740

ELECTROM

AG

NETIC

IND

UCTIO

N

The d

iscovery that an electric current in a w

ire prod

uced m

agnetism

was a turning

p

oint in physics and

the technology that

followed

. The

question

arose as

to w

hether m

agnetism

could p

roduce an electric current in

a wire. In 1831, tw

o physicists, M

ichael Faraday in

England

and Josep

h Henry in the U

nited States, ind

e-p

endently d

iscovered that the answ

er is yes. Until

their discovery, the only current-p

roducing

devices

were voltaic cells, w

hich prod

uced sm

all currents by

dissolving

expensive m

etals in acids. These w

ere the forerunners of our p

resent-day b

atteries. The discov-

ery of Faraday and

Henry p

rovided

a major alternative

to these crude d

evices. Their discovery w

as to change

the world

by m

aking electricity so com

monp

lace that it w

ould p

ower ind

ustries by d

ay and lig

ht up cities

by nig

ht.

Magnetism

can produce electricity, and electricity can produce m

agnetism.

3

Can You C

reate an Electric Current

Without a B

attery?1.

Use tw

o lengths of wire to connect tw

o galvanom

eters.2.

Shake one of the galvanometers w

hile watch-

ing the needle of the other meter.

3.N

ow reverse the roles of the galvanom

eters by shaking the galvanom

eter that was origi-

nally stationary.

Analyze and

Conclud

e1.

Observing D

escribe the reading on a station-ary galvanom

eter as the other galvanometer

is shaken. 2.

Predicting What w

ould happen if you moved

a magnet through the w

ire loops connecting the galvanom

eters?3.

Making G

eneralizations How

could you use m

echanical motion to pow

er an electronic device?

disco

ver!

THE B

IGID

EA

..........

0740_CP

09_SE

_CH

37.indd740

11/13/0711:15:39

AM

0740_CP

09_SE

_CH

37.indd741

74

0

ELEC

TRO

-M

AG

NETIC

IND

UC

TION

Ob

jectiv

es

• Describ

e ho

w vo

ltage is

ind

uced

in a co

il of w

ire. (37.1)

• State and

explain

Faraday’s

law. (37.2)

• Describ

e ho

w a g

enerato

r w

orks. (37.3)

• Describ

e ho

w a m

agn

etic field

affects a mo

ving

charg

e. (37.4)

• Describ

e ho

w a tran

sform

er w

orks. (37.5)

• Explain

wh

y almo

st all electrical en

ergy is so

ld in

the fo

rm o

f altern

ating

curren

t. (37.6)

• Explain

ho

w an

electric field

creates a mag

netic field

. (37.7)

• Describ

e electrom

agn

etic w

aves. (37.8)

disco

ver!M

ATE

RIA

LS two

galvan

om

eters, w

ire

EX

PE

CTE

D OU

TCO

ME Th

e n

eedle o

f the statio

nary

galvan

om

eter will sw

ing

back

and

forth

.

AN

ALY

ZE A

ND C

ON

CLU

DE

See Expected

Ou

tcom

e.

The n

eedle w

ou

ld sw

ing

b

ack and

forth

in b

oth

g

alvano

meters.

Mo

ving

a mag

net th

rou

gh

a w

ire loo

p w

ou

ld create an

electric cu

rrent.

TEA

CH

ING TIP A

mag

net

mo

ved th

rou

gh

a wire lo

op

in

du

ces a voltag

e in th

e wire.

A g

alvano

meter co

ntain

s a m

agn

et, so w

hen

on

e g

alvano

meter is sh

aken th

e o

ther o

ne reg

isters a small

curren

t.

1.2.3.

0740_cp09te_Ch37.indd 740

0740_cp09te_Ch37.indd 740

1/30/08 10:51:54 AM

1/30/08 10:51:54 AM

11/13/0711:15:39

AM

37.1 Electromagnetic Induction

Faraday and Henry both m

ade the same discovery.

Electric current

can be produced in a wire by sim

ply moving a m

agnet into or out of a w

ire coil. No battery or other voltage source w

as needed—only the m

otion of a magnet in a coil or in a single w

ire loop as shown

in Figure 37.1. They discovered that voltage w

as induced by the rela-tive m

otion of a wire w

ith respect to a magnetic field.

