The Project Physics Course: Transparencies Unit 4: Light and Electromagnetism

The Project Physics Course

Unit H-

Transparencies

The Project

Physics V^VyLJi

wW

Transparencies

UNITT"

Light

and Electromagnetism

Published by HOLT,

RINEHART and WINSTON,

Inc.

New

York, Toronto

Project Physics

Overhead Projection Transparencies Unit 4

T30

The Speed of Light

T31

E Field

T32

Magnetic Fields and Moving Charges

T33

Forces Between Current Carriers

T34

The Electromagnetic Spectrum

Inside Conducting Sphere

The Speed

of Light

T30

The Speed

of Light

how the speed of light can be found Romer and from Michclson's rotating mirror apparatus.

This transparency presents a greatly simplified visualization of

from the

celestial

observations of

Romer's Celestial Method Overlay

A

As

Jupiter's innermost

earth.

The period of

moon enters Jupiter's shadow it is no longer visible from moon was found to be 42.5 hours, i.e., it entered eclipse

this

the be-

hind Jupiter or emerged from eclipse every 42.5 hours. However, monthly measure-

ments indicated great variations

Romer

in

this

schedule

— up

to 1320 seconds (22 minutes).

explained this time difference by suggesting that light took longer to reach

when the earth was farther from Jupiter in its orbit around Huygens used Romer's data, together with a new value of 182,000,000 miles

the earth from Jupiter the sun.

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 yield the

more accurate

B

This

value (185,800,000 miles)

Method

Michelson's Terrestrial Overlay

AU

figure of 186,300 miles/second.

a simplified diagram of the apparatus used by Michelson in the late 1920's.

is

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

light travels the 44-mile in the

beam

second diagram. is

found to appear

the mirror

round If

trip

beam was

back, the mirror wheel turned Since one revolution takes

rotated, the

beam

change

in its position

while the

at the detector to shift, as

shown

the wheel rotates at 530 revolutions per second, the light

in exactly the

This means that while the

is

causes the

1

M

same position

as

when

the wheel

was

stationary.

and bottom diagram.

traveling the 22 miles to the distant mirror

of a revolution, as

530 seconds,

K

shown

in

the

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

E

T3 This transparency

Field Inside Conducting Sphere

useful in discussing the electric field strength inside a charged hollow sphere.

is

Applications of shielding techniques can be brought up.

Overlay

A

A

is shown with positive charge spread e\enly over The small black dot represents an arbitrary point within the sphere

hollow metal sphere

surface.

its

entire

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 these patches are proportional to

on the other side. The charges. Q, and and A^:

Q.-,

their areas, .4i

A' Overlav

C

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

Ai —r Ai

The

electric field

also

is

from the chosen point. d'x

= Tr dt

due to each patch

f and .u therefore .

is