(*-«,) 

x sin2(0 2 + o ) tan2(0 2 + o ) v ; 

Here, and in what follows, the suffixes _j_ and || refer to the cases where the 
field E is respectively perpendicular and parallel to the plane of incidence. 
The expressions (66. 10) are unaltered when 2 and 0o are interchanged (but 
the phase of the reflected wave changes by it, as is seen from formulae 
(66. 5) and (66.7)). The reflection coefficient for a wave incident from medium 
1 at an angle 0o is therefore equal to that for a wave incident from medium 2 
at an angle 2 . 

An interesting case is the reflection of light incident at an angle 0o such 
that #o + #2 = h 7 (th e reflected and refracted rays being thus perpendicular). 
Let this angle be 6 V \ sin 6 P = sin {^n— 2 ) = cos 2 , and the law of refrac- 
tion (66.3) gives 

tan 02, = V( £ 2/*i). (66.11) 



f We ignore for the moment the possibility of total reflection (see below). 

X Reflection from an absorbing medium leads in general to the appearance of elliptical 
polarisation. The explicit expressions for the amplitude and phase relations between the 
three waves are then extremely involved ; they are given by J. A. Stratton, Electromagnetic 
Theory, Chapter IX, McGraw-Hill, New York, 1941. 



276 The Propagation of Electromagnetic Waves §66 

For 0o = Op we have tan(02 + 0o) = oo, and R n = 0. Hence, whatever the 
direction of polarisation of light incident at this angle, the reflected light 
will be polarised so that the electric field is perpendicular to the plane of 
incidence. The reflected light is polarised in this way even when the incident 
light is natural: no component with any other polarisation is reflected. The 
angle 6 P is called the angle of total polarisation or the Brewster angle. It 
should be noticed that, whereas natural light can be totally polarised by 
reflection, this effect cannot be produced by refraction, whatever the angle 
of incidence. 

The reflection and refraction of plane-polarised light always results in 
plane-polarised light, but the direction of polarisation is in general not the 
same as in the incident light. Let yo be the angle between the direction of 
Eo and the plane of incidence, and yi, y^ the corresponding angles for the 
reflected and refracted waves. Using formulae (66.5) and (66.7), we easily 
obtain the relations 

cos (0o — 02) 

tanyi = tanyo, 

cos(»„ + * 2 ) 

tan 72 = cos (0o — 02) tan yo. 

The angles yo, y\ and y2 are equal for all angles of incidence only in the 
obvious cases yo = and yo = \n\ they are also equal for normal incidence 
(00 = 02 = 0) and for grazing incidence (0o = %n, in which case there is no 
refracted wave). In all other cases the formulae (66.12) give (by virtue of 
the inequalities < 0o, 02 < fyr an d, as we shall assume, < yo < \n, 
< yi> 72 < t) the inequalities yi > yo, y2 < yo- Thus the direction of E 
is turned away from the plane of incidence on reflection, but towards it on 
refraction. 

A comparison of the two formulae (66.10) shows that, at all angles of 
incidence except 0o = or far, i?„ < R ± . Hence, for example, when the 
incident light is natural the reflected light is partly polarised, and the pre- 
dominant direction of the electric field is perpendicular to the plane of 
incidence. The refracted light is partly polarised, with the predominant 
direction of E lying in the plane of incidence. 

The quantities R n and R ± depend quite differently on the angle of 
incidence. The coefficient R ± increases monotonically with the angle 0o 
from the value (66.8) for 0o = 0. The coefficient J?„ takes the same value 
(66.8) for 0o = 0, but as 0o increases /?„ decreases to zero at 0o = dp before 
monotonically increasing. 

Here two distinct cases occur. If the reflection is from the "optically 
denser" medium, i.e. €2 > ei, then jR„ and R ± increase to the common value 
of unity at 0o = \tt (grazing incidence). If, on the other hand, the reflecting 
medium is "optically less dense" (*2 < ei), both coefficients become equal to 
unity for 0o = 6 r , where 

sin0 r = V( € 2/ € i) = «2/«i; (66.13) 



§66 Reflection and refraction of electromagnetic waves 277 

r is called the angle of total reflection. When 0o = #r the angle of refraction 
02 = far, i-e. the refracted wave is propagated along the surface separating 
the media. 

Reflection from an optically less dense medium at angles 0o > 0r requires 
special consideration. In this case k% z is purely imaginary (see (66.2)), i.e. 
the field is damped in medium 2. The damping of the wave without true 
absorption (i.e. dissipation of energy) signifies that the average energy 
flux from medium 1 into medium 2 is zero (by simple calculation it can 
easily be seen that the vector § giving the average energy flux in medium 
2 is in the ^-direction). That is, all the energy incident on the boundary is 
reflected back into medium 1, so that the reflection coefficients are 
R x = J?,, = 1. This phenomenon is called total reflection.^ The equality 
of R x and R „ to unity can, of course, be obtained directly from FresnePs 
formulae (66.4) and (66.6). 

For 0o > Or the proportionality coefficients between Ei and Eo become 
complex quantities, of the form (a — ib)l(a+ib). The quantities R ± and /?„ 
are given by the squared moduli of these coefficients, which are equal to 
unity. The formulae give, besides the ratio of the magnitudes of the fields 
in the reflected and incident waves, the difference in their phases. For this 
purpose we write Ei x = e-^xEo^ Ei t = e~**".Ebr Then} 

tan|Sj_ = V( € i sin2 0o - € 2 )/\Ai cos ^ ' 

(00.14) 
tan|S„ = \/( € i 2 sin 2 O — n^l^2 cos0 o . 

Thus total reflection involves a change in the wave phase which is in general 
different for the field components parallel and perpendicular to 'the plane of 
incidence. Hence, on reflection of a wave polarised in a plane inclined to 
the plane of incidence, the reflected wave will be elliptically polarised. || 
The phase difference 8 = 8 X — 8 , is easily found to be such that 

cos0oV(eisin2 o -e 2 ) 

tan£S = — . (66.15) 

y €i sm 2 0o 

The difference is zero only for 0o = r or 0o = \n. 

PROBLEMS 

Problem 1. Find the manner in which the reflection coefficient approaches unity near 
the angle of total reflection. 

Solution. We put do — Or— 8, where S is a small quantity, and expand sin 0o and 
cos do in formulae (66.10) in powers of 8. The result is 

# X= 1_4V(28)( W 2-1)-*, 

R t = l-4V(2S)n2(M2-l)-*, 

where n 2 = eijez > 1. The derivatives dR/dS become infinite as 8 _ * when 8 -> 0. 



f It may be mentioned that the reflection coefficient is always equal to unity in reflection 
from a medium with e real and negative. In such a medium there is again no true absorption, 
but the wave cannot penetrate into it. 

X If (a - ib)l(a + ib) = e-«s, then tan £S = b/a. 

|| See The Classical Theory of Fields, §6-5. 



278 The Propagation of Electromagnetic Waves §66 

Problem 2. Find the reflection coefficient for almost grazing incidence of light from a 
vacuum on the surface of a body for which e is almost unity. 

Solution. Formulae (66.10) give the same reflection coefficient: 

R ± £* R t S* foo- V(^o 2 +e-l)]V(e-l) 2 , 

where ^o = i»— 0o. 

Problem 3. Determine the reflection coefficient for a wave incident from a vacuum on 
a medium for which both e and /* are different from unity. 

Solution. Calculations entirely analogous to those given above furnish the result 

|/x cos do — V^M - sin 2 0o)| 2 



R,= 



I/* cos 0o+V( 6 J* — -sin 2 0o)l 
c cos 0o+ \/( e /* — sin 2 #o)| 2 
e cos 0o+ V( e M — sin 2 0o)l 



Problem 4. A plane-parallel layer (region 2) lies between a vacuum (region 1) and an 
arbitrary medium (region 3). Light polarised parallel or perpendicular to the plane of 
incidence falls on the layer from the vacuum. Express the reflection coefficient jR in terms of 
those for semi-infinite media of the substances in regions 2 and 3. 

Solution. We denote by Ao and Ax the amplitudes of the field (E or H, whichever is 
parallel to the layer) in the incident and reflected waves. The field in the layer consists of 
the refracted wave (amplitude A%) and the wave reflected from region 3 (amplitude At.'). 
The boundary condition between regions 1 and 2 gives 

A 2 = a(Ai-ri2Ao) (1) 

where a and rx% are constants. In reflection from a semi-infinite medium of the substance 
in region 2, Az is zero, and so from (1) we have rxz — Ax/Ao, i.e. ri2 is the amplitude of 
reflection in that case. Another equation is obtained from (1) by interchanging A\ and Ao 
and replacing A% by Ai, which corresponds simply to a reversal of the sr-component of the 
wave vector: 

A = a(Ao-ri2Ai). (2) 

In region 3 there is only the transmitted wave, whose amplitude A3 satisfies the conditions 

A 2 eW = aAz, Az'e-W = —arz%Az (3) 

analogous to (1), (2) with Ax = 0. The exponential factors take account of the change in 
the wave phase over the thickness h of the layer, with 

4> = (cohlc)V(ez-sin 2 9 ). (4) 

Eliminating A3 from equations (3), we obtain 

Az'e-W = rzsAzeW, (5) 

where rza = — r32. 

From equations (1), (2) and (5) we find the amplitude of reflection from the layer: 

_ Ai _ me-^+m 
r ~ Ao ~ e-W+rwn ' (6) 

and the reflection coefficient R = \r\ 2 . The significance of r 2 3 is found from the fact that, 
f or Ji = 0, r must be the amplitude of reflection ri3 from a semi-infinite medium of the 
substance in region 3. Hence 

rw — (ri2— ri3)l(ri2ri3— 1). (7) 

Formulae (6) and (7) give the required solution. It should be emphasised that their deriva- 
tion involves no assumptions concerning the properties of regions 2 and 3, which may be 
either transparent or not. 



§67 The surface impedance of metals 279 

If regions 2 and 3 are transparent, then tfi, r\% and r\% are all real, and ras is the amplitude 
of reflection at the boundary between semi-infinite media of the substances in regions 2 and 
3. From (6) we have 

_ (n2+r23) 2 — 4ri2r23 sin 2 tfi 

(ri2r23+l) 2 — 4ri2r23 sin 2 ^ 

As t/t varies, R varies between the limits [(n.2+r23)/(n.2r23+l)] 2 and [(rt2— r23)/(n2r23 — l)] 2 . 
For normal incidence n.2 = (m — W2)/(«i+M2), and ris and r23 are given by similar relations. 
If «2 2 = «iW3, then riz = rzz, and jR may be zero for some value of the thickness of the layer. 
If region 3 is a vacuum, then ri3 = 0, r23 = — *12, and (6) gives 

ri2(«- 21 *-l) sinhi'0 

r = ■ = . (9) 

e -2i<\i _ ri2 2 sjnh j-,-0 _|. l Q g ( — ri2)] 

If also region 2 is transparent, we have 

4i?i2 sin 2 if) 



R 



(l-i?i 2 ) 2 +4i?i2sin 2 1 A 

The transmission coefficient D for the layer (between vacua) is 1 — R only if region 2 is 
transparent. Otherwise D must be calculated from equations (l)-(3), putting 732 =ria. 
The amplitude of transmission d is 

and the transmission coefficient £> = |rf| 2 . 

Problem 5. Determine the reflection and transmission coefficients for light incident 
normally on a slab with a very large complex dielectric permeability e. 

Solution. In this case ri2 = (1 — V e )l(^ + V«0 ~ — (1— 2/\/ e )» and formula (9) of 
Problem 4 gives r = —[1 — (2/V e ) coth #] _1 , ^ = coh^/e/c. If the slab is so thin that 
(oh/c <^ 1/Vl e l> then we can put r = — [l+Hc/eayh] -1 , and distinguish two cases: 

for l/|e| < whle < 1/VM, R = 1 — 4cc"/«A|e|a, 

for coA/c < l/|e|, ii = w 2 /i 2 |e| 2 /4c 2 . 
The transmission coefficient is, by formula (10), 

for oih\c ~ 1/VM, d = — 2/\/e sinh tip, 

for wA/c < 1/VM, rf = (1 —ietoh/lc)- 1 . 
Again two cases can be distinguished : 

for 1/| c| < toA/c < 1/VM, -D = 4c 2 /o) 2 A 2 |e| 2 , 

for a>h/c < l/|e|, D = 1 -e"o>A/c. 

§67. The surface impedance of metals 

The dielectric permeability of metals is, in magnitude, large compared 
with unity at low frequencies (as <o -> 0, it tends to infinity as 1/w). The 
"wavelength" S ~ c/a)^\e\ in metalsf is then small compared with the 
wavelength A ~ c/w in vacuo. If 8 (but not necessarily A) is also small 
compared with the radii of curvature of the metal surface, the problem of 
the reflection of arbitrary electromagnetic waves from the metal can be 
considerably simplified. 



t Large values of \/e(<u) are almost always complex. The electromagnetic field is damped 
inside the body, so that the "wavelength" in the body is also the depth of penetration of the 
field. If e(o>) is expressed in terms of the conductivity by (58.9), the quantity 8 is the same 
as the penetration depth used in §45. 

19 



280 The Propagation of Electromagnetic Waves §67 

The smallness of 8 implies that the derivatives of the field components 
inside the metal along the normal to the surface are large compared with the 
tangential derivatives. The field inside the metal near the surface can 
therefore be regarded as the field of a plane wave, and hence the fields Ef 
and Hf are related by 

E* = vW«)H« * n, (67.1) 

where n is a unit vector along the inward normal to the surface. Since Ej 
and H$ are continuous, their values outside the metal near the surface must 
be related in the same way. As M. A. Leontovich (1948) has pointed out, 
the equation (67.1) may be used as a boundary condition in determining the 
field outside the metal. Thus the problem of determining the external 
electromagnetic field can be solved without considering the field inside the 
metal. 

The quantity ■\/(fJ>/e) is called the surface impedance^ of the metal, and 
we denote it by £ = £' + it," : 

I - Vfc/c). (67-2) 

In the frequency range where e can be expressed in terms of the ordinary 
conductivity of the metal, we have 

I = (1 - i)y/(otnl87ra). (67.3) 

The (time) average energy flux through the surface of the metal is 

§ = (cl&r) re(Et x H t *) = <*'|H«|2n/&r. ( 67 - 4 ) 

This is the energy which enters the metal and is dissipated therein. Hence 
we see, in particular, that 

H > 0. (67.5) 

This inequality determines the sign of the root in (67.2). 

As the frequency increases, the depth of penetration S becomes of the 
same order as the mean free path / of the conduction electrons. J In this case 
the spatial non-uniformity of the field renders impossible a macroscopic 
description of it in terms of the dielectric permeability e, a fact first pointed 
out by H. London (1940). It is remarkable that the condition v/l > to 
(v being the velocity of the conduction electrons), which ensures the absence 
of dispersion of the conductivity and the equality of the latter to its value a 
for a constant field, remains valid. 

It is of importance to note that a boundary condition of the form 

E* = £H t x n (67.6) 



t This name is usually given to the quantity A-n^jc, but we find that convention less 
satisfactory. 

X The mean free path depends considerably on the temperature of the metal. In practice, 
the temperatures considered are usually very low, in the helium range, and the phenomena 
under consideration occur in the range of very short radio waves. 



§67 The surface impedance of metals 281 

still holds at such frequencies. The field inside the metal near the surface 
can again be regarded as a plane wave, although it is no longer described by 
the usual macroscopic Maxwell's equations. In such a wave the fields E and 
H must be linearly related, and the only possible linear relation between the 
axial vector H and the polar vector E is (67.6). The coefficient £. in this 
formula is the only quantity characterising the metal which must be known 
in order to find the external electromagnetic field. 

When the frequency increases further (usually into the infra-red region), 
the macroscopic description of the field again becomes possible, and € is 
again meaningful. The reason is that, on absorbing a quantum ha>, a conduc- 
tion electron acquires a large amount of energy, and its mean free path is 
therefore reduced, the inequality / <^ 8 being consequently again fulfilled. 
The impedance £ is again inversely proportional to \/e. f In this frequency 
range the real part of e(o>) is negative, and its imaginary part is small. The 
inequality / <^ 8 is the condition for both e' and e" to be macroscopically 
significant. The macroscopic significance of the large quantity e' alone, 
however, can be ensured by the fulfilment of the less stringent condition 
v/a> <^ 8, where v is the velocity of the conduction electrons in the metal. 
If this condition holds, the spatial inhomogeneity of the field may be 
neglected in considering the motion of the electrons. 

The inequality £' > is always satisfied by the real part of the imped- 
ance. If formula (67.2) holds, we can also draw certain conclusions concerning 
the sign of £". For example, if the dispersion of e is more important than 
that of fi (i.e. if /x. may be taken as real), the condition e" > gives 
£'£" < and, since £' > 0, £" < 0. This is the most usual case. If the 
dispersion of £ is determined by that of fi, however, a similar argument 
shows that £" > 0. 

The concept of impedance can also be applied to superconductors. A 
characteristic property of superconductors is that the penetration depth 8 is 
small even in the static case (ai = 0). At fairly low frequencies the magnetic 
field distribution can be taken to be the same as the static distribution. To 
determine the electric field we use the equation curl E = iaJH/c, taking the 
sr-axis along the outward normal to the surface of the superconductor. 
Neglecting the tangential derivatives in comparison with the large #-deriva- 
tives, we have dE x /dz = icuHy/c, and similarly for E y . Integrating with 
respect to z through the body gives 

o 



E z {0) = -j H y dz, 



t It should be borne in mind, however, that equation (67.6) can be used as a boundary 
condition only while |e| is large (i.e. £ is small), and certainly does not hold at optical fre- 
quencies. We assume that \t ~ 1, and so small £ correspond to large |e|. If fi^> 1, the 
inequality 8 <^ A must be fulfilled if the boundary condition (67.6) is valid, and therefore 
we must have \/(fie)^> 1, so that £ = V(m/ £ ) may not be small. 



282 The Propagation of Electromagnetic Waves §67 

E x (0) being the value of E x for z = 0, i.e. at the surface of the body. We 
quantitatively define the penetration depth by the relation 

o 

j H v dz = 8H y (0). (67.7) 

-co 

Then £^(0) = ico8Hy(0)/c. Comparing this with the boundary condition 
(67.6), we find that the impedance of a superconductor (in the frequency 
range considered, which in practice extends to about the centimetre wave- 
length region) is given by 

£ = - ico8/c. (67.8) 

This expression is the first term in an expansion of £(a>) in powers of the 
frequency, and the expansion for superconductors thus begins with a term 
in w. The next term, which is in co 2 and real, is the first term in the expansion 

of£'. 

The impedance £(<o), regarded as a function of the complex variable co, 

has many properties analogous to those of the function e(co) (V. L. Ginzburg, 

1954). The boundary condition, which for a wave of a single frequency has 

the form (67.6), must in general be taken as the operator relation 

E t = tVL t X n, (67.9) 

expressing the value of E^ at any instant in terms of the values of H^ at all 
previous instants (cf. §58). As in §62, it therefore follows that the function 
£(a>) is regular in the upper half-plane of co, including the real axis except 
for the point w = 0. The condition that E* is real when H^ is real gives 
£(— co*) = t*{oo). Finally, since the energy dissipation is determined by 
the real part of £(o>) (and not by the imaginary part as for e(o>)), it follows 
that t,'(oi) is positive, and does not vanish for any real w except w = 0. 
Arguments similar to those given in §62 then lead to the conclusion that 
re 1(a)) > throughout the upper half-plane. Hence, in particular, £(a>) 
has no zeros in the upper half-plane. 

The regularity of £(co) in the upper half-plane again leads to Kramers 
and Kronig's formulae. A particularly important formula is 

1 f°£'(*)-l 
£"(o>) = -~P \ ^-^ dx. 

•n J x — co 

—oo 

Using the fact that l'(x) is even, we can also write 

1 ??(x)-l , 1 Fl'(x)-1 
£"(o,) = --P \ -1^ dx + -P — — dx 

JT J X — CO TT J X + CO 



§67 The surface impedance of metals 283 



or 



n«) = — p\ -f^ d *- ( 67 - 10 ) 

77 J X* — CO* 







The term — 1 in the numerator of the integrand may be omitted, since the 
principal value of the integral of l/(x 2 — to 2 ) is zero. 

The above statements concerning the function £(<w) are, of course, equally 
applicable to the reciprocal function l/£(<o); the operator t," 1 converts E$ 
into HiXn. In particular, (67.10) becomes 

2co Ftt-Hx)]' 

K-H«)]" = p\ \^ dx - ( 67 - n ) 

77 J X* — OJ* 



For small £ this formula may be more useful than (67.10). In the form 
(67.11), however, it is not applicable to superconductors, for which £ _1 , 
according to (67.8), has a pole at w = 0. A simple modification in the 
derivation, analogous to that which changes (62.9) into (62.11), gives 

77 J X* — CO* COO 



To conclude this section we shall discuss, as an example of the use of the 
impedance, the reflection of a plane electromagnetic wave incident from a 
vacuum on the plane surface of a metal with surface impedance £. If the 
vector E is polarised perpendicular to the plane of incidence, the boundary 
condition (67.6) gives E + E\ = £(//o — Hi) cos do = £(Eb — E{) cos do, the 
notation being the same as in §66. Hence, since £ is small, we have 
E\JEq = — (1 — 2£ cos do), and the reflection coefficient is 

R ± = 1 - 4£' cos do. (67.13) 

If, on the other hand, Eo lies in the plane of incidence, the boundary 
condition in the form £H$ = nxE ( gives 

Z(H + H{) = (Eo - £i) cos O = (#o - #i) cos Oi 
whence the reflection coefficient is 



R* 



cos 6q — £ 



(67.14) 



cos do + £ 
For angles of incidence not close to |77 

R, = l-4£'sec0 o . (67.15) 



284 The Propagation of Electromagnetic Waves §68 

If, on the other hand, <f>o = \n — 6q <^ 1, then 

4*-Z 



i?„ = 



(67.16) 



fo + t 

This expression takes a minimum value (|£| — £')/(|£| + £') for <f>Q = |£|. 

Except for the special case (67.16), the reflection coefficient for a surface 
with small £ is close to unity. A surface with £ -> is "perfectly conducting" 
and also "perfectly reflecting". The boundary condition at such a surface 
is simply E* = 0, similarly to that for the electrostatic field at the surface of a 
conductor. In a variable field, however, the fulfilment of this condition 
necessarily implies that of a certain condition on the magnetic field : the equa- 
tions icoH/c — curl E and E t — on the surface imply that H n = there. 
Thus the normal component of the magnetic field must be zero on a perfectly 
conducting surface in a variable electromagnetic field. In this respect such a 
surface resembles the surface of a superconductor in a constant magnetic 
field. 

PROBLEM 

Determine the intensity of thermal radiation (of a given frequency) from a plane surface 
of small impedance. 

Solution. According to Kirchhoff 's law, the intensity dl of thermal radiation into an 
element of solid angle do from an arbitrary surface is related to the intensity dlo of radiation 
from the surface of a black body by dl = (1 —R) dlo, where R is the reflection coefficient 
for natural light incident on the surface concerned. Calculating R = $(R X +R t ) from 
formulae (67.13) and (67.14) and using the isotropy of radiation from a black surface 
(d/o = Io do/In), we have 
in 



Hot' J (1 + cog2 tf , ^ 0+ £ /2+ ,,) 



, cos 6 sin 8 dO. 

cos 2 0+2£'cos0+£' 2 +r 2 ) 



Effecting the integration and omitting terms of higher order in £, we find 

In particular, for a metal whose impedance is given by formula (67.3) (n = 1), we have 



§68. The propagation of waves in an inhomogeneous medium 

Let us consider the propagation of electromagnetic waves in a medium 
which is electrically inhomogeneous but isotropic, f In Maxwell's equations 
curlE = iaiH/c, curlH = —iecoE/c (we put everywhere p = 1), € is a 



f For a discussion of wave propagation in an anisotropic medium in a constant magnetic 
field see Ya. L. Al'pert, V. L. Ginzburg and E. L. Feinberg, The Propagation of Radio 
Waves (Rasprostranenie radiovoln), Moscow and Leningrad, 1953. 



§68 The propagation of waves in an inhomogeneous medium 285 

function of the co-ordinates. Substituting for H from the first equation in 
the second, we obtain for E the equation 

AE + ( € o)2/c2)E - grad divE = 0. (68.1) 

Elimination of E gives for H the equation 

AH + (e^2/ c 2)H + (l/ e ) grad e x curlH = 0. (68.2) 

These equations are considerably simplified in the "one-dimensional" 
case, where € varies only in one direction in space. We take this direction 
as the #-axis, and consider a wave whose direction of propagation lies in 
the xsr-plane. In such a wave all quantities are independent of y, and the 
uniformity of the medium in the x- direction means that the dependence on 
x can be taken as being through a factor e iKX , with k a constant. For 
k = the field depends only on z, i.e. we have a wave passing normally 
through a layer of matter in which e = e(z). If #c # 0, the wave is said to 
pass obliquely. 

For k^O two independent cases of polarisation must be distinguished. 
In one, the vector E is perpendicular to the plane of propagation of the wave 
(i.e. it is in the j-direction), and the magnetic field H accordingly lies in 
that plane. Equation (68. 1) becomes 

8 2 E /ecu 2 \ 

a? + hr.-"*)*- a (68 - 3) 

In the other case, the field H is in the ^/-direction, and E lies in the plane of 
propagation. Here it is more convenient to start from equation (68.2), 
which gives 

8 (\dH\ lufi k\ 

We shall call these two types of wave E waves and H waves respectively. 

The equations can be solved in a general form in the important case where 
the conditions of propagation approximate to those of geometrical optics. 
In what follows we shall assume that the function e(z) is real, j In equation 
(68.3) the quantity 2njy/f t where /(#).= ek 2 — k 2 , plays the part of a 
"wavelength" in the -^-direction. The approximation of geometrical optics 
corresponds to the inequality 

iv/ < '■ (68 - 5) 



t Equation (68.3) bears a formal resemblance to Schrodinger's equation for one-dimen- 
sional motion of a particle in quantum mechanics, and the approximation of geometrical 
optics corresponds to the quasi-classical case. Here we shall give the final results ; their 
derivation may be found in Quantum Mechanics, Chapter VII, Pergamon Press, London, 
1958. 



286 The Propagation of Electromagnetic Waves §68 

and the two independent solutions of equation (68.3) are of the form 

constant 

e ±iHfte. (68.6) 

The condition (68.5) is certainly not fulfilled near any point where/ = 0. 
Let -3" = be such a point, with / > for z < and / < for z > 0. At 
sufficiently great distances on either side of z = 0, the solution of equation 
(68.3) is of the form (68.6), but to establish the relation between the coeffi- 
cients in the solutions for z > and z < it is necessary to examine the 
exact solution of equation (68.3) near z = 0. In the neighbourhood of this 
point, f{z) can be expanded as a power series in z : f = —ccz. The solution 
of the equation d 2 E/dz 2 — cczE = which is finite for all z is 



E = (^/a 1 /6)(D(a^), (68.7) 



where 



1 °° 
<J>(!) = cos (^w 3 + ug) du 

s/tt J 



u 

is the Airy function ; we everywhere omit the factor e~ ia>t+iKX in E. The 
asymptotic form of the solution of equation (68.3) for large \z\ is 

E = — cos I ^/fdz + In) for z < 0, 



(68.8) 



E = ~~exp^- JVl/|d*) for*>0, 



with the same coefficient A as in (68.7). The first of these expressions 
represents the stationary wave obtained by superposing the wave incident in 
the positive ^-direction and the wave reflected from the plane z = 0. The 
amplitudes of these waves are both equal to \A\f l > i.e. the reflection coeffi- 
cient is unity. Only an exponentially damped field penetrates into the region 
z > 0. 

As the reflection point is approached, the wave amplitude increases, as is 
shown by the factor /* in the denominators in (68.8). To determine the 
field in the immediate vicinity of that point, the expression (68.7) must 
be used. This function decreases monotonically into the region z > and 
oscillates in the region z < 0, the successive maxima of |2?| continually 
decreasing. The first and highest maximum is reached at a*# = — 1 -02, 
and its value is E = 0-949 A<x~ 1/Q . 



§68 The propagation of waves in an inhomogeneous medium 287 

So far we have spoken of solutions for E waves. It is easy to see that, in 
the approximation of geometrical optics, entirely similar formulae are valid 
for H waves. If we substitute in equation (68.4) H = u^/e, the derivatives 
of e appear as products with u, but not with u'; neglecting therefore the 
terms containing these derivatives, which are small by (68.5), we obtain for 
the function u(z) the equation 

d 2 tt /€(X) \ 

d*2 \ C 2 ; 

which is of the same form as (68.3). Hence the formulae for H differ from 
(68.6)-(68.8) only by a factor y/c. 

A curious difference in the behaviour of the two types of wave occurs 
when an obliquely incident wave (k =£ 0) is reflected from a layer in which 
e(z) passes through zero. The reflection takes place from the plane on which 
f(z) = ek 2 —K 2 = 0, i.e. the wave "does not reach" the plane where € = 0. 
The E wave penetrates beyond the latter plane only as an exponentially 
damped field. When an H wave is reflected, however, there is superposed 
on a similar damped field a strong local field near the plane on which e = 
(see Problem l).f 

PROBLEMS 

Problem 1. Determine the electric field near the point where e = when an obliquely 
(k # 0) incident H wave is reflected. 

Solution. Let e = at the point z = 0. Near this point, we write e = az, and equation 
(68.4) takes the form 

d 2 H ldH 
az* z az 

According to the general theory of linear differential equations, one of the solutions of this 
equation, which we call H\(z), has no singularity at z = 0, and its expansion in powers of 
z begins with z 2 : 

1 1 

Hi(z) = z 2 + -k% 4 - — ak 2 z5+ ... . 

o 15 

The other independent solution has a logarithmic singularity, and its expansion is 

2 2k 2 a 
mz) = Hi(z) logKZ+ — - -j^**+ - • 

The field H(z) is made up of these two solutions, and therefore tends to a constant Ho, say, 
as z ->■ 0. The leading terms in the electric field components are 

i 8H ik 2 Ho 

E x = - — — ££ — log kz, 

ek oz ak 

„ i 8H k „ kHq 

ek dx ek akz 

and become infinite as z -»■ 0. In reality, of course, the absorption which must be present 
in the medium, even though slight (i.e. the fact that the imaginary part of c is not exactly 



t It should be noted that this point is, a singularity of equation (68.4), and the approxi- 
mation of geometrical optics is therefore invalid near it, even though /(ar) does not vanish 
and the condition (68.5) may hold. 



288 The Propagation of Electromagnetic Waves §69 

zero), means that the field attains large but not infinite values compared with the weak field 
in the adjoining regions. 

Problem 2. A "surface" H wave can be propagated along a plane boundary between 
two media whose dielectric permeabilities ei and — |c2| are of opposite signs. The wave is 
damped in both media. Determine the relation between the frequency and the wave number. 

Solution. We take the boundary surface as the acy-plane, the wave being propagated in 
the ^-direction and the field H being in the y-direction. Let the half-space z > contain 
the medium with the positive permeability ei, and the half-space z < that with the nega- 
tive permeability £2. We seek the field in the wave damped as z ->■ ± oo in the form 

Hi = Hoe***-* 12 , «i = V(£ 2 -«J 2 ei/c 2 ) for z > 0, 

H 2 = Hoe ikx+K **, *2 = V^+^M/c 2 ) for z < 0, 

where k, k\ and *2 are real. The boundary condition that H y — H is continuous is already 
satisfied, and the continuity condition on E x gives (1/ej.) BHi/dz = (l/f2) BHz/dz for z = 0, 
or /ci/ei = K2/\ez\. This equation can be satisfied if ei < |e2| (and if ei€2< 0, as has been 
assumed). The relation between k and a> is k 2 = a> 2 ei|e2|/c 2 (|e2|— ei). 
It is easily seen that "surface" E waves cannot be propagated. 

§69. The reciprocity principle 

The emission of electromagnetic waves (of a single frequency) from a 
source consisting of a thin wire in an arbitrary medium is described by the 
equations 

curlE = icoB/c, curlH = — ioiDjc + 4rrj ex jc, (69.1) 

where j ex is the density of periodic currents flowing in the wire which are 
extraneous to the medium. 

Let two different sources (of the same frequency) be placed in the medium ; 
we denote by the suffixes 1 and 2 the fields due to these sources separately. 
The medium may be inhomogeneous and anisotropic. The only assumption 
which we shall make concerning the properties of the medium is that the 
linear relations Di = e^Ejc, Bi = [amHic hold, the tensors e$& and /x^ being 
symmetrical. Under these conditions it is possible to derive a relation 
between the fields of the two sources and the extraneous currents in them. 

We take the scalar products of the two equations curl Ei = i&Bi, 
curl Hi = — ikDi + 477Jex,i/c with H2 and E2 respectively, and of the corres- 
ponding equations for E2 and H2 with —Hi and — Ei. Adding all four 
together, we obtain 

(H2 • curl Ei — Ei • curl H2) + (E2 • curl Hi — Hi • curl E2) 

= (/a)/c)(Bi.H 2 - H1.B2) + (/aVc)(Ei.D 2 - Di-E 2 ) + 

+ (4vr/c)(jex,l • E 2 - jex,2 • Ei). 

But Bi»H2 = iHkHiicH2i = Hi«B2, and Ei«D2 = Di-E2, so that the first two 
terms on the right-hand side are zero. The left-hand side can be trans- 
formed by a formula of vector analysis, and the result is 

div[Ei X H 2 - E 2 X Hi] = (47r/c)(j ex ,l-E2 - jex,2-Ei). 

We integrate this equation over all space ; the integral on the left-hand side 
can be transformed into one over an infinitely remote surface, and is zero. 



§69 The reciprocity principle 289 

Thus we have 

J j ex ,i-E 2 dFi = Jjex,2-EidF 2 . (69.2) 

The integrals are taken only over the volumes of sources 1 and 2 respec- 
tively, since the currents j ex ,i and j ex ,2 are zero elsewhere. Since the wires 
are thin, the effect of each on the field of the other may be neglected, and 
therefore Ei and E 2 in formula (69.2) are the fields due to each of the two 
sources at the position of the other. Formula (69.2) is the required relation; 
it is called the reciprocity theorem. 

If the dimensions of the sources are small compared with the wavelength 
and with the distance between them, this formula can be simplified. The 
field of each source varies only slightly over the dimensions of the other, and 
in (69.2) we can take Ei and E 2 outside the integrals and replace them by 
Ei(2) and E 2 (l), 1 and 2 signifying the positions of the two sources: 

E 2 (l).Jj eX) idFi = E^.Jjex^dJ^. 

The integral J jex dV is just the time derivative of the total dipole moment 
&> of the source. Since & = —iioiP, we have finally 

E 2 (l).^»i = Ei(2).^ 2 . (69.3) 

This form of the reciprocity theorem applies, of course, only to dipole 
emission. If the dipole moment of the source is zero, or very small, the 
approximation made in going from the general formula (69.2) to (69.3) is 
inadequate; see Problem 1. 

PROBLEMS 

Problem 1. Derive the reciprocity theorem for quadrupole emitters and for magnetic 
dipole emitters. 

Solution. If /jex dV = 0, the next terms in the expansion must be taken in the 
integrals (69.2): 

f. dEn f 

ji'Ea dVi ^ — — xtju dFi 

J OXje J 

= 4\~a*~~ ~dx~) J ( x * u+x v 1 *) dVl+ 
l/dEti dE 2 k\ f 

we omit the suffix ex for brevity. The quadrupole moment tensor and the magnetic moment 
tensor are defined by 

t)iic = -iuiDik = J[3(xyfc+»fc/i)-2S iifc r.j] dV, 



= 1 f 

2c J 



M = — rxj dV. 

2c J 

Using the equation curl E = i<a B/c and assuming that e = constant near the sources (and 
so div E = 0), we obtain 

ji-E 2 dV - - — (— - + —^)Di,i k +iu>B2(l)>Jli. 
12\ dxic oxi 1 



290 The Propagation of Electromagnetic Waves §70 

Hence we see that for quadrupole emitters the reciprocity theorem is 

\ dxk dxi ) ' \ Sxjc dxi ) 

and for magnetic dipole emitters 

B 2 (l)-^i = Bi(2).JT 2 . 

Problem 2. Determine the intensity of emission from a dipole source immersed in a 
homogeneous isotropic medium as a function of the permeabilities e and n of the medium. 

Solution. By substituting E = VW^E', H = H', a> = co'/V^m), equations (69.1) are 
brought to the form curl E' = ioiU'/c, curlH' = — ia>'E'/c+4-njexlc, which do not in- 
volve e or ft. The solution of these equations for dipole emission gives a vector field potential 
in the wave region f A' = (l/ci?o)Jjex dV, where Rq is the distance from the source; the 
phase factors are omitted, since they do not affect the calculation of the intensity. Hence 
we see that, for given j e x, we can put A' = Ao, where the suffix signifies the value for the 
source field in a vacuum. The values of H' and E' are 

H' = ik'xA' = i'V(€/t)kxAo = \/( c /*)Ho, E' = H'. 

Hence H = V(*/*)Ho, E = fiEo and I = 7o/x 3/2 e 1/2 . This is the required solution. 

§70. Electromagnetic oscillations in hollow resonators 

Let us consider the electric field in a hollow evacuated resonator with 
perfectly conducting walls. The equations of the field (of a single frequency) 
in the vacuum are 

curl E = icMjc, curlH = - icoE/c. (70.1) 

The boundary conditions on the surface of a perfect conductor (i.e. one 
whose impedance ■£ = 0) are 

E t =0, H n = 0. (70.2) 

To solve the problem, it suffices to consider either E or H. For instance, 
eliminating H from equations (70.1), we find that E satisfies the wave 
equation 

AE + (co2/ c 2)E = 0, (70.3) 

together with the equation 

divE = 0, (70.4) 

which does not follow from (70. 3). Solving these equations with the boundary 
condition E$ = 0, we find the field E, and then H can be derived from the 
first of (70. 1). The boundary condition H n = is automatically satisfied. 

When the shape and size of the cavity are given, equations (70.3) and 
(70.4) have solutions only for certain values of <o, called the eigenfrequencies 
of the electromagnetic oscillations of the resonator concerned. J For £ = 



t See The Classical Theory of Fields, §9-2. 

X The formulae for a resonator filled with a non-absorbing dielectric for which e and /* 
differ from unity are obtained from those for an evacuated resonator by replacing co, E and 
H by (i)V(eft), \/eE and \ZfiHL respectively. This is seen from the fact that the transformation 
just given converts equations (70.1) into the correct Maxwell's equations for the medium: 
curl E = ia)(iH./c, curl H = — iioeE/c. In particular, the presence of the medium reduces 
each eigenfrequency by a factor \/(e[i). 



§70 Electromagnetic oscillations in hollow resonators 291 

the electromagnetic field does not penetrate into the metal, and no loss 
occurs there. All the characteristic oscillations are therefore undamped, 
and all the eigerifrequencies are real. The latter are infinite in number, and 
the order of magnitude of the lowest eigenfrequency coi is c/l, where / is the 
linear dimension of the cavity. This follows immediately from dimensional 
considerations, since / is the only dimensional parameter characterising the 
problem if the shape of the resonator is given. The high eigenfrequencies 
(<d > c/l) lie very close together, and the number of them per unit range of 
(o is V<t) 2 l2ir 2 c 3 , which depends on the volume V of the resonator but not on 
its shape, f 

The (time) average values of the electric and magnetic field energies in 
the resonator are respectively |J(|E|2/8w) dV and $ J(|H| 2 /8tt) dV. We 
shall show that they are equal. Using the first equation (70.1), we write 
JH.H* dV = (c 2 /a) 2 )JcurlE.curlE* dV. The second integral can be 
integrated by parts : 

f curl E- curl E*dF = j> curlE*-df x E + J E- curl curl E*dF. 

Since Et — on the boundary of the volume considered, the surface integral 
is zero, leaving 

J|H|2dF = —A E.curlcurlE*dF 



= - (r2 



(<*/«») J" 



E-AE*dF 



or, by (70.3), 



J|H| 2 dF= f\E\*dV. (70.5) 

This completes the proof, t 

Undamped oscillations in a resonator are obtained if the impedance of its 
walls is assumed to be zero. Let us now ascertain the effect on the eigen- 
frequencies if the impedance of the walls is small but not zero. 