The production of voltage depends only on the relative m

otion of the conductor w

ith respect to the magnetic field. V

oltage is induced w

hether the magnetic field of a m

agnet moves past a sta-

tionary conductor, or the conductor moves through a stationary

magnetic field as show

n in Figure 37.2. The results are the sam

e for the sam

e relative motion.

The am

ount of voltage induced depends on how quickly the m

ag-netic field lines are traversed by the w

ire. Very slow m

otion produces hardly any voltage at all. Q

uick motion induces a greater voltage.

The greater the num

ber of loops of wire that m

ove in a magnetic

field, the greater are the induced voltage and the current in the wire,

as shown in Figure 37.3. Pushing a m

agnet into twice as m

any loops w

ill induce twice as m

uch voltage; pushing it into ten times as m

any loops w

ill induce ten times as m

uch voltage; and so on. 37.1.1

FIGU

RE 37.1 !

When the m

agnet is plunged into the coil, volt-age is induced in the coil and charges in the coil are set in m

otion.

" FIG

URE 37.2

Voltage is induced in the wire

loop whether the m

agnetic field m

oves past the wire or

the wire m

oves through the m

agnetic field.

FIGU

RE 37.3 #

When a m

agnet is plung-ed into a coil of tw

ice as m

any loops as another, tw

ice as much voltage is

induced. If the magnet is

plunged into a coil with

three times as m

any loops, then three tim

es as much

voltage is induced.

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

741

0740_CP

09_SE

_CH

37.indd741

11/13/0711:15:45

AM

741

741

37.1 Electromagnetic

InductionK

ey

Term

electrom

agn

etic ind

uctio

n

Co

mm

on

Misco

ncep

tion

Voltage is produced by a magnet.

FAC

T Vo

ltage is p

rod

uced

by th

e w

ork d

on

e wh

en a m

agn

et and

a clo

sed lo

op

of w

ire are mo

ved

relative to each

oth

er.

$ Teach

ing

Tidb

it A lo

ng

h

elically-wo

un

d co

il of in

sulated

w

ire is called a so

leno

id.

This chapter focuses on the im

portant features of electrom

agnetic induction, and avoids such com

plications as reactance, back em

f, Lenz’s law, and the left- and right-hand rules, which often overwhelm

students. The im

portant concept here is the transm

itting of energy from

one place to another without physical contact. The chapter should be supported by dem

onstrations of electrom

agnetic induction, such as those described in the text.

0740_cp09te_Ch37.indd 741

0740_cp09te_Ch37.indd 741

1/30/08 10:51:58 AM

1/30/08 10:51:58 AM

742

Does it seem

that we get som

ething (energy) for nothing by sim

ply increasing the number of loops in a coil of w

ire? We don’t.

Work is done in pushing the m

agnet into the loop. That’s because the

induced current in the loop creates a magnetic field that repels the

approaching magnet. For exam

ple, in Figure 37.4a the north pole of a bar m

agnet is pushed toward a single loop. T

he current induced in the loop produces a m

agnetic field that repels the approaching bar m

agnet. We see that, in Figure 37.4b, w

hen the magnet is pulled aw

ay from

the loop, the induced current produces a magnetic field that

attracts the receding bar magnet. B

oth cases require work input. If

you try to push a magnet into a coil w

ith more loops, it requires even

more w

ork, as shown in Figure 37.5.

The law

of energy conservation applies here. In Figures 37.3 and 37.4, the force that you exert on the m

agnet multiplied by the distance

that you move the m

agnet is your input work. T

his work is equal to

the energy expended (or possibly stored) in the circuit to which the

coil is connected. If the coil is connected to a resistor, for example,

more induced voltage in the coil m

eans more cur rent through the

resistor, and that means m

ore energy expenditure. 37.1.2T

he amount of

voltage induced depends on how quickly the m

agnetic field changes. Very slow

movem

ent of the magnet into the coil produces hardly any

voltage at all. Quick m

otion induces a greater voltage.It doesn’t m

atter which m

oves—the m

agnet or the coil. It is the relative m

otion of the coil with respect to the m

agnetic field that induces voltage. It so happens that any change in the m

agnetic field around a conductor induces a voltage. T

he phenomenon of inducing

voltage by changing the magnetic field around a conductor is

electromagnetic induction.

CON

CEPT

CHECK

......How

can you create a current using a w

ire and

a mag

net?

FIGU

RE 37.5 !It is m

ore difficult to push the m

agnet into the coil w

ith more loops because

more current flow

s and the coil generates a stronger m

agnetic field that resists the m

otion of the magnet.

