# Full text of "Transparencies Unit 4 - Light & Electromagnetism: Project Physics"

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```The Project Physics Course

Unit H- Transparencies

The

Project

Physics

V^VyLJi wW Transparencies

UNITT"

Light and Electromagnetism

Project Physics

Unit 4

T30 The Speed of Light

T31 E Field Inside Conducting Sphere

T32 Magnetic Fields and Moving Charges

T33 Forces Between Current Carriers

T34 The Electromagnetic Spectrum

The Speed of Light

T30

The Speed of Light

This transparency presents a greatly simplified visualization of how the speed of light can be found
from the celestial observations of Romer and from Michclson's rotating mirror apparatus.

Romer's Celestial Method

Overlay A As Jupiter's innermost moon enters Jupiter's shadow it is no longer visible from the
earth. The period of this moon was found to be 42.5 hours, i.e., it entered eclipse be-
hind Jupiter or emerged from eclipse every 42.5 hours. However, monthly measure-
ments indicated great variations in this schedule — up to 1320 seconds (22 minutes).
Romer explained this time difference by suggesting that light took longer to reach
the earth from Jupiter when the earth was farther from Jupiter in its orbit around
the sun. Huygens used Romer's data, together with a new value of 182,000,000 miles
for the diameter of the earth's orbit, to calculate a value for the speed of light: 138,000
miles/second. Today's time lag value (996 seconds) and 2 AU value (185,800,000 miles)
yield the more accurate figure of 186,300 miles/second.

Michelson's Terrestrial Method

Overlay B This is a simplified diagram of the apparatus used by Michelson in the late 1920's.
The octagonal mirror wheel allowed light to reflect from one surface to a mirror 22
miles away back to another surface on the wheel, and finally to an observer, as shown
in the top diagram. When the mirror is rotated, the change in its position while the
light travels the 44-mile round trip causes the beam at the detector to shift, as shown
in the second diagram. If the wheel rotates at 530 revolutions per second, the light
beam is found to appear in exactly the same position as when the wheel was stationary.
This means that while the beam was traveling the 22 miles to the distant mirror and
back, the mirror wheel turned M of a revolution, as shown in the bottom diagram.
Since one revolution takes 1 530 seconds, K of a revolution takes H x 1/530 second
or 2.36 X 10^ seconds. Dividing 44 miles by this time yields 186,300 miles/ second.

T-30

Romer's Method 1676

Jupiter's Orbit

T-30

Michelson's Method 1924 - 27

^

E Field Inside Conducting Sphere

T3

E Field Inside Conducting Sphere

This transparency is useful in discussing the electric field strength inside a charged hollow sphere.
Applications of shielding techniques can be brought up.

Overlay A A hollow metal sphere is shown with positive charge spread e\enly over its entire
surface. The small black dot represents an arbitrary point within the sphere at which
investigations concerning electric fields can be made.

Overlay B As the double "cone" indicates, a small patch on the surface of the sphere on one
side of the point has a corresponding patch on the other side. The charges. Q, and Q.-,
on these patches are proportional to their areas, .4i and A^:

Overlav C

A'

Since these patches are marked out by the same "cone", their areas are proportional
to the squares of the distances from the chosen point.

Ai d'x . .u f 2i <f'\

—r = Tr and therefore ^ = tt
Ai dt Qi (Pz

The electric field due to each patch is proportional to the charge on the patch and
also is inversely proportional to the square of the distance from the chosen point, so:

- d', ^ d-i

Overlay D

Hence the distance and area factors balance and the E fields due to the two patches at
the point are exactly equal (and opposite).

Using the same argument for other "cones" leads to similar results. Indeed, it is true
for all pairs of charge patches, so the net electric field at the arbitrary point is zero.

T-31

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131

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T4I

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1

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Magnetic Fields and Moving Charges

T32

Magnetic Fields and Moving Charges

This transparency will be useful in discussing a number of phenomena which can occur in magnetic
fields: forces on moving charged particles; forces on charged particles in both magnetic and electric
fields; forces on current carriers; forces on moving conductors. Portions of this transparency are
applicable in Unit 5 also.

Overlay A This shows the poles of a strong magnet producing a magnetic field with a suggestion
of fringing shown at the edges.

Overlay B A negatively charged particle moves in the uniform portion of the magnetic field B
with a velocity V. Cover the upper two representations with an index card and discuss
the consequences of the force acting on the charged particle at right angles to both
V and B. Ask students to predict the behavior of the particle and then reveal the next
two illustrations. Students should quickly realize that the path of the particle must be
a segment of a circle, since the force continually acts at right angles to the velocity.

Overlay C An arrow indicates the curved path that a negatively charged particle might follow
when moving in a uniform magnetic field at right angles to B. Of course, the path
could be a complete circle if the proper conditions are met. Remove overlays A, B,
and C.

Overlay D A set of charged plates produces a strong uniform electric field (without a suggestion
of fringing shown at the edges). Ask students to predict the path that a negative
particle will take when fired into the field with a constant velocity. Ask about a positive
particle, also. The paths of course will be parabolic downward (negative particle) and
upward (positive). Introduce overlay E.

Overlay E This shows the parabolic path taken by a negatively charged particle entering a uniform
electric field at right angles to .E Return overlays A and C and discuss the two forces
due to the magnetic and electric fields which now act on the particle. Remove overlays
C and E and introduce overlay F.