The (time) average energy dissipated in the walls of the resonator in unit 
time can be calculated as the flux of energy into the walls from the electro- 
magnetic field in the cavity. Using the boundary condition (67.6) on the 
surface of a body with impedance £, we write the normal component of the 
energy flux density as S n = (cjSrr) re (E t X H t *) = c£'|Hj| 2 /87r, where £' is 
the real part of £. In this expression, which already contains the small 
factor £', we can as a first approximation take H to be the field obtained 



t See The Classical Theory of Fields, §6-9. 

% By E and H we always mean the fields corresponding to a particular eigenfrequency. 
It is not difficult to show that the fields corresponding to two different eigenfrequencies % 
and ob are orthogonal: 

JE„- E 6 *dF= J"H„-H 6 *dr = 0. 



292 The Propagation of Electromagnetic Waves §70 

by solving the problem with £ = 0. The total energy dissipated is given by 
the integral 



^riHM/, 



(70.6) 



taken over the internal surface of the resonator. The field amplitude is 
damped in time with a decrement obtained by dividing (70.6) by twice the 
total energy of the field, namely £ J*(|E|2+ |H|2) dF/877 = J|H|2 dV/S7r. 

The damping decrement is determined by the imaginary part |co"| of the 
complex .frequency co = co' + za/'.f Writing the formula in the complex 
form 

co — coo = — \ic—, — ; , (70.7) 

¥ J|H|2dF V ' 

co and wo being the frequencies with and without allowance for £, we can 
determine not only the damping decrement but also the change in the eigen- 
frequencies themselves. The latter is seen to be determined by the imaginary 
part of £. We have mentioned in §67 that usually £" < 0, and the eigen- 
frequencies are then reduced. 

In actual calculations, it may be more convenient to transform the volume 
integral in the denominator of (70.7) into one over the surface. The result 
isj 

J|H| 2 dF = £^(|H| 2 - |E|2)r.df. (70.8) 



PROBLEMS 

Problem 1. Determine the eigenfrequencies of a cuboidal resonator with perfectly con- 
ducting walls. 

Solution. We take the axes of x, y, z along three concurrent edges of the cuboid; let 
the lengths of these edges be ax, as, «3- The solutions of equations (70.3) and (70.4) which 
satisfy the boundary condition Et = are 

Ex = A\ cos k x x sin ky y sin k z z . e~ imt (1) 



t In radio engineering the quality of the resonator, defined as the ratio a>72|a>"|, is 
generally used instead of the damping decrement. 

% Since the vector H is tangential to the surface, we have identically 

|(H.H*)(r.df) = |(H-H*)(r.df) -|(H-r)(H*.df) -|(H*.r)(H-df). 

The integrals on the right are transformed by putting df -*■ dFgrad, and using (70.1) we 
obtain 

|(H.H*)(r-df) = t*fr-(H x E* - H* x E) dV + J*H-H* dV. 

Similarly, using the identity r x (E x df) = E(r-df) — (r«E)df = (by the boundary 
condition E* = 0), we have 

|(E.E*)(r.df) = -f(E.E*)(r-df) + |(E.r)(E*.df) + ^(E*.r)(E.df) 
= ikjr-CH. x E* - H* x E) dV - JE-E* dV. 
Subtracting and using (70.5), we obtain formula (70.8). 



§71 The propagation of electromagnetic waves in waveguides 293 

and similarly for E y , E z , where 

k x = nin/ai, k y — 11217/02, k z = 11311/03 (2) 

(«i, «2, «3 being positive integers). The constants Ai, A2, A3 are related by 

Aikx+Azky+Azkz = 0, (3) 

and the eigenfrequencies are to 2 = c 2 (k x 2 -\-k y 2 +k z 2 ). 
The magnetic field is calculated from (1): 

Hz = —(icl<o)(A3k y —A2k z ) sin k x x cos k y y cos kzz-e'^ 1 , 

and similarly for fly, fl z . 

If two or all of the numbers n\, m, m are zero, E = 0. Hence the lowest frequency corres- 
ponds to an oscillation in which one of these numbers is and the other two are 1 . 

Since the relation (3) holds, the solution (1) (with given non-zero «i, 712, m) involves only 
two independent arbitrary constants, i.e. each eigenfrequency is doubly degenerate. The 
frequencies for which one of n\, nz, nz is zero are not degenerate. 

Problem 2. Determine the frequencies of electric dipole and magnetic dipole oscillations 
in a spherical resonator of radius a. 

Solution. In a stationary spherical electric dipole wave, the fields E and H are of the 
formf 

(ctri fey \ /Sin KY \ 
b), H = -ike- iu>t curl I bj, 

where b is a constant vector and k = tojc. The boundary condition nxE = at r = a 
gives cot ka — (ka^—ka. The smallest root of this equation is ka = 2-74. The frequency 
to = 2*74 cja is the lowest eigenfrequency of a spherical resonator. 
In a stationary spherical magnetic dipole wave, we have 

(c«ri fcy \ /sin ky \ 
b], H = e~ iM curl curl I bl. 

The boundary condition on E gives the equation tan ka = ka, whose smallest root is 
ka = 4-49. 



§71. The propagation of electromagnetic waves in waveguides 

A waveguide is a hollow pipet of infinite length, i.e. a cavity infinite in 
one direction, whereas the resonators discussed in §70 are of finite volume. 
The characteristic oscillations in a resonator are stationary waves, but those 
in a waveguide are "stationary" only in the transverse directions; waves 
travelling in the direction along the pipe can be propagated. 

Let us consider a straight waveguide of any (simply-connected) cross- 
section uniform along its length. We shall first suppose that the walls of the 
waveguide are perfectly conducting, and take the s'-axis along the waveguide. 
In a travelling wave propagated in the ^-direction, all quantities depend on z 
through a factor exp (ik z z)> with k z 3. constant. 

The electromagnetic waves possible in such a waveguide can be divided 
into two types: in one, H z = 0, and in the other E z = (Rayleigh, 1897). 



t See The Classical Theory of Fields, §9-6. 

X The formulae below hold for an evacuated waveguide. Those for a waveguide filled 
with a non-absorbing dielectric are obtained by means of the transformation given in the 
first footnote to §70. 



294 The Propagation of Electromagnetic Waves §71 

The former type, in which the magnetic field is purely transverse, are called 
electric-type waves or E waves. The latter, in which the electric field is 
purely transverse, are called magnetic-type waves or H waves, f 

Let us first consider E waves. The x and y components of equations 
(70. 1) give 

dE z a) 8E Z o> 

— ik z E y = i— H x , - — — + ik z E x = i—Hy, 

oy c ox c 

CO CO 

ik z H y = i — E x ikzH x = — i — E y . 



Hence 



c 



ik z 8E ik z 8E Z 

77 17 

&X — i ■"!/ — » 

K* ox k*, oy 

ico dE z ico 8E Z 

IT * TT ■ * 



(71.1) 



ck 2 8y ck 2 8x 



where k 2 = (co 2 /c 2 ) — k z 2 . Thus, in an E wave, all the transverse compo- 
nents of E and H can be expressed in terms of the longitudinal component 
of the electric field. This component must be determined by solving the 
wave equation, which takes the two-dimensional form 

A2E z + k 2 E z = (71.2) 

(A2 being the two-dimensional Laplacian). The boundary conditions for this 
equation are that the tangential components of E should vanish on the walls 
of the waveguide, and can be satisfied by putting 

E z = on the circumference of the cross-section. (71.3) 

According to formulae (71.1), the two-dimensional vector whose components 
are E x , E y is proportional to the two-dimensional gradient of E z . When the 
condition (71.3) holds, therefore, the tangential component of E in the 
xy-plane is also zero. 

Similarly, in an H wave the transverse components of E and H can be 
expressed in terms of the longitudinal component of the magnetic field : 

ik z 8H Z ik z 8H Z 

TT TT 

x 2 8x ' k 2 8y ' 

ico 8H Z ico 8H Z 

E x = — r~ — > Ey — — 



CK 



8y ck 2 8x 



t These types are also known as TM (transverse-magnetic) and TE (transverse-electric) 
waves. 



§71 The propagation of electromagnetic waves in waveguides 295 

The longitudinal field H z is given by the solution of the equation 

A2#* + * 2 H Z = (71.5) 

with the boundary condition 

8H z /dn = on the circumference of the cross-section. (71.6) 

According to formulae (71.4), this condition ensures that the normal compo- 
nent of H is zero. 

Thus the problem of determining the electromagnetic field in a waveguide 
reduces to that of finding solutions of the two-dimensional wave equation 
A2/+/C 2 / = 0, with the boundary condition / = or df /dn = on the 
circumference of the cross-section. For a given cross-section, such solutions 
exist only for certain definite eigenvalues of the parameter k 2 . 

For each eigenvalue k 2 we have the relation 

a? = c 2 (k z 2 + k 2 ) (71.7) 

between the frequency w and the wave number k z of the wave. The velocity 
of propagation of the wave along the waveguide is given by the derivative 

do) ck z c 2 k z 

u z = = = — . (71.8) 

8k z V(kz 2 + x 2 ) *> 

For given k, this varies from to c when k z varies from to 00. 

The (time) average energy flux density along the waveguide is given by the 
^-component of the Poynting vector. A simple calculation, using formulae 
(71.1), gives for an E wave 

S z = -^re(E x H*% = -^4| g rad 2 £ 2 | 2 . 



57T 



8ttk4 



The total energy flux q is obtained by integrating S z over the cross-section 
of the waveguide. We have 

BE 

J|grad 2 £ z |2d/= fa*-^dl- JE z *A2E z df. 

The first integral is taken along the circumference of the cross-section, and 
is zero on account of the boundary condition E z = 0. In the second integral 
we replace A2-Ez by —k 2 E z > and the result is 

q = -—\\E t \*df. (71.9) 

The expression obtained for an H wave is the same with H z instead of E z . 

The electromagnetic energy density W (per unit length of the waveguide) 
may be calculated similarly. It is simpler, however, to derive W directly 
from q, since we must have q = Wu z . From (71.8) and (71.9), therefore, 

w = T^n[\ E #*f- ( 7U °) 



296 The Propagation of Electromagnetic Waves §71 

It follows from (71.7) that, for each type of wave (for a given value of k 2 ) 
there is a minimum possible frequency, namely ck. At lower frequencies 
the propagation of waves of the type concerned is not possible. There is a 
smallest eigenvalue /cmin, which is not zero (see below). We therefore con- 
clude that there is a frequency eo m in = c/c m in below which no waves can be 
propagated along the waveguide. The order of magnitude of a>min is c/a, 
where a is the transverse dimension of the pipe. 

This statement is valid, however, only for waveguides in which the cross- 
section is simply connected (as we have hitherto assumed). When the 
cross-section is multiply connected,! tne situation is quite different. In such 
waveguides not only the E and H waves described above but also another 
type of wave, whose frequency is subject to no restriction, can be propa- 
gated. Such waves (called principal waves) are characterised by the fact that 
k z = k (i.e. k = 0); the velocity of propagation is equal to the velocity 
of light c. We shall derive the chief properties of such waves, and shall see 
why such waves cannot occur when the cross-section of the waveguide is 
simply connected. 

All the field components in a principal wave satisfy the two-dimensional 
Laplace's equation, A2/ = 0- With the boundary condition / = 0, the only 
solution of this equation regular throughout the cross-section (whether or not 
multiply connected) is / = 0. Hence we have E z = in a principal wave. 

With the boundary condition df/dn = 0, a regular solution is 
/ = constant. It is easy to see, however, that when / is H z the constant 
must be zero (by a "constant", of course, we mean a quantity independent 
of x and y, and depending on z and t through the factor exp (ik z z — iwt)). 
For, integrating the equation 

8H X 8H V ioi 
divH = — - + — - + —H z = 
ox cy c 

over the cross-section, we obtain §H n dl+(icoJc) \H Z d/ = 0; since H n = 
on the circumference of the cross-section and H z is constant over its area, 
it follows that H z = 0. 

Thus a principal wave is purely transverse. For E z = H z = 0, the x and 
y components of equations (70. 1) give 

H x = — E y , H y = E x , (71.11) 

i.e. the fields E and H are perpendicular and equal in magnitude. They are 
determined by the equations 

8E Z 8E y 8E y dE x 

divE = — + — - = 0, (curl E) z = — -- — ? = 0, 

dx dy ox cy 

with the boundary condition E< = 0. 



t For example, the space between two pipes one inside the other, or the space outside 
two parallel pipes. 



§71 The propagation of electromagnetic waves in waveguides 297 

We see that the dependence of E, and therefore of H, on x and y is given 
by the solution of a two-dimensional electrostatic problem : E = — grad2 <f>, 
where the potential <f> satisfies the equation A2<£ = with the boundary 
condition <f> = constant. In a simply-connected region, this boundary con- 
dition means that <f> = constant (and so E = 0) is the only solution regular 
throughout the region. This shows that waves of this type cannot be propa- 
gated along a waveguide whose cross-section is simply connected. In a 
multiply-connected region, on the other hand, the constant in the boundary 
condition need not be the same on the various separate parts of the boundary, 
and so Laplace's equation has solutions which are not trivial. The electric- 
field distribution over the cross-section of the waveguide is the same as the 
two-dimensional electrostatic field between the plates of a condenser at a 
given potential difference. 

So far we have assumed the walls of the waveguide to be perfectly con- 
ducting, f If the walls have a small but finite impedance, losses occur and 
the wave is therefore damped as it is propagated along the waveguide. The 
damping coefficient can be calculated in the same way as for the damping in 
time of electromagnetic oscillations in a resonator (§70). 

The amount of energy dissipated in unit time per unit length of the walls 
of the waveguide is given by the integral (c/87r)£'<j>|H| 2 d/, taken along the 
circumference of the cross-section ; H is the magnetic field calculated on the 
assumption that £ = 0. Dividing this expression by twice the energy flux q 
along the waveguide, we obtain the required damping coefficient a. With 
this definition, a gives the rate of damping of the wave amplitude, which 
decreases along the waveguide as e~ az . 

Expressing all quantities in terms of E z or H z by means of formulae 
(71.1) or (71.4), we obtain the following formulae for the absorption coeffi- 
cients : for an E wave 

2K*k z c t\E t \*6f ( ' ' 

and for an H wave 

a = auT Pw ' ( • 3) 

In an actual calculation it may be convenient to transform the surface integ- 
rals in the denominators into integrals along the circumference. The neces- 
sary formulae, whose derivation is similar to that of (70.8), are 



JWd/= ^(n.r)|grad 2 ^|2d/, 
JWd/= — j>(n.r){ K *\H z \2 - | gra d 2 # z |2}a7. 



(71.14) 



f In particular, this assumption is necessary for a rigorous separation of waves with 
Ez = and those with H z = 0. 



298 The Propagation of Electromagnetic Waves §71 

When k z -> 0, i.e. the frequency to ->ck, the expressions (71.12) and 
(71.13) become infinite, but they are then no longer applicable, because their 
derivation presupposes that k is small compared with k z . 

Formulae (71.12) and (71.13) are not valid for a principal wave (in a 
waveguide with a multiply-connected cross-section), in which E z , H z and 
k are all zero. In this case all the field components can be expressed in 
terms of the scalar potential <f>. Using the fact that the fields H and 
E = — grad2<£ in a principal wave are perpendicular and equal in magni- 
tude, we obtain the absorption coefficient 

g'flgradaflSd/ 

a = ; ; . 1/1. 15) 

2J|grad 2 ^|2d/ 

The propagation of a principal wave along a waveguide can be relatively 
simply discussed when its absorption coefficient is not small (so that formula 
(71.15) is inapplicable) but the wavelength c/a> is large compared with the 
transverse dimension of the waveguide. 

As has been mentioned, the transverse electric field in a principal wave at 
any instant corresponds to the electrostatic field in a condenser formed by 
the walls of the waveguide carrying equal and opposite charges. Let these 
charges be ± e(z) per unit length. They are related to the currents ±J{z) 
flowing on the walls by the "equation of continuity" de/dt = — dj/dz, or, 
for a field of a single frequency, icoe = djjdz. Next, let C be the capacity 
per unit length of the waveguide. The "potential difference" <f>2~<f>i be- 
tween its walls is efC; differentiating this with respect to z, we obtain the 
e.m.f. which maintains the current on the walls. (When absorption is 
present, the field is not purely transverse.) Equating the e.m.f. to ZJ, where 
Z is the impedance per unit length, we have 



d 
dz 



(i) - *> 



or 

dz 



(Vi) +i " ZJ = °- (7L16) 

Substituting Z = R-ia)L/c 2 , where R and L are the resistance and self- 
inductance per unit length of the waveguide, we can return from the single- 
frequency current components to currents which are arbitrary functions of 
time. Assuming the capacity C to be constant along the waveguide, we 
arrive at the telegrapher's equation : 

1 d*J BJ L d*J 

-L _ R.± ± = 0. (71.17) 

C dz* 8t c 2 dt* 



§72 The scattering of electromagnetic waves by small particles 299 

If there is no resistance (R — 0), this equation reduces, as it should, to 
the wave equation with a velocity of wave propagation -\/(c 2 /LC) = c.\ 



PROBLEMS 

Problem 1 . Find the values of k for waves propagated in a waveguide whose cross-section 
is a rectangle of sides a and b. Find the damping coefficients. 

Solution. In E waves J E z = constant X sin kxx sin k y y, where k x = mir/a, k y = nzirjb, 
with mi and «2 positive integers. In H waves H z = constant X cos kxx cos k y y, and one of 
«i and M2 may be zero. In both types of wave k 2 = k x 2 +k y 2 = 7r 2 (m 2 /a 2 +«2 2 /6 2 ). The 
smallest value of k corresponds to an Hio wave (the suffixes show the values of «i and 712) 
and is K mln = -n\a (we assume that a > b). 

The damping coefficients are calculated from formulae (71.12) and (71.13) and are: for 
E waves a = 2£'co(k x 2 b+ky 2 a)lcic 2 kzab, for Hn x o waves 

* = —(a+—b), 
ck z ab \ k 2 /' 

and for H ni n 2 waves (m, m *£ 0) 

2ckH'[ , k z 2 



a = 



(okzob 



[«+6+ _±<fe? a +V*)]. 



Problem 2. The same as Problem 1, but for a waveguide whose cross-section is a circle 
of radius a. 

Solution. Solving the wave equation in polar co-ordinates r, <}>, we have for E waves 

E s = constant Xj„(/cr)^n^ 

with the condition J n {ta) = 0, which gives the value of k. In H waves the value of H t is 
given by the same formula, but k is determined by the conditioning fa) = 0. The smallest 
value of k occurs for the Hi wave, and is « mIn = 1 -84/a. 

The damping coefficient is calculated from formulae (71.12)-(71.14). For E waves it is 
a = to^'/cakz, and for H waves 



cX,' K 



T n 2 a> 2 1 

tok z a L c 2 K 2 (a 2 K 2 —n 2 )l 

§72. The scattering of electromagnetic waves by small particles 

Let us consider the scattering of electromagnetic waves by macroscopic 
particles whose dimensions are small compared with the wavelength A ~ c/a> 
of the wave undergoing scattering (Rayleigh, 1871). When this condition 
holds, the electromagnetic field near the particle may be supposed uniform. 
Being in a uniform field periodic in time, each particle acquires definite 
electric and magnetic moments &> and M., whose dependence on time is 
given by factors of the form e _fw *. The scattered wave can be described as 
being the result of emission by these variable moments. At distances R from 



t The equation LC — 1 follows from the mathematical equivalence of the problems of 
determining 1/C and L for a given cross-section. The electric and magnetic fields between 
perfectly conducting surfaces are perpendicular and equal in magnitude (see (71.11)), and 
if this magnitude is given on the surfaces the charge density and the current density are 
respectively determined. Hence the coefficients of proportionality (1/C and L) between the 
field energy and the squared charge and current respectively are the same. 

X We everywhere omit the factor exp(ik z z — teat). 



300 The Propagation of Electromagnetic Waves §72 

the particle which are large compared with A (the wave region), the fields in 
the scattered wave are given byf 

H' = — {n x^ + n x (M x n)}, 
c*R l 

(72.1) 

E' = H'xn, 

where the unit vector n gives the direction of scattering, and the values of 
& and M at the time t-R/c must be taken. We denote the fields in the 
scattered wave by primed letters and those in the incident wave by unprimed 
letters. The (time) average intensity of radiation scattered into a solid angle 
do is d/ = |c|H'|2R 2 do/47r; dividing by the energy flux density in the 
incident wave c|H| 2 /8tt = c|E| 2 /87r, we obtain the effective scattering cross- 
section. 

The calculation of &> and Jl is particularly simple if the dimensions of 
the particle are small in comparison not only with A but also with the "wave- 
length" S corresponding to the frequency to in the material of the particle. 
In this case we can calculate the polarisability of the particle from the 
formulae for an external uniform static field, the only difference being, of 
course, that the values taken for e and /z. are those corresponding to the given 
frequency to, and not the static values. If, as usually happens, /a is close to 
unity, the magnetic dipole term in formula (72. 1) may be omitted. 

For a spherical particle of volume V we have (see (8.9)) 

^ = FaE, a = 3(c - l)/4«(« + 2), (72.2) 

and the effective scattering cross-section is 

do- = (a>/cy\<x.\W 2 sin 2 6 do, (72.3) 

where is the angle between the scattering direction n and the direction of 
the electric field E in the linearly polarised incident wave. The total effective 
cross-section is 

a = 87r|a| 2 co4F 2 /3c4. (72.4) 

The frequency dependence of the effective cross-section is determined by 
the factor to 4 and by the polarisability. At frequencies so low that a shows 
no dispersion, the scattering is proportional to to 4 . It may be noted also that 
the effective cross-section is proportional to the square of the volume of 
the particle. 

If the incident wave is unpolarised (natural light) then the differential 
effective cross-section must be obtained by averaging (72.3) over all direc- 
tions of the vector E in a plane perpendicular to the direction of propagation 
of the incident wave (i.e. perpendicular to its wave vector k). Denoting by & 

t See The Classical Theory of Fields, §9-12. 



§72 



The scattering of electromagnetic waves by small particles 



301 



and <f> the polar angle and the azimuth of the direction of n relative to k (<f> 
being measured from the plane of k and E), we have cos 6 = sin & cos <f> 
(Fig. 31), so that 

da = (co/c) 4 |a| 2 r 2 (l - sin 2 & cos 2 <f>) do. (72.5) 

On averaging over (f>, we obtain the following formula for the effective cross- 
section for scattering of an unpolarised wave : f 

do- = l(w/c)*\<x.\ 2 V 2 (l + cos 2 «•) do, (72.6) 

where & is the angle between the directions of incidence and scattering. 

A 




*-H 



Fig. 31 

From formula (72.5) we can easily find the degree of depolarisation of the 
scattered light. To do so, we notice that, for a given direction of E, E' lies 
in the plane of E and n. The direction of the electric field E' in the scattered 
wave therefore lies in the plane of k and n (the plane of scattering) or perpen- 
dicular to that plane, according as the azimuth <f> of the vector E, measured 
from the plane of k and n, is or £n\ Let 7, and I x be the intensities of 
scattered radiation having these two polarisations. The degree of depolari- 
sation is defined as the ratio of the smaller to the larger of these quantities. 
By (72.5) we have 



IJI ± = cos 2 &. 
If the scattering particle has a large dielectric permeability, 

8 ~ c/aVH < A. 



(72.7) 



The dimensions of the particle may then be small compared with A but not 
small compared with 8. In the first approximation with respect to 1/e, the 
electric moment of the particle may be calculated as simply the moment of 
a conductor (e -> oo) in a uniform constant external field. In calculating the 
magnetic moment, however, the induced currents in the particle are of 



t For future reference we may note the formula sin 2 8 — i(l + cos 2 £). 



302 The Propagation of Electromagnetic Waves §72 

importance, and the problem cannot be taken as static; instead, we must 
seek a solution of equation (63.2) (with fx = 1) : 

AH + € W 2H/c2 = (72.8) 

which becomes the field of the incident wave far from the particle. The 
magnetic and electric moments are of the same order of magnitude, and both 
terms in formula (72.1) must be retained. The angular distribution and 
amount of scattering are different from those discussed above (see Problem 
2). 

PROBLEMS 

Problem 1. Linearly polarised light is scattered by randomly oriented small particles 
whose electric polarisability tensor has three different principal values. Determine the 
depolarisation coefficient for the scattered light. 

Solution. Neglecting, as above, the magnetic moment, we have from (72.1) 

E' = (to 2 /c 2 i?)(n x 0>) x n. 

The required depolarisation coefficient is given by the ratio of the principal values of the 
two-dimensional tensor I a g = E' a E'p*, where the bar denotes an averaging over orienta- 
tions of the scattering particle for a given direction of scattering n, and the suffixes a and 
P take two values in the plane perpendicular to n. f It is more convenient, however, to aver- 
age the three-dimensional tensor tPi&'k* and then project it on the plane perpendicular to 
n; these components of the tensor SP^k* are proportional to the corresponding compo- 
nents 7 a p. Substituting 0>i = cttkEik, we have 



W»* = «UXicm*EiEm*. 
In effecting the averaging we use the formula 



attain,* = A8ik8im+B(8ii8}em+ SirnSkl)' 

This is the most general tensor of rank four which is symmetrical in i, k and /, m and contains 
only scalar constants. These constants are determined from two equations obtained by 
contracting the tensor, firstly with respect to i, k and /, m, secondly with respect to i, / and 
k, m. They are 

A = 2 ' ai<|2 i ~ |gi * 12 = l{|« 1 |«+|a a |»+|«,|«+4rc(aia«*+ai«a*+ai«,*)}, 

B = 3 l a ^-1 a **l 2 = ^{|ai|2+|a 2 |2+|a3| 2 -re( ai a2* + aia 3 * + a 2 a3*)}, 

where ai, 0C2, a3 are the principal values of the tensor aj». 

In a linearly polarised wave, the field amplitude E<*> (we omit the time factor e -<0> <) can 
always be defined so as to be real. Then we have 



fe * = (A+B)EiE k +B8i k E*. (1) 

Let the 2-axis be in the direction of n, and the »sr-plane contain the directions of n and E; 
these axes are the principal axes of the tensor 7 a p. Taking the appropriate components of 
the tensor (1), we find the depolarisation coefficient I v /I x = Bj[(A+B) sin 2 6+B], where 
is the angle between E and n. 

Problem 2. Determine the effective cross-section for scattering by a sphere of radius a, 
for which e is large; it is assumed that A^> a~S. 



t See The Classical Theory of Fields, §6-7. 



§73 The absorption of electromagnetic waves by small particles 303 

Solution. The problem of calculating the magnetic moment acquired by a sphere with 
given e (and n — 1) in a variable magnetic field H is the same as that solved in §45, Problem 
1, except that k in the formulae derived there must be replaced by co-y/e/c. Thus 
M = —a 3 yH., where 



*/< 3 3 \ 

Y = 2\ 1+ ka COtka --w)' 



The electric moment can be calculated, in the first approximation with respect to 1/e, as 
simply the moment of a conducting (e -> co) sphere in a uniform constant electric field: 
0» = a 3 E. 

Taking into account the fact that E and H are perpendicular, we have after a simple 
calculation, using (72.1), the following formula for the effective scattering cross-section: 

da = (a 6 <o 4 /c 4 ){|y| 2 cos 2 ^+sin 2 <£-(y+y*) cos S-+cos 2 &(cos 2 <f>+\y\ 2 sin 2 <f>)} do, 

where <f> and •& are the angles shown in Fig. 31. In scattering of unpolarised light we have 

da = (a 6 a)4/c4){i[l + |y|2][l+ CO s 2 *]-(y+y*) cos &} do, 

and the degree of depolarisation of the scattered light is IJI X = |(y— cos ft)/(l — y cos &)| 2 . 
The total scattering cross-section is a = 8wa 6 o> 4 (l + |y| a )/3c*. 

In the limit ka -> co (i.e. when A ^> a ^> 8) we have y — \, corresponding to scattering 
by a perfectly reflecting sphere into which neither the electric nor the magnetic field 
penetrates. 

§73. The absorption of electromagnetic waves by small particles 

The scattering of electromagnetic waves by particles is accompanied by 
absorption. The effective absorption cross-section is given by the ratio of 
the mean energy Q dissipated in a particle per unit time to the incident 
energy flux density. To calculate Q we can use the formula 

Q = -&>.&-Ji-h, (73.1) 

where &> and M are the total electric and magnetic moments of the particle, 
and the external fields © and ^ are replaced by the fields E and H in the 
scattered wave; cf. (45.21). 

Using the complex representation of quantities, we can write (see the 
penultimate footnote to §45) 

Q = - * re(^.E* + ^f .ft*) = faV(ae" + *m")\E\*, 

where a 6 and ocm are the electric and magnetic polarisabilities of the particle. 
Dividing by the incident energy flux, we obtain 

a = 4™F(or e " + *m")lc. (73.2) 

Let us apply this formula to absorption by a sphere of radius a <^ A, 
assuming it non-magnetic (p = 1). The nature of the absorption depends 
considerably on the magnitude of the dielectric permeability. 

If c is small, then we have both a <^ A and a <^ S. In this case the magnetic 
polarisability may be neglected in comparison with the electric polarisability. 
With the latter given by (72.2), we have 

a = 127rcoa^"/c[(e' + 2)2 + ( e ")2]. (73.3) 

If, on the other hand, JeJ > 1, the electric part of the absorption becomes 
small, and the magnetic absorption may be important even if we still have 



304 The Propagation of Electromagnetic Waves §74 

8 > a. When this last condition holds (i.e. \ka\ <^ 1), the magnetic 
polarisability is ccm = (&z) 2 /407r = a 2 to 2 e/4(kTC 2 and the effective absorption 
cross-section is 

1277coa 3 e' 
(7 = 



7 1 o> 2 a 2 \ 

b + 9^)- (73 - 4) 



When e increases further, the electric part of the absorption becomes 
small compared with the magnetic part. In the limit S <^ a (i.e. \ka\ > 1, 
cotka -> — i) we have ccm = 9i/87rka = 9«V£/877o>a, where £ = l/\/ € is the 
surface impedance of the sphere. Hence 

a = 67ra2£". (73.5) 

It may be noticed that this formula could have been obtained more directly, 
without using the general expression for the magnetic polarisability a TO (o>) 
of the sphere. When £ is small, the energy dissipation Q can be calculated 
by "integrating" the mean Poynting vector (67.4) over the surface of the 
sphere, the distribution of the magnetic field over the surface being given 
by the solution (42.3) of the problem of a superconducting (£ = 0) sphere in 
a uniform magnetic field. 

Knowing the effective absorption cross-section of the sphere, we can 
immediately determine the intensity of the thermal radiation emitted from 
the sphere. According to KirchhofTs law, f the intensity 6.1 in a frequency 
range do> is given in terms of a(w) by d/ = 47rCG(a))eo((o) du>, where 
eo((t)) = #<o 3 /47r 3 c 3 [exp (hto/T)— 1] is the spectral density of black-body 
radiation per unit volume and unit solid angle. 

§74. Diffraction by a wedge 

The ordinary approximate theory of diffraction^ is based on the assump- 
tion that the deviations from geometrical optics are small. It is thereby 
assumed, firstly, that all dimensions are large compared with the wavelength; 
this applies both to dimensions of bodies (screens) or apertures and to 
distances of bodies from the points of emission and observation of the light. 
Secondly, only small angles of diffraction are considered, i.e. the distri- 
bution of light is examined only in directions close to the edge of the geo- 
metrical shadow. Under these conditions, the actual optical properties of 
the substances involved are of no importance ; all that matters is that they 
are opaque. 

If these conditions are not fulfilled, the solution of the diffraction problem 
requires an exact solution of the wave equation, taking into account the 
appropriate boundary conditions on the surfaces of the bodies, which depend 
on their properties. The finding of such solutions offers exceptional mathe- 
matical difficulties, and has been effected for only a small number of prob- 
lems. A simplifying assumption is usually made concerning the properties 



t See Statistical Physics, §60, Pergamon Press, London, 1958. 
J See The Classical Theory of Fields, §§7-7 to 7-9. 



§74 Diffraction by a wedge 305 

of the body at which the diffraction occurs, namely that it is perfectly con- 
ducting, and therefore perfectly reflecting. 

The following remark may be made here. It might seem reasonable to 
solve the diffraction problem on the assumption that the surface of the body 
is "black", i.e. completely absorbs light incident on it. In reality, however, 
such an assumption concerning the body in stating the exact problem of 
diffraction would involve a contradiction. The reason is that, if the sub- 
stance of the body is strongly absorbing, the coefficient of reflection is not 
small but, on the contrary, almost unity (see §67)). Hence a reflection 
coefficient close to zero implies a weakly absorbing substance and a thickness 
of the body which is large compared with the wavelength. In the exact 
theory of diffraction, parts of the surface of the body at a distance from its 
edge of the order of the wavelength are necessarily of importance ; but the 
thickness of the body near its edge is always small, so that the assumption 
that it is "black" is certainly untenable. 

Considerable theoretical interest attaches to the exact solution (first 
obtained by A. Sommerfeld, 1894) of the problem of diffraction at the edge 
of a perfectly conducting wedge bounded by two intersecting semi-infinite 
planes. A complete exposition of the very complex mathematical theory, 
which involves the use of special devices, is beyond the scope of this book. 
Here we shall give, for reference, the final results, f 

We take the edge of the wedge as the sr-axis in a system of cylindrical 
co-ordinates r, <f>, z. The front surface (OA in Fig. 32) corresponds to 




Fig. 32 



f A detailed account of the calculations is given by A. Sommerfeld, Optics, Academic 
Press, New York, 1954; P. Frank and R. von Mises, Differential and Integral Equations in 
Physics (Differential- und Integralgleichungen der Physik), part 2, Chapter XX, 2nd ed., 
Vieweg, Brunswick, 1935. Another method of solution, due to M. I. Kontorovich and 
N. N. Lebedev, is given by G. A. Grinberg, Selected Problems in the Mathematical Theory 
of Electric and Magnetic Phenomena (Izbrannye voprosy matematicheskol teorii elektricheskikh 
i magnitnykh yavlenvC), Chapter XXII, Moscow, 1948. 

A modified solution for diffraction of a cylindrical wave emitted by a line source parallel 
to the edge of the wedge has been given by H. S. Carslaw, Proceedings of the London Mathe- 
matical Society 30, 121, 1899. 



306 



The Propagation of Electromagnetic Waves 



§74 



tj> = 0, and the rear surface (OB) to (f> = y, where 2ir—y is the angle of 
the wedge, the region outside it being < <f> < y. Let a plane wave of a 
single frequency and unit amplitude be incident in the r^-plane on the 
front surface at an angle ^o to the surface; by symmetry, it is sufficient to 
consider angles <£o < iy. We shall distinguish two independent modes of 
polarisation of the incident wave, and therefore of the diffracted wave also : 
the edge of the wedge (the #-axis) may be parallel to either E or H. The 
letter u will denote E z and H z respectively. 

The electromagnetic field is then given in all space by the formula (the 
time factor e~* wi being everywhere omitted) 

u{r, <f>) = v(r, <f>-<f>o)+ v(r, j> + <&>), (74.1) 

where the upper and lower signs correspond to the polarisations with E and 
H respectively in the ^-direction, and the function v(r, if/) is given by the 
complex integral 

1 r d£ 
v(r,ili\ = — e-*frrcos£ (74.2) 



where k = cojc. The path of integration C in the £-plane consists of the 
two loops Ci and C% shown in Fig. 33. The ends of these loops are at infinity 




Fig. 33 

in parts of the £-plane (shaded in Fig. 33) where im cos £ < 0, and so the 
factor e-**rcos£ tends to zero at infinity. The integrand in (74.2) has poles 
on the real £-axis at the points £ = -ifi + 2ny, where n is any integer. The 
integration may be taken along a path D = D1+D2 (Fig. 33) instead of C, 
adding to the integral the residues of the integrand at the poles, if any, in 
the range — it ^ £ ^ it. We write v as 

v(r,+) = vo(r,if J ) + v d (r,M, (74.3) 

where v& is the integral (74.2) taken along the path D, and vq is the contri- 
bution from the residues at these poles. Each pole gives rise to a term 
exp [ — ikr cos (ip — 2ny)] in £>o, which represents either the incident wave or 



§74 Diffraction by a wedge 307 

one of the waves reflected from the surface of the wedge in accordance with 
the laws of geometrical optics. The function v& represents the diffractive 
distortion of the wave. The field at distances from the edge of the wedge 
large compared with the wavelength is of the greatest interest. When 
kr > 1, the asymptotic formula f 

7T sin(7r 2 /y) timA jv 

v * r > *> = //o u^ +M < 2;/ / n\ ( 74 ' 4) 

y^(2irkr) COS (7r 2 /y) - COS (Triply) 

holds, provided that the angle «/r satisfies the condition 

[cos {rfljy) - cos (77^/y)]2 > 1/kr. (74.5) 

The dependence on r of the function va, and therefore of the field 
u d (r, <f>) = v d (r,<f>-<f>o) + v d (r,<f)+(f)o), is given by a factor eMr/<y/r, i.e. this 
field resembles a cylindrical wave emitted by the edge of the wedge. 

In the form given above, (74. l)-(74. 5) are valid for any angles y and fo. 
The more detailed discussion of these formulae will be effected on the 
assumption (for definiteness) that the angles <f>o and y are so related 
(y > 7r+(f>o) that, in geometrical optics, two boundaries are formed: the 
boundary Ob of the complete shadow (region III in Fig. 32), and the boun- 
dary Oa of the "shadow" of the wave reflected from the surface OA.% 

In regions I, II, III the function uo(r, <f>) = vo(r, <f> — <f>o) + vo(r, <f> + (f)o) has 
the following forms : 

Region I : hq = e~ ikr cos ^ ~ $o> + e~ ikr cos ^ + ^o), 

Region II: u = e - ikrco * &-*<>), ) ( 74 - 6 ) 

Region III: wo = 0. 

These expressions, which do not vanish as kr -> oo, describe the incident 
(region II) or incident and reflected (region I) waves, undistorted by dif- 
fraction. The diffractive distortion of the field is given by formula (74.4), 
but the condition (74.5) ceases to hold when «/r approaches tt and the dif- 
ference \ift — 7r\ is no longer large compared with \j^/(kr). 

The values <f> + <f>o = tt correspond to the geometrical boundaries of the 
shadow ; for <f> — <f>o = tt we have the boundary of the complete shadow, and 
for <j> + (f>Q = tt that of the shadow of the reflected wave. In the immediate 
neighbourhood of these values a different asymptotic expression must be 
used, which is valid if the inequality \rp — Tr\ <^ 1 holds. This condition, 



t The next terms in this asymptotic expansion have been given by W. Pauli, Physical 
Review 54, 924, 1938. 

% In Fig. 32, <f>o < \tt. If <£o > \t, the boundary Oa lies to the right of the direction of 
the incident wave. 

If y < 7T + <f>o, there is no region of complete shadow, and reflection (single or multiple) 
takes place from both sides of the wedge. 



308 The Propagation of Electromagnetic Waves §75 

together with kr > 1, ensures the validity of the usual approximate theory 
of Fresnel diffraction, f Accordingly we have near the boundary Ob of the 
complete shadow the asymptotic expression 

w 

i -i r 

V W V(2w)J ( 74 - 7 ) 

—oo 

w = - ((f> - (f> - ir)^Qkr). 
Similarly, near the boundary Oa of the "shadow" of the reflected wave 

w 
1 -i r 
u(r, <f>) = e~ ikr cos W ~ $o> + e~ ikr cos ^ + 0o> e ir > 2 diy, 

— oo 

(74.8) 
V) = - (<f> + <f>o - ■n-)-V / (P0- 

In this approximation the diffraction pattern is independent of the angle of 
the wedge and of the direction of polarisation of the wave. 

The ranges of applicability of formulae (74.4) and (74.7), (74.8) partly 
overlap. For example, near the boundary of the complete shadow the com- 
mon range of applicability is given by 

i>\<f>-<h-ir\> i/V(H 

and in this range 

u{r,<f>) = u (r,cf>) + _-J_««*r + *r>_! , (74.9) 

yylTTkr) <f> — <po — tt 

with «o given by (74.6). This expression can be obtained from (74.7) by 
using the asymptotic formulae for the Fresnel integral with large |«j| : 

w 

f e^dr) = (1 + i)V(br) + e iu ?l2iw for to > 0, 

—00 

w 

f e^dt] = e iw *!2iw for tv < 0. 



§75. Diffraction by a plane screen 

The exact formula (74.2) for diffraction by a wedge can be brought to a 
comparatively simple form in the particular case of diffraction by a half- 

t See The Classical Theory of Fields, §7-8. 