FIGU

RE 37.4 "a. C

urrent induced in the loop pro-duces a m

agnetic field (suggested by the im

aginary yellow bar m

agnet), w

hich repels the approaching bar m

agnet.b.When the bar m

agnet is pulled aw

ay from the loop, the

induced current is in the opposite direction and produces a m

agnetic field that attracts the receding bar m

agnet.

0740_CP

09_SE

_CH

37.indd742

11/13/0711:15:46

AM

0740_CP

09_SE

_CH

37.indd743

74

2

Prod

uce m

otio

n o

f a wire lo

op

w

ith resp

ect to a m

agn

et as sh

ow

n in

Figu

res 37.1 and

37.2. U

se a large classro

om

d

emo

nstratio

n g

alvano

meter

to sh

ow

the in

du

ced vo

ltage

that resu

lts from

the relative

mo

tion

.

Plunge a bar magnet into a

coil as in Figure 37.3. Show the

twice-as-m

uch deflection for a coil w

ith twice as m

any turns, and so on. Establish the directly proportional relationship

betw

een induced voltage and

number of turns in the coil.

If you

have a Tesla co

il, d

emo

nstrate in

du

ction

by

ligh

ting

up

a no

n-co

nn

ected

fluo

rescent lam

p a m

eter or

mo

re away. Th

is is imp

ressive.

Dem

on

stratio

ns

Dem

on

stratio

ns

Electric cu

rrent can

b

e pro

du

ced in

a w

ire by sim

ply m

ovin

g a m

agn

et in

to o

r ou

t of a w

ire coil.

Te

ac

hin

g R

es

ou

rc

es

• Reading and Study W

orkbook• Transparencies 87, 88• Presentation

EXPR

ESS

• Interactive Textbook• N

ext-Time Q

uestion 37-1 • Conceptual Physics A

live! D

VD

s Mag

netism

and

In

du

ction

CO

NCEP

TCH

ECK

......

CO

NCEP

TCH

ECK

......

0740_cp09te_Ch37.indd 742

0740_cp09te_Ch37.indd 742

1/30/08 10:52:00 AM

1/30/08 10:52:00 AM

11/13/0711:15:46

AM

37.2 Faraday’s LawFaraday’s law

describes the relationship between induced voltage

and rate of change of a magnetic field.

Faraday’s law states that

the induced voltage in a coil is proportional to the product of the num

ber of loops, the cross-sectional area of each loop, and the rate at w

hich the magnetic field changes w

ithin those loops.Voltage is one thing, and current is another. T

he amount of cur-

rent produced by electromagnetic induction depends not only on the

induced voltage but also on the resistance of the coil and the circuit to w

hich it is connected. 37.2 For example, you can plunge a m

agnet in and out of a closed rubber loop and in and out of a closed loop of copper. T

he voltage induced in each is the same, providing each

intercepts the same num

ber of magnetic field lines. B

ut the current in each is quite different—

a lot in the copper but almost none in the

rubber. The electrons in the rubber sense the sam

e electric field as those in the copper, but their bonding to the fixed atom

s prevents the m

ovement of charge that occurs so freely in the copper.

CON

CEPT

CHECK

......What d

oes Faraday’s law

state?

37.3 Generators and

Alternating C

urrentO

ne way to generate a current is to plunge a m

agnet into and out of a coil of w

ire. As the m

agnetic field strength inside the coil is increased (m

agnet entering), the induced voltage in the coil is directed one w

ay. When the m

agnetic field strength diminishes (m

agnet leav-ing), the voltage is induced in the opposite direction. T

he greater the frequency of field change, the greater the induced voltage. T

he fre-quency of the induced alternating voltage equals the frequency of the changing m

agnetic field within the loop.

Rather than m

oving the magnet,

it is more practical to m

ove the coil. This

is best accomplished by rotating the coil

in a stationary magnetic field, as show

n in Figure 37.6. A

machine that produces

electric current by rotating a coil within

a stationary magnetic field is called

a generator. It is essentially the opposite

of a motor.

Whereas a m

otor converts electrical energy into m

echanical energy, a generator converts m

echanical energy into electrical energy.

If you push a magnet

into a coil connected to a resistor, as show

n in Figure 37.5, you’ll feel a resistance to your push. For the sam

e pushing speed, w

hy is this resis-tance greater in a coil w

ith more loops?