Overlay F The path that a negatively charged particle will take in the combined magnetic and
electric fields is a straight line if the forces caused by the respective fields are equal-
Remove overlay F.

Magnetic Fields and Moving Charges (continued)

T32

Magnetic Fields and Moving Charges (continued)

Overlay G This is a detachable overlay which illustrates the mutually perpendicular vectors F, V,
and B which operate on a moving negatively-charged particle in a magnetic field
(according to the left hand rule). Use it with overlay H to illustrate the generator and
motor principles. Overlays G and H can be made easily detachable by carefully cutting
the binding ring as shown in this sketch.

Do not cut here

Cut along this line

Overlay H This detachable overlay representing a segment of metallic wire. With overlay A in
place on the stage, align overlays G and H so that the charged particle is positioned
inside the wire. Now assume that electrons are flowing to the right through the wire.
Since the magnetic field is perpendicular to the velocity of the electrons, there will be
a force exerted on the electrons in an upward direction according to the (left) hand rule.
Such a force on the flowing electrons pushes the entire wire upward. You can illustrate
this phenomenon by carefully sliding the overlays in the proper directions as indicated
in the diagram. (The arrow for G shows its motion relative to the moving H.)

The Motor Principle

The Generator Principle

When a wire is moved at right angles to B through a magnetic field there will be
produced a deflecting force on the free electrons in the wire thus producing an elec-
tron displacement. If the wire is part of a closed loop, a current is produced as me-
chanical energy is converted into electrical energy. (If the loop is not closed, the
displacement will produce an excess of electrons at one end of the mo\ing wire and a
deficiency of electrons at the other.) You can illustrate the operation of this principle
by orienting overlays G and H as shown in the diagram and move them in the direc-
tions indicated. (The arrow for G shows its motion relative to the moving H.)

T-32

T-32

T-aa

T-32

T32

T-M

T32

A F

T'32

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Forces Between Current Carriers

T33

Forces Between Current Carriers

This transparency provides an account of the forces produced between two parallel current carriers,

based on the principles governing moving charged particles in magnetic fields (see T32). It should

prove very useful when used in connection with the Current Balance Experiment.

Overlay A The enlarged segments of two parallel conductors.

Overlay B A battery and connection complete a circuit. The arrows indicate the direction of
electron flow. In this circuit, the electron flow in the parallel conductors is in opposite
directions.

Overlay C Magnetic field lines surround the left wire as determined by the (left) hand rule. An
electron is shown moving to the right in the field created by the left wire. The force
on the electron, and consequently on the entire wire, will be outward, that is, away
from the other wire. Remove this overlay and introduce overlay D.

Overlay D The magnetic field produced by the right wire will cause an outward force on the
moving electron in the left wire. Return overlay C and note that wires with anti-
parallel currents will repel each other. Remove overlays B, C, and D.

Overlay E In this difi"erent completed circuit the electron flow is now in the same direction in the
two wires.

Overlay F Magnetic field lines surround the left wire as determined by the (left) hand rule. An
electron is shown moving to the left in the field created by the left wire. The force on
the electron, and consequently on the entire wire, is seen to be inward, that is, toward
the other wire. Remove this overlay and introduce overlay G.

Overlay G The magnetic field produced by the right wire will have an efi'ect on the moving elec-
tron in the left wire. Return overlay F and note that wires with parallel currents
attract each other.

T-33

T-33

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T^33

A
B
C

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T-33

T-33

T-33

T'33

A F

T-33

T-13

A F

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The Electromagnetic Spectrum

T34

The Electromagnetic Spectrum

This transparency may be used extensively both in Unit 4 and in Unit 5. It presents a diagram of the
continuum of the electromagnetic spectrum with a full color reproduction of the visible spectrum.
In addition several spectra of elements are presented.

Overlay A The full electromagnetic spectrum is shown in perspective with a missing slot repre-
senting the visible light segment.

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Overlay B The visible spectrum with an Angstrom wavelength scale.

Overlay C Some of the principal Fraunhofer lines in the solar spectrum. Remove this overlay
and introduce each of the successive overlays separately.

Overlay D The principal lines in the Hydrogen emission spectrum.

Overlay E The principal lines in the Helium emission spectrum.

Overlay F The principal lines in the Mercury emission spectrum.

Overlay G The principal lines in the Sodium emission spectrum.

Overlay H The principal lines in the Sodium absorption spectrum.

T34

VISIBLE LIGHT

T34

VISIBLE LIGHT

I I I I I I

7500

7000

I I I I I I I I I I I I I I I I I
6500 6000 5500 5000

Wavelength in Angstroms (lO~ m)

A
B

4500

4000

T34

VISIBLE LIGHT

Fraunhofer Lines

A
C

134

7500

VISipijE 1\CjHJ

7000 6500 6000 5500 5000

Wavelength in Angstroms { 10" m)

4500

4000

A
B

7500 7000

6500 6000 5500 5000

Wavelength in Angstroms (lO~ m)

4500 4000

i

i?S4

VISIBLf I IGHT

r-p-T

7500

7000 6500 6000 5500 5000

Wavelength in Angstroms ( 10" m)

4500 4000

I

ifm

VISIBLE LIGHT

7500

7000

6500 6000 5500 5000

Wavelength in Angstroms ( 10" m)

4500 4000

f

fM

Absorption Spectrum of xxJium

7500 7000

6500 6000 5500 5000

Wavelength in Angstroms ( 10" m)

4500 4000

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