§75 Diffraction by a plane screen 309 

plane (y = 2tt).\ The complex integral in (74.2) can be reduced to a Fresnel 
integral : 



v(r,tfi) = e -«fcrcos^ + iw) ^ 2 dry, 

V' 7r J 

— oo 

(75.1) 
zv = y/{lkr) COS |^r. 

This formula holds for any values of r and ip. For kr > 1 and angles 
|^t — it\ > lj\/(kr) the asymptotic expression 

tto(r, 0) = - ««*+* > \ — (75.2) 

2 y \2jnkr) cos f^ 

(formula (74.4) with y = 2tt) holds.J 

Using formula (75.2), the solution of the problem of diffraction by a plane 
perfectly conducting screen of any shape can be obtained in closed form. 
The only assumptions are that the dimensions of the screen and the distance 
from it are large compared with the wavelength, and that the angles of 
diffraction are moderately large (this region overlaps the region of small 
angles in which the ordinary Fresnel-diffraction formulae are valid). The 
result is in the form of an integral along the edge of the screen, analogously 
to the expression of the diffraction field, in the ordinary approximate theory, || 
as an integral over a surface spanning the aperture in a screen. We shall not 
pause to give the calculations here. 

In the exact theory of diffraction by plane perfectly conducting screens, 
there is a theorem (due to L. I. Mandel'shtam and M. A. Leontovich) in 
some ways analogous to Babinet's theorem in the approximate theory. 

Let us consider a plane screen with an aperture of any shape, and take the 
plane of the screen as z = 0. Let an electromagnetic wave be incident from 
the side z < 0, and let Eo, Ho be the total fields in the incident wave and the 
wave which would be reflected from the screen if there were no aperture. 
We assume the field continued beyond the screen (z > 0). Since H z = 0, 
Ef = for z = (by the boundary conditions at a perfectly conducting 
surface), the values of Eo and Ho f or z > and z < are related by 

Eoz{x, y, z) = E 0z (x, y, - z), Eo*(*, y, z) = - Eo ( (x, y, - *), 
H 0z (x, y,z)= - H 0z (x, y, - z), Hot(*, y, z) = Ho*(* , y, - z). 

Next, let E' and H' be the fields which would occur if a flat plate corres- 
ponding to the aperture in size, shape and position, and having infinite 



f See the references quoted in the second footnote to §74. 

X Yu. V. Vandakurov (Zhurnal Sksperimental'noi i teoreticheskoi fiziki 26, 3, 1954) has 
obtained the exact solution of the three-dimensional problem of the diffraction by a half- 
plane of electromagnetic waves emitted from an arbitrarily oriented electric or magnetic 
dipole at a finite distance from the edge of the half-plane. 

|| See The Classical Theory of Fields, §7-7. 



310 The Propagation of Electromagnetic Waves §75 

magnetic permeability, were placed in the field Eo, Ho. Then the solution 
of the diffraction problem for the aperture in the screen is given by 

E = i(Eo+E'), H = KHo + H') for * < 0, 

E = i(E - E'), H = |(Ho - H') for z > 0. K 

To show this, we notice that the fields E', H' have the same symmetry 
(expressed by formulae (75.3)) as the fields E , Ho. They therefore satisfy 
on the plane z = the conditions 

E' t = 0, H'z = outside the aperture, 

E'a = - E'j 2 , H' z i = - H' Z 2 on the aperture, 

the suffixes 1 and 2 corresponding to z -> ± . They also satisfy the further 
conditions 

E' z = 0, H't — on the aperture, 

since the boundary conditions on the surface of a body with [a = oo are 
obtained from those for a perfectly conducting body (e = oo) by inter- 
changing E and H. Hence it is clear that the fields (75.4) satisfy the necessary 
conditions E t = 0, H z = on the surface of the screen (z -> — ) outside 
the aperture, and are continuous on the aperture. Finally, since E', H' 
tend to Eo, Ho at infinity, the fields (75.4) tend to E , H as z -> -oo and 
to zero as z -» + oo. They therefore satisfy all the conditions of the problem. 
This proves the theorem. 

Thus the problem of diffraction by an aperture in a screen with e = oo 
is equivalent to a problem of diffraction by a complementary screen with 
/x = oo. 

PROBLEMS 

Problem 1. A plane wave of a single frequency is incident normally on a slit cut in a 
perfectly conducting screen, the width la of the slit being large compared with the wave- 
length. Determine the distribution of light intensity beyond the slit, at large distances from 
it and for large angles of diffraction. 

Solution. For a > A, the diffraction field beyond the slit can be regarded as a super- 
position of fields arising from independent diffraction at each of the two edges of the slit 
and determined by means of the asymptotic formula (75.2). When the distances AP = n 
and BP = n from the edges of the slit to the point of observation (Fig. 34) are large com- 
pared with a, we can put, in the factors e ikr i and e ikr *, r\ = r—a sin x, ^2 = r+a sin x, and 
elsewhere r\ = r% S r; the angles between the s-axis and AP, OP, BP can all be taken as 
the angle of diffraction x- 

The result is 

gHkr+in) t s [ n (& a s j n x ) cos (ka sin x) \ 

~ V(2nkr)\ sin^x ~ cos ix > 
Hence the intensity of light diffracted into an angle dx is (relative to the total intensity of 
light incident on the slit) 



1 ( [" sin (ka sin x) l 2 [ cos (ka sin x) I 2 ] d 
_ 47r£aU sinix J L cos ix J' 

_ feqfr sin(feasinx) -| 2 1 J d 

~ Al kasinx J C ° SX+ [2*acosix] 2 l * 



§75 



Diffraction by a plane screen 



311 



For small x this expression becomes the formula for Fraunhofer diffraction by the slit: 



d/ = 



1 sin 2 kax 
■nka y 2 



d* 



Problem 2. A plane wave is incident on a perfectly conducting plane with a circular 
aperture whose radius a is small compared with the wavelength. Determine the intensity 
of diffracted light passing through the aperture (Rayleigh, 1897). 

Solution. As stated above, this problem is equivalent to that of diffraction by a circular 
disc with ft = co, and, since a <^ A, we have the case of scattering by a small particle. 
According to §72, in order to solve the latter problem it is necessary to determine the static 
electric and magnetic polarisabilities of the disc. The field Eo is perpendicular to the plane 
of the disc, and the boundary condition E' z = is formally identical with the condition in 
electrostatics at the boundary of a body with e = 0. The field Ho is parallel to the disc, 
and the boundary condition H't = corresponds to a magnetostatic problem with n = co. 
Hence the electric and magnetic moments of the disc are (see §4, Problem 4, and §42, Prob- 
lem) & — — 2a 3 Eo/37r, M = 4a 3 Ho/37r. In going to the problem of diffraction by an aper- 
ture we must, in accordance with formulae (75.4), divide these expressions by 2 and then 
substitute them in the scattering formula (72.1). 




Thus the intensity of radiation diffracted into a solid angle do is t 



d/ = 



4tt 9tt 2 c 4 
c to 4 a 6 



{nXEo— 2nX(H Xn)} do 



{(n x Eo) 2 +4(n x H ) 2 +4n-H x Eo} do. 



4ir 9n 2 c* 

The total diffracted intensity is obtained by integration over a hemisphere, and is 

c 4a> 4 a 6 

4w 27ttc 4 

The "effective diffraction cross-section" may be defined as the ratio of the intensity of dif- 
fracted radiation to the energy flux density in the incident wave cE 2 \4tt (letters without 
suffixes refer to the incident wave). Two modes of polarisation of the incident wave may be 
distinguished: 

(a) the vector E in the incident wave is perpendicular to the plane of incidence (the xz- 
plane), i.e. it is parallel to the plane of the screen (the ay-plane). The sum of the fields in 
the incident and reflected waves at the surface of the screen is 

Eo = 0, Hox = 2H cos a = 2E cos a 



t The factor e-" * is omitted; E and H are real. 

21 



312 The Propagation of Electromagnetic Waves §75 

(a being the angle of incidence). Hence 

16a 6 a> 4 
da = cos 2 a (1 —sin 2 & cos 2 4>) do, 

97Hc 4 

where & is the angle between the direction of diffraction n and the normal to the screen (the 
z-axis), and <f> is the azimuth of the vector n with respect to the plane of incidence. The 
total cross-section is a = (64w 4 a 6 /277rc 4 ) cos 2 a. 

(b) the vector E lies in the plane of incidence. Then JEb = Eoz = — IE sin a, 
= Hoy = 2H — 2E. The differential effective cross-section is 

16a 6 w 4 
da = cos 2 &+sin 2 & (cos 2 ^+1 sin 2 a)— sin & sin a cos ^} do, 

9?Hc 4 

and the total cross-section is a = (64w 4 a 6 /277rc 4 )(l +i sin 2 a). 
For natural incident light a = (64a> 4 a 6 /27irc 4 )(l — | sin 2 a). 



CHAPTER XI 

ELECTROMAGNETIC WAVES IN ANISOTROPIC MEDIA 

§76. The dielectric permeability of crystals 

The properties of an anisotropic medium with respect to electromagnetic 
waves are denned by the tensors e^(w) and fiik(<*>), which give the relation 
between the inductions and the fields :f Di = €u{<o)Ek t Bi = ]ii]c{o>)H]c. In 
what follows we shall, for definiteness, consider the electric field and the 
tensor e^; all the results obtained are valid for the tensor fine also. 

As to -> 0, the em tend to their static values, which have been shown in 
§13 to be symmetrical with respect to i and k. The proof was thermo- 
dynamical, and therefore holds only for states of thermodynamic equili- 
brium. In a variable field, a substance is of course not in equilibrium, and 
the proof in §13 is consequently invalid. To ascertain the properties of the 
tensor e^ we must use the generalised principle of the symmetry of the 
kinetic coefficients. 

It is easy to see that the components of the tensor e^ come under the 
general definition of the quantities <x a & :J the rate of change of the energy 
in a variable electric field is given by the integral 



/ 



1 dD 

-E.-dF. (76.!) 



A comparison with SP (124.7) shows that, if the components of the vector E 
at each point are taken as the quantities x a , the corresponding quantities f a 
will be the components of D. (The suffix a takes a continuous series of values, 
denumerating both the components of the vectors and the points in the 
body.) The coefficients a a & are then the components of the tensor e -1 ^. 
The symmetry properties of e ik are, of course, identical with those of 
its inverse. 

It should be noted that the components of the polarisability tensor for the 
whole body, i.e. the coefficients in the equations ^ = V<x.ij&ic, also come 
under the general definition. For the rate of change of the energy of a body 
placed in a variable external field © is 

-^.d(£/d*. (76.2) 

t It should be recalled that these quantities refer to the variable fields in the wave; the 
possible presence of a constant induction (in a pyroelectric or ferromagnetic crystal) is 
irrelevant to this discussion. 

t See Statistical Physics, §124, Pergamon Press, London, 1958. Formulae in this book 
will be referred to by means of the prefix SP. 

313 



314 Electromagnetic Waves in Anisotropic Media §76 

Hence we see that, if the x a are the three components of the vector ^, then 
the corresponding^ are those of the vector (£, so that the coefficients a a & are 
in this case Va.^. 

Thus we can use the generalised principle of the symmetry of the kinetic 
coefficients (SP (124.13)) to deduce that the tensor e^ is symmetrical: 

Uk = m- (76.3) 

Here it is assumed that the body is not in an external magnetic field, f 

Repeating for the anisotropic case the derivation of formula (61.4), we 
find that the electric losses are given by 

io) 

-(**** - m)EiE k *. (76.4) 

The condition that absorption is absent is e^* = e^ = e^, i.e. the e^ must 
be real, as in an isotropic medium. 

When absorption is absent, the internal electromagnetic energy per unit 
volume can be defined as shown in §61. The formula for an anisotropic 
medium corresponding to (61.9) is 

V = __(*>€«.)£«♦£*. (76.5) 

l07r dco 

In §67 we used the surface impedance £, in terms of which the boundary 
conditions at the surface of a metal can be formulated even if the dielectric 
permeability is no longer meaningful. At the surface of an anisotropic body 
the boundary condition corresponding to (67.6) is 

E a = UH x n)„ (76.6) 

where £, a p(a>) is a two-dimensional tensor on the surface of the body. It 
should be borne in mind that the value of this tensor depends, in 
general, on the crystallographic direction of the surface concerned. 

The energy flux into the body is (c/4tt)ExH«ii = (c/47r)E«Hxn 
= (c/47r)i? a (Hxn) a . (Here E and H are real.) Hence we see that if, in 
applying the principle of the symmetry of the kinetic coefficients, we take 
the components E a as the x a , then the corresponding f a will be — (Hxn) a , 
i.e. f a will be -(i/w)(Hxn) a (returning to the complex form). The coeffi- 
cients a.ab are therefore the same, apart from a factor, as the components 
£ a/? , and we conclude that 

U = U (76.7) 

in the absence of an external magnetic field. 



t The properties of the tensor eoc in the presence of an external magnetic field will be 
discussed in §82. 



§77 A plane wave in an anisotropic medium 315 

PROBLEM 

Express the components of the tensor £ a p m terms of those of i? a 3 — e_1 ap> assuming 
that the latter exists and that the body is non-magnetic (mk = Sik)- 

Solution. In an anisotropic medium, the equation £ 2 = 1/e (67.2) becomes £ a y£yp = y^. 
In components this gives f 

£ll 2 +£l2£21 = 1711. £22 2 +£l2£21 = ^22, 

£l2(£ll+£22) = 1712, £2l(£l +£22) = 1?21. 

The solution of these equations is 

£l2 = 1712/1, £21 = W£> 

£11 = [1711 ± VCmviM— wm)]/^ £22 = [•>?22±'v / ( l ?ii l ?22— yiW2i)]l£, 

i 2 = 1711 + >?22± 2 \/( '71122 — '?12'?2l). 

The choice of signs is determined by the condition that the absorption of energy must be 
positive. 

§77. A plane wave in an anisotropic medium 

In studying the optics of anisotropic bodies (crystals) we shall take only 
the most important case, where the medium may be supposed non-magnetic 
and transparent in a given range of frequencies. Accordingly, the relation 
between the electric and magnetic fields and inductions is 

D t = e«E to B = H. (77.1) 

The components of the dielectric tensor e^ are all real, and its principal 
values are positive. 

Maxwell's equations for the field of a wave of a single frequency w are 

icoH. = c curlE, icoD = - c curlH. (77.2) 

In a plane wave propagated in a transparent medium all quantities are pro- 
portional to e ik ' r , with a real wave vector k. Effecting the differentiation 
with respect to the co-ordinates, we obtain 

coH/c - k x E, coD/c = - k x H. (77.3) 

Hence we see, first of all, that the three vectors k, D, H are mutually 
perpendicular. Moreover, H is perpendicular to E, and so the three vectors 
D, E, k, being all perpendicular to H, must be coplanar. Fig. 35 shows the 
relative position of all these vectors. With respect to the direction of the 
wave vector D and H are transverse, but E is not. The diagram shows also 
the direction of the energy flux S in the wave. It is given by the vector 
product ExH, i.e. it is perpendicular to both E and H. The direction of S 
is not the same as that of k, unlike what happens for an isotropic medium. 



t We do not assume £12 = £21, and thereby allow for the presence of an external magnetic 
field. 



316 



Electromagnetic Waves in Anisotropic Media 



§77 



Clearly the vector S is coplanar with E, D and k, and the angle between S 
and k is equal to that between E and D. 
We can define a vector n by 

k = conic. (77 A) 

The magnitude of this vector in an anisotropic medium depends on its 
direction, whereas in an isotropic medium n = \/e depends only on the 
frequency. f Using (77.4), we can write the fundamental formulae (77.3) as 

H=nxE, D = - n x H. (77.5) 

£ O 




Fig. 35 



The energy flux vector in a plane wave is 

S = cEx H/4tt = (c/47r){£ 2 n - (E.n)E}; (77.6) 

in this formula E and H are real. 

So far we have not used the relation (77. 1) which involves the constants 
e ik characterising the material. This relation, together with equations (77.5), 
determines the function w(k). 

Substituting the first equation (77.5) in the second, we have 

D = n X (E x n) = n 2 E - (n.E)n. (77.7) 

If we equate the components of this vector to e^is* in accordance with (77.1), 
we obtain three linear homogeneous equations for the three components of 
E: n 2 Ei-n(n]cE]c = €«£* or 

(n 2 8ijc - mnjc - € ik )E k = 0. (77.8) 

The compatibility condition for these equations is that the determinant of 
their coefficients should vanish: 



\n 2 8ik - ninic — c{*| = 0. 



(77.9) 



In practice, this determinant is conveniently evaluated by taking as the 
axes of x, y, z the principal axes of the tensor c^ (called the principal di- 
electric axes). Let the principal values of the tensor be e (a;) , e { y\ e (z) . 



f The magnitude n is still called the refractive index, although it no longer bears the 
same simple relation to the law of refraction as in isotropic bodies. 



§77 A plane wave in an anisotropic medium 317 

Then a simple calculation gives 

n\<&n£ + c^V + € ( 2 ) Wz 2) _ [n x ^x)^y + € te)) + 

+ n y ^y\eW + e<*>) + w z 2 e (z)( e ^) + €<y>)] + e te) e (J/) € fe) = 0. (77.10) 

The sixth-order terms cancel when the determinant is expanded; this is, of 
course, no accident and is due ultimately to the fact that the wave has two, 
not three, independent directions of polarisation. 

Equation (77.10), called Fresnel's equation, is one of the fundamental 
equations of crystal optics. It determines implicitly the dispersion relation, 
i.e. the frequency as a function of the wave vector. (It should be recalled 
that the principal values e W) are functions of frequency, and so are, in some 
cases (see §79), the directions of the principal axes of the tensor €*#.) For 
waves of a single frequency, however, to, and therefore all the € (<) , are usually 
given constants, and equation (77.10) then gives the magnitude of the wave 
vector as a function of its direction. When the direction of n is given, 
(77.10) is a quadratic equation, for w 2 , with real coefficients. Hence two dif- 
ferent magnitudes of the wave vector correspond, in general, to each direc- 
tion of n. 

Equation (77.10) (with constant coefficients c w) ) defines in the co-ordinates 
n x > n y , n z the "wave-vector surface", f In general this is a surface of the 
fourth order, whose properties will be discussed in detail in the following 
sections. Here we shall mention some general properties of this surface. 

We first introduce another quantity characterising the propagation of light 
in an anisotropic medium. The direction of the light rays (in geometrical 
optics) is given by the group velocity vector dw/dk. In an isotropic medium, 
the direction of this vector is always the same as that of the wave vector, but 
in an anisotropic medium the two do not in general coincide. The rays may 
be characterised by a vector s, whose direction is that of the group velocity, 
while its magnitude is given by 

n-s = 1. (77.11) 

We shall call s the ray vector. Its significance is as follows. 

Let us consider a beam of rays (of a single frequency) propagated in all 
directions from some point. The value of the eikonal (which is, apart from 
a factor to/c, the wave phase ; see §65) at any point is given by the integral 
Jn*dl taken along the ray. Using the vector s which determines the direc- 
tion of the ray, we can put if/ = Jn»dl = J(n«s/*) d/ = J" dlfs. In a homo- 
geneous medium, s is constant along the ray, so that ip = LJs, where L is 
the length of the ray segment concerned. Hence we see that, if a segment 
equal (or proportional) to s is taken along each ray from the centre, the 
resulting surface is such that the phase of the rays is the same at every 
point. This is called the ray surface. 



t A much less convenient concept called the "surface of normals" or "surface of indices" 
has often been used; it is obtained by taking a point at a distance \\n (instead of n) in each 
direction. 



318 Electromagnetic Waves in Anisotropic Media §77 

The wave-vector surface and the ray surface are in a certain dual relation- 
ship. Let the equation of the wave- vector surface be written f{k Xi k y , k z , o>) 
= 0. Then the components of the group velocity vector are 

do) dfjdki 

= - -^-— , (77.12) 

dki df/dco 

i.e. they are proportional to the derivatives dfjdki, or, what is the same thing 
(since the derivatives are taken for constant w), to the derivatives df/dni. 
The components of the ray vector, therefore, are also proportional to 
df\dn%.\ But the vector df/dn is normal to the surface / = 0. Thus we 
conclude that the direction of the ray vector of a wave with given n is that 
of the normal at the corresponding point of the wave- vector surface. 

It is easy to see that the reverse is also true : the normal to the ray surface 
gives the direction of the corresponding wave vectors. For the equation 
s«Sn = 0, where Sn is an arbitrary infinitesimal change in n (for given a>), 
i.e. the vector of an infinitesimal displacement on the surface, expresses the 
fact that s is perpendicular to the wave-vector surface. Differentiating 
(again for given co) the equation n«s = 1, we obtain n«Ss + s«Sn = 0, and 
therefore n«Ss = 0, which proves the above statement. 

This relation between the surfaces of n and s can be made more precise. 
Let no be the radius vector of a point on the wave-vector surface, and so 
the corresponding ray vector. The equation (in co-ordinates n x , %, n z ) of 
the tangent plane at this point is so'( n_n o) = 0, which states that so is 
perpendicular to any vector n — no in the plane. Since so and no are related 
by so* no = 1, we can write the equation as 

s -n = 1. (77.13) 

Hence it follows that l/j is the length of the perpendicular from the origin 
to the tangent plane to the wave- vector surface at the point no. 

Conversely, the length of the perpendicular from the origin to the tangent 
plane to the ray surface at a point so is l/«o- 

To ascertain the location of the ray vector relative to the field vectors in 
the wave, we notice that the group velocity is always in the same direction 
as the (time) average energy flux vector. For let us consider a wave packet, 
occupying a small region of space. When the packet moves, the energy 
concentrated in it must evidently move with it, and the direction of the 



f Differentiating the left-hand side of equation (77.10) with respect to m and determining 
from the condition n«s = 1 the proportionality coefficient between Si and df/dm, we obtain 
the following relations between the components of s and n: 

e (s)( € (j/) _j_ e (z)) _ 2e(*> Ws 2 — (e<*> + e (y) )Wj/ 2 — (e ( *> + e (z) )w 2 2 
Sx = Hx 2e<*> e <!/> e <z) _ nx 2 € (s)( € (y) + e (z>) _ % 2 e (2/)( e (s) + € (z)) _ Mi 2 € ( Z )( € (z) + 6 <v))' 

(77.12a) 
and similarly for s v , s z . 



§77 A plane wave in an anisotropic medium 319 

energy flux is therefore the same as the direction of the velocity of the 
packet, i.e. the group velocity, f 

Since the Poynting vector is perpendicular to H and E, the same is true 
of s: 

sH = 0, s-E = 0. (77.14) 

A direct calculation, using formulae (77.5), (77.11) and (77.14), gives 

H = s x D, E=-sxH. (77.15) 

For example, sxH = sx(nxE) = n(s«E)-E(n«s) = -E. 

If we compare formulae (77.15) and (77.5), we see that they differ by the 
interchange of 

E and D, n and s, e ik and e -1 ^ (77.16) 

(the relation n«s = 1 remaining valid, of course). The last of these pairs 
must be included in order that the relation (77. 1) between D and E should 
remain valid. Thus the following useful rule may be formulated: an 
equation valid for one set of quantities can be converted into one valid for 
another set by means of the interchanges (77.16). 

In particular, the application of this rule to (77.10) gives immediately an 
analogous equation for s : 

_ L% 2( e (y) + e (z)) + % 2( e te) + e (z)) + Sz 2( € (x) + e (2/))] +1 = 0. (77.17) 

This equation gives the form of the ray surface. Like the wave- vector sur- 
face, it is of the fourth order. When the direction of s is given, (77.17) is a 
quadratic equation for s 2 , which in general has two different real roots. 
Thus two rays with different wave vectors can be propagated in any direc- 
tion in the crystal. 

Let us now consider the polarisation of waves propagated in an aniso- 
tropic medium. Equations (77.8), from which Fresnel's equation has been 
derived, are unsuitable for this, because they involve the field E, whereas 
it is the induction D which is transverse (to the given n) in the wave. In 
order to take account immediately of the fact that D is transverse, we use 
for the time being a new co-ordinate system with one axis in the direction 



f It is easy to demonstrate mathematically that the group velocity is in the same direction" 
as the Poynting vector. Differentiating formulae (77.5) (for given co), we obtain 

SD = SH x n + H x Sn, SH = n x SE + Sn x E. 

We take the scalar product of the first equation with E and of the second with H, obtaining 

E-SD = H-SH + ExH-Sn, H-SH = D-SE + ExH-Sn. 

But D« SE = HkEichEi = E-SD, and so, adding the two equations, we have E X H* Sn = 0, 
i.e. the vector E X H is normal to the wave-vector surface. This is the required result. 

The result thus obtained relates to the instantaneous, as well as to the average, energy 
flux. In this proof, however, the symmetry of the tensor ejfc is vital. The result is therefore 
not valid in the above form for media in which eat is not symmetrical (gyrotropic media, 
§82). The statement is still valid, however, for the average value of the Poynting vector. 



320 Electromagnetic Waves in Anisotropic Media §77 

of the wave vector, and denote the two transverse axes by Greek suffixes, 
which take the values 1 and 2. The transverse components of equation 
(77.7) give D x = n 2 E a ; substituting E a = e-^D^, where e~\ fi is a component 
of the tensor inverse to e^, we have D a — n 2 e- 1 afi Dp = 0, or 

(^- € -i a ^ = 0. (77.18) 

The condition for the two equations (a = 1, 2) in the two unknowns Z)i, 
Z>2 to be compatible is that their determinant should be zero. This condi- 
tion is, of course, the same as FresnePs equation, which was written in the 
original co-ordinates x, y, z. We now see also, however, that the vectors D 
corresponding to the two values of n are along the principal axes of the sym- 
metrical two-dimensional tensor of rank two e -1 ay? . According to general 
theorems it follows that these two vectors are perpendicular. Thus, in the 
two waves with the wave vector in the same direction, the electric induction 
vectors are linearly polarised in two perpendicular planes. 

Equations (77.18) have a simple geometrical interpretation. Let us draw 
the tensor ellipsoid corresponding the tensor e -1 ^, returning to the principal 
dielectric axes, i.e. the surface 

X 1/ 2 

***«*- -JS + -& + -0-1- < 77 - 19 ) 

(Fig. 36). Let this ellipsoid be cut by a plane through its centre perpendicular 
to the given direction of n. The section is in general an ellipse ; the lengths 
of its axes determine the values of n, and their directions determine the 
directions of the oscillations, i.e. the vectors D. 




Fig. 36 

From this construction (with, in general, e te) , c^, e^ different) we see at 
once, in particular, that, if the wave vector is in (say) the ^-direction, the 
directions of polarisation (D) will be the y and z directions. If the vector n 
lies in one of the co-ordinate planes, e.g. the ary-plane, one of the directions 
of polarisation is also in that plane, and the other is in the ^-direction. 

The polarisations of two waves with the ray vector in the same direction 
have entirely similar properties. Instead of the directions of the induction 



§78 Optical properties of uniaxial crystals 321 

D, we must now consider those of the vector E, which is transverse to s, and 
equations (77. 1 8) are replaced by the analogous equations 

(^-^)^ = 0. (77.20) 

The geometrical construction is here based on the tensor ellipsoid 

mXiX k = e<*>* 2 + e<yy + e®Z 2 = 1, (77.21) 

corresponding to the tensor cm itself (called the Fresnel ellipsoid). 

It should be emphasised that plane waves propagated in an anisotropic 
medium are completely linearly polarised in certain planes. In this respect 
the optical properties of anisotropic media are very different from those of 
isotropic media. A plane wave propagated in an isotropic medium is in 
general elliptically polarised, and is linearly polarised only in particular 
cases. This important difference arises because the case of complete isotropy 
of the medium is in a sense one of degeneracy, in which a single wave vector 
corresponds to two directions of polarisation, whereas in an anisotropic 
media there are in general two different wave vectors (in the same direction). 
The two linearly polarised waves propagated with the same value of n 
combine to form one elliptically polarised wave. 

§78. Optical properties of uniaxial crystals 

The optical properties of a crystal depend primarily on the symmetry of 
its dielectric tensor e^. In this respect all crystals fall under three types: 
cubic, uniaxial and biaxial (see §13). In a crystal of the cubic system 
Hk = eSflb, i.e. the three principal values of the tensor are equal, and the 
directions of the principal axes are arbitrary. As regards their optical proper- 
ties, therefore, cubic crystals are no different from isotropic bodies. 

The uniaxial crystals include those of the rhombohedral, tetragonal and 
hexagonal systems. Here one of the principal axes of the tensor e^ coincides 
with the axis of symmetry of the third, fourth or sixth order respectively ; in 
optics, this axis is called the optical axis of the crystal, and in what follows 
we shall take it as the ^-axis, denoting the corresponding principal value of 
*ik by €,,. The directions of the other two principal axes, in a plane per- 
pendicular to the optical axis, are arbitrary, and the corresponding principal 
values, which we denote by e ± , are equal. 

If in Fresnel's equation (77.10) we put € te) = e ( i/> = e ± , e (2 > = e„, the 
left-hand side is a product of two quadratic factors : 

(w 2 - e ± )[ € ^n z 2 + e ± (n x 2 + V) ~ fi«i] = °- 

In other words, the quartic equation gives the two quadratic equations 

n 2 = e ±i (78.1) 

nl + n l± n l=i 



322 



Electromagnetic Waves in Anisotropic Media 



§78 



Geometrically, this signifies that the wave- vector surface, which is in general 
of the fourth order, becomes two separate surfaces, a sphere and an ellipsoid. 
Fig. 37 shows a cross-section of these surfaces. Two cases are possible: if 
€ x > e„, the sphere lies outside the ellipsoid, but if e x < e„ it lies inside. 
In the first case we speak of a negative uniaxial crystal, and in the second 
case of a positive one. The two surfaces touch at opposite poles on the n z - 
axis. The direction of the optical axis therefore corresponds to only one 
value of the wave vector. 





Fig. 37 

The ray surface is exactly similar in form. By the rule (77.16), its equation 
is obtained from (78.1) and (78.2) by replacing n by s and e by 1/e: 

s* = l/ ej , (78.3) 

^ 2 + e„(% 2 + % 2 ) = 1. (78.4) 

In a positive crystal the ellipsoid lies within the sphere, and in a negative 
one outside. 

Thus we see that two types of wave can be propagated in a uniaxial crystal. 
With respect to one type, called ordinary waves, the crystal behaves like an 
isotropic body of refractive index n = V € _l- The magnitude of the wave 
vector is wn/c whatever its direction, and the direction of the ray vector is 
that of n. 

In waves of the second type, called extraordinary waves, the magnitude of 

the wave vector depends on the angle 6 which it makes with the optical 

axis. By (78.2) 

1 sin 2 cos 2 
- = + • (78.5) 

n* e € ± 

The ray vector in an extraordinary wave is not in the same direction as the 
wave vector, but is coplanar with that vector and the optical axis, their com- 
mon plane being called the principal section for the given n. Let this be the 
zx-phne; the ratio of the derivatives of the left-hand side of (78.2) with 
respect to n z and n x gives the direction of the ray vector: s x \s z = ejix/e^ng. 
Thus the angle 8' between the ray vector and the optical axis and the angle 6 
satisfy the simple relation 

tan0' = (€ x /e„)tan0. (78.6) 



§78 Optical properties of uniaxial crystals 323 

The directions of n and 1 are the same only for waves propagated along or 
perpendicular to the optical axis. 

The problem of the directions of polarisation of the ordinary and extra- 
ordinary waves is very easily solved. It is sufficient to observe that the four 
vectors E, D, s and n are always coplanar. In the extraordinary wave s and 
n are not in the same direction, but lie in the same principal section. This 
wave is therefore polarised so that the vectors E and D lie in the same princi- 
pal section as s and n. The vectors D in the ordinary and extraordinary 
waves with the same direction of n (or E, with the same direction of s) are 
perpendicular. Hence the polarisation of the ordinary wave is such that E 
and D lie in a plane perpendicular to the principal section. 

An exception is formed by waves propagated in the direction of the optical 
axis. In this direction there is no difference between the ordinary and the 
extraordinary wave, and so their polarisations combine to give a wave which 
is, in general, elliptically polarised. 

The refraction of a plane wave incident on the surface of a crystal is dif- 
ferent from refraction at a boundary between two isotropic media. The laws 
of refraction and reflection are again obtained from the continuity of the 
component n^ of the wave vector which is tangential to the plane of separa- 
tion. The wave vectors of the refracted and reflected waves therefore lie in 
the plane of incidence. In a crystal, however, two different refracted waves 
are formed, a phenomenon known as double refraction. They correspond to 
the two possible values of the normal component n n which satisfy FresnePs 
equation for a given tangential component n^. It should also be remembered 
that the observed direction of propagation of the rays is determined not by 
the wave vector but by the ray vector s, whose direction is different from that 
of n and in general does not lie in the plane of incidence. 

In a uniaxial crystal, ordinary and extraordinary refracted waves are 
formed. The ordinary wave is entirely analogous to the refracted wave in 
isotropic bodies; in particular, its ray vector (which is in the same direction 
as its wave vector) lies in the plane of incidence. The ray vector of the 
extraordinary wave in general does not lie in the plane of incidence. 

PROBLEMS 

Problem 1 . Find the direction of the extraordinary ray when ligjit incident from a vacuum 
is incident on a surface of a uniaxial crystal which is perpendicular to its optical axis. 

Solution. In this case the refracted ray lies in the plane of incidence, which we take as 
the xsr-plane, with the s-axis normal to the surface. The ^-component of the wave vector 
n x = sin ft (ft being the angle of incidence) is continuous ; the component n z for the refracted 
wave is found from (78.2): 



** = A /(ejL--^sin2^. 



The direction of the refracted ray is given by (78.6): 

Cx n x V e x sin ft 



tan ft' 



e, n 2 V[ e «( e « — sin 2 ft)] 
where ft' is the angle of refraction. 



324 Electromagnetic Waves in Anisotropic Media §79 

Problem 2. Find the direction of the extraordinary ray when light is incident normally 
on a surface of a uniaxial crystal at any angle to the optical axis. 

Solution. The refracted ray lies in the ars-plane, which passes through the normal to 
the surface (the #-axis) and the optical axis. Let a be the angle between these axes. The 
ray vector s, whose components are proportional to the derivatives of the left-hand side of 
equation (78.2) with respect to the corresponding components of n, is proportional to 






where 1 is a unit vector in the direction of the optical axis. In the present case the wave 
vector n is in the ^-direction, so that 






sin" 5 a cos" a 

s e 1 . 



Hence we find 

s x ( e i~ e x) sin 2a 



tan $' 



Sz € i + c x+( e i~~ e x) cos 2a 



§79. Biaxial crystals 

In biaxial crystals the three principal values of the tensor e^ are all dif- 
ferent. The crystals of the triclinic, monoclinic and rhombic systems are of 
this type. In those of the triclinic system, the position of the principal di- 
electric axes is unrelated to any specific crystallographic direction; in 
particular, it varies with frequency, as do all the components e^. In crystals 
of the monoclinic system, one of the principal dielectric axes is crystallo- 
graphically fixed ; it coincides with the second- order axis of symmetry, or is 
perpendicular to the plane of symmetry. The position of the other two 
principal axes depends on the frequency. Finally, in crystals of the rhombic 
system, the position of all three principal axes is fixed: they must coincide 
with the three mutually perpendicular second-order axes of symmetry. 

The study of the optical properties of biaxial crystals involves the con- 
sideration of Fresnel's equation (77.10) in its general form. We shall assume 
for definiteness that 

ete) < e (y) < €<*>. (79.1) 

To ascertain the form of the fourth-order surface defined by equation 
(77.10), let us begin by finding its intersections with the co-ordinate planes. 
Putting in equation (77.10) n z = 0, we find that the left-hand side is the 
product of two factors : 

( W 2 _ € te))( e te) Wa .2 + € (y) ny 2 _ € (x) e (2/)) = 0. 

Hence we see that the section by the xy-plane consists of the circle 

«2 = € te) (79.2) 

and the ellipse 

n<p w« 2 

— + — = 1, (79.3) 

€ (y) € te) ' v ' 



§79 



Biaxial crystals 



325 



and by the assumption (79.1) the ellipse lies inside the circle. Similarly we 
find that the sections by the yz and xz planes are also composed of an 
ellipse and a circle ; in the js-plane the ellipse lies outside the circle, and in 
the xs-plane they intersect. Thus the wave-vector surface intersects itself, 
and is as shown in Fig. 38, where one octant is drawn. 




Fig. 38 

This surface has four singular points of self-intersection, one in each 
quadrant of the xs-plane. The singular points of a surface whose equation 
is /far, %> n z ) = are given by the vanishing of all three first derivatives of 
the function/. Differentiating the left-hand side of (77.10), we obtain the 
equations 

n x [&\&> + e (2) ) - e (a % 2 - (e^Wa; 2 + €<2% 2 + e^n^)] = 0, 

% [ € W( € (*> + € <z>) - e (J/) W 2 _ ( € U) Wa .2 + € (y) ny 2 + € (z) nz 2fl = (), L (79.4) 

w 2 [ e (z)( e te) + &)) _ e (%2 _ ( € (x) nx 2 + € (y) % 2 + € fe) Wz 2)] = 0; 

the equation (77.10) itself must, of course, be satisfied also. Since we know 
that the required directions of n lie in the #*-plane, we put % = 0, and the 
two remaining equations give immediately f 



n x 2 = 



€ (z)( e (y) _ € te)) 
€ (z) _ € (x) ' 



«z 2 = 



e te)( e (z) _ € <i/)) 
€ (z) _ € (x) ' 



(79.5) 



The directions of these vectors n are inclined to the 0-axis at an angle /? 
such that 

i € (z)( € (y) _ e te)\ 

* - ± J ^ M J - (79-6) 



n x 



= + tani 



n z 



e te)( € (z> _ e (y)) 



t It is easy to see that the solution thus found is the only real solution of equations (79.4). 
If none of n x , n y , n t is zero, the three equations (79.4) are inconsistent: they then involve 
only two unknowns, namely rfi and e^n* 2 + ^n y 2 + e(^n z 2 . Un x or n z is zero the solutions 
are imaginary. 



326 Electromagnetic Waves in Anisotropic Media §79 

This formula determines lines in two directions in the tf.s'-plane, each of 
which passes through two opposite singular points and is at an angle j8 to 
the sr-axis. These lines are called the optical axes or binomials of the crystal ; 
one of them is shown dashed in Fig. 38. The directions of the optical axes 
are evidently the only ones for which the wave vector has only one magni- 
tude, f 

The properties of the ray surface are entirely similar. To derive the corres- 
ponding formulae, it is sufficient to replace n by s and e by 1/e. In particular, 
there are two optical ray axes or biradials, also lying in the xs-plane and at 
an angle y to the #-axis, where 

y e i.y) — e te) / e te) 

Since e (a! > < e< 3 >, y < £. 

The directions of corresponding vectors n and s are given by the general 
formulae (77. 12a). Their directions are the same only for waves propagated 
along one of the co-ordinate axes (i.e. the principal dielectric axes). If n lies 
in one of the co-ordinate planes, s lies in that plane also. This rule, however, 
is subject to an important exception for wave vectors in the direction of the 
optical axes. 

When the values of n given by (79.5) are substituted in formulae (77.1.2a), 
the components of s take the indeterminate form 0/0. The origin and mean- 
ing of this indeterminacy are quite evident from the following geometrical 
considerations. Near a singular point, the inner and outer parts of the wave- 
vector surface are cones with a common vertex. At the vertex, which is the 
singular point itself, the direction of the normal to the surface becomes in- 
determinate ; and the direction of s as given by formulae (77.12a) is just the 
direction of the normal. In fact the wave vector along the binormal corres- 
ponds to an infinity of ray vectors, whose directions occupy a certain conical 
surface, called the cone of internal conical refraction. 

To determine this cone of rays, we could investigate the directions of the 
normals near the singular point. It is more informative, however, to use a 
geometrical construction from the ray surface. 

Fig. 39 shows one quadrant of the intersection of the ray surface with the 
xs"-plane (continuous curves), and also the intersection of the wave-vector 
surface, on a different scale. The line OS is the biradial, and ON the bi- 
normal. Let n# be the wave vector corresponding to the point iV. It is 
easy to see that the singular point N on the wave-vector surface corresponds 
to a singular tangent plane to the ray surface. This plane is perpendicular to 
ON, and touches the ray surface not at one point but along a curve, which is 
found to be a circle. In Fig. 39 the trace of this plane is shown by ab. This 
follows at once from the geometrical correspondence between the wave- vector 



t In the tensor ellipsoid (77.19) the binormals are the directions perpendicular to the 
circular sections of the ellipsoid. An ellipsoid has two such sections. 