Answer: 37.2

thin

k!

FIGU

RE 37.6 !

A sim

ple generator turns m

echanical energy into electrical energy. Voltage is induced in the loop w

hen it is rotated in the m

agnetic field.

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

743

0740_CP

09_SE

_CH

37.indd743

11/13/0711:15:47

AM

743

Farad

ay’s law states

that th

e ind

uced

vo

ltage in

a coil is p

rop

ortio

nal to

th

e pro

du

ct of th

e nu

mb

er of

loo

ps, th

e cross-sectio

nal area o

f each

loo

p, an

d th

e rate at wh

ich

the m

agn

etic field ch

ang

es w

ithin

tho

se loo

ps.

Te

ac

hin

g R

es

ou

rc

es

• Problem-Solving Exercises in

Physics 18-2

CON

CEPT

CHEC

K

......

CON

CEPT

CHEC

K

......

37.2 Faraday’s LawK

ey

Term

Faraday’s law

" Teach

ing

Tip Exp

lain ag

ain

that th

e mag

net is n

ot a so

urce

of vo

ltage, b

ut rath

er the vo

ltage

is ind

uced

wh

en w

ork is d

on

e to

pu

sh th

e mag

net in

to th

e coil.

It may seem

that o

ne in

creases vo

ltage b

y simp

ly increasin

g th

e n

um

ber o

f loo

ps in

a coil, b

ut it is

at the exp

ense o

f add

ed d

ifficulty

in p

ush

ing

the m

agn

et into

mo

re lo

op

s (Figu

re 37.5). The cu

rrent

ind

uced

is surro

un

ded

by its o

wn

m

agn

etic field, w

hich

resists th

e mag

net yo

u are p

ush

ing

or

pu

lling

. The g

reater the cu

rrent

ind

uced

by th

e action

, the m

ore

resistance m

et. This is evid

ent

in cran

king

a gen

erator, w

hen

ad

ditio

nal electrical lo

ad is

sud

den

ly intro

du

ced.

" Teach

ing

Tip D

iscuss th

e o

peratio

n o

f traffic con

trol

sign

als that are activated

by

the p

assing

of m

etal vehicles

over w

ire loo

ps em

bed

ded

in

the ro

ad su

rface and

of th

e m

etal detecto

rs used

in airp

orts.

The p

assage o

f ferrom

agn

etic m

aterial thro

ug

h o

r over th

ese lo

op

s alters (chan

ges) th

e m

agn

etic field an

d in

du

ces vo

ltage in

the loops.

0740_cp09te_Ch37.indd 743

0740_cp09te_Ch37.indd 743

1/30/08 10:52:04 AM

1/30/08 10:52:04 AM

744

Simple G

enerators When the loop of w

ire is rotated in the mag-

netic field, there is a change in the number of m

agnetic field lines w

ithin the loop, as shown in the diagram

above. In Figure 37.7a, the loop has the largest num

ber of lines inside it. As the loop rotates

(Figure 37.7b), it encircles fewer of the field lines until it lies along

the field lines and encloses none at all (Figure 37.7c). As rotation con-

tinues, it encloses more field lines (Figure 37.7d) and reaches a m

axi-m

um w

hen it has made a half revolution (Figure 37.7e). A

s rotation continues, the m

agnetic field inside the loop changes in cyclic fashion.T

he voltage induced by the generator alternates, and the current produced is alternating current (A

C). T

he current changes magnitude

and direction periodically, as shown in Figure 37.8. T

he standard alternating current in N

orth Am

erica changes its magnitude and

direction during 60 complete cycles per second

—60 hertz.

FIGU

RE 37.7 !

As the loop rotates, there is a change in the num

ber of m

agnetic field lines it encloses.

Guitar pickups are tiny

coils with m

agnets inside them

. The mag-

nets magnetize the

steel strings. When

the strings vibrate, voltage is induced in the coils and boosted by an am

plifier, and sound is produced by a speaker.

a.The loop starts by enclosing the m

aximum

num

ber of field lines.

b.As the loop rotates,

fewer field lines pass

through it.

c.In this position, the loop encloses no field lines.

d.Now

the loop encloses m

ore field lines again.

e.After half a turn, the loop

again encloses the maxi-

mum

number of field lines.

0740_CP

09_SE

_CH

37.indd744

11/13/0711:15:48

AM

0740_CP

09_SE

_CH

37.indd745

74

4

37.3 Generators and

Alternating Current

Ke

y Te

rmg

enerato

r

Retu

rn to

the m

oto

r you

d

emo

nstrated

in C

hap

ter 36, an

d sh

ow

that w

hen

you

app

ly m

echan

ical energ

y, the m

oto

r b

ecom

es a gen

erator. U

se a larg

e classroo

m d

emo

nstratio

n

galvan

om

eter to sh

ow

the

effect. Dem

on

stratio

nD

em

on

stratio

n

0740_cp09te_Ch37.indd 744

0740_cp09te_Ch37.indd 744

1/30/08 10:52:06 AM

1/30/08 10:52:06 AM

11/13/0711:15:48

AM

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

745

Metal Detectors W

alk through a m

etal detector in an airport, and you’re w

alking through a coil of w

ire that carries a small

electric current. In the opening in the coil there is a m

agnetic field. Any change in this field is sensed by the coil. If you carry iron into the coil, you change this m

agnetic field. A changing m

agnetic field induces a change in the current in the coil. The change sets off an alarm

.

Complex

Generators T

he generators used in power plants are

much m

ore complex than the m

odel discussed here. Huge coils m

ade up of m

any loops of wire are w

rapped on an iron core, to make an

armature m

uch like the armature of a m

otor. They rotate in the very

strong magnetic fields of pow

erful electromagnets. T

he armature is

connected externally to an assembly of paddle w

heels called a tur-bine. Energy from

wind or falling w

ater can be used to produce rota-tion of the turbine, but as show

n in Figure 37.9, most com

mercial

generators are driven by moving steam

. At the present tim

e, a fossil fuel or nuclear fuel is used as the energy source for the steam

.It is im

portant to emphasize that an energy source of som

e kind is required to operate a generator. Som

e fraction of energy from the

source, usually some type of fuel, is converted to m

echanical energy to drive the turbine, and the generator converts m

ost of this to elec-trical energy. T

he electricity that is produced simply carries this

energy to distant places. Some people think that electricity is a source

of energy. It is not. It is a form of energy that m

ust have a source. 37.3

CON

CEPT

CHECK

......How

is a generator d

ifferent from a m

otor?

! FIG

URE 37.8

As the loop rotates, the m

ag-nitude and direction of the induced voltage (and current) change. O

ne complete rota-

tion of the loop produces one com

plete cycle in volt-age (and current).

FIGU

RE 37.9 "

Steam drives the turbine,

which is connected to the

armature of the generator.

Link

to

TEC

HN

OLO

GY

0740_CP

09_SE

_CH

37.indd745

11/13/0711:15:48

AM

745

" Teach

ing

Tip In

discu

ssing

th

e op

eration

of a g

enerato

r via Fig

ures 37.6, 37.7, an

d 37.8,

po

int o

ut th

at maxim

um

voltag

e is in

du

ced n

ot w

hen

the lo

op

co

ntain

s the m

ost m

agn

etic field

lines b

ut w

hen

the g

reatest n

um

ber o

f field lin

es are “clip

ped

” (chan

ged

) as the lo

op

is tu

rned

. Hen

ce in Fig

ure 37.8

the vo

ltage is at a m

aximu

m

wh

en th

e loo

p p

asses thro

ug

h

the zero

-nu

mb

er-of-lin

es po

int.

Its rate of ch

ang

e of m

agn

etic field

lines is g

reatest at this p

oin

t.

" Teach

ing

Tip C

on

tinu

e with

a h

istorical th

eme: M

entio

n th

at w

ith th

e adven

t of th

e gen

erator

the task w

as to d

esign

meth

od

s o

f mo

ving

coils o

f wire p

ast m

agn

etic fields, o

r of m

ovin

g

mag

netic field

s past co

ils of w

ire. Pu

t turb

ines b

eneath

waterfalls,

and

bo

il water to

make steam

th

at turn

s turb

ine b

lades—

enter

the In

du

strial Revo

lutio

n.

W

hereas a m

oto

r co

nverts electrical

energ

y into

mech

anical en

ergy, a

gen

erator co

nverts m

echan

ical en

ergy in

to electrical en

ergy.

Te

ac

hin

g R

es

ou

rc

es

• Reading and Study W

orkbook• Presentation

EXPR

ESS

• Interactive Textbook

CON

CEPT

CHEC

K

......