§79 



Biaxial crystals 



327 



surface and the ray surface (§77): if the tangent plane is drawn at any point 
s of the ray surface, then the perpendicular from the origin to this plane is 
in the same direction as the wave vector n corresponding to s, and its length 
is 1/n. In our case there must be an infinity of vectors s corresponding to 
the single value n = n^; hence the points on the ray surface which repre- 
sent these vectors s must lie in one tangent plane, which is perpendicular 
to nzy. Thus in Fig. 39 the triangle Oab is the section of the cone of internal 
conical refraction by the xs-plane. 




Fig. 39 



There is no especial difficulty in carrying out a quantitative calculation 
corresponding to this geometrical picture, but we shall not do so here, and 
give only the final formulae. The equations of the circle in which the cone 
of refraction cuts the ray surface are 



(< 



(z) 



Ax) 



w 



+ 



+ {SxVb (X \^ Z) ~ &>)] ~ S z V[e {z \* iy) ~ * te) )]} X 
e (z) _ € (y) J € (y) _ e (x) ' 



(-J- 



y € (y) — e (x> \ 



5 C*> W e<*> ' - (79 ' 8) 

SxVb (z \e (y) ~ ^ x) )] + W[ ete) ( ete) - * {y) )] = VW Z) ~ e {x) \ (79.9) 

The first of these equations is the equation of the cone of refraction if s x , 
s y , s z are regarded as three independent variables. The second is the equa- 
tion of the tangent plane to the ray surface. In particular, for s y = equation 
(79.8) gives the two equations 

Sx j&\<&> - «te>) s x ltte\t&> - €<*>) 

S Z ~ A/ e (a;) ( e(z) - e(v) )' ~S~z ~ N e (z) (e fe) - e^>) ' 

which determine the directions of the extreme rays (respectively Oa and Ob 



22 



328 



Electromagnetic Waves in Anisotropic Media 



§79 



in Fig. 39) in the section by the .x^-plane. The former is along the binormal 
(cf. (79.6)), which is perpendicular to the tangent ab. 

Similar results hold for the wave vectors corresponding to a given ray 
vector. The vector s along the biradial corresponds to an infinity of wave 
vectors, whose directions occupy the cone of external conical refraction. In 
Fig. 39 the triangle Oa'b' is the section of this cone by the xs-plane. The 
corresponding formulae are again obtained by substituting n for s and 1/e 
for e in the formulae (79.8), (79.9), and are 

€ (2/)( e (z) _ e (z)) % 2 + [ Wa V(e (z) - * (2/) ) ~ «2\/( e(2/) - e (x) )] X 

x [n z eW^/( € (z) - € (y>) - n z eU>^/(z{y) _ e (*))] = 0, 

«aV( e(2/) ~ * (aj) ) + «zV( ete) - * {y) ) = V[e (y K* {z) ~ ete) )]- 

In observations of the internal conical refraction f we can use a flat plate 
cut perpendicular to the binormal (Fig. 40). The surface of the plate is 
covered by a diaphragm of small aperture, which selects a narrow beam from 
a plane light wave (i.e. one whose wave vector is in a definite direction) inci- 
dent on the plate. The wave vector in the wave transmitted into the plate 
is in the direction of the binormal, and so the rays are on the cone of internal 
refraction. The wave vector in the wave leaving the other side of the plate 
is the same as in the incident wave, and so the rays are on a circular cylinder. 



Fig. 40 

To observe the external conical refraction, the plate must be cut per- 
pendicular to the biradial, and both its surfaces must be covered by dia- 
phragms having small apertures in exactly opposite positions. When the 
plate is illuminated by a convergent beam (i.e. one containing rays with all 
possible values of n), the diaphragms admit to the plate rays with s along 
the biradial, and therefore with directions of n occupying the surface of the 
cone of external conical refraction. The light leaving the second aperture 
is therefore on a conical surface, although this does not exactly coincide with 
the cone of external refraction, on account of the refraction on leaving the 
plate. 

The laws of refraction at the surface of a biaxial crystal for an arbitrary direc- 
tion of incidence are extremely complex, and we shall not pause to discuss 



f We shall describe only the principle of the experiment. 



§80 Double refraction in an electric field 329 

them here.f but only mention that, unlike what happens for a uniaxial 
crystal, both refracted waves are "extraordinary" and, in particular, the 
rays of neither lie in the plane of incidence. 

§80. Double refraction in an electric field 

An isotropic body becomes optically anisotropic when placed in a constant 
electric field. This anisotropy may be regarded as the result of a change in 
the dielectric constant due to the constant field. Although this change is 
relatively very slight, it is important here because it leads to a qualitative 
change in the optical properties of bodies. 

In this section we denote by E the constant electric field in the body,J 
and expand the dielectric tensor e^ in powers of E. In an isotropic body in 
the zero-order approximation, we have e^ = e (0) S^. There can be no terms 
in eik which are of the first order in the field, since in an isotropic body there 
is no constant vector with which a tensor of rank two linear in E could be 
constructed. The next terms in the expansion of e^ must therefore be 
quadratic in the field. From the components of the vector E we can form 
two symmetrical tensors of rank two, E 2 8m and EfEk. The former does not 
alter the symmetry of the tensor e^Snc, and the addition of it amounts to a 
small correction in the scalar constant e (0) , which evidently does not result 
in optical anisotropy and is therefore of no interest. Thus we arrive at the 
following form of the dielectric tensor as a function of the field : 

€« = «i* + x£i£*, (80.1) 

where a is a scalar constant. 

One of the principal axes of this tensor coincides with the direction of the 
electric field, and the corresponding principal value is * 

€„ = e«»+a£2. ( 8 0.2) 

The other two principal values are both equal to 



C _L 



= e<o>, (80.3) 



and the position of the corresponding principal axes in a plane perpendicular 
to the field is arbitrary. Thus an isotropic body in an electric field behaves 
optically as a uniaxial crystal (the Kerr effect). 

The change in optical symmetry in an electric field may occur in a crystal 
also (for example, an optically uniaxial crystal may become biaxial, and a 
cubic crystal may cease to be optically isotropic), and here the effect may be 
of the first order in the field. This linear effect corresponds to a dielectric 
tensor of the form € ik = eui {0) + at&iEi, where the coefficients <x. ik i form a 
tensor of rank three symmetrical in the suffixes i and k (a«i = a*«). The 



t A detailed account of the calculations may be found in the article by G. SziVESSY 
Handbuch der Physik, vol. XX, Chapter 11, Springer, Berlin, 1928. ' 

X Not to be confused with the variable field of the wave, which is usually very weak. 



330 Electromagnetic Waves in Anisotropic Media §81 

symmetry of this tensor is the same as that of the piezoelectric tensor. The 
effect in question therefore occurs in the twenty crystal classes which admit 
piezoelectricity. 



§81. Mechanical-optical effects 

Besides the Kerr effect discussed in §80, there are other ways in which the 
optical symmetry of a medium can be changed by external agencies. These 
include, first of all, the effect of elastic deformations on the optical properties 
of solids. In particular, such deformations may render an isotropic solid body 
optically anisotropic. Such phenomena are described by the inclusion in 
Hk{u>) of additional terms proportional to the components of the deforma- 
tion tensor. The corresponding formulae are exactly the same as (16.1) and 
(16.6) for the static dielectric permeability, except that the coefficients are 
now functions of frequency. In the deformation of an isotropic body, for 
example, we have 

eoc = e (0) S^ + aiUijc + a^uahtk. (81.1) 

The coefficients «i(a>) and a^ico) are called elastic-optical constants. 

Another case is the occurrence of optical anisotropy in a moving fluid. 
Here we do not refer to the relativistic effects described by the equations 
of §57; the effects to be considered are due to the presence of velocity 
gradients in the fluid. The corresponding general expression for the 
dielectric tensor is 

m . = Ma + J^ + ^) + J^.^\ (81 . 2) 

and represents the first terms in an expansion of e^ in powers of the deriva- 
tives of the velocity. In an incompressible fluid dvijdxi = div v = 0, and 
the last two terms in (81.2) give zero on contraction; e (0) is the dielectric 
permeability of the fluid at rest.f The second and third terms in (81.2) 
are respectively symmetrical and antisymmetrical in the suffixes i and k. 
For uniform rotation of the fluid we have v = Slxr, where £1 is the angu- 
lar velocity of rotation, and the symmetrical term is zero.J 

These phenomena are of practical importance only in such systems as 
suspensions and colloidal solutions of anisotropically shaped particles. 



f To avoid misunderstanding, it should be emphasised that the symmetry relations dis- 
cussed in Statistical Physics §124 (the generalised principle of symmetry of the kinetic 
coefficients) do not apply to the expression (81.2). These relations are derived on the assump- 
tion that the processes corresponding to the coefficients under consideration are the only 
cause of energy dissipation in the system. In the present case, however, besides the dissipation 
in the variable electromagnetic field in the wave, there is another mode of dissipation, which 
is unrelated to the field, namely the internal friction in the non-uniform fluid stream. 

t See §82 for the optical properties resulting from the antisymmetrical part of the 
tensor etfc. 



§82 Magnetic-optical effects 331 

In this case the effect (called the Maxwell effect) is due primarily to the 
orienting of particles suspended in the fluid by the action of the velocity 
gradients. Since a uniform rotation cannot orient the particles, it follows 
that A2 <^ Ai, and we can write simply 

€ut = ^B fk + X 1 (—=- + -^\. (81.3) 



§82. Magnetic-optical effects 

In the presence of a constant magnetic field H, f the tensor e^ is no longer 
symmetrical. The generalised principle of symmetry of the kinetic coeffi- 
cients given by SP (124.14) requires that 

e«(H)-€«(-H). (82.1) 

The condition that absorption is absent requires that the tensor should be 
Hermitian : 

Hk = e&f*, (82.2) 

as is seen from (76.4), but not that it should be real. Equation (82.2) implies 
only that the real and imaginary parts of em must be respectively symmetrical 
and antisymmetrical : 

Hk = ejw\ Hk" = — ***"• (82.3) 

Using (82.1), we have 

efjt'(H) = e W '(H) = e ik \- H), 

€**"(H) = - €«"(H) = - €«"(- H), C ' j 

i.e. in a non-absorbing medium e^' is an even function of H, and €&" an 
odd function. 

The inverse tensor e -1 ^ evidently has the same symmetry properties, 
and is more convenient for use in the following calculations. To simplify 
the notation we shall writet 

e~Hk = ?)ik = rut' + irjik". (82.5) 

Any antisymmetrical tensor of rank two is equivalent (dual) to some axial 
vector; let the vector corresponding to the tensor 17^" be G. Using the anti- 
symmetrical unit tensor em, we can write the relation between the compo- 
nents 7]nc" and Gt as 

W = e ikiG h (82.6) 



t Not to be confused with the weak variable field of the electromagnetic wave. 
X Of course, ij**' and t\ik" are not the tensors inverse to euc' and eue". 



332 Electromagnetic Waves in Anisotropic Media §82 

or, in components, r) xy " = G z , rj zx " = G y , t] yz n = G x . The relation 
Ei = -qijcDjc between the electric field and induction becomes 

E t = {7) ik ' + ie m G t )D k = rjmDjc + «(D X G) t . (82.7) 

A medium in which the relation between E and D is of this form is said to be 
gyrotropic. f 

We may give a general discussion of the nature of waves propagated in 
an arbitrary gyrotropic medium, assumed anisotropic, with no restriction on 
the magnitude of the magnetic field. J 

We take the direction of the wave vector as the .sr-axis. Then equations 
(77.18) become 

\Vcl0 ~ -£«iA D e = Viae + i7 l*e" ~ -^Ke) D e = °> ( 82 - 8 ) 

where the suffixes a, jS take the values x, y. The directions of the x and y 
axes are taken along the principal axes of the two-dimensional tensor t]^ ; and 
we denote the corresponding principal values of this tensor by l/«oi 2 and 
l/«02 2 . Then the equations become 

(— - -)d x + iG z D y = 0, 

W nJ (82.9) 

/ 1 1\ K } 

-iG z D x +i—---)Dy = 0. 

The condition that the determinant of these equations vanishes gives an 
equation quadratic in n 2 : 

\ n 2 «oi 2 / \n £ nmrl 
whose roots give the two values of n for a given direction of n:j| 

l.i(JL + _L) + I\H±.±)\gA. (82.11) 

n* 2\»oi 2 n 2 2 / ~ V L4\«oi 2 «02 2 / J 



f The gyration vector is the vector g in the opposite relation 

Dt = € ik f E k + i(E x g) f . (82.7a) 

The coefficients in (82.7) and (82.7a) are related as follows (cf. §21, Problem): 

r)ik = {WW- 1 * -gigk)lU\, ,„ _ . 

^ / /l l (»2.7b) 

where |e| and |€'| are the determinants of the tensors eik and eik'. 

X The medium is again assumed non-magnetic with respect to the variable field of the 
electromagnetic wave, i.e. fiik(,o) = S^. This, however, does not exclude a constant field 
magnetising the medium (i.e. the static permeability may differ from unity). 

The properties derived for 6fjfc(a>) are equally applicable to the tensor fJ4k(<o) in a frequency 
range where the dispersion of the magnetic permeability is of importance. 

|| When there is no field, G = and n = noi or no2. It should be remembered, however, 
that when the field is present noi and M02 in equation (82.10) are not in general the values 
of n f or H = 0, since not only G but also the components i)ik depend on the field. 



§82 



Magnetic-optical effects 



333 



Substituting these values in equations (82.9), we find the corresponding 
ratios D p /D x : 

^ = _wi(_L _J_U /ri/j__ ±\\<u]\ (8 2.i2) 

D x G z l2\«oi2 «02 2 / V L4Ui 2 «02 2 / II 

The purely imaginary value of the ratio D y /D x signifies that the waves 
are elliptically polarised, and the principal axes of the ellipses are the * and y 
axes. The product of the two values of the ratio is easily seen to be unity. 
Thus, if in one wave D y = ipD x , where the real quantity p is the ratio of the 
axes of the polarisation ellipse, then in the other wave D y — — iD x \p. This 
means that the polarisation ellipses of the two waves have the same axis 
ratio, but are 90° apart, and the directions of rotation are opposite (Fig. 41). f 




Fig. 41 



The components Gi and rjut' are functions of the magnetic field. If, as 
usually happens, the magnetic field is fairly weak, we can expand in powers 
of the field. The vector G is zero in the absence of the field, and so for a 
weak field we can put 



Gt = focHjc, 



(82.13) 



where/ifc is a tensor of rank two, in general not symmetrical. This dependence 
is in accordance with the general rule (82.4) whereby, in a transparent medium, 
the components of the antisymmetrical tensor rjm" (and €*&") must be odd 
functions of H. The symmetrical components r)uc' are even functions of the 
magnetic field. The first correction terms (which do not appear in the 
absence of the field) in t\iu are therefore quadratic in the field.? 

In the general case of an arbitrarily directed wave vector, the magnetic 
field has little effect on the propagation of light in the crystal, causing only 



t If the vectors D in the two waves are denoted by Di and D2, these relations may be 
written Di-Dg* = DixDix* + Di y D2y* = 0. This is a general property of the eigenvectors 
on reduction to the principal axes of a Hermitian tensor (in this case, the tensor r}a&'). 

X When second-order quantities are neglected, formulae (82.7b) reduce to the simpler 
forms 



Vile 



= €~h 



Gt = — ei*'£*/ie'| 



(82.13a) 



334 Electromagnetic Waves in Anisotropic Media §82 

a slight ellipticity of the oscillations, with an axis ratio of the polarisation 
ellipse which is small (of the first order with respect to the field). 

The directions of the optical axes (and neighbouring directions) form an 
exception. The two values of n are equal in the absence of the field when 
the wave vector is along one of these axes. The roots of equation (82.10) 
then differ from these values by first-order quantities,! and the resulting 
effects are analogous to those in isotropic bodies, which we shall now con- 
sider. 

The magnetic-optical effect in isotropic bodies (and in crystals of the 
cubic system) is of particular interest on account of its nature and its com- 
paratively large magnitude. 

Neglecting second-order quantities, we have 77^' = e _1 S^, where e is the 
dielectric permeability of the isotropic medium in the absence of the magnetic 
field. The relation between D and E is 

E = -D + iD x G, (82.14) 

€ 

or 

D = eE + iE x g; (82.15) 

in the same approximation, the vectors g and G are related by 

G = - g/e 2 . (82.16) 

The dependence of g (or G) on the external field reduces in an isotropic 
medium to simple proportionality: 

g=/H, (82.17) 

in which the scalar constant / may be either positive or negative. 

In equation (82.10) we now have «oi = «02 = «o = \A> tne refractive 
index in the absence of the field. Hence 1/n 2 = + G z + lfn 2 or, to the same 
accuracy, 

«+ 2 = wo 2 ±«o 4 £z= wo 2 +g z . • (82.18) 

Since the #-axis is in the direction of n, we can write this formula, to the same 
accuracy, in the vector form 

(n ± ^g) 2 = no*. (82.19) 

Hence we see that the wave- vector surface in this case consists of two spheres 
of radius no, whose centres are at distances ±g/2no from the origin in the 
direction of G. 



t It should be noticed that the two roots of (82.10) do not become equal. The geometrical 
significance of this is that the two parts of the wave-vector surface are separated. 



§82 Magnetic-optical effects 335 

A different polarisation of the wave corresponds to each of the two values 
of n : we have 

D x = +iD y , (82.20) 

where the signs correspond to those in (82.18). The equality of the magni- 
tudes of D x and D y , and their phase difference of + \tt, signify a circular 
polarisation of the wave, with the direction of rotation of the vector D respec- 
tively anticlockwise and clockwise looking along the wave vector (or, to use 
the customary expressions, with right-hand and left-hand polarisation 
respectively). 

The difference between the refractive indices in the left-hand and right- 
hand polarised waves has the result that two circularly polarised refracted 
waves are formed at the surface of a gyrotropic body. This phenomenon is 
called double circular refraction. 

Let a linearly polarised plane wave be incident normally on a slab of thick- 
ness /. We take the direction of incidence as the #-axis, and that of the vector 
E ( = D) in the incident wave as the x-axis. The linear oscillation can be 
represented as the sum of two circular oscillations with opposite directions 
of rotation, which are then propagated through the slab with different wave 
vectors k ± = ojn ± /c. Arbitrarily taking the wave amplitude as unity, we 
have D x = i[exp (ik + z)+exp (ik-z)] t D y = %i[— exp (ik + z) + exp (ik-z)], or, 
putting k = %(k + + k-) and k = %(k + — k-), 

D x = l e ikz( e iK Z + e -iK Z } _ e ikz cos KZy 

D y = \ie ikz { - e iKZ + e~ iKZ ) = e ikz sin kz. 

When the wave leaves the slab we have D y /D z = tan kI = tan (la)g/2cno). 
Since this ratio is real, the wave remains linearly polarised, but the direc- 
tion of polarisation is changed (the Faraday effect). The angle through which 
the plane of polarisation is rotated is proportional to the path traversed by 
the wave; the angle per unit length in the direction of the wave vector is 
( wg/2cno) cos 6, where 6 is the angle between n and g. 

It should be noticed that, when the direction of the magnetic field is 
given, the direction of rotation of the plane of polarisation (with respect to 
the direction of n) is reversed (left-hand becoming right-hand, and vice 
versa) when the sign of n is changed. If the ray traverses the same path twice 
in opposite directions, the total rotation of the plane of polarisation is there- 
fore double the value resulting from a single traversal. 

For 6 = \n (the wave vector perpendicular to the magnetic field), the 
effect linear in the field given by formulae (82.18) disappears, in accordance 
with the general rule stated above that only the component of g in the direc- 
tion of n affects the propagation of light. For angles 6 close to \n we must 
therefore take account of the terms proportional to the square of the field, 
and in particular these terms must be included in the tensor 77^'. By virtue 
of the axial symmetry about the direction of the field, two principal 



336 Electromagnetic Waves in Anisotropic Media §82 

values of the symmetrical tensor ii«/ are equal, as for a uniaxial crystal. We 
shall take the #-axis in the direction of the field, and denote by 77 , and 77 x 
the principal values of 77^' in the directions parallel and perpendicular to 
the magnetic field. The difference rj^—r]^ is proportional to H 2 . 

Let us consider the purely quadratic effect (called the Cotton-Mouton 
effect) which occurs when n and g are perpendicular. In equations (82.9) 
and (82.10) we have G z = 0, and l/«oi 2 > V w 02 2 are respectively 77 „, i] x . 
Thus in one wave we have 1/n 2 = 77 „ , D y = 0; this wave is linearly polarised, 
and the vector D is parallel to the #-axis. In the other wave l[n 2 = rj ±y 
D x = 0, i.e. D is parallel to the j-axis. Let linearly polarised light be inci- 
dent normally on a slab in a magnetic field parallel to its surface. The two 
components in the slab (with vectors D in the xz and yz planes) are propa- 
gated with different values of n. Consequently the light leaving the slab is 
elliptically polarised. 

PROBLEMS 

Problem 1. Determine the directions of the rays when a ray incident from a vacuum is 
refracted at the surface of an isotropic body in a magnetic field. 

Solution. The direction of the ray vector s is given by the normal to the wave-vector 
surface. Differentiating the left-hand side of equation (82.19) with respect to the compo- 
nents of the vector n, we find that s is proportional to n±g/2wo. The square of the latter 
expression is wo 2 , and so the unit vector in the direction of the ray is given by 

:_i/. ± JLA a) 

* «o\ 2«o / 

Let the angle of incidence be 9. The refracted rays do not in general lie in the plane of 
incidence, and their directions are given by the angle 6' to the normal to the surface and the 
azimuth ^' measured from the plane of incidence. We take the latter as the as-plane, with 
the ar-axis perpendicular to the surface. The components n x and n v of the wave vector are 
unaltered by refraction. In the incident ray they are n x = sin 0, n y = 0. Substituting these 
values in (1), we find the x and y components of the unit vector s/s, which give immediately 
the directions of the refracted rays : 

1 1 

sin 6' cos </>' = — sin 6 ± - — -g x , 
no 2m 

1 

sin 6' sin j>' — ± — — -gy. 
2mo z 

When the angle of incidence is not small, the azimuth <f>' is small, and we can write 

<f>' = ±g v /2no sin 0, 

• a' sin e _ L 8* 
sinfl = ±-^-T- 

wo 2m £ 

For normal incidence (6 = 0) we take the xs-plane through the vector G; then ^' = 0, 
and 6' = sin 6' = ±gx/2no 2 . Although this formula does not involve g g , it is not valid if 
gz = 0, since the approximation linear in the field is inadequate when n and g are perpen- 
dicular. 

Problem 2. Determine the polarisation of the reflected light when a linearly polarised 
wave is incident normally from a vacuum on the surface of a body rendered anisotropic by 
a magnetic field. 

Solution. For normal incidence the direction of the wave vector is unaltered by the 
passage of the wave into the medium. In all three waves (incident, reflected and refracted) 



§83 Natural optical activity 337 

the vectors H are therefore parallel to the surface (the ay-plane). The electric vector E in 
the incident and reflected waves is also parallel to the *-y-plane; in the refracted wave 
Ez t 6 0, but the relation between the x and y components of E and H is the same as in an 
isotropic body (H x = —nE y , H y = nE x ). If the polarisation of the incident wave is the 
same as that of one of the two types of wave which can be propagated in the anisotropic 
medium concerned, with the given direction of n, then there is only one refracted wave, 
which has this polarisation. The problem is then formally identical with that of reflection 
from an isotropic body, and the fields Ei and Eo in the reflected and incident waves are re- 
lated by 

Ei=(l-n)Eo/(l+«), (1) 

where n is the refractive index corresponding to this polarisation. 

The linear polarisation can be regarded as resulting from the superposition of two circular 
polarisations with opposite directions of rotation. If Eo in the incident field is in the re- 
direction, we put Eo = Eo + +Eo~, where Eo + x = iEo + y = \Eo, Eo~x = —iEo~y = $Eo. 
Using formula (1) for each wave, with n ± given by (82.18), we obtain 

Ei x = ±Eo\— + — ^Eo- 

ll+n+ 1 + n-J 1 



1+mo 

E ly = ttEo\— — — £*iE tt 

Ll+«_ 1+n+J 



1 — n- 1 — M+"| ^ g cos 6 

1 + n+ J «o( 1 + wo) 2 ' 



where is the angle between the direction of the incidence and the vector g. Hence we see 
that the reflected wave is elliptically polarised, the major axis of the ellipse being in the 
^-direction, and the ratio of the minor and major axes being (g cos 0)/«o(«o 2 — 1). 

Problem 3. Determine the limiting form of the frequency dependence of the gyration 
vector at high frequencies. 

Solution. The calculations are similar to those in §59, except that the electron equation 
of motion must include the Lorentz force due to the constant external magnetic field H: 

dv' 

m— — = eEoe-iM+ev'xHIc. 
dt 

If <o ^> eHlmc, this equation can be solved by successive approximations. As far as terms 
of the first order in H we have 

ic &^ 

v' = E — — E x H, 

mm m?co 2 c 

and the induction is then 

D = < w )E+ty(«)ExH, 
where e(a>) is given by (59.1) and/(a>) = —4irNe 3 /cm 2 a> 3 . 

§83. Natural optical activity 

The frequency dispersion of the dielectric (and magnetic) permeability 
results from the dependence of the macroscopic properties of matter on the 
time variation of the electromagnetic field. The dependence on its spatial 
variation has been ignored up to this point. The condition for this treatment 
to be valid is that the atomic dimension a should be small compared with the 
wavelength A (see §58). 

The inequality a <^ A is a necessary condition for the macroscopic theory 
to be applicable at all. When the quantities involving the small ratio a/A 
are entirely neglected, however, certain kinds of effect are overlooked which 
make their appearance when the next terms in a/A after the zero-order 
approximation are included. We shall now discuss these effects. 



338 Electromagnetic Waves in Anisotropic Media §83 

The expansion in powers of a/X corresponds, in the macroscopic theory, 
to an expansion of the induction D in powers of the spatial derivatives of the 
field E as well as of E itself. Taking only the first-order terms, we must 
include terms proportional to the first powers of the first-order derivatives. 
For a field of a single frequency u> we can write the expansion as 

Di = e ik «»E k + yiadEtldxh (83.1) 

where e^ (0) and yaa are functions of frequency. 

Before proceeding to investigate this expression, we should make the 
following remark. To the accuracy used here there is no physical significance 
in separating the mean value of the microscopic current density pv into the 
two parts BPJdt and c curl M. In the present theory, therefore, it is appro- 
priate to write Maxwell's equations as 

10B 
curlE = , 

c dt 

(83.2) 

curl B = , 

c dt 

without introducing H as well as the mean microscopic magnetic field E = B. 
Instead, all terms resulting from the averaging of the microscopic currents 
are supposed included in the definition of D. 

The symmetry properties of the tensor yun in (83.1) are determined by 
applying the generalised principle of symmetry of the kinetic coefficients, 
as was done in §76 for the tensor e ik . We saw in §76 that, if the components 
of the vector E at each point in the body are taken as the quantities x a , then 
the corresponding f a will be the components of the vector D. The presence 
of the spatial derivatives in the relations (83.1), however, interferes with the 
direct application of the symmetry principle, which is best used as follows. 
Let x a and x' a be two different sets of values of the x a , and f a ,f' a the corres- 
ponding sets of values of the f a . By the symmetry (a a& = a a& ) of the coeffi- 
cients in the relations 

x a = Sa«&/&, x'a = 2<W'& 

6 



we have 



Vxaf'a = Vx f af a . (83.3) 



In the present case, this equation takes the form JE<D'i dV = JjE'iD* dV. 
Substituting (83.1) and using the known symmetry of €*>«, we obtain 

f ^'* JT r f v> dEk Ajr 
YiiclEi— — dV = J- yociEi—dV 



§83 Natural optical activity 339 

or, integrating by parts on one side of the equation, 

f 8E' k r dE'i r dE ' k 

J OXl J OXi J OXl 

Since the functions E and E' are arbitrary, we therefore have the required 
symmetry property : 

Ym = — ym- (83.4) 

We shall also suppose that no absorption takes place in the medium. Let 
us ascertain what conditions are thereby imposed on the tensor yaa. The 
dissipation of energy in a periodic field is given by the (time) average value 
of the integral -(1/4tt) JE-(dD/&) dV. Here E and D are real; if we 
use the complex representation, the integral to be averaged can be written 

1 r i 8D* 0D\ ioi r 

E + E* dV = (E.D* - E*.D)dF. 

16tt J \ 8t 8tl 16ttJ k } 

Substituting(83. l)and using the fact that e (0) ^ is real in a transparent medium, 
we obtain 



16tt) V 



8E k * 8E k \ 

YiM*Ei— nuEi*-- — dV 

OXl OXl I 



16ttJ \ 



dE k * dE t *\ 

YikfEi— — + y ik iE k -- — \dV 
oxi oxi l 



iu) r dE k * 

= - 77" I (y*w* + Ykil)Ei— — dV. 

l07T J OXl 



8xi 

This expression is zero identically if yi k f = —y k u = yi k i. Thus we con- 
clude that, if absorption is absent, the tensor yiu must be real. 

For a plane wave, with wave vector k = <oti/c, we have 8E k /dxi 
= i<oE k ni/c, so that Di = €i k E k , where 

Hk = e (0) ta + icoytkini/c (83.5) 

is the dielectric permeability tensor, which now exhibits dispersion with 
respect to both frequency and wave vector, f 

Instead of the antisymmetrical tensor of rank two yiuni, we shall use the 
gyration vector g, which is dual to it. This vector is given by 

coyikim/c = e ik igi, (83.6) 

i.e. 

Hk = e m ik + iemgi, (83.7) 



f When a> -> the quantities yuci, which do not pertain to the expansion in powers of 
w, tend to constants. The imaginary part of cm therefore tends to zero as the first power of 
the frequency. 



340 Electromagnetic Waves in Anisotropic Media §83 

which is formally the same as the expression used in §82. The only difference 
is that in §82 the vector g depended only on the properties of the medium 
(and on the applied magnetic field), whereas here the gyration vector depends 
also on the wave vector of the field. According to (83.6) the components of 
this vector are linear functions of the components of n, i.e. 

gi = gikfik. (83.8) 

Substituting (83.8) in (83.6), we find wyucinifc = ei]e m g m inu or, since n is 
arbitrary, 

wynci\c = eocmgmi, (83.9) 

which gives the relation between the components of the tensor yaa of rank 
three and the pseudotensor guc of rank two. f 

The particular crystallographic symmetry of the body places certain restric- 
tions on the components of the tensor yua (or gut) and, in particular, may have 
the result that all the components are zero. For example, the tensor yua 
cannot exist in bodies having a centre of symmetry: when the sign of each 
co-ordinate is changed (inversion), all the components of a tensor of rank 
three (and of a pseudotensor of rank two) change sign, whereas by the sym- 
metry of the body they must remain unchanged by this transformation. 

Bodies in which the tensor gat is not zero are said to have natural optical 
activity. Thus the existence of optical activity certainly implies that the 
body has no centre of symmetry. 

Let us first consider the natural optical activity of isotropic bodies. If a 
liquid or gas consists of a substance having no stereoisomer, it is symmetrical 
not only with respect to any rotation but also with respect to reflection (inver- 
sion) about any point, and can have no optical activity. Such activity can 
occur only in fluids having two stereoisomeric forms, and the two forms must 
be present in different quantities. The fluid then has no centre of symmetry. 

In an isotropic body, and in crystals of the cubic system, the pseudotensor 
gik reduces to a pseudoscalar : 

gm=f8m; '(83.10) 

the tensor yua is given in terms of /by yua = cfeua/a). A pseudoscalar is a 
quantity which changes sign on inversion of the co-ordinates. The two 
stereoisomers are converted into one another by the operation of inversion, 
and so their values of /are the same with opposite signs. 

Thus, in an optically active isotropic body, the gyration vector g = /n, 
and the relation between the electric induction and field in the wave is given 

by 

D = ffi + pxn. (83.11) 

Since D^n = 0, it follows that E«n = 0. That is, in such a wave not only 
the induction D (as in any medium) but also the field E is transverse to the 
direction of n. 



t In components gxx = <*>Yyzx\c, g X y = o>Yyzy\c, gyx — toyzxxjc, etc. 



§83 Natural optical activity 341 

The change in the refractive index n when allowance is made for the natural 
optical activity is a small quantity. In determining this change we can there- 
fore put n = no = \A (0) m tne small term Exg in (83.11). Then the prob- 
lem of calculating the difference n — no is formally identical with that con- 
sidered in §82 of the change in n due to the magnetic field, except that g 
has a different meaning and is always parallel to n (the #-axis in §82). By 
analogy with (82.18) we can therefore derive immediately the equation 

» ± 2 = «o 2 ± g = «o 2 ± fno. (83.12) 

These two values correspond (cf. (82.20)) to the following ratios of the two 
components of E (or D) : 

E z = ± iE y , (83.13) 

i.e. to waves which are left-hand and right-hand circularly polarised. It 
may also be noted that the magnitude of n is independent of its direction, 
and therefore the direction of n is the same as that of the ray vector s. 

Thus we see that the optical properties of a naturally active isotropic body 
resemble those of an inactive body in a magnetic field: it exhibits double 
circular refraction, and when a linearly polarised wave is propagated in it 
the plane of polarisation is rotated. The angle of rotation per unit path 
length of the ray is <nf\2c. 

The sign of the constant g, and therefore the direction of rotation, are 
opposite for the two stereoisomers, and we therefore speak of dextrorotatory 
and laevorotatory stereoisomers. 

Unlike the rotation of the plane of polarisation in a magnetic field, the 
magnitude and sign of the rotation in naturally active substances do not 
depend on the direction of propagation of the ray. Hence, if a linearly 
polarised ray traverses the same path in a naturally active medium twice in 
opposite directions, the plane of polarisation is unchanged. 

Let us now consider naturally active crystals. We shall not give here a 
systematic analysis of all possible cases of symmetry (see the Problem), 
but simply note that natural activity is impossible if a centre of symmetry 
is present, but possible if there is a plane of symmetry or a rotary-reflection 
axis. It should be emphasised that the conditions for the existence of natural 
activity in crystals are not the same as those which allow the existence of 
crystals in two mirror-image (enantiomorphic) forms; the latter conditions 
are more stringent, and require the absence of both a centre and a plane of 
symmetry. Thus a crystal can be optically active and yet be identical with 
its mirror image. 

In a naturally active crystal (uniaxial or biaxial), when light is propa- 
gated with an arbitrary direction of the wave vector, we have essentially 
ordinary double refraction of linearly polarised waves; the allowance for 
the activity would amount to replacing the strictly linear polarisation by an 
elliptical polarisation with an axis ratio of the first order of smallness. 

The only exception is formed by the directions of the optical axes, along 



342 Electromagnetic Waves in Anisotropic Media §83 

which, if the activity is neglected, the two roots of Fresnel's equation coin- 
cide. In these directions the phenomenon of natural activity of crystals is 
analogous to that of isotropic bodies : double circular refraction of the first 
order occurs, with a corresponding rotation of the plane of polarisation of 
linearly polarised waves. These phenomena rapidly disappear as the wave 
vector deviates from the direction of the optical axis. 

For a quantitative calculation of natural activity in crystals it is more 
convenient to use, not the expression giving D in terms of E, but the inverse, 
as in §82. As far as first-order quantities this is 

Ei = ^- 1 aD t + (D x G) h (83.14) 

where the vector G is related to the g previously used by Gf = — e (0) ^fc/|e (0) | ; 
see (82.13a). Owing to the formal correspondence between this expression 
and (82.7), the equations (82.9) and (82.10) are again valid. In these equations 
G z is the component of G in the direction of n. If we write G in the form 

Gt = G iJc n k , (83.15) 

in analogy with (83.8), the component is proportional to 

n-G = Gncimje. (83.16) 

This quadratic form determines the optical properties of a naturally active 
crystal. The tensor G^ itself need not be symmetrical, but if it is separated 
into symmetrical and antisymmetrical parts the latter does not appear in 
the form (83.16). Thus we conclude that the tensor Guc may be assumed 
symmetrical in discussing the optical properties of naturally active crystals. 

PROBLEM 

Find the restrictions imposed by crystal symmetry on the components of the tensor 

Solution. Under any rotation, the pseudotensor Guc behaves as a true tensor ; in parti- 
cular, the presence of an axis of symmetry of order higher than the second results, as for a 
true symmetrical tensor of rank two, in complete isotropy in a plane perpendicular to the 
axis. The behaviour of the pseudotensor Gac under reflection is determined by the fact 
that it is dual to a true tensor of rank three : under any reflection which changes the sign of 
a given component of a true tensor of rank two, the corresponding component of Gac remains 
unchanged, and vice versa. For example, on reflection in the ysr-plane the components 
Gxx, G yy , Gzz, G yz change sign, but G X y, Gxz do not. 

We give below the non-vanishing components of the tensor Gm for all crystal classes 
which allow natural activity. The sr-axis is taken along the axis of symmetry of the third, 
fourth or sixth order or (in the classes Cz, C% v ) along the only second-order axis of symmetry 
or (in the class C s ) perpendicular to the plane of symmetry. When three mutually perpendicu- 
lar axes of symmetry are present, they are the co-ordinate axes. 

Class C%: all. 

Class Cz: Gxx, Gyy, Gzz, G X y, the last of which may be made to vanish by a suitable 
choice of the x and y axes. 

Class C s : G X i, Gyz, one of which may be made to vanish by a suitable choice of the x 
and y axes. 

Class Czv' G X y (the xz and yz planes being planes of symmetry). 

Class Dz' Gxx, Gyy, Gzz. 

Classes Cs, C4, Ce, Da, D4, D6*. Gxx = G yy , Gzz- 



§83 Natural optical activity 343 

Class S4: Gxx — —Gy y> G X y, one of which may be made to vanish by a suitable choice 
of the x and y axes. 

Class D2d- G X y (the * and y axes being in vertical planes of symmetry). 

Classes T, O: Gxx = G yy = G sz . 

It may be noted that, in uniaxial crystals of the classes S4 and D2&, the scalar (83.16) is 
zero if the vector n is in the sr-direction, since G zl = 0. This means that in these crystals 
there is no natural-activity effect in the direction of the optical axis. 

In a biaxial crystal of the class C% v the optical axes are in one of the planes of symmetry. 
For vectors n lying in the xz or yz plane the scalar (83.16) is again identically zero, so that 
here also there is no effect in the direction of the optical axes. The only crystal class which 
allows rotation of the plane of polarisation along the optical axis but not enantiomorphism 
is the monoclinic class C». 



23 



CHAPTER XII 

THE PASSAGE OF FAST PARTICLES THROUGH 

MATTER 

§84. Ionisation losses by fast particles in matter: the non-relativistic 
case 

A fast charged particle, in passing through matter, ionises the atoms and 
thereby loses energy, f In gases, the ionisation losses can be regarded as 
being due to collisions between the fast particle and individual atoms. In 
a solid or liquid medium, however, several atoms interact simultaneously 
with the particle. The effect of this on the energy loss by the particle can 
be macroscopically regarded as resulting from the dielectric polarisation of 
the medium by the charge. Let us first consider this effect for non-relativistic 
velocities of the particle. We shall see that the polarisation of the medium 
then has only a slight effect on the losses. The derivation of this result is 
of interest because the method can be extended to other cases. 

Let us first of all ascertain the conditions under which the phenomenon 
can be macroscopically considered. The spectral resolution of the field 
produced at a distance r from the path of a particle moving with velocity v 
consists chiefly of terms whose frequency is of the order vfr (the reciprocal 
of the "collision time"). The ionisation of an atom can be effected by field 
components of frequency a> ^ a>o, where a>o is some mean frequency cor- 
responding to the motion of the majority of the electrons in the atom. The 
particle therefore interacts simultaneously with many atoms if vjcoo is large 
compared with the distances between the atoms. In solids and liquids 
these distances are of the same order of magnitude as the dimension a of the 
atoms themselves. Thus we obtain the condition v > acoQ, i.e. the velocity 
of the ionising particle must be large compared with the velocities of the 
atomic electrons (or at least of the majority of them).t 

Let us now determine the field produced by a charged particle moving 
through matter. In the non-relativistic case it is sufficient to consider only 
the electric field, defined by the scalar potential <f>. This potential satisfies 
Poisson's equation 

£A<£ = - 47reS(r - vt), (84.1) 



f We speak, as is customary, of "ionisation losses", but these are, of course, understood 
to include losses due to the excitation of atoms to discrete energy levels. 