CON

CEPT

CHEC

K

......

0740_cp09te_Ch37.indd 745

0740_cp09te_Ch37.indd 745

1/30/08 10:52:09 AM

1/30/08 10:52:09 AM

746

37.4 Motor and G

enerator C

omparison

In Chapter 36 you saw

how an electric current is deflected in a m

ag-netic field, w

hich underlies the operation of the motor. T

his discov-ery occurred about 10 years before Faraday and H

enry discovered electrom

agnetic induction, which underlies the operation of a gen-

erator. Both of these discoveries, how

ever, stem from

the same single

fact: M

oving charges experience a force that is perpendicular to both

their motion and the m

agnetic field they traverse. We w

ill call the deflected w

ire the motor effect and the law

of induction the generator effect. Each of these effects is sum

marized in Figure 37.10.

Study them. C

an you see that the two effects are related?

The m

otor effect occurs when a current m

oves through a mag-

netic field. In the figure, the current is moving to the right, and the

magnetic field creates a perpendicular upw

ard force on the electrons. B

ecause the electrons can’t leave the wire, the entire w

ire is tugged upw

ard along with the electrons. In the generator effect, a w

ire with

no current is moved dow

nward through a m

agnetic field. The elec-

trons in this wire experience a force perpendicular to their m

otion, w

hich is along the wire. So a current begins to flow

.A

striking example of a device functioning as both m

otor and generator is found in hybrid autom

obiles. When extra pow

er for accelerating or hill clim

bing is needed, this device draws current from

a battery and acts as a m

otor to assist the gasoline engine. When

braking or rolling downhill causes the w

heels to exert a torque on the device, it acts as a generator and recharges the battery. T

he electrical part of the hybrid engine is both a m

otor and a generator.

CON

CEPT

CHECK

......How

does a m

agnetic field

affect a moving

charge?

FIGU

RE 37.10 !The figure show

s the motor effect

and the generator effect. a. When

a current moves to the right,

there is a force on the electrons, and the w

ire is tugged upward.

b.When a w

ire with no current is

moved dow

nward, the electrons in

the wire experience a force, creat-

ing current.

0740_CP

09_SE

_CH

37.indd746

11/13/0711:15:50

AM

0740_CP

09_SE

_CH

37.indd747

74

6

37.4 Motor and

Generator Com

parison!

Teachin

g Tip

Co

mp

are m

oto

rs and

gen

erators—

in

prin

ciple th

ey are the sam

e. A

mo

tor co

nverts electrical en

ergy

into

mech

anical en

ergy. A

g

enerato

r con

verts mech

anical

energ

y into

electrical energ

y. In

fact, a mo

tor acts also

as a g

enerato

r, wh

ich creates a

“back vo

ltage” (b

ack emf) an

d

an o

pp

osin

g cu

rrent. Th

e net

curren

t in a m

oto

r is the in

pu

t cu

rrent m

inu

s the g

enerated

b

ack curren

t. The n

et curren

t in

a po

wer saw

will n

ot cau

se it to

overh

eat and

dam

age its m

oto

r w

ind

ing

s—so

lon

g as it is ru

nn

ing

an

d g

eneratin

g a b

ack curren

t th

at keeps th

e net cu

rrent lo

w.

Bu

t if you

sho

uld

jam th

e saw

so th

at it can’t sp

in, th

e back

curren

t wo

uld

cease, causin

g

the n

et curren

t to b

ecom

e d

ang

erou

sly hig

h an

d p

ossib

ly b

urn

ou

t the m

oto

r.

! Teach

ing

Tip M

entio

n th

at electric m

oto

rs are used

in d

iesel-p

ow

ered railro

ad en

gin

es. The

com

bu

stion

eng

ine alo

ne can

no

t m

ove a h

eavy load

from

rest, bu

t w

hen

it is cou

pled

to an

electric m

oto

r, it can. W

hen

the arm

ature

in a m

oto

r is no

t turn

ing

, the

curren

t in th

e win

din

gs is h

ug

e, w

ith a co

rrespo

nd

ing

ly hu

ge

force. A

s bo

th th

e train an

d

the m

oto

r gain

speed

, the b

ack cu

rrent g

enerated

by th

e mo

tor

brin

gs th

e net cu

rrent in

the

mo

tor d

ow

n to

no

n-o

verheatin

g

levels.