X The corresponding condition for the energy E of the particle is Ep> MI/m, where M is 
the mass of the particle, m that of the electron, and / some mean ionisation energy for the 
majority of the electrons in the atom. 

344 



§84 Ionization losses by fast particles in matter 345 

in which the "dielectric constant" is written as an operator, and the expres- 
sion e8(r—vt) on the right-hand side is the density due to a point charge e 
moving with constant velocity v. f 

We expand <f> as a Fourier space integral : 

00 

<£ = f 0kexp(fk-r)dk. (84.2) 

—oo 

Taking the Laplacian of this equation, we have 

00 

/\,<f> = — I <f>kk 2 exp(fk«r)dk, 

—00 

whence it is seen that the Fourier component of A<£ is (A^)k = —k 2 <f>^. 
Taking the Fourier component of equation (84.1) gives 

£(A<£)k = - -— f ±neh{r - v*) exp(- ik-r)dV 

e 
= exp ( — itv-k). 

Thus €^t = (e/2jr 2 k 2 )exp(-itv»k), and <£ k therefore depends on time 
through a factor exp ( — itv-k). The operator e acting on a function exp ( — iatt) 
multiplies it by «(to). Hence 

e 
fa = o 2M ,u ^ ex P(~ itv ' k )' 

27T 2 « 2 €(k«V) 

The Fourier components of the field and of the potential are related by 
Ejj exp (ik-r) = -grad [«^exp (ik-r)] = -ik<f> k exp («k-r), or E k = -ik<f>^ 
Thus 

iek 

27T 2 /J 2 e(k«v) 

The total field strength is obtained by inverting the Fourier transform: 

00 

E = J Ekexp(fk.r)dk. (84.4) 

—00 

The energy loss by the moving particle is just the work done by 
the force cE exerted on the particle by the field which it produces. Taking 
the value of the field at the point occupied by the particle, namely 

t We assume that the particle moves in a straight line, and thereby neglect scattering, as 
is always permissible in problems of this type. 

If the charge on the particle is ze, then all the formulae pertaining to energy loss in this 
and the following sections should be multiplied by z z . 



346 The Passage of Fast Particles Through Matter §84 

r = vt, we obtain in the integrand in (84.4) a factor exp (itv-k) which 
cancels with the factor exp ( — itv>k) in the expression (84.3) for E^. Hence 
the force F is 



F=-_J_— dk. 



ie i 

~2n 2 J k 2 e(k>v) 



— 00 



It is evident that the direction of the force F is opposite to that of the 
velocity v; let the latter be the ^-direction. Putting k x v = a>,q = -\/(ky 2 + kz 2 ) 
and replacing d&^d&z by 2-nq dq, we can write the magnitude of F as 

00 Qo A A 

* * (84.5) 



-tJJ 



(co)(q 2 v 2 + co 2 ) 
-oo o 

The choice of qo is discussed below. 

The following remark should be made concerning the integration with 
respect to o> in formula (84.5). As to -> oo the function e(co) -> 1, and the 
integral is logarithmically divergent. This happens because we ought to 
have subtracted from the field E the field which would be present if the 
particle were moving in a vacuum (i.e. if € = 1); this field evidently does 
not affect the energy lost by the particle in matter. 

If this subtraction were effected, 1/e in the integrand of (84.5) would 
be diminished by unity, and the integral would converge. The same result 
can be obtained by taking the integration from — R to +R and then letting 
R tend to infinity. Since the function e'(a>) is even, the real part of the 
integrand is an odd function of the frequency, and gives zero. The integral 
of the imaginary part of the integrand converges. 

In what follows we shall sometimes find it convenient to use the notation 

l/.e(a>) = 7j(a>) = rj' + it{\ (84.6) 

with -q'(w) and i/'(ai) respectively even and odd functions, and -q" = — e"/|c| 2 
< 0. Formula (84.5) can be rewritten in the explicitly real form 

7T J J (q 2 V 2 + a> 2 ) 


The energy loss per unit path length is the work done by the force over 
that distance, which is just F; it is called the stopping power of the substance 
with respect to the particle. 

According to the general rules of quantum mechanics, the Fourier com- 
ponent of the field whose wave vector is k transmits to the S-electron released 
in ionisation a momentum hk. For sufficiently large q ( > coo/v) we have 
k 2 = q 2 + to 2 /v 2 x q 2 , so that the momentum transferred is approximately 
/*q. A given value of q corresponds to collisions with impact parameter 
~ 1/q. Hence the condition for the macroscopic treatment to be valid is 
l/q p a. Accordingly, we take as the upper limit of integration a value qo 



§84 Ionisation losses by fast particles in matter 247 

such thaf wqJv <^ go <^ l/«. The quantity F(qo) is the energy loss of a fast 
particle with transfer of momentum not exceeding hqo to the atomic electron. 
Integrating with respect to q in (84.7), we obtain 

2e 2 r Gov 

F(qo) = — HV'HI log— dco. (84.8) 

77^ J CO 



This formula cannot be further transformed in a general manner, but it 
can be written in a more convenient form as follows. We first calculate the 
integral 

00 00 

co?/'(co) da> = — \l ' I (co/e) da>. 
—oo 

To do so, we notice that, if the integration is taken in the complex co-plane 
along a contour consisting of the real axis and a very large semicircle a in 
the upper half-plane, the integral is zero, since the integrand has no poles 
in the upper half-plane. For large values of to, the function e(o>) is given by 
formula (59.1): 

Aire^N 
<co) = 1 ~. (84.9) 

The integration along the large semicircle a can be carried out by using this 
formula, and the result isf 

f liriNe 2 C dco 

- cotj'Xw) dw = = 27rW* 2 /w. (84.10) 

J m J co 

a- 

We define a mean frequency of the motion of the atomic electrons by 

oo 

coi/'(co) log a) dco 



logcS = 



00 

J arq" '(co) dco 


oo 

f co|V'(co)l log co dco. (84.11) 



277We 2 





t This is the same as the value of the integral 

CO 

I <oe"(<o) dco 
o 
(see (62.14)), as it should be, since, as |a>| -*- oo, |e| -*■ 1 and 17" -> — e" 



348 The Passage of Fast Particles Through Matter §84 

Then formula (84.8) can be written • 

F(q ) = (47TNe*lmv*)log(qovla>). (84.12) 

The following remark should be made here. It might seem from the form 
of (84.7) or (84.11) that the main contribution to the ionisation losses 
(84.12) comes from frequencies at which there is considerable absorption. 
This is not so; these formulae may contain a considerable contribution 
from ranges in which e" is small. The reason is that in such ranges the 
function e(o>) « e'(a>) may pass through zero. It is seen from formula 
(84.5) that the zeros of e(a)) are poles of the integrand. In reality, of course, 
€"(<o) is not exactly zero, and so the zeros of e(a>) are not on the real axis but 
just below it. Hence, when the expression used for c(a>) is real and passes 
through zero, the contour must be indented upwards at the: pole of the 
integrand, and so a contribution to the integral occurs. For example, if the 
function e(to) is given by (64.5), the contribution to the retardation (84.12) 
from the poles ± a)\ (where e(a>i) = 0) is easily seen, by direct calculation 
from (84.7), to be (4nNe /l /mv 2 a 2 ) log (qiv/wi). 

In order to find the energy loss F(qi) with transfer of momentum not 
exceeding some value hqi > hqo, we must "join" formula (84.12) to that 
given by the quantum theory of collisions, corresponding to energy loss by 
collisions with single atoms. This can be done by using the fact that the 
ranges of applicability of the two formulae overlap. As we know from the 
theory of collisions, the energy loss with transfer of momentum in a range 
of hdq is 

dF= (47TNe*lmvZ)dqlq, (84.13) 

and this formula is applicable (in the non-relativistic case) for any value of 
q > (oo/v which is compatible with the laws of conservation of momentum 
and energy, provided that the energy transferred is small compared with 
the initial energy, of the fast particle, f The energy loss with all values of q 
between qo and q\ is accordingly (^irNe^lmv 2 ) log (qi/qo)> When this quantity 
is added to formula (84.12), qo is replaced by q\, so that 

F( qi ) = (^Ne^/mv 2 ) log(^/d>). (84.14) 

If a momentum hq\ large compared with the atomic momenta is given to 
an atomic electron, its energy is E\ = h 2 qi 2 /2m. Thus we can write 

F(£i) = {2TrNet\mv 2 )\og(2mv 2 E x \h 2 ib 2 ). (84.15) 



f See Quantum Mechanics, §121, Pergamon Press, London, 1958. The "effective retarda- 
tion" used there differs from Fbya factor N. 

Formula (84.13) applies to collisions with free electrons. Its range of applicability as 
hitherto determined (q ^> am/v), however, extends to values of q for which the atomic 
electrons cannot be regarded as free. The condition for this is q % wo/wo, where vo is the 
order of magnitude of the velocity of the majority of the atomic electrons ; the energy tfiq z l2m 
of the S-electron is then large compared with atomic energies. 



§85 Ionisation losses by fast particles : the relativistic case 349 

This formula gives the energy loss of a fast particle (an electron, for 
example) by ionisation with a transfer of energy not exceeding E\. It differs 
from the usual formula derived from a microscopic discussion of collisions, 
neglecting interactions between atoms, f only by the definition of the "ionisa- 
tion energy", which is here represented by hw. The mean (with respect to 
the electrons) ionisation energy of an atom is usually almost independent 
of its interaction with other atoms, being determined mainly by the electrons 
of the inner shells, which are almost unaffected by that interaction. More- 
over, this quantity appears here only in a logarithm, and so the exact definition 
of it has even less effect on the magnitude of the energy loss. 

The maximum energy which can be transmitted to an atomic electron in 
its interaction with a fast heavy particle is 2mv 2 , and is small compared with 
the original energy of the heavy particle.^ Substituting this value for E\ 
in (84.15), we obtain the total ionisation losses of a heavy particle: 

F = (47rNe i lmv 2 ) \og(2mv 2 lhw). (84.16) 

This differs from the usual formula || only in the definition of the ionisation 
energy as hoi. 

§85. Ionisation losses by fast particles in matter: the relativistic case 

At velocities comparable with that of light, the effect of the polarisation 
of the medium on its stopping power with respect to a fast particle may 
become very important even in gases, f f 

To derive the appropriate formulae, we use a method analogous to that 
used in §84, but it is now necessary to begin from the complete Maxwell's 
equations. When extraneous charges are present with volume density p ex , 
and extraneous currents with density j ex , these equations arett 

1 /YFT 

divH = 0, curlE = , (85.1) 

c dt 

iaiE Att 

div£E = 47rp ex , curlH = -— + — j ex . (85.2) 

c ot c 

In the present case the extraneous charge and current distribution are 
given by 

pex = eh{r - \t), jex = ev 8(r - vt). (85.3) 



t See Quantum Mechanics, formula (121.13). 

X When a heavy particle collides with an electron, the maximum transferable momentum 
Hqm&x is small compared with the momentum Mv of the heavy particle. The change in the 
energy of this particle is therefore v«/jq, and equating this to the electron energy we have 
H^/lm = fiv«q ^ hvq, whence hqmax. — 2mv, Ei,max = 2mv 2 . 

|j See Quantum Mechanics, formula (122.10). 

f t This effect was pointed out by E. Fermi (1940), who performed the calculation for the 
particular case of a gas whose atoms are regarded as harmonic oscillators. The general 
derivation given here is due to L. Landau. 

JJ We put /x(a>) = 1 , since matter does not exhibit magnetic properties at the frequencies 
important as regards ionisation losses. 



350 The Passage of Fast Particles Through Matter §85 

We introduce scalar and vector potentials, with the usual definitions : 

1 BA 

H = curlA, E = grad<£, (85.4) 

c dt 

so that equations (85.1) are satisfied identically. The additional condition 

ldty 

divA + -=0 (85.5) 

c dt 

is imposed on the potentials A and <f>; this is a generalisation of the usual 
Lorentz condition in the theory of radiation. Then, substituting (85.4) in 
(85.2), we obtain the following equations for the potentials: 

£ d 2 A 4tt 

AA "T^i" = --*vS(r-vf), 
c l ct l c 

(85.6) 

We expand A and <f> as Fourier space integrals. Taking the Fourier com- 
ponents of equations (85.6), we have (cf. §84) 

£ 8 2 A k ev 
k 2 A* -\ = exp(- itV'k), 

/ e 8 2 d> k \ e 

T Vt ' + ^l^) = 2^ exp( -' Yv - k) - 

Hence we see that A k and <f> k depend on time through a factor exp ( — itv*k). 
We again put to = k»v = k x v, and obtain 

e v 

A k = e~ iu>t , 

2rr*c & - w 2 e(a>)/c 2 

v ;/ (85.7) 

e 1 

x _ e -i*>t m 

Y 2tt 2 6(co) & - a> 2 e(oi)/c 2 

The Fourier component of the electric field is 

Ek = io*A k \c — ik<f> k . (85.8) 

From these formulae the force F = eE acting on the particle is found in 
the same way as in §84. f Using the same notation, we now have 

ie 2 



a 2 



(i-^) <0 * d * d<u 

(85.9) 



i'l**^-*)] 



t The magnetic force ev X H/e is seen by symmetry to be zero, and in any case is per- 
pendicular to the velocity of the particle and so does no work on it. 



§85 



Ionisation losses by fast particles : the relativistic case 



351 



As c -> oo this formula tends, of course, to (84.5). 

Let us first carry out the integration with respect to frequency. In order 
to effect an integration in the complex o>-plane, we first ascertain the poles 
of the integrand in the upper half-plane. The function €(o>) has no singu- 
larity and no zero in this half-plane, and so the required poles can only be 
the zeros of the expression 

to*l -q 2 . 

U 2 W 

We shall show that, for any value of the positive real quantity q 2 , this expres- 
sion vanishes for only one value of to. 
The proof is as follows. Let 

We consider the integral 

1 r df(to) dco 

2m J dco f(to) - a! 

taken along a contour C consisting of the real axis and a large semicircle 
(Fig. 42). The function f(w) has no pole in the upper half-plane or on the 
real axisjf the integral in question therefore gives the number of zeros of 
the function /(o>)- a in the upper half-plane. To calculate its value, we 
write it as 

(85.10) 



1 f d/ ^ 
lid J /— a 





Fig. 42 



t For metals e(o>) has a pole at to — 0, but to 2 e always tends to zero with at. 



352 The Passage of Fast Particles Through Matter §85 

For o} = 0, / = 0. For positive real a> we have im/ > 0, and for negative 
real o> im/ < 0. At infinity / tends to -a> 2 [(l/s> 2 )-(l/c 2 )], and therefore 
/ goes round a large circle when to goes round the large semicircle. Hence 
we see that the path of integration C" in the /-plane is of the kind shown 
in Fig. 42. Let a — q 2 be a positive real number, as in Fig. 42. Then, 
in going round C", the argument of the complex number/— a changes by 
2tt, and the integral (85.10) is equal to unity. This completes the proof, f 

Furthermore, it is easy to see that this single root of the equation 
f(<o) — q 2 = lies on the imaginary a>-axis: for purely imaginary <o the 
function /(eu), like e(o>), is real and takes all values from to oo, including q 2 . 

Let us now return to the integral with respect to co in (85.9): 




(J--i), 

\ ev 2 c 2 / 



\c 2 V 2 ] 

This can be written as the difference between the integral along the contour 
C and that along the large semicircle. The latter is jdco/a) = iir, and the 
former is 2ni times the residue of the integrand at its only pole. Let a>(q) 
be the function defined by the equation 

a>*(— ) = q 2 . (85.11) 

\ c 2 v 2 l 

Then, since the residue of an expression f(z)/<f>(%) at a pole z = zq is 
f(zo)/<l>'(zo), the integral along C is 



f-L-i) J-i-i) 

\ €V 2 C 2 / \ eV 2 C 2 J 

r Vd 2 ~ v 2 ) J 



2mi — - — = 2mi- 



d f J* 1\1 -d? 2 /d 

dco 



Collecting these expressions and substituting in (85.9), we have 

Qo 



F = e 2 







,(_L-I) 

\ ev 2 c 2 J 
q dq/dco 



+ 1 



qdq 



t If a is negative the argument of / — a changes by 4«- on going round C", so that the 
integral (85.10) is equal to 2, i.e. the function / (<a) + \a\ has two zeros in the upper half- 
plane. 



§85 Ionisation losses by fast particles: the relativistic case 353 

or, replacing the integration with respect to q in the first term by one with 
respect to co, 



p = e 2 f [ 1 o> do> + \e 2 q$ 



«(0) 

UQo) 1 

= — f [ lLdco + |eV + 

V 2 J L^w) J 



-K0) 

+ ¥*(-, - \)[« 2 (<lo) - ^(0)]. (85.12) 

\ V 1 C l I. 

Large values of q correspond to large absolute values <o of the root of equa- 
tion (85.11). Using therefore the expression (84.9) for e(w), we find 

where we have put j8 = ^[\-v 2 Jc 2 ]. Substitution in (85.12) gives 

e* iV Tt 1 1 2nN* e 2 B 2 

F= 6 - U--lLd«, -^(0); (85.13) 

v 2 J Le(co) J mc 2 2v 2 

in the integral, only the leading term ivqojp need be retained in <o(qo). 

The integration in (85.13) is over purely imaginary values of a>. We use 
the real variable co" = to/i, with the lower limit £ = o>(0)/f, and again put 
1/e = r) (84.6). The required integral is 

vqlB 
- J fo(lV)- l]co"dw". 

I 

The values of the function 77(0) on the imaginary axis can be expressed in 
terms of its imaginary part on the real axis: 



V (ia>")-1=- f- 

77 J X 



X7)"(X) 

ax 



:2 + a/' 2 




(cf. (62.17)). Hence the integral is (if we neglect x in comparison with vqo) 

2 rf° %"(*)i«>"<w-d« _ 1 ? »y dx 

w J J *» + »"« ~ » J *™ ( " g W* 2 + « 



/32(* 2 + fO 
of 



354 The Passage of Fast Particles Through Matter §85 

We substitute this result in (85.13), and for simplicity put 



logQs J log («« + £*), (85.14) 

where the bar denotes an averaging with weight io\rj" (co)|, as in (84.11). 
Then 

inNe* q v 2ttN(A e 2 B 2 
F{qo) = ^ logf- — + -tp. (85.15) 

Two cases must be considered in the further examination of this formula. 
Let us first suppose that the medium is a dielectric, and that the velocity of 
the particle satisfies the condition 

v* < c 2 /e , (85.16) 

where eo = e (0) is the electrostatic value of the dielectric permeability. On 
the imaginary axis the function e(a>) decreases monotonically from eo > 1 
for co = to 1 for <a = zoo. The expression on the left-hand side of equa- 
tion (85.11) therefore increases monotonically from to go, and for q = 
(85.11) gives tu = 0. Thus we must put £ = in (85.15); then Q, becomes 
the mean atomic frequency d> (84. 1 1), and 

4ttA^ 4 r Qov v 2 1 
F(qo) = i-logfr-^" • (85-17) 

For v <^ c this formula becomes (84.12), as it should. 

The value of qo is such that qo <4 1/a, where a is the order of magnitude 
of the distances between the atoms (in solids and liquids equal to the dimen- 
sion of the atoms). In order to extend the formula to higher values of the 
transferred momentum and energy, it must be "joined" to the formulae 
of the ordinary theory of collisions, as in §84, but the joining must now be 
carried out in two stages. First, using formula (84.13), we enter the range 
of q corresponding to energy transfers large compared with atomic energies 
but not yet relativistic. Formula (85.17) is unchanged in form, but now 
involves the S-electron energy H 2 q 2 j2m. Calling this E± f we have 

2irNe*[ 2mv 2 Ei v 2 l /f ^,^ 

F(Ei) =^h^~d- (85 - 18) 

We can now go on to the relativistic values of E\ by using a formula of relati- 
vistic collision theory, according to which the stopping power with energy 
transfer between E' and E' + dE' is 

(27rNe*/mv 2 ) dE'/E' (85.19) 

if E' is small compared with the maximum transfer -Ei >m ax compatible with 
the laws of conservation of momentum and energy for a collision between 
the fast particle concerned and a free electron. (In the non-relativistic 
case, this formula is the same as (84.13).) Since the integration of (85.19) 



§85 Ionisation losses by fast particles: the relativistic case 355 

gives a term in log E\ it is clear that formula (85.18) is unchanged in form, 
and it is therefore valid for all E\ <^ iE^max. 

The maximum energy transfer to an electron from a heavy particle isf 
Ex max « 2mv 2 jp 2 . If £i >m ax is small compared with the total energy E of 
the fast particle (i.e. if ' E <4 M 2 c 2 \m), the differential expression for the 
energy lost by free electrons is 

mv 2 \E' 2mc 2 / 

for all E\ whatever the kind of heavy particle concerned. The energy loss 
additional to (85.18), with energy transfer from Ex to tfi.max (with 
E\ <^ .E^max) is then 

2^/ ZW /PW\ = ^fl/w 2 ^ _ t_\ (8 5.20) 

mv 2 I 8 E x 2mc 2 ) mv 2 \ S j8*£i c*/ 

Adding this to (85.18), we find the total stopping power with respect to the 
heavy particle : 

F^^L^-t). (85.21) 

mv 2 \ pzkcb c 2 } 

Formulae (85.18) and (85.21) differ from those of the usual theory only in 
that the "ionisation energy" is hd>. 

Let us now turn to the second case, namely that where 

v* > c 2 le , (85.22) 

which, in particular, always holds for metals, where e = oo. The expression 
a> 2 ( € /c 2 -l/v 2 ) on the left-hand side of equation (85.11) then has two zeros 
on the imaginary w-axis, one at o> - and the other at a = i£, where £ is 
defined by 

«(#) = c 2 /v 2 . (85.23) 

In the range from to i£ the expression o> 2 (e/c 2 - 1/v 2 ) is negative, and for 
| <u | > I it takes all positive values from to oo. As q -> 0, therefore, the 
root of equation (85.11) in this case tends to £, which is the value to be 
substituted in (85.14) and (85.15). 

Two limiting cases may be considered. If | is small compared with the 
atomic frequencies w , then the last term in (85.15) may be neglected, and 
Q « cD. Thus we return to formula (85.17). The opposite limiting case, 
where £ > co , is of particular interest. Since, for large £, the function 
e(i£) tends to 1, it is evident from (85.23) that this case corresponds to 
ultra-relativistic velocities of the particle. Using formula (84.9) for e(to), we 
can write equation (85.23) as l + ^rrNe 2 /m£ 2 = c 2 /v 2 , whence 
£2 = 47rNe 2 v 2 /mc 2 p 2 « ^Ne 2 \m^ 2 . 

t See The Classical Theory of Fields, §2-5, Addison- Wesley Press, Cambridge (Mass.), 
1951 ; Pergamon Press, London, 1959. 



356 The Passage of Fast Particles Through Matter §85 

As the velocity of the particle increases, the condition £ > o>o is ulti- 
mately fulfilled in any medium, i.e. whatever the electron density N (even 
in a gas). The velocity required is, however, the greater, the smaller N, i.e. 
the more rarefied the medium. 

From (85.14) we then have simply Q « £. Putting also v « c, we find 
that the last two terms in (85.15) cancel, leaving 

F(q ) = (2irNe*/mc2) log (m^q^/AvN^). 

Extending this formula, in the same manner as above, to large values of 
the momentum and energy transfer, we find the following expression for 
the energy loss of an ultra-relativistic particle with an energy transfer not 
exceeding E 1 (^ E lmax ): 

F(Ei) = (2wiVe 4 /»ic a )log(m2ca£ , i/27riVca* 2 ). (85.24) 

This result is considerably different from that obtained in the ordinary 
collision theory, which neglects the polarisation of the medium. According to 
that theory, in the ultra-relativistic range the stopping power F(E{) continues 
to increase (though only logarithmically) with the energy of the particle. 
The polarisation of the medium results in a screening of the charge, and 
the increase in the losses is thereby finally stopped; it tends to the 
constant value (independent of jS) given by formula (85.24). 

For heavy particles a formula can also be derived for the total stopping 
power with any energy transfer up to J5i f max (if the latter is small compared 
with the energy of the particle itself). Again using the expression (85.20), in 
which we can now put v = c, we find 

27rAfc4 r nPc* 1 

= ~lnc2~[ ° g 7rNe 2 h^ ~ } ( • ) 

We see that the total stopping power continues to increase with the velocity of 
the particle, owing to "close" collisions with a large energy transfer, for 
which the polarisation of the medium has no screening effect. This increase, 
however, is rather slower than that given by the theory when the polarisation 
is neglected. 

It may also be noted that the presence of the electron density iV in the 
argument of the logarithm in formulae (85.24) and (85.25) results in the 
following property of energy losses of ultra-relativistic particles : when such a 
particle passes through different substances containing the same number of 
electrons per unit surface area, the losses are smaller in media with 
larger N. 

Finally, we may point out that a measurement of the energy losses of fast 
particles in matter makes possible, in principle, the determination of the 
function e(i£j) for the substance concerned. It is easy to show that the exact 
expression for F for the case (85.22) is such that 

d(©2) 2c 2 ' v ' 



§86 



Cherenkov radiation 357 



where F is the quantity given by formula (85.18) or (85.21). F is measured; 
the derivative d( J F ^ 2 )/d(^ 2 ) contains only the known quantities N and v, and 
can be calculated. Thus, using (85.26), each value of £ can be related to a 
value of v, and the value of e(i£) can then be calculated from (85.23). 



§86. Cherenkov radiation 

A charged particle moving in a transparent medium emits, in certain cir- 
cumstances, an unusual type of radiation, first observed by P. A. Cherenkov 
and S. I. Vavilov, and theoretically interpreted by I. E. Tamm and I. M. 
Frank (1937). It must be emphasised that this radiation is entirely unrelated 
to the bremsstrahlung which is almost always emitted by a rapidly moving 
electron. The latter radiation is emitted by the moving electron itself when 
it collides with atoms. The Cherenkov effect, however, involves radiation 
emitted by the medium under the action of the field of the particle moving 
in it. The distinction between the two types of radiation appears with 
particular clarity when the particle has a very large mass : the bremsstrahlung 
disappears, but the Cherenkov radiation is unaffected. 

The wave vector and frequency of an electromagnetic wave propagated 
in a transparent medium are related by k = nw/c, where n = \/e is the 
refractive index, which is real.f We have seen that the frequency of the 
Fourier component of the field of a particle moving uniformly in the x- direction 
in a medium is related to the ^-component of the wave vector by a> = k x v. 
If this component is a freely propagated wave, these two relations must be 
consistent. Since k > k x , it follows that we must have 

v > cjn{w). (86.1) 

Thus radiation of frequency o> occurs if the velocity of the particle exceeds 
the phase velocity of waves of that frequency in the medium concerned. 

Let 6 be the angle between the direction of motion of the particle and 
the direction of emission. We have k x = k cos 6 = (weo/c) cos and, since 
k x = <o/v, we find that 

cos# = c]nv. (86.2) 

Thus a definite value of the angle 6 corresponds to radiation of a given fre- 
quency. That is, the radiation of each frequency is emitted forwards, and 
is distributed over the surface of a cone of vertical angle 20, where 6 is 
given by (86.2). The distributions of the radiation in angle and in frequency 
are thus related in a definite manner. 



f We again suppose the medium isotropic and non-magnetic. The Cherenkov radiation 
in an anisotropic medium has been discussed by V. L. Ginzburg, Zhurnal eksperimental'noi 
i teoreticheskot fiziki 10, 608, 1940; A. A. Kolomenskii, Doklady Akademii Nauk SSSR 86, 
1097, 1952; M. I. Kaganov, Zhurnal tekhnicheskoi fiziki 23, 507, 1953. 

A review of various cases in the theory of Cherenkov radiation and an extensive biblio- 
graphy is given by B. M. BoLOTOvSKii, Uspekhi fizicheskikh nauk 67, 201, 1957. 



358 The Passage of Fast Particles Through Matter §86 

The emission of electromagnetic waves, if it occurs, involves a loss of 
energy by the moving particle. This loss forms part, through a small part, 
of the total losses calculated in §85. f In this sense the name "ionisation 
losses" is not quite accurate. We shall now find the corresponding part of 
the total losses, and thus determine the intensity of the Cherenkov radiation. 

According to (85.9), the energy loss in the frequency interval dto is 



/e 2 ^ /l 1 \ 

dF = - dco— > o> 

77 ^ \c 2 eW 



qdq 



\c 2 v 2 / 



where the summation is over terms with at = ±|to|. We introduce as a 
new variable 



£ = q 2 



\c 2 v 2 / 



Then 



dF= -dJ~yJ ) f 

2tt^ U 2 eW J 






In integrating along the real |-axis we must pass round the singular point 
f = (for which q 2 +k x 2 = k 2 ) in some manner, which is determined by 
the fact that, although we suppose e(w) real (the medium being trans- 
parent), it actually has a small imaginary part, which is positive for w > 
and negative for to < 0. Accordingly, $ has a small negative or positive 
imaginary part, and the path of integration ought to pass below or above 
the real axis respectively. This means that, when the path of integration is 
displaced to the real axis, we must pass below or above the singular point 
respectively. This gives a contribution to dF, and the real parts cancel in 
the sum. Indenting the path of integration with infinitesimal semicircles, 
we find 



Jd|/| = 2/7 



Thus the final formula is 



e 2 l c 2 \ 

dF = - l-- T Udto, (86.3) 

which gives the intensity of the radiation in a frequency interval dto. 
According to (86.2), this radiation is emitted in an angle interval 

c dn 

d0 = . dto. (86.4) 

vn 2 sin 6 dco 



f The bremsstrahlung is not included therein. 



Rg6 Cherenkov radiation 359 

The total intensity of the radiation is obtained by integrating (86.3) over 
all frequencies for which the medium is transparent. 

It is easy to determine the polarisation of the Cherenkov radiation. As 
we see from (85.7) the vector potential of the radiation field is parallel to 
the velocity v. The magnetic field 1^ = ikx\ is therefore perpendicular 
to the plane containing v and the ray direction k. The electric field (in the 
"wave region") is perpendicular to the magnetic field, and therefore lies in 
that plane. 

In connection with our discussion of the radiation emitted by a particle 
moving in matter, we may mention another effect whose existence has been 
deduced by V. L. Ginzburg and I. M. Frank: a particle must emit radiation 
on passing from one medium to another. This "transition" radiation is in 
principle different from the Cherenkov radiation, in that it must occur for 
any velocity of the particle, not necessarily exceeding the phase velocity of 
light in the medium. It is also unrelated to the bremsstrahlung which also 
occurs when charged particles are incident on a surface separating two 
media. As with Cherenkov radiation, the distinction is particularly clear 
for a particle of infinite mass, for which the bremsstrahlung is zero but the 
transition radiation is not. f 



t A simple derivation of the formulae for the transition radiation is given by G. M. 
Garibyan, Zhurnal eksperimentaVnoi i teoreticheskoi fiziki 33, 1403, 1957; Soviet Physics 
JETP 6 (33), 1079, 1958. 



24 



CHAPTER XIII 

ELECTROMAGNETIC FLUCTUATIONS 

§87. Current fluctuations in linear circuits 

We may apply the general theory of fluctuations f to the interesting problem 
of current fluctuations in linear electric circuits, first considered by H. 
Nyquist (1928). 

The current fluctuations are free electrical oscillations in the conductor 
(i.e. they occur in the absence of any externally applied e.m.f.). In a closed 
linear circuit the oscillations of greatest interest are, of course, those in 
which a non-zero total current J flows in the conductor. In what follows 
we shall assume that the condition for a quasi-steady state holds : the dimen- 
sions of the circuit are small compared with the wavelength A ~ cjto. Then 
the total current J is the same at every point in the circuit, and is a function 
of time only. 

In order to find the spectral resolution of the current fluctuations, we take 
/ as the quantity x which appears in the general formulae of SP §124. In 
order to ascertain the corresponding meaning of a, let us suppose that an 
external e.m.f. <f acts on the circuit. Then the rate of dissipation of energy 
in the circuit is Q = J $ . A comparison with SP (124.7) shows that/ = — <f, 
or, if in this linear relation we take / and & proportional to e- i<ot , $ = io>f. 
But the current and the e.m.f. in a linear circuit are related by $ = ZJ, 
where Z(<o) is the impedance of the circuit; see §47. Hence / = 6\Z 
= iwf/Z, whence we conclude that <x(a>) = ta>/Z(a>). Its imaginary part is 
a" = im (twJZ) = ojR/\Z\ 2 , where R(w) = re Z(a>). The formulae below 
do not depend on the particular nature of the phenomena which result in 
the dispersion of the circuit resistance. 

From SP (123.8) we now find for the required current fluctuations 

hco hco 

■ ^ = 2tt|Z| 2 COt 2T' 

This formula can be written in another form by regarding the current 
fluctuation as the result of a "random" e.m.f. S m = Z(w)J 0) . This is given 
by 

tioi hixi ,„„ „ v 

0^ = — -#( w )coth-— • (87-2) 

Lit Ll 



t See Statistical Physics, §123, Pergamon Press, London, 1958. Sections and formulae 
in this book will be referred to by means of the prefix SP. 

360 



§88 Electromagnetic field fluctuations 361 

In the classical case (hw <4 T) we have 

(**). = TR{a>)l>*. (87.3) 

§88. Electromagnetic field fluctuations 

The electromagnetic quantities E, H, . . . which appear in macroscopic elec- 
trodynamics are obtained by an averaging process, which can be regarded 
as consisting of two operations. If we take, for clarity, the classical view, 
then we have the averaging over a physically infinitesimal volume, the particles 
in that volume being in fixed positions, followed by the averaging with 
respect to the motion of the particles. In considering electromagnetic fluc- 
tuations we are concerned with the oscillations in time of quantities averaged 
over physically infinitesimal volumes, and the quantities discussed below 
will be of this kind. 

It should be noted that, if we take the quantum view, we can consider the 
volume average of the operator of a quantity, but not that of the quantity 
itself, and the second stage in the averaging consists in determining the 
expectation value of this operator by the use of quantum probabilities. 
Strictly speaking, therefore, the quantities E, H, . . . mentioned below should 
be regarded as operators of quantum mechanics, but this does not affect the 
final results, and to simplify the formulae we shall regard E, H, . . . as classi- 
cal quantities. 

As a result of fluctuations in the position and motion of the charges in a 
body, spontaneous local electric and magnetic moments occur in it; let the 
values of these moments per unit volume be respectively K/4rr and L/47T. 
They are in a sense analogous to the spontaneous polarisation of pyroelec- 
trics and the spontaneous magnetisation of ferromagnetics, but of course 
differ in that they give zero on averaging. The relation between the induc- 
tion and the field for these electric and magnetic fluctuations is given by 

Di = etkEjc + Ku Bi = (LiicHjc + Li, (88.1) 

and their "Fourier components" are 

Di(o = e«fc(">)-Ek> + Kia,, 
Bia = IHki^Hjca + Liu. 

Maxwell's equations are 

(curlEJ* = (ia>/c)(fi ik H]ca, + L io} ), (88.3) 

(curlHJf = - (ia>lc)(e ik E km + Ku,). (88.4) 

We call K and L the extraneous fluctuating inductions, but this name is, of 
course, conventional and refers to the way in which these quantities are 
formally defined rather than to their physical nature. 

In order to use the general formulae derived in SP §124 we must establish 
the relations between the electromagnetic quantities under consideration and 



362 Electromagnetic Fluctuations §88 

the quantities x a , f a which appear in the general theory. This is done as 
follows. We suppose, in a purely formal manner, that the quantities K and 
L are not spontaneously arising moments but the result of an external action, 
namely the placing of certain extraneous electric charges and currents in the 
body, and calculate the consequent change in the energy of the body. 

To do this we observe that the equation of conservation of energy, in the 
form which follows from Maxwell's equations, is 



/ 



1 ( dD dB\ c 

• 



— »E + H \dV= (fcExH-df, 

M dt dt I 4tt ^ 



or, substituting from (88. 1), 

f 1 ( d d \ 

J U E % { * ikEk) + H *dP aHk) ) 

r 1 ff 1 dl.\ 

J)ExH.df E + H-— \dV. 

J 4vr J \ dt dt) 



c 



Hence we see that the change in energy due to the "external action" con- 
sidered is 

E 3 + H 3) dF . (88.5) 

at H) 



-=/( 



In SP §124 we considered a discrete series of fluctuating quantities x a , 
whereas here we have a continuous series (the values of the fields at every 
point in the body). We shall evade this unimportant difficulty in a purely 
formal manner, by dividing the volume of the body into small but finite 
portions AF and taking some mean values of the fields in each portion; the 
passage to infinitesimal portions will be made in the final formulae. Thus the 
integral in (88.5) is replaced by the sum 



1 W_ dK __ SL 
4^ 



f dK dL) r 

E + H-— AK, 

I dt dt 



taken over all the portions AV. 

Comparing this expression with SP (124.7), we see that, if the x a are 
taken to be the components of the vectors EAF/4tt, HAF/4tt in each portion 
AV, then the corresponding f a will be the components of the vectors K and 
L: 

^^EAF/477, HAF/47T; 



The relations SP (124.11) 



f ao) = S <x.- 1 ab{o))x b(l)y (88.7) 

b 



§88 Electromagnetic field fluctuations 363 

which give the relation between/ w and # w , correspond to Maxwell's equations 
(88.3), (88.4), and so we have 

K ib) = - e m E kb) + (*V/o>)(curlH w ) f , 

Liu = - WkHk» ~ (ic/coXcurlEJ,. 

The coefficients or\ b are found by comparing (88.8) with (88.7), using the 
definitions (88.6) ; the suffixes a and b denumerate both the components of the 
vectors E, H and the portions AV. 

The curl operators in (88.8) are to be regarded as difference operators, 
defined with respect to a discrete set of points (say the centres of the portions 
AV). The actual form of these operators is, however, of no importance, 
since it is here sufficient to note that the operators acting on H w and E w in 
(88.8) are purely imaginary and differ only in sign. This means that the 
relations a -1 ,* = (orha)* are satisfied by the coefficients a -1 ^ which relate 
the values of K and L at a given point to those of H and E respectively at 
various points. 

According to the general formula SP (124.12) it therefore follows at once 
that 

{K tl L K ) m = 0, (88.9) 

where the suffixes 1 and 2 signify that the quantities are taken at the points 
ri and r 2 respectively. Formula (88.9) is valid whether ri = r 2 or not. 

Next, from the first equation (88.8), using (88.6), we see that the coefficients 
ar^a which relate K io) and Ejc w are -e ik 4tt/AV if K w and E w refer to the 
same point in space, and zero otherwise. By SP (124.12) we therefore have 

(KaKn). = (n # r 2 ), 

(KtKjc^ = ih(ctt* - ^)-^r coth(hojj2T). 

Passing now to the limit AV -> 0, we can evidently write both these 
formulae together as 

(KaKn) m = ih{e ki * - €«) S(r 2 - n) coth (ha>/2T), (88.10) 

where ri and r 2 refer to any two points in the body. In what follows we shall 
assume that the body is not in an external magnetic field. Then e^ = e k i, 
and (88.10) can be written 

(KaKn) m = 2WS(r 2 - n) coth(feo>/2T). (88.11) 

In an entirely similar manner we may derive the formula 

(LaL M ). = lh m " S(r 2 - n) coth (hco/2T). (88.12) 

Thus the fluctuations of the "extraneous" inductions at two points in the 
body are correlated only in the limit when the two points coincide (r 2 -> ri). 
This limit, of course, must be taken in the macroscopic sense, and the 
above statement really means that the correlation extends only to distances 



364 Electromagnetic Fluctuations §88 

comparable with the dimension of the atoms. It is most important to observe 
that the correlation formulae for the extraneous inductions do not depend 
on the shape of the body ; in this sense the formulae are universally valid. 