M

ovin

g ch

arges

experien

ce a force

that is p

erpen

dicu

lar bo

th to

th

eir mo

tion

and

the m

agn

etic field

they traverse.

CO

NCEP

TCH

ECK

......

CO

NCEP

TCH

ECK

......

0740_cp09te_Ch37.indd 746

0740_cp09te_Ch37.indd 746

1/30/08 10:52:12 AM

1/30/08 10:52:12 AM

11/13/0711:15:50

AM

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

747

37.5 Transformers

Consider a pair of coils, side by side, as show

n in Figure 37.11. One is

connected to a battery and the other is connected to a galvanometer.

It is customary to refer to the coil connected to the pow

er source as the

primary (input), and the other as the secondary (output). A

s soon as the sw

itch is closed in the primary and current passes through its

coil, a current occurs in the secondary also—even though there is no

material connection betw

een the two coils. O

nly a brief surge of cur-rent occurs in the secondary, how

ever. Then w

hen the primary sw

itch is opened, a surge of current again registers in the secondary but in the opposite direction.

The explanation is that the m

agnetic field that builds up around the prim

ary extends into the secondary coil. Changes in the m

ag-netic field of the prim

ary are sensed by the nearby secondary. These

changes of magnetic field intensity at the secondary induce voltage in

the secondary, in accord with Faraday’s law

.If w

e place an iron core inside the primary and secondary coils

of the arrangement show

n in Figure 37.11, the magnetic field w

ithin the prim

ary is intensified by the alignment of m

agnetic domains in

the iron. The m

agnetic field is also concentrated in the core, which

extends into the secondary, so the secondary intercepts more of the

field change. The galvanom

eter will show

greater surges of current w

hen the switch of the prim

ary is opened or closed. Instead of opening and closing a sw

itch to produce the change of m

agnetic field, suppose that alternating current is used to power

the primary. T

hen the rate at which the m

agnetic field changes in the prim

ary (and hence in the secondary) is equal to the frequency of the alternating current. N

ow w

e have a transformer,as show

n in Figure 37.12. A

transformer is a device for increasing or decreasing volt-

age through electromagnetic induction.

A transform

er works by

inducing a changing magnetic field in one coil, w

hich induces an alternating current in a nearby second coil.

! FIG

URE 37.11

Whenever the prim

ary switch

is opened or closed, voltage is induced in the secondary circuit.

When the sw

itch of the prim

ary in Figure 37.11 is opened or closed, the galvanom

eter in the secondary registers a cur-rent. But w

hen the switch

remains closed, no current

is registered on the galva-nom

eter of the secondary. W

hy?Answ

er: 37.5.1

thin

k!

FIGU

RE 37.12 "

A sim

ple transformer

arrangement using an iron

core creates greater current in the secondary coil.

0740_CP

09_SE

_CH

37.indd747

11/13/0711:15:51

AM

747

37.5 Transformers

Ke

y Te

rmtran

sform

er

Co

mm

on

Misco

ncep

tion

A transform

er can step up energy, or step up pow

er.

FAC

T As a tran

sform

er steps u

p

(or d

ow

n) vo

ltage, it tran

sfers en

ergy fro

m o

ne co

il to th

e o

ther, alw

ays ob

eying

the law

of

con

servation

of en

ergy.

# Teach

ing

Tip Exp

lain h

ow

a tran

sform

er wo

rks, and

d

emo

nstrate th

e setup

sho

wn

in

Figu

re 37.11.

Ligh

t a bu

lb w

ith a h

and

-cran

ked g

enerato

r and

sho

w

ho

w th

e turn

ing

is easier wh

en

the b

ulb

is loo

sened

and

the

load

remo

ved. A

llow

stud

ents

to try th

is them

selves du

ring

o

r at the en

d o

f class.

TEA

CH

ING TIP Stress ag

ain

the fact th

at we d

on

’t get

som

ethin

g fo

r no

thin

g w

ith

electrom

agn

etic ind

uctio

n,

and

refer back to

Figu

re 37.5.

Dem

on

stratio

nD

em

on

stratio

n

Te

ac

hin

g R

es

ou

rc

es

• Reading and Study W

orkbook• Laboratory M

anual 99• Transparency 89• Presentation

EXPR

ESS

• Interactive Textbook

0740_cp09te_Ch37.indd 747

0740_cp09te_Ch37.indd 747

1/30/08 10:52:15 AM

1/30/08 10:52:15 AM

748

A m

ore efficient arrangement for a transform

er is shown in

Figure 37.13, where the iron core form

s a complete loop to guide all

the magnetic field lines through the secondary. A

ll the magnetic field

lines within the prim

ary are intercepted by the secondary.