Formulae (88.11) and (88.12) can be put in another form for fairly low 
frequencies (the quasi-static range), when the tensor e^ can be expressed in 
terms of the constant (frequency-independent) conductivity tensor ai k by 

*ik — 4-7rt(T{fc/a>. (88.13) 

We then introduce the quantity j = (l/47r)dK/*fr, or 

j w = - ioKJ4ir. (88.14) 

The significance of this quantity is seen from the resulting form of equation 
(88.4): 

(curlHJ« = {^jc){a ik E kb) +ju,). (88.15) 

From this we find that the total fluctuation of the current density is at k E k +ji, 
so that the vector j is the "extraneous" current which is not related to the 
electric field E. For frequencies at which (88.13) holds and not too low 
temperatures we have T > h<o, so that coth (hwjlT) « 2T/ha>. Formula 
(88.11) thus becomes 

GW«). = (TMa ik S(r 2 - n). (88.16) 

Formula (88.16) was derived, in another manner, by M. A. Leontovich 
and S. M. Rytov (1952), and formulae (88.11) and (88.12) by S. M. Rytov 
(1953). Together with equations (88.3) and (88.4), these equations in prin- 
ciple solve the problem of calculating the electromagnetic fluctuations in 
any body. The solution proceeds as follows. Regarding K w and L w as 
known functions of the co-ordinates, we solve equations (88.3), (88.4) for 
E w and H w , taking into account the appropriate boundary conditions: the 
tangential components of E w and H w are continuous at the surface of the 
body (outside the body, of course, K = L = 0, but E and H are not zero). 
We thus obtain E w and H w as linear functionals of K w and L w . Accordingly, 
any quantity quadratic (or bilinear) in E w and H w can be expressed in terms 
of quadratic functionals of K w and L w , and the mean values are calculated 
from formulae (88.11), (88.12); K w and L w do not appear in the final result. 

As an example, let us consider electromagnetic fluctuations in an infinite 
isotropic medium (S. M. Rytov, 1953). We assume the magnetic per- 
meability of the medium to be unity. Then p" = 0, and we must put L = 
also. Thus equations (88.3) and (88.4) become 

curlE w = io)UJc, 

curlH w = - /a>(eE w + KJ/c, 



(88.17) 



and 

(K a K k2 ) u = 2»c"a«8(i* - ri) coth(ha>l2T). (88.18) 



§88 Electromagnetic field fluctuations 365 

We write K w as a spatial Fourier integral: 

K w (r) = J g w (p) exp(/p.r)dp. (88.19) 

—oo 

Then 

00 

gJP) = ^f\ K-M exp( ~ /p * r)dr ' 

—oo 
g/(P) = g-i-P)- 

Let us determine the correlation function for the components gjp). To 
do so, we write the product gtMgkAp') ^ a double integral: 

giMgkofip') 



00 00 

= 7^ f f ^ari)^^(r2)exp[-/(p.ri + p'.r 2 )]dridr 2 . 
(2tt) 6 J J 

—oo —oo 

Averaging by means of the formula 

K i<0 {ri)K kb ,(r 2 ) = (K a K k2 )J(co + a/), 

substituting (88.18) and effecting the integration over either n or r 2 , we 
obtain, on account of the factor S(r 2 -ri) in the integrand, 

giMg*o>(P') = 2fte"8(a> + a/) 3a; coth(ka>l2T) x 



(2*) 
or, finally, 



00 

exp [ - i(p + p') «r] dr 

(277)6 J FL ^ V) J 



giMgic^l = -r*"ZacK«> + ")S(P + P') coth(^co/2T). (88.20) 

47T 5 

Equations (88.17) may be solved by Fourier's method. As well as repre- 
senting K w as the integral (88.19), we put 



00 

E w = Ja(p)exp(/p-r)dp, 

—oo 

00 

H w = - J p X a exp (*p-r) dp. 



366 Electromagnetic Fluctuations §88 

Then the first equation (88.17) is satisfied, and substitution in the second 
equation gives px(pxa) = — A 2 (ea+g), whence 

a = [k*eg - (p-g)p]/<£ 2 - €#). 

Using this expression and formula (88.20), we obtain for the correlation 
of the Fourier components of the electric field 

he" hco 

atjpfajp') = 3(p + p')S(o> + to') coth — x 

&\e\* h ilc -p iP lc [k\*+e*)-p*] 

x . (88.21) 

Finally, the spatial correlation of the fluctuation of the electric field is obtained 
by inverting the Fourier transformation: 

Efa(ri)£*,*(ra) 

00 00 

= j / %(p)«^(p') exp |>'(p-ri + p'-r 2 )] dp dp'. 

—oo —oo 

One integration can be effected immediately, because of the delta function 
in (88.21). To carry out the second integration we must expand (88.21) in 
partial fractions, and then use the formulae 



oo 

J 



exp(»p.r) dp = 2 y- 



—oo 



p^exp(sp.r) = _ 2j7 , g2 ( e - Kr \ 
J p* + k* dxidxjc \ r J 

—oo 

The first of these is obtained by taking the Fourier component of the well- 
known relation 



e -KT 



( A _ K 2) = _ 4^S( r ), (88.22) 

r 



and the second by differentiating the first. 
The result is 



h hut i k? 

(EnEtt^ = — coth-- — (e-^-e - e -*r<-e* )8ik + 
47« 21 { r 

+ — ?!_[l( e * c -fcrv- e _ ce -*rV-e*)l I (88.23) 

lei 2 dxidxjc Yr j) 



§89 Black-body radiation in a transparent medium 367 

where r = |r 2 -ri|, and the root y/-c is taken ^^ ihe "S 11 for which 
reA /- e > 0. In particular, contracting with respect to the suffixes i and k 
and using the relation (88.22), we find 



(El .E 2 ) w = A cothgl^^rv- _ e -*rv- e *) + 




S(r)j. 



(88.24) 



We can similarly calculate the correlation of the various components H w , 
both among themselves and with the E w , but shall not pause to do so here. 

§89. Black-body radiation in a transparent medium 

The presence of the factors e" and /*" in formulae (88.11) and (88.12) 
emphasises the relation between electromagnetic fluctuations and absorption 
in a medium. If we take the limit as e" -» (with e > 0) in formulae 
(88.23) and (88.24), the result is not zero. This arises from a difference in 
the order of passage to two limits, those of infinite medium and zero e". 
Since, in an infinite medium, any non-zero value of e", no matter how small, 
causes absorption, the result obtained by passing to the limits in our order 
pertains to a transparent medium in which, as in any actual medium, there 
is in fact some absorption. 

For example, let us find the limiting form of formula (88.24). To do so, we 
note that, for small e", y/-e = V(- € '-*' e ") * -W € '( l + ¥*"/*), V~ € * 
= ^/(_ e ' + tV') &iy/€'{\-\U"le). The choice of signs is determined by 
the condition that the real part should be positive. In the limit e" -> 0, 
therefore, 

co 2 h sin comic . hw 
(Ei-Ea). = — coth— , 

7TC 2 r 11 

where n = -yA * s ^ refractive index. Since there is no delta-function term, 
this expression remains finite when ri = r 2 : 

(E2) w = (cu3^«/ttc3) coth(ha>l2T). (89.1) 

The spectral density of the electric field energy per unit volume is (cf. (61.10) 
and SP (118.6)) 

1 d(« 2 co) 

-2(E2) W -1_^. 

57T ClCU 

Substituting (89.1), we have 

ufihn d(w 2 co) hoj 

coth- 



4tt 2 c3 do> IT 



368 Electromagnetic Fluctuations §90 

The mean square of the magnetic field can be calculated similarly. It is found 
that (H 2 ) w = e(E 2 ) w , and so the magnetic energy is 

1 o) z hn z hco 
— 2(H 2 ) W = coth . 

We shall not give the detailed calculations here, since the final result which is 
obtained below is in any case obvious. 

Thus the total spectral density of the electromagnetic energy density in 
the fluctuation fields is 

co z h l d(» 2 a>) \ h 



(d^co) \ hat 
n h n z I coth 
dco J IT 



hco \ co 2 n 2 d 



(Hco \ coW d 
hhco H ) (nco). 



The first term in the parentheses relates to the zero-order oscillations of the 
field; the second term gives the energy of thermodynamic-equilibrium 
electromagnetic radiation in a transparent medium (i.e. the energy of black- 
body radiation) : 

hco co 2 n 2 d 

— (nco). (89.2) 



e h<o/T _ 1 ^2^3 dco 

This formula could also be obtained, without considering fluctuations, 
by an appropriate generalisation of Planck's formula for black-body radia- 
tion in a vacuum. According to this formula, the energy of black-body radia- 
tion per unit volume having wave-vector components in an interval dk x , 
dky, dk z is 

hco 2dkxdkydk z 

e*»l T - 1 (2^)3 * 

the factor 2 arises because of the two directions of polarisation. To obtain 
the spectral energy density, we must replace dk x dk y dk z by 4irA 2 dk and 
substitute k = cojc. To go from a vacuum to a transparent medium it is 
sufficient to put k = nco/c instead of k = to/c, i.e. k 2 dk = k 2 (dk/dco) dco 
= (afinZjc 3 ) d(nco)fdco, which gives formula (89.2). It should be noted that 
this formula remains valid even if p ^ 1, provided that n is interpreted as 

§90. Forces of molecular attraction between solid bodies 

The theory of electromagnetic fluctuations can be used to calculate the 
forces of interaction between any two macroscopic bodies whose surfaces 
are a very short distance apart (E. M. Lifshitz, 1954). In what follows we 
assume only that this distance is large compared with those between the 
atoms. In this case the problem can be treated macroscopically. 



§90 Forces of molecular attraction between solid bodies 369 

We may consider the interaction between bodies as resulting from the 
fluctuations of the electromagnetic field which always occur in an absorbing 
medium and also outside it. If the space between the surfaces is a vacuum, 
this means of interaction is evidently the only one. 

Let us regard the interacting bodies as two media occupying half-spaces 
with plane parallel boundaries at a distance / apart. The fluctuations of the 
electromagnetic field in the two media and in the space between them can be 
found by solving equations (88.17)f with the appropriate boundary condi- 
tions (continuity of the tangential components of E w and HJ at the two 
surfaces. The solution is most conveniently effected by expanding the 
required functions (and the quantity K w ) as Fourier integrals with respect 
to the transverse co-ordinates y, z (the #-axis being normal to the surfaces) ; 
this gives a system of linear inhomogeneous ordinary differential equations 
to determine the fields as functions of x. Solving these equations, we obtain 
integral expressions for E w and H w , whose integrands contain the "external" 
fluctuation fields Ki w and K 2w in the two media. Actually, it is sufficient 
to obtain the explicit forms of E w and H w in the space between the media, 
since the mutual attraction force F w per unit area of either surface can be 
calculated as the ^-component of the Maxwell stress tensor, statistically 
averaged in accordance with formula (88.18). Since the calculations involve 
only the spectral components of the fields, the required total attraction force 
F must be obtained by integrating F w over all frequencies .% 

The calculations are somewhat laborious; we shall not go through them 
here, but give only the final result. The force is|| 



f T ha)l[(si+ p)(s2 + p) at , A 

pafi coth —J ; F {) ^ er*tP" /• - 1 + 

J J F 2T\Y{ Sl -p){s2-p) J 



+ \(*i + *iP)^+<*P) e - 2ip „ l/e _ {\ - 1 ) dp d(x)> (9 o.!) 

L(s 1 -e 1 p)(s 2 -€2p) J ) 

where ei(<o), e 2 (to) are the dielectric permeabilities of the two media, and 

*i = #iH - 1 +P 2 l *2 = VM*>) - 1 +p*], (90.2) 

the signs of the roots being taken so that the imaginary parts of *i and $2 
are positive, ff The paths of integration are shown in Fig. 43a. The integra- 



t The magnetic permeabilities of the two bodies are assumed to be unity. 

X The value of F M resulting from the calculation includes a term which diverges on 
integration with respect to <o. This term, however, is independent of I, and represents the 
pressure exerted on the surface by the black-body radiation in the vacuum. In an actual 
problem these forces act on all sides of a body and cancel out, so that the term in question 
does not relate to the mutual attraction under consideration, and must be omitted. 

|| SeeE. M. Lifshitz, Zhurnal eksperimental'noi i teoretichesko! fiziki 29, 94, 1955; Soviet 
Physics JETP 2, 73, 1956. . 

1 1 Since the imaginary part of the radicand (e") is positive, im s > implies re s > 0. 



370 



Electromagnetic Fluctuations 



§90 



tion with respect to to is over real values from to oo ; that with respect to p 
is along the real axis from 1 to 0, and thence along the imaginary axis 
to «oo. 

If the temperature of the bodies may be taken as zero (see below), 
cotti(hco/2T) in (90.1) is replaced by unity. We shall begin by discussing 
formula (90. 1) for this particular case. 



(a) 



© 



® 



(b) 



© 



® 



(0 



® 



Fig. 43 

Both the complex form of (90.1) and the appearance of the expression 
g-Zipai/c^ which oscillates on the real part of the path of integration with 
respect to p, are inconvenient. The latter fact, in particular, hampers the 
calculation of the integral for large values of /, when the oscillation becomes 
very rapid. This difficulty can be removed by appropriate changes in the 
paths of integration in the planes of the complex variables a> and/> : they must 
be displaced in such a way that the integration with respect to p is taken over 
only real values, and that with respect to a> over only imaginary ones (Fig. 
43b). Then the exponent —lipwljc is always real. f 

The result is the following expression for the interaction force (when 
T= 0): 



F = 



2ttW 



J J F \1(S1-P)(S2-P) J 



1 



(S1-P)(S2~P) 



if\ 



+ 



(si + eip) (s 2 + € 2 p) 



L(*l - ev 



(*1 ~ cip) (S2 ~ €2p) 



e 2pUc -I 



] 'jd/^. 



(90.3) 



f This transformation is permissible if there is some way of simultaneously displacing 
both paths without passing through a pole of the integrand. A detailed investigation, using 
the properties of the function e(a>) given in §62, shows that this is so. 



§90 Forces of molecular attraction between solid bodies 371 

Here we have put a = % for imaginary co; ei and e 2 denote the real functions 
€i(/£) and € 2 (t£). The sign re has been omitted, since the expression (90.3) 
is evidently real. This formula makes possible, in principle, a calculation of 
the force F for any distance I if the functions e(*|) for the two bodies are 
known. These functions can be expressed in terms of the imaginary part 
of e(a>) for real o>, by formula (62.17). Thus we can say that the law of 
interaction of the bodies is entirely determined when the functions e"(w) in 
them are given. 

Let us consider the limiting case of distances / which are small compared 
with the wavelengths A of greatest importance in the absorption spectra of 
the bodies. The temperatures which occur in solids and liquids are always 
small compared with the important values of ho) ~ hwo (where a>o ~ 27rc/Xo). 
Hence we can assume that T = and use formula (90.3). 

On account of the exponentially increasing factor e 2 ^ l/c in the denomina- 
tors in the integrand, the main contribution to the integral with respect to 
p comes from values such that p£ljc ~ 1. Then p > 1, and hence we can 
put 5i « S2 « p in determining the leading terms. In this approximation 
the first term in square brackets in (90.3) is zero. The second term gives 



F = 



h ?? * 2 d*d| (9Q4) 



I6772/3 







m&" 



where the new variable of integration x = 2pgl/c, and the lower limit of 
integration 2£ljc has been replaced in this approximation by zero. 

Formula (90.4) gives the force of attraction in the limiting case of small /. 
It is inversely proportional to the cube of the distance. The function e(*'£) — 1 
decreases monotonically to zero with increasing £. Values of $ exceeding 
some go therefore make no important contribution to the integral ; the condi- 
tion for / to be small is / <^ c/|o- 

We shall show how the passage to the limit of the interaction between indi- 
vidual atoms is made in (90.4). To do so, we formally assume both media 
to be so rarefied that the differences ei- 1 and e%— 1 are almost zero, and(90.4) 
gives with sufficient accuracy 



F = 



64t7 2 /3 





00 00 

j J *V-*(ei - l)(e 2 - l)dtf d| 



OD 

= 32^ J t^-™' 1 ^- 



372 Electromagnetic Fluctuations §90 

Expressing €(ii-) in terms of e"(a>) on the real axis by (62.17) we obtain 

00 



oo oo oo 

coia)2ei"(a>i)e 2 "(a> 2 ) 

a£ atuidco2 



-Jill 



("l 2 + | 2 )("2 2 + £ 2 ) 




oo oo 



2 f f ei"(a)i)e 2 "(w2) 

acuiaa)2, 



-:// 







a>i + OJ2 



and the force F is 



00 00 



16tt3/3 





f f " N " (Ma V j^ (90.5) 

J J Ct»l + C02 



The force of interaction between rarefied media may be regarded as the 
resultant of the interactions between pairs of molecules. Then the force 
(90.5) corresponds to an interaction of molecules with an energy which 
depends on the distance jR between them: 



00 00 



V = 3 JL- f f 1^M^l dwidw2 , (9 o. 6) 







where N is the number of atoms per unit volume. Formula (90.5) is obtained 
from this by integration over the two half-spaces separated by a gap of 
width /, followed by differentiation of the total energy with respect to /. 
Formula (90.6) is the same as that derived by F. London by applying the 
ordinary perturbation theory of quantum mechanics to the dipole interaction 
between atoms. In making the comparison, it should be borne in mind that 
e"(w) is related to the spectral density of "oscillator strengths" /(a>) by 
(62.13). The oscillator strengths are expressed, in the usual manner, in 
terms of the squared matrix elements of the dipole moment of the atom. 
Thus we see that the microscopic formula can be derived from the macro- 
scopic theory. 

Let us now consider the opposite limiting case, where the distance / is 
large compared with the wavelengths Ao which are of greatest importance in 
the absorption spectra of the bodies. The temperature will at first be sup- 
posed zero, as before. 



§90 Forces of molecular attraction between solid bodies 373 

In the general formula (90.3) we again introduce as a new variable of 
integration x = 2p$lc, but leave p instead of £ as the second variable : 

F = Jl. f p*fr fr + *>fr + *W i"r+ 

32tt2/4 J J pA L(*i - />) (*a - *) J 



o l 



r(*i + nP)(*2 + «?f)^_ n * 



L(*l - €lp) (S2 ~ Vp) J ' 

e = e(ixc/2pl), S = y/Wixcftpl) - 1 + /> 2 ]. 

On account of the factor e* in the denominators, the important values of x 
are - 1, and, since p > 1, the argument of e is almost zero (for large /) 
throughout the important range of values of the variables. We can there- 
fore replace ei and c 2 by their values for to = 0, i.e. the electrostatic dielectric 
permeabilities, which we denote by eio and € 20 . For metals, the function 
€(co) tends to infinity as <o -> 0, and so e = oo. 
Thus we have finally 



he r° r°V j T (sio + p)(s2o+p) gX _ J 
F = M J J p* \[{sio -P){S20-P) 6 J 



-1 
+ 



i 



+ [-(no + *ioj>)(*2o + wP^A- 1 }^^ (9 o.7) 

L ($10 - n0^)(*20 - €20/>) J ) 

S10 = V(«10 - 1 + P 2 \ 5 20 = \/( e 20 ~ 1 + P 2 )' 

Here the force of attraction is inversely proportional to Z 4 . It is noteworthy 
that, in this limiting case, the force depends only on the electrostatic dielec- 
tric permeabilities of the two media. 

Let us consider some special cases. A particularly simple result is obtained 
when both media are metals. Putting in (90.7) eio = e 2 o = 00, we find 

fc [ F x* d P dx = fc^_ (90 gv 

167r2/4j J /*(«*- 1) Z 4 240* 
1 

This force is independent of the kind of metal concerned (a property which 
ceases to hold at small distances, the interaction then depending on the values 
of €(/£) for all £ and not only for | = 0).t 



t Formula (90.8) had previously been derived in another manner by H. B. G. Casimir 
(1948). 



374 



Electromagnetic Fluctuations 



§90 



For two media of the same dielectric (eio = €20 = eo), numerical integration 
of (90.7) gives 



he 7T 2 / eo — 1 \ 2 



F = 1 

P 240\ € + 



1 r 



(90.9) 



where <£(eo) is a function shown graphically in Fig. 44. 



06 

0-6 

w° 0-4 










































02 





















0-2 



0-4 
Fig. 44 



0-6 



0-8 



Finally, we can make the transition in formula (90.7) to the case of inter- 
action between individual molecules. As previously, we assume the two 
media to be so rarefied that the differences eio — 1 and €20 — 1 are small. 
Retaining only the first non- vanishing term in an expansion of the integrand 
in (90.7) in powers of these differences, we obtain 

00 00 

he r f 1 - 2/> 2 + 2/> 4 

F = X?e~*dx — d/>(eio - l)(e 20 - 1) 



or 



he 23 
F = ( ei0 - l)( €20 - 1). 

This force corresponds to a molecular interaction with energy 
23/*c (eio - l)(f20 - 1) 23hc 



U = 



647r3#7 



N^ 



4ttR7 



aia 2 , 



(90.10) 



(90.11) 



where ai and cx.2 are the static polarisabilities of the two molecules. This 
formula is the same aS that obtained by H. B. G. Casimir and D. Polder 



§90 Forces of molecular attraction between solid bodies 375 

(1948), using quantum methods, for the attraction between two molecules 
at a large distance apart, when the delay in interaction becomes important. 

To obtain formulae which take account of the temperature we return to 
the initial expression (90. 1) and see how the transformation which for T = 
leads to (9.0.3) must be modified when T ^ 0. The function coth (hwjlT) 
has an infinite number of poles on the imaginary axis, at the points 

con = Hn = 1™Tn\K (90.12) 

where n is any integer. When the path of integration with respect to w is 
moved on to the imaginary axis, therefore, it must be indented by semicircles 
at these poles, and by a quarter-circle at a> = (Fig. 43c). This gives rise 
to contributions to the real part of the integral, equal to in {^iri for n = 0) 
times the residues of the integrand at the poles. The integration along the 
imaginary axis between the poles gives only imaginary quantities, which 
do not affect the real part. 
Thus we have 

00 

"* ^=0 J U (il. " P)(S2n - P) J 

+ r(i. + i^X*. + ^P\mniic -\VW (90.13) 

Y{sin - *lnp){s2n — ^2np) J J 

% = V( e n - 1 + p% en = e(*fn). 

The prime indicates that the term with n = is halved. As T -> 0, the 
distances between the poles tend to zero, the summation over n can be re- 
placed by an integration with respect to £, and we return to formula (90.3), 
which does not involve T. 

Whereas, in the limiting case of small distances, the temperature can 
always be taken as zero in determining the interaction force, at large distances 
the effect of the temperature may be considerable. The characteristic tem- 
perature for a distance / is hcjl, and the condition for T to be negligible is, 
roughly, lT\hc <^ 1. At sufficiently low temperatures this condition is 
compatible with / > A , but at high temperatures the two conditions may be 
contradictory, and the limiting form (90.7) is then never applicable. 

Let us consider the limiting case of distances so large that IT J he > 1. 
Then only the first term in the sum in (90.13) need be retained. We cannot 
at once put n = 0, however, because an indeterminacy results: the factor 
i n z is zero, but the integral with respect to p diverges. This difficulty may 
be circumvented by replacing p by a new variable of integration x = 2p£ n l/c 
(the factor £ n 3 being thereby removed). Putting now n = we obtain 



F = 



16ttP 
o 



00 

[4 ( " +lfa + 1) .-ll"V (90.14) 

J L(cio-l)(«ao-l) J 



25 



376 Electromagnetic Fluctuations §90 

Thus, at sufficiently large distances, the force of interaction decreases less 
rapidly and returns to the l// 3 law, with a coefficient which depends on the 
temperature and on the static value of the dielectric permeability. The re- 
maining terms in the sum (90.13) all diminish exponentially when IT/hc is 
large. 



CHAPTER XIV 

SCATTERING OF ELECTROMAGNETIC WAVES 

§91. The general theory of scattering in isotropic media 

In the theory of propagation of electromagnetic waves in transparent media 
discussed in Chapters IX-XI, a phenomenon has been neglected which, 
though not prominent, is of fundamental importance : scattering. Scattering 
results in the appearance of scattered waves of small intensity, whose fre- 
quencies and directions are not those of the main wave. 

Scattering is ultimately due to the change in the motion of the charges in 
the medium under the influence of the field of the incident wave, resulting 
in the emission of the scattered waves. The microscopic mechanism of 
scattering must be investigated by quantum methods, but this investigation 
is not needed in developing the macroscopic theory described below. We 
shall therefore give only some brief remarks on the nature of the processes 
which cause the change in the wave frequency on scattering. 

The basic scattering process consists in the absorption of the original 
quantum ho> by the scattering system and the simultaneous emission by that 
system of another quantum hat'. The frequency to of the scattered quantum 
may be either less or greater than <o ; these two cases are called respectively 
Stokes scattering and anti-Stokes scattering. In the former case the system 
absorbs an amount of energy h((o — a/); in the latter case it emits h(<o' — <o) 
and makes a transition to a state of lower energy. In the simple case of a 
gas, for example, scattering takes place at individual molecules, and the 
change in frequency may be due either to a transition of the molecule to 
another energy level or to a change in the kinetic energy of its motion. 

Another kind of process occurs when the primary quantum hto remains 
unchanged but causes the scattering system to emit two quanta: one of 
energy h<o, with the same frequency and direction, and a "scattered" quan- 
tum hoi . The energy h(to+u>') is obtained from the scattering system. 
Processes of this type, however, are, under ordinary conditions, very rare 
in comparison with those of the first type, and are of little importance as 
regards the phenomenon of scattering, f 

Proceeding now to consider the macroscopic theory of scattering, we 
must first make precise the meaning of the averaging processes performed 
in that theory. As already mentioned at the beginning of §88, the averaging 
of quantities in macroscopic electrodynamics can be regarded as comprising 



f We shall see in §92 that this stimulated emission is unimportant at all temperatures 
T <^ H(a>+ <a). It may become significant for radio waves. 

377 



378 Scattering of Electromagnetic Waves §91 

two operations, that of averaging over a physically infinitesimal volume with 
a given position of all the particles in it, followed by that of averaging the 
result with respect to the motion of the particles. In the theory of scatter- 
ing, however, this procedure is impossible, because the averaging with respect 
to the motion of the particles annuls the very phenomenon which is to be 
discussed. Thus (e.g.) the field and induction of the scattered wave which 
appear in the theory of scattering must be taken to be those resulting from 
the first averaging only. The single-frequency components of the fields in 
the scattered wave, taken in this sense, will be denoted in this section by 
E\ H', D' and B'. 

The fields in the incident wave will be denoted by the unprimed letters 
E, H. In the present chapter we always suppose the incident wave to be of a 
single frequency w. 

In the propagation of the scattered wave we have the relation D' = e(o/)E' 
between the electric induction and field (the scattering medium being 
assumed isotropic), but this relation does not reveal the phenomenon of 
scattering, i.e. the formation of the scattered wave from the incident wave. 
To describe this, additional small terms must be included in the expression 
for D'. In the first approximation, these terms must be linear in the field of 
the incident wave. The most general form of the relation is then. 

D'i = e'EU + a. ik E k + PaEjc*. (91.1) 

Here e denotes e(o/); on k and jS^ are tensors which characterise the scatter- 
ing properties of the medium. In general they are not symmetrical, and their 
components are functions both of the frequency to of the scattered wave and 
of the primary frequency w. f 

The last term in (91.1) pertains to the part of the scattering which results 
from processes of stimulated emission. All the terms on the right-hand side 
of equation (91.1) must correspond to the same frequency a> as D on the 
left-hand side. Since E* has the frequency — w, the frequency of the quanti- 
ties fiijc must be cj + oj' to make the frequency of the products Pi k E k * equal to o>' . 
But co + <o' is the frequency which characterises processes of stimulated emis- 
sion. Because this effect is small, as mentioned above, we can neglect the 
corresponding term in (91.1), and in what follows we shall write 

D'i = e'E'i + * ik E k . (91.2) 

Similar formulae give the relation between B' and H'. We shall, however, 
neglect the magnetic properties of the medium, which are usually of no 
importance as regards the scattering of light, and therefore put B' = H'. 

Maxwell's equations for the field in the scattered wave are curl E' 
= ito'H'/c, curl H' = — ico'D'/c. Eliminating H' from these equations, we 



f The fact that a and /? are tensors does not, of course, contradict the assumed isotropy 
of the medium. Only the fully averaged properties of the medium are isotropic; the local 
deviations from the average properties, which include the additional terms in (91.1), need 
not be isotropic. 



§91 The general theory of scattering in isotropic media 379 

find curl curl E' = a>' 2 D'/c 2 . Substituting from (91.2) E' = D'/e'-a-E/e', 
where a«E denotes the vector whose components are a^&> and using the 
equation div D' = 0, we obtain for D' the equation 

AD' + &' 2 D' = - curl curl (a- E), (91.3) 

where k' = oi^/e'jc is the wave number of the scattered wave. 

For an exact formulation of the conditions under which equation (91.3) 
is to be solved, we divide the scattering medium into small regions (whose 
dimensions are still large compared with molecular distances). On account 
of the molecular nature of the scattering processes, their correlation at dif- 
ferent points in the medium (assumed non-crystalline) extends in general 
only to molecular distances, f Hence the scattered light from the various 
regions is non-coherent. We can therefore treat scattering from one region 
as if the light were not scattered at all in the remainder of the medium. In 
this way we calculate the field of the scattered wave at a large distance from 
the scattering region. Using a well-known approximation for the retarded 
potentials at a large distance from the source,* we can immediately derive 
the required solution of equation (91.3): 

D' = Icurlcurl ^^^ f oc-E exp(- zk'.r)dF. (91.4) 

4-tt Ro J 

Here Ro is the radius vector from some point within the scattering volume 
(the integration being over that volume) to the point where the field is to be 
calculated; the vector k' is in the direction of Ro. The integral in (91.4) is 
independent of the co-ordinates of the point considered; retaining in the 
differentiation, as usual, only terms in l/i?o, we obtain 

D = - eXp(ik ' Ro) k'x[k'x fa.Eexp(-ik'.r)dF]. 
47tRo j 

Since, at the point considered, the medium is regarded as not scattering, the 
relation between D' and E' there is given by D' = e'E' simply. In the field 
of the incident wave E we separate a factor periodic in space, putting E 
= Eoexp (zk«r). Then, with the notation 

G = J*<x.E exp(-*q.r)dF, (91.5) 

where q = k' — k, we have 

£ , _ _exp(,*'flo) k , x (k , x G) (91 6) 

\ttRqz 



t Exceptions may occur for particular cases of scattering, which will be discussed in §94. 
In such cases the dimensions of the scattering regions must be supposed large in comparison 
with the wavelength of the light. 

X See The Classical Theory of Fields, §9-2, Addison- Wesley Press, Cambridge (Mass.), 
1951 ; Pergamon Press, London, 1959. 



380 Scattering of Electromagnetic Waves §91 

The vector E' is perpendicular to the direction k' of the scattered wave, and 
is given by the component of the vector G perpendicular to k'. 

Having thus determined the non-averaged field in the scattered wave, we 
can now investigate the intensity and polarisation of the scattered light. To 
do so, we form the tensor 



hk = E' t E' k *, (91.7) 

where the bar denotes the final averaging over the motion of the particles, 
which so far has not been carried out. The averaging of a quadratic expres- 
sion gives, of course, a result which is not zero. Since E' is perpendicular to 
k\ the tensor Im has non-zero components only in the plane perpendicular 
to k'. These components form a two-dimensional tensor 7 ay? in that plane 
(Greek suffixes take two values). The tensor I a/S is, by definition, Hermitian: 
I a p = I fi( *. It can be reduced to "principal axes", and the ratio of its two 
principal values gives the degree of depolarisation, while their sum is pro- 
portional to the total intensity, f 

The products E'iE'k* involve products of integrals Gi, which must also 
be averaged. Writing the product as a double integral, we have 

GiG k * = EoiEo m * j j ao.iafa.,2* exp [- *q.(n - r 2 )] dFi dF 2 . (91.8) 

The suffixes 1 and 2 indicate that the values of a are taken at two different 
points in space. 

In averaging the integrand it must be remembered that the correlation 
between the values of a at different points in the body extends in general only 
over molecular distances. After averaging, therefore, the integrand will be 
appreciably different from zero only for |r2 — ri| ~ a, where a is of the order 
of molecular distances. The exponent is ~ a/A, where A is the wavelength 
of the scattered wave ; but a/X <^ 1 if the macroscopic theory is applicable, 
and so we can replace the exponential factor by unity.J 

Next, the integration with respect to the co-ordinates ri and r 2 can be 
replaced by one with respect to |(ri + r 2 ) and r = ri - r 2 . Since the integrand 
depends, after averaging, on r only, we have 

G^*> = VEoiEom* / oni,^ km ,2*dV, (91.9) 

where V is the volume of the scattering region. It is evident a priori that the 
scattering must be proportional to V. It should be noted that the direction 
of the wave vector k in the incident wave appears neither in (91.9) nor, 
consequently, in the following formulae. 



f See Classical Theory of Fields, §6-7. The reduction of an Hermitian tensor to principal 
axes means putting it in the form hk = hmmki* + ^msnta*, where m, m are, in general, 
perpendicular complex "unit" vectors: m»nj* = 1, ri2»n2* = 1, ni»na* = 0. The prin- 
cipal values Ai, A2 of an Hermitian tensor are real. 

% This procedure requires further discussion in the case of Rayleigh scattering (§94). 



§91 The general theory of scattering in isotropic media 381 

The integrals in (91.9) form a tensor of rank four, which depends only on 
the properties of the scattering medium. Since the medium is isotropic, 
the most general form of this tensor is 

f a«,ia fcm>2 * dV = \{a + c)8 a 8 km + \{a - c)h im hi + bo ik 8i m , (91.10) 

where a, b and c are scalar functions of a> and to. This tensor is automatically 
symmetrical with respect to an interchange of the suffixes i, / and k, m\ 
this interchange is equivalent to taking the complex conjugate, since the points 
1 and 2 are equivalent; the tensor (91.10) is therefore real, and so are a, b } c. 
Substituting (91.10) in (91.9) we obtain 

G^* = V{\{a + c)E 0i E ok * + \{a - c)E Qi *E 01c + bE 0i Eoi*8i k }. (91.11) 
This expression could have been written down at once, since it is the most 
general Hermitian tensor of rank two which is quadratic in Eo and involves 
no other particular directions. This tensor is, of course, not transverse to 
k\ The required general form of the tensor/^ is obtained by "projecting" 
the tensor (91.11) on a plane perpendicular to k'; to do this, it is sufficient 
to take a co-ordinate system with one axis in the direction of k' and find the 
components of the tensor along the other two axes. 

Let us consider the scattering of a linearly polarised wave. The amplitude 
of the field E can be defined as a real quantity, f The components of the 
tensor I afi for the scattered light are therefore also real. This means that the 
scattered light is partially polarised, and can be divided into two independent 
(non-coherent) waves, each of which is linearly polarised. Since there are only 
two distinctive directions (those of Eo and k') on which the tensor I afi can 
depend, it is evident that one of these waves must be polarised with E' in 
the plane of E and k\ and the other with E' perpendicular to this plane. The 
intensities of the two scattered-light components will be denoted by Jiand h\ 
they are the principal values of the tensor J a/ff . 

For real E , the expression (91.11) becomes 

~G&* = V{aE 0i Eo k + bE^hac). (91.12) 

We may note first of all that the scattering of linearly polarised light is deter- 
mined by two, not three, independent constants. To find h and h, we 
take the components of E in the two directions mentioned. The correspond- 
ing components of the tensor (91.12) give the result 

h-asitfO + b, h~b, (91.13) 

the coefficients of proportionality being the same ; 6 is the angle between Eo 
and the direction of scattering k'. The intensity of the scattered-light compo- 
nent whose electric field is polarised perpendicular to the plane of E and 
k' is independent of the direction of scattering. 



t See The Classical Theory of Fields, §§6-5, 6-7. We shall not consider here the scattering 
of elliptically polarised light, on account of the complexity of the formulae. 



382 



Scattering of Electromagnetic Waves 



§91 



When natural light passes through a medium, the scattered light is par- 
tially polarised, and it is evident from symmetry that the two non-coherent 
components are linearly polarised, with their electric fields parallel and per- 
pendicular to the scattering plane (the plane of k and k'). Let the intensi- 
ties of these components be 7„ and 7 X respectively. To determine these, 
we average (91. 1 1) over all directions of the vector Eo in the plane perpendicu- 
lar to k. The averaging of the product EqiEq^ gives 



EoiEo k * = J|Eo| 2 (S^ - nm), (91.14) 

where n is a unit vector in the direction of k. This is a tensor of rank two 
which depends only on the direction of k, gives |Eo| 2 on contraction, and 
satisfies the condition ntEoiEok* = (n^Eo^ofc* = 0. Thus we have, when 
natural light is scattered, 

G^ = V\E \ 2 {ka(8 ik - mn k ) + b8 ik }. (91.15) 

Finally, taking the components of this tensor in the two directions of polarisa- 
tion, we obtain the required formulae : 



\a cos 2 & + b, I ± ~ \a + b, 



(91.16) 



where & is the scattering angle (i.e. that between k and k'). 

Let us return to formula (91.10), which relates the scalar quantities a, b and 
c to the tensor a^. Like any tensor of rank two, a** can be written, in general, 
as a sum of three independent parts : 

a-ik = l<x<>ik + Hk + dik, (91.17) 

where a = a^ is a scalar, syc a symmetrical tensor whose trace is zero 
(sac = s k i, Hi = 0) and am an antisymmetrical tensor. We substitute this 
in (91.10) and contract with respect to various pairs of suffixes, obtaining the 
three equations f 

6a + 3b + Zc = Ja Wj iaM J 2*dF = Jaia 2 *dr, 



3a + % = jaik,i«4k,2*dV 

= ijaia 2 * dV + Js ik ,iSik,2*dV + JW,i«M;,2* dV, 
6a + 3b - 3c = \oi ik> i(x. k i t 2*dV 

= jjaia 2 * dV + js ik>1 Si kt 2* dV - ja ik> ia ik , 2 * dV. 



► (91.18) 



t The integrals on the right-hand sides of these equations are positive, since each can 
be written as a square by a transformation inverse to that whereby (91.8) becomes (91.9). 
Expressing the three integrals in terms of a, b, c (i.e. solving equations (91.18) for these in- 
tegrals) we obtain the inequalities 2a + b + c > 0, 2b + c — a > 0, 2b + a — c > 0. From 
these, in particular, it follows that b > 0. 



§92 The principle of detailed balancing applied to scattering 383 

The right-hand sides of these equations, and therefore their solutions for 
a, b, c, do not involve cross-products of a, s ijc and am. This means that 
scattering can always be regarded as a superposition of three types of process, 
which may be called scalar, symmetrical and antisymmetrical scattering. We 
shall discuss each of these in turn. 

Retaining only the first terms on the right of equations (91.18), we have 

a = c = -[^wFdV, 6 = 0. (91.19) 

It is seen from (91.13) that, in scalar scattering of polarised light, the scattered 
light is itself completely polarised, and its angular intensity distribution is 
given by I = (3/2) sin 2 0. (Here and henceforward the expressions for I are 
normalised so as to give unity on averaging over directions.) In scattering of 
natural light, however, the angular distribution of the total intensity and the 
degree of depolarisation of the scattered light are given, according to (91.16), 
by I = I ± + I^ = f(l + cos 2 &), IJI ± = cos 2 &; see the second footnote 
to §72. 

For symmetrical scattering, equations (91.18) give 



= -b = - -c = — sa,isa,2* dV. 
3 5 30 J 



(91.20) 



In scattering of polarised light we have I = h + h = in>(6 + sin 2 0), 
hjh = 3/(3+ sin 2 0), and in scattering of natural light I = f^(14-sin 2 ^), 
IJI ± = l-£sin 2 &. 

Finally, for antisymmetrical scattering we obtain 

b = c = - a = - f aa,iaa,2*dV; (91.21) 

in scattering of polarised light / = f (1 + cos 2 0), hjh = cos 2 0, and in scatter- 
ing of natural light I = |(2+ sin 2 &), IJI t = 1/(1+ sin 2 &). 

§92. The principle of detailed balancing applied to scattering 

The general principle of detailed balancing f can be used to obtain a 
relation between the intensities in various scattering processes. 

Let dwi2 be the probability that a quantum %oi\ is scattered (on a path of 
unit length) and gives rise to a quantum /za> 2 in the solid angle element dor, 
let dw 2 i be the probability of the converse process, in which a quantum ha) 2 
yields a quantum hw\ in the solid angle element doi. According to the 
principle of detailed balancing we have dw\i\h<^doi = dw2ijki 2 doi, where 
ki and k 2 are the wave numbers of the two quanta. Substituting k± 2 = eicoi 2 /c 2 , 
& 2 2 = €2( o2 2 Jc 2 (where €1 = e(o>i), e 2 = <o> 2 )), we obtain 

eicoi 2 do>i2/do2 = £20>2 2 dw>2i/doi. (92.1) 

t See Quantum Mechanics, §116, Pergamon Press, London, 1958. 



384 Scattering of Electromagnetic Waves §92 

Here it is assumed that the initial and final states of the scattering system 
correspond to discrete energy levels E\ and E 2 , related by E\ + h w\ = E 2 +hw 2 . 
This statement of the problem is not quite true to reality, since the energy 
levels of a macroscopic body are extremely closely spaced and can be regarded 
as quasi-continuous. 

Instead of the scattering probability dw±2 with an exactly determined fre- 
quency change, we must therefore use the probability of scattering into a 
frequency range da>2, i.e. of the body's entering a state whose energy lies 
in a range dE 2 = hda) 2 . Denoting this probability (again per unit path 
length) by dhi 2 , we have dhi 2 = dzo 12 dT 2 = dwi 2 (dr 2 /dE 2 )h doj 2 , where dr 2 
is the number of quantum states of the body in the energy range dE 2 . Instead 
of (92. 1), we therefore have 

dl\ dh 12 dr 2 dA 2 i 
eio>i' s — = - — e 2 u)2r 



d£i do 2 da>2 dE 2 doida>i 

According to a well-known relation between the statistical weight of a 
macroscopic state of a body and its entropy Sf, the derivative dT/dE is 
essentially exp ST, so that (dTi/cLEi) :(dT 2 ldE 2 ) = exp (S^i - Sf 2 ). Since the 
relative change in the energy of the body resulting from the scattering of one 
quantum is negligible, the relative change in entropy is also small, and can 
be taken as ^i-^ 2 = {d^ldE){E x - E 2 ) = (E 1 -E 2 )/T = tya*- an)/T. 
Using this result, we can write the final expression of the principle of detailed 
balancing for scattering in the form 

d/h2 dh 2 \ 

e-^^eim 2 — = e-^2/T €2C02 2 . (92.2) 

do2 do) 2 doi dct>i 

The quantity dh, whose dimensions are cm -1 , is called the differential 
extinction coefficient for scattering of light. It can also be defined as follows : 
dh is the ratio of the number of quanta scattered in the direction do and the 
frequency range dw per unit time and volume to the incident photon flux 
density. By integrating dh over all directions and frequencies of the scat- 
tered light, we obtain the total extinction coefficient, which represents the 
damping decrement of the photon flux density as the light passes through the 
scattering medium. 

Let w 2 < <oi. The relation (92.2) connects the intensities (extinction 
coefficients) of Stokes (1 ->2) and anti-Stokes (2 -+ 1) scattering. We see 
that the latter is in general less than the former by approximately the factor 
e -h{o x -o 2 )iT % Thi s is a very general result, and corresponds to the fact that 
the transfer of energy from the body to the electromagnetic field reduces the 
probability of the process by a factor e~^ EIT , where A£ is the energy trans- 
ferred. In particular, the stimulated emission, in which the body gives up 
an energy ^(oii+oj 2 ) in each scattering process, is therefore usually very 
weak. The probability of such a process, when /*(oii+ u> 2 ) > T, contains the 
small factor e - h ^+ (a » )/T . 



§93 Scattering with small change of frequency 385 

The general relation (92.2) is much simplified in the important case of 
scattering with a relatively small change in frequency. We shall denote o>i 
by co simply, and the small difference w^-wy by Aco(<^co), and put for 
brevity d/n 2 /do 2 dco 2 = I(a>, Aai). In the non-exponential factors e<f in 
(92.2) we can neglect the difference A to; these factors then cancel, leaving 

I(a>,Aco)e-WT = /(a, + Aco, - Aco>r*<" + ^' T . 

In the first argument of the function /(co+Aco, -Aco), which gives the 
initial frequency of the light, we can neglect Aco, i.e. refer the scattered 
intensity to a somewhat displaced frequency of the incident light. Then 

/(«, Aco) = /(co, - A«)e-*A-/r (92.3) 

In this approximation I on each side of the equation refers to the same 
frequency of the incident light. In other words, the relation (92.3) gives a 
simple relation between Stokes and anti- Stokes scattering of the same light 
with the same magnitude of the frequency change Aco. 

§93. Scattering with small change of frequency 

The theory given in §91 is entirely general, and is applicable to all cases 
of scattering in an isotropic medium, whatever the mechanism of scattering. 
Such a general discussion, of course, cannot proceed very far, and a further 
investigation of the phenomenon of scattering requires some restrictive 
assumptions. 

In most practical cases the scattering of light involves only a relatively 
small change in frequency, Aco = to' -co. The calculations given below 
pertain to this case. Besides the condition A to <^ co, we shall suppose that 
the relative change in the refractive index of the medium over the frequency 
range Aco is small. This condition means that the frequency <o must riot 
lie close to a range in which the scattering medium is also absorbing. 

If o) is in the optical range, the microscopic mechanism of scattering with 
small Aco may involve various kinds of motion of atoms and molecules (as 
opposed to the purely electronic motions which give rise to optical transi- 
tions), including intramolecular vibrations of atoms, rotations or vibrations 
of molecules, etc. 

Let q = q(t) denote the set of co-ordinates describing the motion which 
causes the scattering, f Since this motion is relatively slow, the macroscopic 
description of the motion can be regarded from a different standpoint by 
introducing the dielectric permeability tensor € ik (q), whose components at 
any instant depend only on the values of the co-ordinates q at that instant as 
parameters. This property follows from the assumed smallness of the 
relative change in e. The dielectric permeability thus defined pertains to 
the field averaged with respect to the electron motion for a given position of 



t For simplicity, we shall give a classical discussion. The results are actually still valid 
when quantum mechanics is used to describe the motion of the nuclei. 



386 Scattering of Electromagnetic Waves §93 

the nuclei. When the averaging of the field with respect to the motion of 
the nuclei is carried out, the dielectric permeability reduces to the scalar 
€(a>). Let the deviation of e^ from this value be Se^: 

ea(q) = At* + 8e ik (q). (93.1) 

The tensor e ik gives the relation between the field and the induction as 
functions of time. It should be emphasised that the incident wave is still 
assumed to have a single frequency a), but the field E' in the scattered wave 
is now regarded as a function of time, not resolved into single-frequency 
components. The total field consists of the field E in the incident wave and 
the field E' in the scattered wave. Thus Dt + D\ = e ik (Ek + E'jc). Cancelling 
Di = cEi and omitting the second-order term he^E'^ we obtain 

D'i = cE't + 8e ik (q)E k . (93.2) 

The relation (93.2) is of the same form as (91.2). There is a difference, 
however, in that with this approach it is clear that the tensor km = Sea is 
symmetrical. This follows at once from the general theorem concerning the 
symmetry of the dielectric permeability tensor. Furthermore, since this 
tensor is real for a transparent medium, the tensor 8e^ is also real. 

Since the tensor cum has no antisymmetrical part, there is no antisym- 
metrical scattering (§91) with small change in frequency. 

Let us calculate the total scattered intensity with all frequency changes 
A to <^ o>. This can easily be done as follows. In equation (91.3) for the 
field in the scattered wave we can replace k' by k = ofy/ejc (and take the 
value of a for a/ = a>) ; this equation does not then involve a/, i.e. it is the 
same for every component of the spectral resolution of the field. The equa- 
tion is therefore valid for the unresolved field in the scattered wave, which 
we shall denote by the same letter E'. Using the solution (91.6), we obtain 

A 4 — w 4 — 

lE'l 2 = |G| 2 sin2 = |G2sin 2 0, 

where 6 is the angle between k and G, and the bar denotes, as in §91, the 
final average with respect to the motion of the particles (i.e. with respect to 
the time dependence of q). 

We define the extinction coefficient h as the ratio of the total intensity of 
light scattered in all directions per unit volume of the scattering medium to 
the incident flux density :f 

|Gp 
h = 



Ljww- 



F|E|2J ' ' 6ttc*V |E|2 



t This definition differs by a factor ai'lm from the general definition (in terms of the 
number of scattered quanta) given in §92. In the present case this factor may be taken as 
unity, and the two definitions are equivalent. 



§94 Rayleigh scattering in gases and liquids 387 

As we have seen in §91, in calculating the mean value |G[ 2 we can replace 
the exponential factor in the integrand in G by unity, so that 

[Gp = EotEm? j Sen dV j SeikdV. 

The expression to be averaged is a tensor of rank two and, since the medium 
is isotropic, the result of the averaging is 

J 8e U dV j" Se lk dV = i8«( j Bem.dV) . 

Thus we have finally 

h = ^LU\8e lm dv) 2 , (93.3) 



or 



h = (o>4/1877c4)F(S^ m )r 2 , ( 93 - 4 ) 

where the suffix V denotes an averaging over the volume V. 

The mean value of the squared integral can be written as the mean value of 
a double integral, and is found to be proportional to the volume V (cf. §91). 
Hence the value of the extinction coefficient is independent of the scattering 
volume, as it should be, and also of the polarisation of the incident light. 

Formula (93.4) can be regarded in the following way. We can say formally 
that scattering would not occur in a completely homogeneous medium (i.e. 
one whose dielectric permeability is exactly constant). The scattering can be 
macroscopically described as resulting from inhomogeneities in the medium. 
The variation of these inhomogeneities with time, when resolved into spectral 
components, gives the change in frequency of the light when it is scattered. 

§94. Rayleigh scattering in gases and liquids 

Two types of scattering can be distinguished, depending on the change in 
frequency of the light: (1) combination scattering, which is the Raman- 
Landsberg-MandeVshtam effect and results in the appearance in the scattered 
light of lines whose frequency differs from that of the incident light, (2) 
Rayleigh scattering, in which the frequency is essentially unchanged. 

Combination scattering in gases results from a change, due to the incident 
light, in the vibrational, rotational or electronic state of the molecule, f 
Rayleigh scattering, on the other hand, does not involve a change in the 
internal state of the molecule. In the limiting case of a rarefied gas, when 
the mean free path / of the molecules is large compared with the wave- 
length A of the light, scattering takes place independently at each molecule, 
and can be discussed microscopically, using quantum mechanics. 



t Under ordinary observational conditions, electronic transitions are unimportant. 



388 Scattering of Electromagnetic Waves §94 

Here we shall discuss the opposite limiting case, where / < A, f and the 
Rayleigh scattering in gases can be divided into two parts. One part is due 
to irregularities in the orientation of the molecules (called fluctuations of 
anisotropy). The other part is scattering by fluctuations in the gas density. 
The orientation of the molecules is entirely changed by a few collisions, i.e. 
after a time of the order of the mean free time t. Hence the scattering by 
fluctuations of anisotropy results in the appearance of a relatively broad line 
with its peak at a/ = w and width ~ h/r. The scattering by fluctuations of 
density gives a much sharper line superposed on the other. As we shall see 
below, fluctuations of density in volumes ~ A 3 are of importance in the 
scattering of light with wavelength A. Since these volumes are large, the 
fluctuations in them occur comparatively slowly, and so the scattered line 
is narrow. In what follows we shall regard this sharp line as being undis- 
placed. 

The scattering by density fluctuations is scalar scattering (see the end of 
§91): since the density p is a scalar, so is the change in the dielectric perme- 
ability Se resulting from a change in p. The change in the dielectric 
permeability in fluctuations of anisotropy, on the other hand, is described by 
a symmetrical tensor Sew with zero trace. The latter property follows from 
the fact that the effect must vanish on averaging over all directions. Thus 
the scattering by anisotropy fluctuations is symmetrical scattering. 

In liquids the situation is less simple. Combination scattering can arise 
only from a change in the vibrational or electronic state of the molecule; 
rotational combination lines do not occur for scattering in liquids. The 
reason is that, because of the strong interaction between molecules in a 
liquid, they cannot rotate freely so as to acquire discrete rotational energy 
levels. The rotation of the molecules, therefore, like any motion in which 
their relative position changes, contributes in a liquid only to the relatively 
broad scattering line at <o' = w, which in this case may be regarded as the 
effect of Rayleigh scattering. The relaxation time of such motions depends 
on the viscosity of the liquid. 

The possibility of separating from the total Rayleigh scattering in a liquid 
a part due to thermodynamic fluctuations (of density or temperature) depends 
on the magnitudes of the various relaxation times. It is necessary that the 
relaxation times of all processes of establishment of equilibrium in the liquid 
should be small in comparison with the times characterising the fluctuations 
concerned. In this case a narrow "undisplaced" line and a broader one are 
observed. The undisplaced line corresponds to scalar scattering. The 
broader background, however, does not in general correspond, as it does in 
gases, to purely symmetrical scattering with no scalar part. 

The total intensity of the undisplaced line is easily calculated by means of 



t More precisely, the necessary condition is / <^ A sin i&, where & is the scattering angle. 
This is because the expression (94.4) which gives the scattered intensity involves the fre- 
quency only in the expression q = (2cd/c) sin i&. 



§94 Rayleigh scattering in gases and liquids 389 

the general formula (93.4). For scalar scattering Be ik = Se8 ifc , and the 
extinction coefficient is therefore 

h . -?Lv&y. (94.D 

677c 4 
If 8 P and ST are the changes in density and temperature, then 

be = (ae/0/>)r8p + {dejdT) p hT. 

The fluctuations of density and temperature are statistically independent 
(EfSp = 0), and their mean squares are 

(MV = Tt/pCvV, (W = (TplV)(8pldp) T , 
where c v is the specific heat per unit mass. Thus we have finally 

This formula was first derived by A. Einstein (1910). 

For gases formula (94.2) becomes much simpler. The dielectric perme- 
ability of a gas (at optical frequencies) is almost independent of temperature, 
and hence the second term in the brackets can be neglected. The density 
dependence is that e- 1 is proportional to p, and hence 

p(de/dp) T « e - 1 « 2(w - 1), 

where n = V € is th e refractive index. Since, from the equation of state of a 
perfect gas, (l/p)(8pldp) T = 1/NT, where N is the number of particles in 
unit volume, we find that 

h = 2a>*(n - \flZTTC*N. (94.3) 

This formula was first derived by Rayleigh (1881). 

Let us now examine the fine structure of the undisplaced line. This 
requires a consideration of the time variation of the fluctuations. In this 
respect, thermodynamic fluctuations fall into two classes.t Adiabatic fluctua- 
tions of pressure in a fluid are propagated as undamped waves with the 
velocity of sound u\ we here neglect the absorption of sound, since it causes 
only a broadening of the line (see below). Fluctuations of entropy at constant 
pressure, however, are not propagated relative to the fluid, and are damped 
only gradually as a result of thermal conduction. 

The time variation of the intensity (not averaged with respect to time) is 
given by the squared modulus of the integral 

G(*) = J* §€(*). exp(- fq.r)dF.Eo, (94.4) 



t See Statistical Physics, §111, Pergamon Press, London, 1958. 
X See Fluid Mechanics, §79, Pergamon Press, London, 1959. 



390 Scattering of Electromagnetic Waves §94 

in which he is regarded as a function of time. In order to determine the shape 
of the scattering line, G(t) must be resolved into spectral components (i.e. 
Se(t) must be so resolved); the distribution of intensity as a function of Aco 
will then be given by the squared modulus of the component G Aw . However, 
the factor exp( — /q»r) in (94.4) cannot be replaced by unity, as we have 
done hitherto. The reason is that the quantity |G A J 2 depends markedly on 
the correlation of the time variation of the fluctuations at different points in 
space. This is clear when |G A J 2 is written as a double integral 

j j 8e(t)8e(f)exp[- /q-(r - r')] exp[*Aeo(* - t')]dVdV dtdt'. 

On account of the wave propagation of sound disturbances, the time variation 
of pressure fluctuation is correlated even at great distances. This fact was of 
no importance in determining the total intensity of the line, which is obtained 
by averaging the square |G(?)| 2 with respect to time; since, in this case, 
G(t) and G*(t) are taken at the same instant, it follows that only the correla- 
tion between the values of Sc at different points at the same instant is of 
importance, and this correlation extends only over short distances. 

Let us first consider the changes Se which result from pressure fluctuations. 
The quantity (94.4) is the Fourier space component of the fluctuation he 
whose wave vector is q; its time dependence is given by e-^ Aw , where 
Aw = ±qu. Since to « a/, we have q = |k' — k| = (2cu/c) sin-|&, where & 
is the angle between k and k'. If the corresponding value of Aw is denoted 
by Actio, then 

Ao>o = qu = ± (2cou/c) sin^&. (94.5) 

Thus the scattering by pressure fluctuations results in the appearance of a 
doublet (called the MandeV shtam-Brillouin doublet), the distance 2Atuo 
between whose components depends on the angle of scattering. 

The fluctuations of entropy have zero frequency, as stated above, and so 
scattering by them gives a central line with A to = 0. 

Let us determine the intensities of the doublet and the central line. The 
total intensity of the undisplaced line is given by formula (94.2), so that it is 
sufficient to determine, say, Jdoubiet/itotai (where /doublet is the combined 
intensity of the two components of the doublet, i.e. twice the intensity of each 
component f). Since the doublet lines are due to scattering by adiabatic pres- 
sure fluctuations, their intensity is given by the mean square (de/dp)s 2 (8p)v 2 - 

Using the formula for adiabatic pressure fluctuations and a simple trans- 
formation by means of the formula for the ratio of adiabatic and isothermal 



f The difference between the intensities of the two components is, according to formula 
(92.3), usually negligible, since /jAwo <C T. 



§94 Rayleigh scattering in gases and liquids 391 

compressibilities, we obtain 



/d€\ 2 pT/dp\ /a € \a 

V \8p)s\dp)s 

Vc v \dp) T \dpJ s 

The adiabatic derivative {d€Jdp) s can be expressed in terms of more con- 
venient quantities by transforming it to the variables p and T: 
{d€ j dp ) s = {deldp) T + {Tlc vP *){dpldT) p {d € ldT) p . 

The required ratio of intensities is given by the ratio of (94.6) to the mean 
square total fluctuation (the expression in brackets in (94.2)). We shall not 
give the cumbersome general formula, but only the simpler form obtained 
when the temperature dependence of e is neglected : 

/*dout)let//*total = C v \c v ("4.7) 

(L. Landau and G. Placzek, 1933). 

To determine the shape of the lines, it is necessary to consider the dissipa- 
tive processes which result in the "decay" of the fluctuations. These 
processes cause a damping of the fluctuation amplitude as «-?', where y is 
a definite constant. If the "eigenfrequency" of the oscillations is Aa> , the 
total time dependence is given by e-W A «*>+y). The intensity distribution in 
the line is proportional to the squared moduli of the Fourier components of 
this factor, i.e. 

d/ = — dAc, (94.8) 

it (Aco - Ao; ) 2 + y 2 

where 7 is the total intensity of the line. This is called the dispersion form 
of the line. The "width" is y. 

According to formulae derived in the theory of absorption of sound, f the 
damping coefficient for sound fluctuations with wave vector q is 

where 77, £ are the viscosity coefficients of the fluid and k its thermal conduc- 
tivity. Substituting q 2 = 2(<o/c) 2 (l-cos &), we obtain the following ex- 
pression for the width of the doublet components : 

y = -^(1 - cos *) [1 + £ + *(1 - 1)1 . (94.9) 

pc 2 L3 \c v Cp/ J 



t See Fluid Mechanics, §77. 
26 



392 Scattering of Electromagnetic Waves §94 

The damping of isobaric fluctuations of entropy (and therefore of tempera- 
ture) is determined by the heat-conduction equation dTjdt = xAT, where x 
is the thermometric conductivity. For fluctuations with wave vector q (i.e. 
spatial variation as exp(*q»r)), we therefore have 

y = xq 2 = 2x(w 2 /<: 2 )(l - cos 9). (94.10) 

The shape of the central line is given by (94.8) with Awo= 0, the width y 
being (94.10). 

As already mentioned at the beginning of this section, the above theory is 
applicable to scattering in a liquid if all the relaxation times in it are small 
compared with those characterising the fluctuations. It should be borne in 
mind that, in any liquid, there are relaxation times of various orders of 
magnitude. The most rapid relaxation process, apparently, is the "decay" of 
elastic stresses in the liquid. The corresponding Maxzoellian relaxation time 
is r M ~ r)/G, where G is the modulus of rigidity, f The reorientation of the 
molecules, i.e. the "decay" of the anisotropy fluctuations, takes place less 
rapidly. The corresponding Debye relaxation time is r D ~ 7)a 3 /kT, where a 
is the dimension of the molecule; the difference between r M and r D is 
particularly large in liquids with large molecules. Finally, various other 
slow relaxation processes leading to the dispersion of sound are also possible 
(e.g. chemical reactions, slow transfer of energy to vibrational degrees of 
freedom of the molecule). The important processes as regards scattering are 
those for which 1/t is comparable with the frequency of the "sound" distur- 
bances which cause the scattering. There is as yet no complete survey of all 
the possible cases, and we shall not give one here, but merely mention that, 
when the viscosity of the liquid is sufficiently high, and so 

tm > 1/<Z M ~ c/cjdu sin^&, 

the liquid behaves as an amorphous solid with respect to the scattering of 
light. 

Finally, we may note an unusual type of scattering which occurs at the free 
surface of a liquid. The fluctuations have the result that this surface is no 
longer perfectly plane, and the consequent "roughness" causes a partial 
scattering of the light reflected from it (L. I. Mandel'shtam, 1913)4 

PROBLEM 

Light is scattered in a gas whose molecules are linear, with polarisabilities a, and <x ± 
along and across the axis respectively. Determine the intensity resulting from the various 
types of scattering. 

Solution. The total intensity of scattered light (for given vibrational and electronic 
states of the molecules) includes the Rayleigh scattering and the rotational part of the com- 
bination scattering. Since the scattering takes place at the individual molecules of the gas, 



t See Theory of Elasticity, §31, Pergamon Press, London, 1959. 

t See L. I. Mandel'shtam, Annalen der Physik 41, 609, 1913, where a calculation is given 
for light scattered in the plane of incidence. 



§95 Critical opalescence 393 

the total extinction coefficient is most simply obtained from formula (72.3), by multiplying 
by the number of particles per unit volume N and replacing the squared polarisability by 
Ja«2 = £(«. 2 + 2a x 2): 

A = ^ ai * + 20. (1) 

9c* 

The undisplaced Rayleigh line is due to the scalar part of the polarisability, i.e. it is the 
same as if the polarisability tensor of the molecule were ianSi*. The same formula, (72.3), 
therefore gives 

Aundisp = *" («i+2a x ) 2 . (2) 

27c* 

The difference Motai— Aundisp includes the "background" (scattering by anisotropy fluctua- 
tions) and the rotational combination scattering. In order to separate the former, we must 
first average the polarisability tensor of the molecule with respect to rotation about some 
particular axis (perpendicular to the axis of the molecule). The polarisability along the axis 
of rotation averaged in this way is evidently oc ± , and that along any direction in a plane 
perpendicular to the axis of rotation is l( a x+ a i)- In other words, a molecule rotating 
about a given axis is to be regarded as a particle for which the principal values of the polaris- 
ability tensor are a ± , K a x+ a i)> i( a x+ a i)- Using these, [we calculate the symmetrical 
tensor oc«r— $olu8uc, whose trace is zero, and then a procedure similar to the derivation of 
formulae (1) and (2) gives 

8™*N (ax-«.) 2 ,,, 

Abackg - —^ £ • V) 

Finally, the intensity of the rotation combination scattering is obtained by subtracting (2) 
and (3) from (1): 

Zmo*N (tt x -a,) 2 

Acombin — — — 2 * 

§95. Critical opalescence 

The isothermal compressibility (dp/dp)T increases without limit as the 
critical state is approached. The expression (94.2) for the total intensity due 
to scalar Rayleigh scattering therefore increases also. This indicates a marked 
increase in scattering near the critical point, called critical opalescence, f The 
formula (94.2) itself is, however, inapplicable, because the expressions for 
the thermodynamic fluctuations used in its derivation are no longer correct. 

The increase in intensity does not take place for all three components of 
the fine structure of the Rayleigh line, but only for the central component. 
According to (94.2) and (94.7), the intensity of the doublet is 

c p \dp/T\dp/T 



"doublet = 



67TC 4 

The thermodynamic formula 

Cp C<Q = 



T (dp/dT)/ 
p 2 {dp\dp) T 



t A similar phenomenon occurs for scattering in a solid near the critical point of a second- 
order phase transition. It has been discussed by V. L. Ginzburg, Doklady Akademii Nauk 
SSSR 105, 240, 1955. 



394 Scattering of Electromagnetic Waves §95 

gives near the critical point 

" doubiet = ^jm&WT - (95 - 1} 

As we shall see below, the factor exp( — /q»r) in (94.4) cannot be replaced 
by unity near the critical point, even in calculating the total scattered 
intensity. Let dh be the differential extinction coefficient, relating to scatter- 
ing into a given solid angle do (corresponding to a given value of q = k — k'). 
Considering, for definiteness, the scattering of unpolarised light, and using 
the result that the angular dependence (for scalar scattering) is given by the 
expression f(l +cos 2 -8-), we have 



dh = 



A 1 



6ttc4 V 



J Seexp(— iq>r)dV 



2 do' 

. f(l + cos 2 a-) . (95.2) 

4?r 



Near the critical point, the density fluctuations increase but the tem- 
perature fluctuations remain finite. It is therefore sufficient to consider 
8e — (de/dpJTSp, so that 



o> 4 /8e\2 1 I , 



2 A ' 

.|(1 + cos 2 vJL. (95.3) 

477 



According to the theory of fluctuations, the mean square density fluctua- 
tion near the critical point can be expressed in terms of the coefficients a 
and b in the formula 

F - F = |a(o»2 + iftgrad op) 2 , (95.4) 

where F is the free energy per unit volume, f 

This formula gives the leading terms in an expansion of the change in the 
free energy in powers of 8p and of its gradient; the latter has to be taken into 
account because of the amplification of local inhomogeneities in the body 
near the critical point. The constant a is expressed in terms of the ordinary 
thermodynamic quantities byt 

a = (l/p)(dpl8p) T . (95.5) 

The mean square in (95.3) can be expressed in terms of a and b by 



|Spexp(-/q.r)dr = VT/(a + btf). (95.6) 



f See Statistical Physics, §116. 

% The derivative {dFjdp)T is the thermodynamic potential per unit mass, and the second 
derivative is therefore a = (8 2 F[8p 2 )t = (3$/3p)r = (l/p)(S/)/3p)2'. 



§96 Scattering in amorphous solids 395 

Substituting in (95.3), we obtain the final result 

ih = _^(t\* 1 + C °f 9 do'. (95.7) 

p\dp/T & 

This formula was first derived by L. S. Ornstein and F. Zernike (1914). 
When the angle & is not small, the first term in the denominator may be 
neglected, and 

a? /0e\2 l + cos2& 

dA = — I do • (95.8) 

647T 2 c%\dp/ T 1 -cos& 

The total intensity scattered in all directions is obtained by integrating 
(95.7) with respect to o. When (dp/8p) T = 0, i.e. at the critical point, the 
integral is logarithmically divergent for small angles. In reality, the integra- 
tion should be extended only to angles of the order of the diffraction angle 
( ~ A/L, where L is the dimension of the body). The total intensity therefore 
depends logarithmically on the dimension of the scattering body. 

§96. Scattering in amorphous solids 

Rayleigh scattering in amorphous solids differs considerably from that in 
fluids. In an isotropic solid there are two velocities of propagation of 
sound, ui (longitudinal) and u t (transverse). The fine structure of the 
Rayleigh line therefore includes not one but two Mandel'shtam-Brillouin 
doublets. They are due to scattering by transverse and longitudinal "sound 
waves", and their distances from the centre of the line are respectively 
±Ao>j, ±A(o t , where A^ = qui, A<*>t = qut. Since ui > ut, it follows that 
Awi > Aw*. The central component of the line is again due to fluctuations 
which are not propagated relative to the medium. In this case the main 
fluctuations of the latter type are those of structure. In an amorphous body, 
where the atoms are not arranged in an ordered manner, these fluctuations 
are comparatively large and vary only slowly with time (on account of the 
extreme slowness of the diffusion processes in a solid). Scattering by these 
fluctuations leads to a strong line whose width is almost zero. As regards 
polarisation and angular distribution, this line results from a superposition 
of scalar and symmetrical scattering. 

Next, let us consider the doublet components of the Rayleigh line in 
amorphous bodies. Here we cannot put exp( — *q-r) = 1 in the integral G, 
as we did for fluids, even in calculating the total intensity (and polarisation) 
of the scattered light ; moreover, the scattering cannot be classified according 
to dependence on angle as in §91. The reason is that, in a solid, the effect of 
any deformation (in this case, fluctuations) extends to considerable distances. 
Hence the fluctuations at different points in the body at the same instant are 
correlated even at distances large compared with 1/q. 



396 Scattering of Electromagnetic Waves §96 

The field in the scattered wave is 

w 2 exp(j&i? ) 

*---S&r in ' x < n ' xG >' (9U) 

where 

Gt = J* Se^ exp( - iq . r) d V . E ok , (96.2) 

and n' is a unit vector in the direction of scattering. The change in the 
dielectric permeability resulting from the deformation of an isotropic body is 

<W = aiunc + azuuSijc, (96.3) 

where ui k is the strain tensor (see (81.1)). Since the integral (96.2) isolates 
from Setjc the Fourier space component with wave vector q, ut k in (96.3) 
must be taken as the deformation in a sound wave with this wave vector. We 
therefore write the displacement vector as 

u = re{uoexp(/q.r)} = £[uoexp(/q-r) + uo*exp(- *q«r)], (96.4) 
whence the strain tensor is 

1 idui 8ujc\ 
2\dx k Bxil 
= re{fy(u iq k + woA#i)exp(zq.r)}, 
and the volume integral is 

j u ik exp( - iq • r) d V = liV{uQiq k + u 0k qi). (96.5) 

Let us first consider scattering by transverse "sound" waves. Since in a 
transverse wave u is perpendicular to q, and uu = 0, 8e ik = a\Ui k . Using 
(96.5), we therefore have 

G = i*rai{uo(q-Eo) + q(uo-E )}. (96.6) 

A transverse sound wave can have two independent directions of polari- 
sation: the vector u may be in the plane of k and k', or perpendicular to 
that plane. Since E is perpendicular to k, it is easy to see that in the first 
case the component of G in the plane perpendicular to k' is zero. Thus 
transverse sound waves "polarised" in the plane of k and k' do not scatter 
light. 

If the vector u is perpendicular to the plane of k and k', a simple calcu- 
lation, using (96.1) and (96.6), gives for the field in the scattered wave 

a) 2 exp (ikRo) .„ , „ 

E\ = F „ V . \axiVquo cos \* . E ±y 

^ RoC (96.7) 

oj 2 exp (ikRo) . r „ 

E' = L± -.lanVquocosl&.Et. 

AttRoC 2 - 



§96 Scattering in amorphous solids 397 

Here & is, as usual, the angle between k and k', and the suffixes || and J_ 
denote components in the plane of scattering and perpendicular to that 
plane. The coefficient of proportionality in these two formulae involves the 
same fluctuation mo. This means that no depolarisation occurs on scattering: 
linearly polarised light remains so (though it is polarised in a different 
plane). 

Since the coefficients in formulae (96.7) are exactly the same, the extinc- 
tion coefficient dh does not depend on the state of polarisation of the incident 
light, and is 

dn = iSH^\ FSpcosH^-do. (96.8) 

\ 167TC 2 / 

It remains to determine the mean square amplitude of the fluctuation uq. 

From the point of view of the general theory of thermodynamic fluctuations, 
the sound wave (96.4) may be regarded as a combination of two classical 
oscillators (waves propagated to the right and to the left), each having a 
mean kinetic energy %T. Since the frequency of the oscillations is here 
Aw = quu the m.?an kinetic energy is $Vpu 2 = \Vp{utqf\iio\ 2 . Equating 
this to 2 . %T, we have 

|^j2 = iTJVpuPq 2 . (96.9) 

Finally, substituting (96.9) in (96.8), we obtain 

dh = — cos*i&do. (96.10) 

The angular dependence of the scattering is totally different from that which 
occurs in fluids. 

Let us now consider scattering by longitudinal "sound" waves. In these 
waves u is parallel to q, and from (96.3) and (96.4) we find 

n x .„ / q(q-Eo) , v \ 
G = fyVuoqlai — + a 2 E [. 

A simple calculation gives for the field in the scattered wave 



to 2 exp (ikRo) 

co 2 exp (ikRo) 

E i = — a n o -hiVuoqtkai + (z a i + fl 2) cos &]E t . 

'hrRoC 2 



(96.11) 



In this case also there is no depolarisation on scattering. The angular dis- 
tribution and the extinction coefficient, however, depend on the state and 
direction of the polarisation of the incident light. We shall not pause to 
write out the relevant formulae, which are somewhat cumbersome. The 
calculations are wholly similar to those given above, and the expression for 
|mq| 2 differs only in that ut is replaced by ui in (96.9). 



CHAPTER XV 

DIFFRACTION OF X-RAYS IN CRYSTALS 

§97. The general theory of X-ray diffraction 

The phenomenon of X-ray diffraction in crystals occupies a special place 
in the electrodynamics of matter, since the wavelengths concerned are 
comparable with the distances between atoms. For this reason the usual 
macroscopic approach to matter as a continuous medium is entirely invalid, 
and we must begin by considering scattering by individual charged particles, 
and essentially by electrons ; the scattering by nuclei is unimportant, because 
of their much greater mass. 

The frequencies of the motion of electrons in the atom are of order 
wo ~ v/a, where v is their velocity and a the dimension of the atom. If 
A ~ a, then, since v <^ c, these frequencies are small compared with the 
X-ray frequency a> ~ cjX. This makes it possible to write the equation of 
motion of an electron in the field of the electromagnetic wave as 

mv' = eE, (97.1) 

i.e. the electrons may be regarded as free (see §59). 

From (97. 1) we find the additional velocity acquired by the electron under 
the action of the wave field: v' = ieE/ma). 

Let n(x, y, z) be the number density of electrons in a crystal, averaged 
over the quantum states of the electrons and over the statistical distribution 
of the thermal motion of the nuclei in the lattice. It should be emphasised 
that the usual macroscopic averaging over physically infinitesimal volume 
elements is not included, i.e. n(x, y, z) is the actual density of the "electron 
cloud" in the crystal lattice. The corresponding current density due to the 
wave field is : 

j' = en\' — ie 2 JiE/mco. (97.2) 

We substitute this current in the microscopic Maxwell's equations : 

curlE = iwH/c, (97.3) 

curlH = — icoE/c + 4-ny/c 
iw I ^ne 2 n 

--7 1 



i/ 4ire 2 n\ 

-1 r )E. (97.4) 



We thereby take account of its reciprocal effect on the field, i.e. scattering. 

398 



§97 The general theory of X-ray diffraction 399 

It is, of course, assumed that this effect is small, i.e. that the inequality 

47re%/ma> 2 <^ 1 (97.5) 

holds. Putting D = cE, where 

47re 2 » 

e = 1 -, (97.6) 

mm 1 

in accordance with the usual definition of the induction, we reduce equation 
(97.4) to the usual form curl H = -ioiDjc. Thus, in this sense, the expres- 
sion (97.6) for the dielectric permeability (cf. (59.1)) can be used even for 
wavelengths A ~ a, though it must of course be remembered that the 
symbols E and D no longer retain their previous meanings : they now pertain 
to the field which has not been averaged over physically infinitesimal volumes, 
and e is accordingly a function of the co-ordinates. 

In the scattering of X-rays by heavy atoms it may happen that the condi- 
tion co > coq is fulfilled for the outer electron shells but not for the inner 
ones, where to < coo and so the inequality A > a holds. In this case the 
dielectric permeability can still be regarded as the coefficient of propor- 
tionality between D and E, but the formula corresponding to (97.6) gives 
only the contribution of the outer electrons. That of the inner electrons must 
in principle be calculated by averaging over the volume of their shells. Thus, 
if we put D = eE with e a function of the co-ordinates, all possible cases 
are allowed for. In what follows we shall, for definiteness, use the expression 
(97.6). 

In effecting the averaging of the electron density in (97.2) to obtain 
n(x, y, z) independent of time, we exclude a possible change of frequency on 
scattering. That is, we consider only strictly coherent scattering, with no 
change in frequency. 

Eliminating H from the two equations (97.3) and (97.4), we obtain 
curl curl E = ccFD/c 2 . Here we substitute E = T>+47re 2 nE/mco 2 and ex- 
pand the expression curl curl E, using the fact that div D = 0, as follows 
from (97.4). Then 

AD + u> 2 D/c 2 = curl curl (47re%E/wa> 2 ). (97.7) 

On the right-hand side of this equation, which already contains the small 
quantity 4r7re 2 njmco 2 , E must be taken as the given field of the incident wave. 
Let us find the solution of equation (97.7) in the region outside the scattering 
crystal and at large distances from it. f Since this equation is of the same 
form as equation (91.3), the required solution is obtained immediately by 



t In solving equation (91.3) it was not possible to consider the field outside the body, 
since the boundary conditions on the surface would have had to be taken into account (the 
quantity e' on the left-hand side being different inside and outside the body). The left-hand 
side of equation (97.7), however, is the same in all space. 



400 Diffraction of X-rays in Crystals §97 

analogy with (91.4) : 

? 2 exp (ikRo) 



E = 



men 2 Rq 



k' X (k' X E ) f n exp(- *q.r)dF. (97.8) 



Here R is the distance from the origin, which is within the crystal, to the 
point considered; q = k' — k; k = k' = wfc; Eo is the amplitude of the 
incident wave. We put E instead of D on the left-hand side because the two 
are equal in the vacuum outside the crystal. 

To characterise the intensity of X-ray diffraction we use an effective cross- 
section or, defined as the ratio of the intensity diffracted into a solid angle 
do' to the energy flux density in the incident wave. By (97.8) we have 



da = ( 1 sin 2 nexp(-iq>r)dV 



do', (97.9) 



where 9 is the angle between Eo and k'. If the incident radiation is "natural" 
(not polarised), the factor sin 2 6 in this formula becomes |(l+cos 2 &), 
where & is the angle between k and k' (see the second footnote to §72) : 



1/ e 2 \ 2 f 

da = -( (l + cos 2 &) nexp(-*q.r)dF 

2\mc 2 J J 



do'. (97.10) 



In what follows we shall, for definiteness, consider this particular case. 

We see that the intensity of radiation diffracted in a given direction is 
essentially proportional to the squared modulus of the integral 

jnexp(-iq-r)dV, (97.11) 

i.e. the Fourier space component (with the appropriate value of q) of the 
electron density. As q .-> this integral becomes simply the electron density 
n averaged over a lattice cell. If n is replaced by n in equations (97.3) and 
(97.4), we obtain the usual macroscopic Maxwell's equations, with dielectric 
permeability e(a>) = \-ATte 2 n\ma> 2 . According to these equations, when 
X-rays pass through a crystal they are refracted according to the ordinary 
laws of refraction, with refractive index V € - Thus diffraction through small 
angles amounts to ordinary refraction, which is of no interest here. In what 
follows we shall always assume that q is appreciably different from zero. 

The electron density, like any function of position in a crystal lattice, can 
be expanded as a Fourier series : 

n = S n b exp (2mb • r), (97. 12) 

b 

where the summation is taken over all periods b of the reciprocal lattice, f 



t See Statistical Physics, §132, Pergamon Press, London, 1958. 



§97 The general theory of X-ray diffraction 401 

When (97.12) is substituted in (97.11) and the result is integrated over the 
volume of the crystal, we obtain practically zero except for values of q close 
to some 2-nh. Between these values the intensity is negligible. We can 
therefore consider each diffraction maximum separately, putting 

n = «bexp(27rtb«r) 

with the appropriate value of b. Substitution in (97. 10) gives 

l/e 2 \ 2 

da = - (1 + cos 2 $)» b 2 x 

2\mc 2 / 



x I f exp [- t(k' - k - 27rb).r] dV 



do'. (97.13) 



The strongest maxima occur in directions for which the equation 

k' - k = 2db (97.14) 

(Laue's equation) is exactly satisfied, and are called principal maxima. For 
given b, however, a principal maximum does not occur for an arbitrary 
direction and frequency of the incident radiation. If the equation (97.14) 
is written as k' = k + 2?rb and squared, and we use the fact that k 2 = k' 2 , 
we have 

bk = - 7tb\ (97.15) 

This equation determines the values of the wave vector k for which principal 
maxima occur with the given value of b. Geometrically, equation (97.15) 
represents a plane in k-space perpendicular to the vector b at a distance irb 
from the origin. In particular, we see that k ^ irb. 
Since |k' - k| = 2k sin $&, it follows from (97. 14) that 

ksm%d- = 7rb (97.16) 

(Bragg and Vul'f's equation), which determines the angle of diffraction at 
the principal maximum. 

Any vector b of the reciprocal lattice determines a family of crystal planes 
represented by the equations r»b = constant integer. These planes are 
perpendicular to b, and the vectors k and k' corresponding to the condition 
(97.14) make equal angles of incidence and reflection with the planes (Fig. 
45). For this reason, diffraction at a principal maximum is sometimes spoken 
of as "reflection" from the corresponding crystal planes. 

The total intensity of the diffraction "spot" near a maximum is obtained 
by integrating (97.13) over a solid angle about the direction of k\ Let us 
determine the intensity near a principal maximum. We denote by k'o the 
value of k' corresponding to Laue's equation for a given k: k'o = k + 27rb, 
and put also x = k' — k'o. Near the maximum, x is small; since k' and k'o 



402 



Diffraction of X-rays in Crystals 



§97 



differ only in direction, x is perpendicular to k'o. The solid angle element 
can therefore be written 



do' = dK X dKy/k' 2 = d.K X di<ylk 2 , 
where the sr-axis is taken in the direction of k'o. Thus 



(97.17) 



1 / e 2 ^2 
° ~ 2k 2 [mc* 



)2 
(1 + cos 2 &)|« b | 2 j f dK x d Ky I f exp(- i*..r)dV\ 





Zirb 




/' 


N 




/ 


\ 




/ 


\ 




/ 


\ 




/ 


\ 


/ 




\ 


/ 




\ 


/ 




\ 


/ 




\ 


/ 




\ 


/ 




\ 


^v 




JLr 




\5 


%/ 



Fig. 45 



In the volume integral we can effect the integration with respect to z, since 
exp( — /x»r) is independent of z: j exp(-iycr) dV = /Zexp(— zx»r)d/, 
where d/ = dx dy and Z = Z(x, y) is the length of the body in the direction 
of k'o. Finally, using a well-known formula in the theory of Fourier integrals : 



f \<t> K \*dK X d Ky = — - U 2 dxdy, (97.18) 

J \LttY j 



(2n) 



where 



<l>* 



-&vl«* y) 



exp(— ix*r)dxdy 



are the two-dimensional Fourier components, we obtain 
27T 2 / e 2 \ 2 



= ^(^)V + cos 2 &)|« b | 2 fZ 2 d/ 
k 2 \mc 2 ] J 

= —(—) sin 2 ^(l + cos 2 $)|" b | 2 ( Z 2 df. 
b 2 \mc 2 / J 



(97.19) 



The integral is of the order of Z, 4 , where L is the linear dimension of the body. 
Thus the total effective cross-section, and therefore the total intensity of the 
spot, are proportional to F 4/3 , where V is the volume of the body. The 
maximum intensity, however, follows a different law. For k' — k = 2rrb, 



§97 The general theory of X-ray diffraction 403 

the integral in (97.13) is just V, and so da is proportional to V 2 : 

(—) = U—)\l + cos* &)\n h \*V*. (97.20) 

\do'/max 2\mc 2 / 

The sharpness of the maximum is shown by the fact that the maximum 
intensity is proportional to a higher power of V than the total intensity. The 
"width" of the peak is evidently proportional to V 4/3 fV 2 = V~ 2/3 . 

The theory given above is valid only if the diffraction effect is small. We 
now see that this requirement imposes a certain condition on the dimension 
of the crystal: a must be small compared with the geometrical cross-section 
of the body ( ~ L 2 ), whence 

<? 2 L 
— T |«b| <^ 1. (97.21) 

PROBLEMS 
Problem 1. Determine the intensity distribution in the diffraction spot round a principal 
maximum in diffraction by a crystal in the form of a cuboid of sides L x , L y , L z . 

Solution. As above, we use the vector X = k'— k'o, and take the axes of co-ordinates 
parallel to the sides of the cuboid, with the origin at its centre. 

The integral Jexp(— t'X«r) dV becomes a product of three integrals of the form 

*£* 2 

exp(— Ik x x) dx — — sin \kxL x . 

-L* 



Thus 



/ e 2 \ 2 1 

da = 321 — -1 (1 + cos 2 &)|« b | 2 — - — - — - sin 2 \k x L x sin 2 \k v L v sin 2 \kzL z do'. 

XmC*/ K x i K V i Kz i 



K x "Ky'K Z 

The components of the vector x are not independent, being related by the condition 
x-k'o = 0. 

Problem 2. The same as Problem 1, but for diffraction by a spherical crystal of radius a. 

Solution. We again put x = k'— k'o, and take the sr-axis in the direction of x, with 
the origin at the centre of the sphere. Then 

a 
J exp(— i/<z) dV = j" 7r(a 2 — z 2 ) exp(— Ikz) dz 

—a 

= — r(sin Ka — ko cos ko). 

Thus 

/ e 2 \ 2 1 

da = Sir 2 \ — -) (1 + cos 2 *)|mJ 2 — (sin ko — ko cos ko) 2 do'. 
\mcv " k 6 

Problem 3. Determine the total intensity of the diffraction spot round a subsidiary 
maximum. 

Solution. In this case the wave vector k of the incident wave does not satisfy the condi- 
tion (97.15). As shown above (97.15) is the equation of a plane perpendicular to the vector 
b. Let the small displacement of the terminus of the vector k from this plane be i?b, where 
i) <^ 1. That is, we put k = ko+i?b, where ko satisfies equation (97.15) (Fig. 46). 



404 



Diffraction of X-rays in Crystals 



§98 



The maximum intensity in the spot occurs for a direction of k' for which the difference 
k'— (k+27rb) has its least magnitude (so that the integral in (97.13) has its maximum 
value). The magnitude of the difference of two vectors, one of which is arbitrary in direc- 
tion, has its least value when they are parallel. Hence, since k' — k, we have 

|k'-k-2db| m m = £-|k+2i7b| 
# s -(k+2db) 2 




Since k is close to ko and we are considering the region near the maximum, k' ^ k+2wb 
and the denominator can be replaced by 2k. In the numerator, we expand the squared 
parenthesis and obtain 

-2k-2irb-(2irb) 2 = [-2ko-2»rb-(2wb) 2 ]-2 1? b-27rb = -4i7ij& 2 . 

Thus |k'-k-27rb|min ^ -2nr)b*lk. 
Next, we put 

2itrjtP\ 



k' = (k+27rb)(l- -^-Wk, 



and take the sr-axis in the direction of k+27rb. This reduces the problem to the calculation 
of the integral (cf. the derivation of formula (97.19)) 



J $dK X d* y \j exp{2mr)b 2 /k-z-Wr} dV\ 2 

ff, j If , .„ sin (-n^ZIk) I 
= J J d^d^J « P (-,x.r) ^^ d/| 



Finally, using formula (97.18), we obtain 



— Sirb) <1+C0,W)W, J 



sin 2 (7rr)b 2 Z/k) 



d/. 



(Trr)b 2 lk)* 

As i) -> this formula becomes (97.19). If 7n?6 2 Z/&> 1 (which is compatible with t\ < 1), 
the squared sine can be replaced by its mean value \, and we have 



/ e 2 \ 2 l+cos 2 £ 






K\ 2 S, 



where S is the area of the "shadow", i.e. the projection of the body on the xy-plane. 



§98. The integral intensity 

The formulae derived in §97 give the diffracted intensity when a plane 
wave of a single frequency is incident on a crystal. Let us now consider 
some cases where these conditions are not fulfilled. 



§98 The integral intensity 405 

First, let the incident wave be plane but not of a single frequency,! its 
spectral resolution including waves with wave vectors k whose directions are 
the same but whose magnitudes k = wjc are not. Let p(k) be the frequency 
distribution of the incident radiation intensity, normalised by the condition 
j P (k) dk=h 

The total intensity of the diffraction spot is determined by the effective 
cross-section, which is obtained by multiplying the expression (97.13) by 
p(k) and integrating with respect to d and k : 

a = 2 W/ l%|2 J J I J 6XP C ~ ^ " k " 2db) ' r] dV \ X 

x (1 + cos2&) P (£)do'd&. (98.1) 

We put temporarily K = k' — k — 2irb and write the squared modulus as a 
double integral: 

| Jexp(- JK.r)dF|2 = j Jexp[tK.(r 2 - ri)]dFidF 2 . 

Using instead of ri and r 2 the variables |(ri + r 2 ) and r = r 2 — riand integrat- 
ing with respect to the first gives | Jexp(iK«r) dV\ 2 = V Jexp(jK-r) dV. In 
the remaining integral we can effect the integration over all space, J and the 
result is 

| J exp (tK-r) dF|2 = (2tt)3F8(K). (98.2) 

Substituting this result in (98.1), we obtain 

/ e 2 \ 2 , 

a = 4tt3 |« b \W{\ + cos 2 Sx>) x 

\mc 2 / ' 

x J J S(k' - k - 27rb)/)(A) do' d& ; (98.3) 

on account of the presence of the delta function, the factor 1 + cos 2 & in the 
integrand can be replaced by its value at & = &q, where &o is the angle 
between the k and k' which satisfy Laue's condition (denoted by ko and 
k' = k + 27rb). 

The integration with respect to d can be carried out by noticing that it 
is equivalent to an integration with respect to 

dk' = k'*dk'do' = ££'d(A'2)do\ 



f Corresponding to Laue's method in the X-ray analysis of crystals. 

% This is possible because we require only the total intensity of the diffraction spot, and 
not its width. 



406 Diffraction of X-rays in Crystals §98 

if an additional factor (2/k)S(k' 2 -k 2 ) is included in the integrand. Thus the 
integral in (98.3) becomes 



// 



2 

-8(k' - k - 27rb)S(£' 2 - &)p(k)dk'dk. 

k 



Effecting the integration with respect to k' by means of the first delta func- 
tion, we can replace k' 2 by (k + 2rrb) 2 in the second delta function, and the 
result is 

f -3(4,^2 + 4nh.k) P (k) dk = f S(b-k + irW)p{k) dk, 

so that 

a = 2tt4^—\ \n*\W{\ + cos 2 # ) f -o(b-k + 7rb 2 ) P (k)dk. (98.4) 

\mc 2 / J k 

Finally, we have to carry out the integration over k (the direction 
n = k/k being given). The argument of the delta function is zero for 
k = ko, and the integral is p(ko)/ko\b*n\ = p(ko)/\b'ko\ = p(ko)J7rb 2 . Thus 

a = 2tt(—) \m\W(l + cos 2 & )p(koW. (98.5) 

Let us now consider another case, where the incident wave is of a single 
frequency but its components have varying directions of k which differ by 
rotation about some axis;f let 1 be a unit vector along that axis, and «/rthe 
angle of rotation about it. Let p(ip) be the angular distribution of the incident 
radiation intensity, normalised by the condition 



The calculations leading to formula (98.4) are valid in this case also, 
except that the integration with p(k) dk must be replaced by one with 
p(+)d+: a ■ 

a = 2tt 2 (— ) \n h \W(l + cos 2 & ) f -S(b-k + nrb*)p{<p) dtp. (98.6) 

\mc 2 / J k 

We again denote by ko the value of k for which the argument of the delta 
function is zero, and measure \p from the plane of 1 and ko. For small «/», 
k = k + (lxk )^. Then the integral in (98.6) becomes 



I 



-S(b-1 x k ^)/#)d«£ = />(0)/*|b-l X ko| 

k = />(0)/& 2 |b.l x n | 



= p(0)sva*(l$f i )l'n z lfi\b'l X no|. 



t Corresponding to Bragg's method (the rotation method) in X-ray analysis. The rotation 
referred to is that of the crystal about 1, not that of the direction of k. 



§99 Diffuse thermal scattering of X-rays 407 

Thus 

2 / *2 \2 p(0) 

CT = » (— H sinHa (l + cos^o)«b 2 F |u ^ (98.7) 

b 2 \mc 2 ) |b*lxno| 

Finally, let us consider the diffraction of a plane wave, of a single fre- 
quency, from a body consisting of crystallites arranged at random, t 

Let k'o and bo be values of k' and b such that Laue's condition 
k'o = k+27rbo is satisfied. The directions of k'o and bo are not uniquely 
determined, since Laue's condition is, of course, still fulfilled when the 
triangle k, 27rbo, k'o is rotated about the direction of k. Thus the principal 
maximum corresponds to directions of k' occupying a conical surface of 
vertical angle 2&o- Instead of a diffraction "spot" we now have a "ring". 

The required total effective cross-section is determined by a formula which 
differs from (98.4) only in that the integration with p(k)dk is replaced by 
an averaging over the directions of b : 

/ e 2 \2 /• 1 do b 

a = 2tt 2 V\ n b 2(l + cos2& ) -S(b • k + tt6 2 )— -, (98.8) 

\ mc 2 / J k 47T 

where do b is an element of solid angle about the direction of b. Denoting 
by a the angle between k and b, we can write the integral in (98.8) as 



s 



1 27rrfcosa I 1 

-8(bk cosa+7r6 2 ) = = sinH&o- 

k K ' 4tt 2bk 2 2b*ir 2 * 



Each of the three cases considered in this section corresponds to a particular 
method of averaging the diffraction pattern. The dependence of the total 
averaged diffraction intensity on the volume of the body reduces, as we 
should expect, to a simple proportionality. In the pattern which is not 
averaged, the intensity and its distribution over the spot depend more 
markedly on the volume. 

§99. Diffuse thermal scattering of X-rays 

In §§97 and 98 we have taken n(x, y, z) to be the time average electron 
density in the crystal: various density oscillations were thereby excluded, 
and consequently so was the corresponding (non-coherent) scattering of 
X-rays. One cause of non-coherent scattering is the thermal fluctuations of 
density. This scattering is "diffusely" distributed in all directions, but it is 
characterised by a relatively high intensity near directions corresponding to 
the sharp lines of the "structural" scattering described in the preceding 
sections. Here we shall discuss these maxima of the thermal scattering 
(W. H. Zachariasen, 1940). 

The thermal oscillations of the crystal lattice can be represented as com- 
binations of "sound" waves. As we shall see, the maxima of the thermal 
scattering arise from wavelengths large compared with the lattice constant. 



t Corresponding to Debye and Scherrer's method (the powder method) in X-ray analysis. 
27 



408 Diffraction of X-rays in Crystals §99 

The change in the electron density due to such a wave can be regarded, at 
any point, as due to a simple displacement of the lattice by an amount equal 
to the local value of the displacement vector u in the wave. Thus the change 
in the density {not averaged with respect to time) when a given sound wave 
passes can be expressed in terms of the mean density by 

Sn = «(r— u) — n(r) ^ —u-dn/dr. 

In considering diffuse scattering near a given line, we must replace n by 
« b exp(27rtb«r) with the appropriate b, so that 

8n = - 2Wb • u« b exp(27rtb • r). (99.1) 

The scattering by density fluctuations is, of course, not coherent with that 
by the mean density, and the two therefore do not interfere. Hence the 
effective cross-section for diffuse scattering can be obtained from (97.10), 
substituting 8n for n and then carrying out the statistical averaging over 
fluctuations : 

da = 27T 2 ( |« b | 2 (l+ cos 2 #)x 

x | j b • u exp( - iK • r)d V\ 2 do', (99.2) 

where K = k' — k — 27rb. The scattered intensity is large for directions where 
K ^ 27rb. 

The integral j" u exp( — iK-r) dV gives the Fourier space component of u 
whose wave vector is K, and we can therefore take u to be simply the dis- 
placement vector in a sound wave having this wave vector. The inequality 
K <^ 2rrb therefore implies that the wavelength of the scattering sound wave 
is large compared with the dimension of the crystal lattice cell. 

Thus we can put 

u = i[uo exp(*K • r) + uo* exp( - iK • r)], (99.3) 

so that J(b«u) exp( — z*K*r) dV = ^Fb»uo and the effective cross-section is 

2 / 2 \ 2 

da = ^-(-^r-l \n b \ 2 (l + cos2&)bib k ^ k V2do'. (99.4) 

2 \ mc 1 } 

The products of the components of uo are averaged as in §96 for a sound 
wave in an isotropic medium. The elastic energy per unit volume of a 
deformed crystal is WnimUikUim, where uoc is the strain tensor and Xikim the 
elastic modulus tensor, t Hence the mean elastic energy of the whole crystal 
is ^V^ncimiHkUim- We substitute 

1 / dm dujc \ 
2\ dxjc dxi J 
= £re {(iKkUQi + iKiUok) exp(^K • r)}. 



t See Theory of Elasticity, §10, Pergamon Press, London, 1959. 



§99 Diffuse thermal scattering of X-rays 409 

The terms containing exp(±2iK«r) give zero on averaging. Using also the 
symmetry of the tensor Xikim with respect to interchange of i, k, or /, m, or 
i, k and /, m, we obtain iV\ ik i m K k K m uoiiioi* or £Vg ik uoiUok* , where 

gik = hlkmKiK m . (99.5) 

According to the general theory of thermodynamic fluctuations, we can at 
once write down the required mean values : f 



uowojc* = (4T!V)g~i ik , (99.6) 

where g^m is the tensor inverse to gt kl and the effective scattering cross- 
section is 



da = 2n*(—) TV\n b \2(l + cos?d-)bib k g-i ik od'. (99.7) 

\mc 2 } 

Thus the diffusely scattered intensity is, as we should expect, proportional 
to the volume of the crystal. A characteristic feature of this scattering is the 
way in which its intensity is distributed over the area of the spot. Apart 
from the factor 1 + cos 2 &, which is almost constant for a given spot, the 
intensity is given by the expression g'hkhh- This expression is the product 
of 1/K 2 and a fairly involved function of the direction of the vector K with 
respect to the crystal axes. For scattering near a principal maximum the 
diffusely scattered intensity is itself a maximum where K = (the expres- 
sion (99.7) becomes infinite for K = and is, of course, invalid). If the 
condition (97.15) b«k = — <nb 2 is not satisfied, however, K cannot be zero, 
and the maximum of the diffusely scattered intensity lies at some K different 
from zero, which in general does not coincide with the maximum of the 
structural scattering. In either case the diffuse scattering forms a background 
whose intensity falls off essentially as 1/K 2 , that is, considerably more 
slowly than the intensity in the sharp structural-scattering line superposed 
upon it. 



f See Statistical Physics, §110. If the prob abilit y distribution for fluctuating quantities 
xi, X2, ... is of the form exp(— ^Xncx t xje), then xtx* = A-^fc. A factor 2 in (99.6) appears 
because each of the complex moi involves two independent quantities. 

27* 



APPENDIX 

CURVILINEAR CO-ORDINATES 

We give below, for reference, certain formulae relating to vector operations 
in curvilinear co-ordinates, both general and particular. 

In an arbitrary system of orthogonal curvilinear co-ordinates «i, u 2 , «3, 
the squared element of length is d/ 2 = h^&ufi + htf'&wg' + htf'&uz 2 , where 
the hi are functions of the co-ordinates. The element of volume is 

d V = hihzhz d«i du 2 dwg. 

The various vector operations can be expressed in terms of the functions 
h i as follows. For vector operations on a scalar : 

1 8f 
(grad/), = -/-, 
hi cm 

1 v d /h 2 h 3 3f 



1 y /h 2 h 3 g/\ 
hxhzhz^- 1 dui\ hi du\l' 



where the summation is over cyclic interchanges of the suffixes 1, 2, 3. For 
vector operations on a vector : 

divA = irirSir^ 3 ^' 

1 f 8 d I 

(curl A)! = — U(M,) - -r-(M2) • 

«2«3 L <7W2 ^3 J 

The remaining components of curl A are obtained by cyclic interchanges 
of the suffixes. 

Cylindrical co-ordinates r } <f>, z. 

Element of length: d/ 2 = dr 2 + r 2 d<£ 2 + da; 2 ; 

h r = 1, hf = r, h z = 1. 
Vector operations : 

_ 1 d I df\ i a 2 / a 2 / 

A ' = rSU/ + r 2 " &£2 + d^ 2 ' 
_ 1 a 1 dAf dA z 

r dr r d(f> dz 

410 



Curvilinear Co-ordinates 411 

1 dAz 8A, 

3A r dA z 

(curl AY* = — , 

v } * 8z Br 

18 1 8A r 

(curlA), = -— (r4,) — , 

r or r c<p 

A r 2 8A 6 

(AA) r =AA r -— - 



r 2 r 2 8<f> 
A, 2 8A r 

(AA) Z = AA Z . 

In the expressions for the components of AA, /\Ai signifies the result of 
the operator A acting on A\ regarded as a scalar. 

Spherical co-ordinates r, 6, </>. 

Element of length: d/2 = dr 2 + r 2 d0 2 + r 2 sin* d<f> 2 ; 

hr = 1, h e = r, h$ = r sin0. 
Vector operations : 

1 8 ( 8f\ 1 8 l . 8f\ 1 a 2 / 

A/ = \r 2 — \ + sin0— + -, 

J r 2 8r\ 8rJ r 2 sin 9 80\ 86/ r 2 sin 2 8cf> 2 

\ 8 \ 8 1 ZA* 

divA = ——(f*Ar) + ——-(A e sm0) + 



r 2 8r r sin 89 r sin 8<j> 

1 [8 8A e -\ 

1 8A r 1 8 
(curl A), = -r-~ ~ ~ T (rA^ 
r sin 9 8<f) r or 

Ira 8A r l 

2r 1 8 1 8A 6 -\ 

(AA) r = A ^r--Ur + -— -(Agsm0)+ . ' , 

r 2 L sin0 80 sinS a<£ J 

2 ra^ r ^ C os0 a^i 

r 2 I 80 2 sin 2 sin 2 8<f> ] 

2 [8A r 8A e A 6 I 

r 2 sin L a<6 a^ 2 sin J 



INDEX 



Absorption coefficient, 265 

Absorption of electromagnetic waves by 

small particles, 303f. 
Acceleration, excitation of currents by, 210ff. 
Airy function, 286 
Anisotropic media, electromagnetic waves 

in (XI), 313ff. 
Antiferromagnetic Curie point, 165f. 
Antiferromagnetics, 117 

Barnett effect, 144 

Bindrmal, 326 

Biot and Savart's law, 122 

Biradial, 326 

Black-body radiation in a transparent 

medium, 367f. 
Bragg and Vul'f 's equation, 401 
Bragg's method, 406n. 
Brewster angle, 276 



Capacity, 4, 8f., 19f., 62, 202ff. 

coefficients, 4, 6, 8 

of conducting ellipsoid, 24f. 

of conducting sphere, 62 

mutual, 8, 17 
Causality, 257, 260 
Charge distribution 

on conducting disc, 27, 28 

on conducting ellipsoid, 24, 28 

on conductors, Iff., 18 
Charges 

in a dielectric, 68 

extraneous, 37 
Cherenkov radiation, 357ff. 
Conduction current density, 120 
Conductivity, electrical, 92 

tensor, 93, 96 
Conductors, Iff. 

cylindrical, 16, 17, 18, 125, 194 

disc-shaped, 27, 28 

electrostatics of (I), Iff. 

electrostriction of, 33ff. 

ellipsoidal, 20ff. 

forces on, 31ff., 62, 142ff., 221 

motion in a magnetic field, 205ff. 
turbulent, 234ff. 

spherical, 15f., 34f., 95f., 193ff., 209f., 212 
Conformal mapping, 14 

method of, 12ff. 
Conical refraction 
external, 328 
internal, 326, 328 

413 



Contact potential, 100 
Cotton-Mouton effect, 336 
Critical field, 173ff. 
Critical opalescence, 393ff. 
Critical state, 82f. 
Crystals 

biaxial, 59, 324ff. 

dielectric properties of, 58ff., 313ff. 

enantiomorphic, 341 

magnetic properties of, 116ff. 

natural optical activity of, 341 f. 

piezoelectric, 76 

pyroelectric, 60 

uniaxial, 59, 321ff. 
negative, 322 
positive, 322 

X-ray diffraction in (XV), 398ff. 
general theory of, 398ff. 
Curie point 

antiferromagnetic, 165f. 

ferroelectric, 83 

ferromagnetic, 146 
Curie— Weiss law, 148 
Current 

boundary conditions for, 92f. 

conduction, 120 

constant (III), 92ff. 
magnetic field of, 119ff. 

in a crystal, 398 

density, 92 

eddy, 189ff. 

electric, 1 

excitation by acceleration, 210ff. 

linear, 122; see also Linear current 

molecular, 120n. 

in a moving conductor, 206 

superconductivity, 169ff. 

surface, 115 
Curvilinear co-ordinates, 410f. 
Cylinder 

conducting, 16, 17, 18, 125, 194 

dielectric, 43, 57, 58 

Debye relaxation time, 392 
Debye and Scherrer's method, 407n. 
Demagnetisation coefficients, 44n., 169 
Depolarisation coefficients, 26f., 302 
Depolarising field, 44n. 
Dielectric axes, principal, 316 
Dielectric constant, see Dielectric permea- 
bility 
Dielectric crystals, 58ff., 313ff. 



414 



Index 



Dielectric cylinder, 43, 57 
Dielectric disc, 43, 57, 58, 62 
Dielectric ellipsoid, 42ff., 56f. 
Dielectric fluid, forces in, 64ff. 
Dielectric permeability 

analytical properties of, 256ff. 

of crystals, 313ff. 

dispersion of, 247ff. 

electrostatic, 38 

at high frequencies, 251 

at low frequencies, 250 

of a mixture, 45ff. 

spatial variation of, 337ff. 

tensor, 58, 69ff., 329ff., 339, 385f. 
Dielectric polarisation, 36 
Dielectrics, 1 

electrostatics of (II), 36ff. 

electrostriction of isotropic, 55ff. 

moving, 243ff. 

boundary conditions, 245f. 

thermodynamics of, 47ff. 

total free energy of, 52ff. 
Dielectric solid, forces in, 69ff. 
Dielectric sphere, 42f., 45, 61, 62, 73, 246f. 
Dielectric susceptibility, 38 

sign of, 63f. 
Dielectric tensor, 58; see also Dielectric 

permeability tensor 
Diffraction 

by a plane screen, 308ff. 

by a wedge, 304ff. 

of X-rays in crystals (XV), 398ff. 
general theory of, 398ff. 
Diffusion phenomena, HOff. 
Dipole moment 

of conducting cylinder, 16, 18f. 

of conducting disc, 28 

of conducting sphere, 16 

of conductor, 7 

of dielectric, 36f., 54 
Disc 

conducting, 27, 28 

dielectric, 43, 57, 58, 62 

superconducting, 173 
Discontinuities in a magnetic fluid, 224ff. 
Discontinuity 

contact, 225 

rotational, 226ff. 

tangential, 225ff. 
stability of, 227ff. 
Dispersion relation, 220 
Displacement, electric, 37n. 
Dissipative function, 204 
Domains, 87ff., 152ff., 158ff., 179ff. 
Double circular refraction, 335 
Double refraction, 323 

in an electric field, 329ff. 

Easy magnetisation, direction of, 150 
Eddy currents, 189ff. 



Eikonal, 269, 317 
Einstein-de Haas effect, 145 
Elastic constant tensor, 75 
Elastic-optical constants, 330 
Electric displacement, 37n. 
Electric field, constant 
of conductors, Iff. 

boundary conditions, 2f., 39f., 92 
energy of, 3ff. 
in dielectrics, 36ff. 

boundary conditions, 37ff. 
thermodynamics of, 47ff. 
See also Electromagnetic field 
Electric induction, 37 
Electric moment, 37 
Electrocaloric effect, 56ff. 
Electrocapillarity, 103f. 
Electromagnetic field 

boundary conditions, 273, 280f., 290 
fluctuations, 361ff. 
in moving media, 243ff. 
quasi-static (VII), 186ff. 

in dielectrics, 239ff. 
variable, 247ff. 
Electromagnetic fluctuations (XIII), 360ff. 
Electromagnetic wave equations (IX), 239ff 
Electromagnetic waves, 239ff. 

absorption of, by small particles, 303f. 
in anisotropic media (XI), 313ff. 
extraordinary, 322ff. 
ordinary, 322 
plane 

in an absorbing medium, 264f. 
in anisotropic media, 315ff. 
homogeneous, 264 
inhomogeneous, 263 
of a single frequency, 263ff., 268 
in a transparent medium, 264 
propagation of (X), 269ff. 

in an inhomogeneous medium, 284ff. 
in waveguides, 293ff. 
reflection and refraction of, 272ff. , 283f. 
in resonators, 290ff. 
scattering of (XIV), 377ff. 
by small particles, 299ff. 
Electromotive force, 101 
Electrostatic induction coefficients, 4, 6 
Electrostatics 
of conductors (I), Iff. 
of dielectrics (II), 36ff. 
Electrostriction 
of conductors, 33ff. 
of dielectrics, 55ff. 
Ellipsoid 

conducting, 20ff. 
dielectric, 42ff., 56f. 
ferromagnetic, 157f. 
superconducting, 169f., 182 
Ellipsoidal co-ordinates, 20ff. 
E.m.f., 101 



Index 



415 



Energy 

of conductors, 3ff., 124 
of a system of currents, 13 Iff. 
of dielectrics, 48ff., 52ff., 79ff. 
of fields in dispersive media, 253ff. 
flux (Poynting vector) 124, 191, 242, 253, 
271,274,280,314 
in a plane wave, 264, 315f. 
in a resonator, 291f. 
in a waveguide, 295f. 
free self-, 132 
interaction, 132 

of magnetic substances, 129ff., 149ff., 156, 
159ff. 
Ettingshausen effect, 109 
E waves, 285f., 294ff. 
Extinction coefficient 
differential, 384 
total, 384 
Extraneous charges, 37 
Extraordinary waves, 322ff. 

Faraday effect, 335 
Faraday's law, 207 
Fast particles, passage of through matter 

(XII), 344ff. 
Fermat's principle, 270 
Ferroelectric axis, 83 
Ferroelectrics, 83ff. 

domains in, 87f. 
Ferromagnetics, 117 

near the Curie point, 146ff. 

domain structure of, 152ff., 158ff. 

thermodynamics of, 147 
Ferromagnetism (V), 146ff. 
Fluctuations 

of anisotropy, 388 

of current in linear circuits, 360f. 

electromagnetic (XIII), 360ff. 

electromagnetic field, 361ff. 
Fresnel ellipsoid, 321 
Fresnel's equation, 317, 324ff. 
Fresnel's formulae, 273ff. 

Galvanic cell, lOlff. 
Geometrical optics, 269ff. 
Group velocity, 220, 270, 318 
Gyration vector, 332n., 339 
Gyromagnetic coefficients, 145 
Gyromagnetic phenomena, 144f. 
Gyrotropic media, 319n., 332ff. 

Hall effect, 97 

Hall's constant, 98 

if waves, 285, 287f., 294ff. 

Hydromagnetic waves, 221n. 

absorption of, 223 
Hysteresis, 151 



Image, 9 

force, 10, 40 
Images, method of, 9ff. 
Impedance, 197ff. 

matrix, 201 

surface, 280ff., 314f. 
Inductance 

mutual, 132 

self-, 132 
Induction 

electric, 37 

extraneous fluctuating, 361 
correlations of, 363ff. 

unipolar, 208f. 
Inversion 

method of, llf. 

radius of, 12 

transformation, 12 
Ionisation losses by fast particles, 344ff. 

relativistic, 349ff. 

Joule's law, 93 

in a moving conductor, 206 

Kerr effect, 329 
Kinetic coefficients, 94 

symmetry of, 94, 96, 314, 331, 338f. 
Kramer's and Kronig's formulae, 259ff., 

282f. 

Laue's equation, 401 
Laue's method, 405n. 
Leduc-Righi effect, 109 
Linear currents, 122ff., 133ff., 197ff., 210, 
212 

fluctuations of, 360f. 

mutual inductance of, 133 

self-inductance of, 136ff. 
Lorentz condition, 350 
Losses 

electric, 254 

magnetic, 254 

Magnetic anisotropy energy, 146n., 148n., 

149ff. 
Magnetic crystal classes, 118 
Magnetic field, 114 

boundary conditions, 115, 187f. 

conductor moving in, 205ff. 

constant (IV), 113ff. 

of constant current, 119ff. 

conducting fluid moving in, 213ff. 

forces on matter in, 141ff., 221 

thermodynamics of, 126ff. 

See also Electromagnetic field 
Magnetic fluid dynamics (VIII), 213ff. 
Magnetic flux, 134 
Magnetic induction, 113 
Magnetic moment, 114 

in variable field, 252 



416 



Index 



Magnetic-optical effects, 3 3 Iff. 
Magnetic permeability, 114 

analytic properties of, 262 

of crystals, 313 

dispersion of, 25 Iff. 

tensor, 119 
Magnetic polarisability tensor, 192 
Magnetic space groups, 117 
Magnetic structure, 116 
Magnetic susceptibility, 114f. 

sign of, 115, 129 
Magnetisation, 114 

by rotation, 144f. 

regions of spontaneous, 152 
Magneto-elastic energy, 156 
Magnetostatics and electrostatics compared, 

115, 116, 126ff., 141f., 146 
Magnetostriction, 155ff. 
Mandershtam-Brillouin doublet, 390 
Maxwell effect, 331 
Maxwellian relaxation time, 392 
Maxwell's equations, 2, 113, 315, 349 
Maxwell stress tensor, 31 
Mechanical-optical effects, 330f. 
Molecular attraction between solid bodies, 

368ff. 
Momentum density, 242 
Mutual inductance, 132 

Natural optical activity, 248n., 337ff. 

of crystals, 341f. 
Nernst effect, 109 

Ohm's law, 92, 200 
Optical axis, 321, 326 
Optical frequencies, 248 
Optical ray axis, 326 
Ordinary waves, 322 
Oscillator strength, 261 



Peltier coefficient, 108 
Peltier effect, 107f. 
Penetration depth, 189, 279f. 

in a superconductor, 167 
Phase velocity, 219, 270 
Piezoelectricity, 60n., 70n., 73ff. 
Piezoelectric tensor, 74, 76ff. 
Piezomagnetism, 119 
Polarisability tensor, 7f., 192 
Polarisation, 36 

coefficient, 38 

of dielectric in variable field, 249 

of electromagnetic waves in anisotropic 
medium, 319ff. 

in geometrical optics, 271 

in gyrotropic medium, 335ff. 

in uniaxial crystal, 323 
Ponderomotive forces , 64 



Potential 
contact, 100 
electric 

complex, 13 
scalar, 2f., 350 
vector, 13, 350 
magnetic, vector, 120 
Powder method, 407n. 
Poynting vector, see Energy flux 
Principal section, 322 
Principal waves in waveguides, 296ff. 

damping of, 297ff. 
Pyroelectricity, 60f. 



Quadrupole moment tensor, 28 

of conducting ellipsoid, 28 
Quality of a resonator, 292n. 
Quasi-static fields, 186, 364 



Raman-Landsberg-Mandel'shtam effect, 

387 
Ray surface, 317ff. 
Ray vector, 317ff. 
Reactance, 198n. 
Reciprocity theorem, 289f. 
Reflection coefficient, 274, 277ff., 283f. 
Refractive index, 264, 265, 269, 316n., 322, 

368 
Resistance, 198n. 

complex, 197 
Resonators, 290ff. 

quality of, 292n. 
Rotation method, 406n. 



Scattering 

in amorphous solids, 395ff. 

anti-Stokes, 377 

antisymmetrical, 383 

combination, 387 

effective cross-section for, 300ff. 

of electromagnetic waves (XIV), 377ff. 
by small particles, 299ff. 

by fluctuations, 388ff. 

principle of detailed balancing for, 383 ff. 

Rayleigh, in gases and liquids, 387ff. 

scalar, 383 

with small change in frequency, 385ff. 

Stokes, 377 

symmetrical, 383 
Self-inductance, 132 

of linear circuits, 136ff. 

of superconductors, 171 
Shock waves in a magnetic fluid, 229ff. 

weak, 231 

in weak magnetic fields, 231f. 
Skin effect, 136n., 195ff. 
Solenoid, 137, 140f. 



Index 



417 



Sphere 

conducting, 15f., 34f., 62, 95f., 193ff., 

209f., 212 

dielectric, 42f., 45, 61, 62, 73, 246f. 
Spheroidal co-ordinates 

oblate, 22 

prolate, 22 
Stewart-Tolman effect, 212 
Stimulated emission, 377n. 
Stopping power, 346 
Stress tensor, 65ff., 70f. 
Superconductivity (VI), 167ff. 

destruction of, 173ff. 

transition point, 167 
Superconductors 

critical field in, 173ff. 

currents in, 168ff. 

ellipsoidal, 169f., 182 

impedance of, 28 If. 

intermediate state of, 179ff. 

magnetic properties of, 167ff. 

multiply connected, 170ff. 

rotating, 212 

self-inductance of, 171 

thermodynamics of, 173ff. 

Telegrapher's equation, 298 
Tensor ellipsoid, 59n. 
Thermodynamic inequalities, 81ff. 
Thermoelectric phenomena, 104ff. 
Thermoelectromotive force, 108 
Thermogalvanomagnetic effects, 109f. 
Thomson coefficient, 107 
Thomson effect, 107 
Thomson's formula, 203 
Thomson's relations, 108 



Thomson's theorem, 7 

Total polarisation, angle of, 276 

Total reflection, 277 

angle of, 277 
Transparency ranges, 254 
Transparent media, 266ff. 

black-body radiation in, 367f. 
Turbulence in conducting fluid, 234ff. 
Two-dimensional field, 12f. 

Unipolar induction, 208f. 

Velocity of light in moving medium, 271f. 

Waveguides, 293ff. 
Waves 

on a charged liquid surface, 35 

electric-type, 294 

electromagnetic, see Electromagnetic 
waves 

extraordinary, 322ff. 

hydromagnetic, 221n. 
absorption of, 223 

magnetic-type, 294 

ordinary, 322 

principal, 296 

shock, see Shock waves 
Wave-vector surface, 317ff., 322, 324ff., 

334n. 
Wedge problem, 14f. 
Work function, 99 

X-ray diffraction in crystals (XV), 398ff. 

general theory of, 398ff. 
X-rays, diffuse thermal scattering of, 407ff. 



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physics on which the authors are not very well informed. " Nature 

"The remarkable nine-volume Course of Theoretical Physics . . . the clearness and accuracy of the 

authors' treatment of theoretical physics is well maintained. " 

Proceedings of the Physical Society 



Landau 
Lifshitz 



Of individual volumes, reviewers have written : 

MECHANICS 

"The entire book is a masterpiece of scientific writing. There is not a superfluous sentence and the 
authors know exactly where they are going. ... It is certain that this volume will be able to hold its 
own amongst more conventional texts in classical mechanics, as a scholarly and economic exposition 
of the subject." Science Progress 

QUANTUM MECHANICS (Non-relativistic Theory) 

". , . throughout the five hundred large pages, the authors' discussion proceeds with the clarity and 
succinctness typical of the very best works on theoretical physics." Technology 

FLUID MECHANICS 

"The ground covered includes ideal fluids, viscous fluids, turbulence, boundary layers, conduction 
and diffusion, surface phenomena and sound. Compressible fluids are treated under the headings of 
shock waves, one-dimensional gas flow and flow past finite bodies. There is a chapter on the fluid 
dynamics of combustion while unusual topics discussed are relativistic fluid dynamics, dynamics of 
superfluids and fluctuations in fluid dynamics ... a valuable addition to any library covering the 
mechanics of fluids." Science Progress 

THE CLASSICAL THEORY OF FIELDS (Second Edition) 

"This is an excellent and readable volume. It is a valuable and unique addition to the literature of 
theoretical physics." Science 

"The clarity of style, the concisement of treatment, and the originality and variety of illustrative problems 
make this a book which can be highly recommended." Proceedings of the Physical Society 

STATISTICAL PHYSICS 

". . . stimulating reading, partly because of the clarity and compactness of some of the treatments put 
forward, and partly by reason of contrasts with texts on statistical mechanics and statistical thermo- 
dynamics better known to English sciences. . . . Other features attract attention since they do not 
always receive comparable mention in other textbooks." New Scientist 

THEORY OF ELASTICITY 

"1 shall be surprised if this book does not come to be regarded as a masterpiece." 

Journal of the Royal Institute of Physics (now the Physics Bulletin) 

". . . the book is well constructed, ably translated, and excellently produced." 

Journal of the Royal Aeronautical Society 

ELECTRODYNAMICS OF CONTINUOUS MEDIA 

"Within the volume one finds everything expected of a textbook on classical electricity and magnetism, 
and a great deal more. It is quite certain that this book will remain unique and indispensable for many 
years to come." Science Progress 

"The volume on electrodynamics conveys a sense of mastery of the subject on the part of the authors 
which is truly astonishing." Nature 




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08 016019 



Pergamon