Voltage

Voltages may be stepped up or stepped dow

n with a trans-

former. To see how

, consider the simple case show

n in Figure 37.14a. Suppose the prim

ary consists of one loop connected to a 1-V alter-

nating source. Consider the sym

metrical arrangem

ent of a secondary of one loop that intercepts all the changing m

agnetic field lines of the prim

ary. Then a voltage of 1 V

is induced in the secondary.If another loop is w

rapped around the core, as shown in Figure

37.14c, the induced voltage will be tw

ice as much, in accord w

ith Faraday’s law

. If the secondary is wound w

ith three times as m

any loops, or turns as they are called, then three tim

es as much volt-

age will be induced. If the secondary has a hundred tim

es as many

turns as the primary, then a hundred tim

es as much voltage w

ill be induced, and so on. T

his arrangement of a greater num

ber of turns on the secondary than on the prim

ary makes up a step-up trans-

former. Stepped-up voltage m

ay light a neon sign or operate the pic-ture tube in a television receiver.

FIGU

RE 37.14 !

a. The voltage of 1 V induced in the secondary equals the voltage of the prim

ary. b. A voltage of

1 V is induced in the added sec-ondary also because it intercepts the sam

e magnetic field change

from the prim

ary. c.The voltages of 1 V induced in each of the tw

o one-turn secondaries are equivalent to a voltage of 2 V induced in a single tw

o-turn secondary.

FIGU

RE 37.13 !

The iron core guides the changing m

agnetic field lines, which m

akes a m

ore efficient transformer.

Transformers convert

voltage from high to

low (from

120 V to 6 V for your laptop) or from

low to high

(from 120 V to 220 V,

for your Hong-Kong

hair dryer).

0740_CP

09_SE

_CH

37.indd748

11/13/0711:15:52

AM

0740_CP

09_SE

_CH

37.indd749

74

8

! Teach

ing

Tip Stress th

at vo

ltage m

ay be step

ped

up

, an

d cu

rrent m

ay be step

ped

up

, b

ut th

e pro

du

ct of cu

rrent an

d

voltag

e cann

ot b

e stepp

ed u

p.

Caution: Wear g

og

gles

and

heat-resistan

t glo

ves w

hen

perfo

rmin

g th

is d

emo

nstratio

n. W

eld a p

air o

f nails to

geth

er with

a step-

do

wn

transfo

rmer. Th

is is a sp

ectacular d

emo

nstratio

n

wh

en yo

u casu

ally place yo

ur

fing

ers betw

een th

e nail en

ds

befo

re they m

ake con

tact. Th

en rem

ove yo

ur fin

gers an

d

brin

g th

e po

ints to

geth

er, allo

win

g th

e sparks to

fly w

hile th

e nails q

uickly b

ecom

e red

and

then

wh

ite ho

t.

Dem

on

stratio

nD

em

on

stratio

n

The conservation of energy reigns!

! Teach

ing

Tip M

entio

n

the ro

le of th

e transfo

rmer in

step

pin

g d

ow

n vo

ltages in

toy

electric trains, electric calcu

lators,

mo

bile p

ho

ne ch

argers, an

d

po

rtable m

usic p

layers, and

the

role o

f stepp

ing

up

voltag

es in

television

sets and

variou

s electrical d

evices.

0740_cp09te_Ch37.indd 748

0740_cp09te_Ch37.indd 748

1/30/08 10:52:18 AM

1/30/08 10:52:18 AM

11/13/0711:15:52

AM

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

749

If the voltage in a transform

er is stepped up, then the current is stepped dow

n. Ohm

’s law

says that increased voltage w

ill produce increased current. Is there a contradiction here, or does O

hm’s Law

not apply to transform

ers?Answ

er: 37.5.2

thin

k!

If the secondary has fewer turns than the prim

ary, the alternat-ing voltage produced in the secondary w

ill be lower than that in the

primary. T

he voltage is said to be stepped down. If the secondary has

half as many turns as the prim

ary, then only half as much voltage is

induced in the secondary. So electrical energy can be fed into the prim

ary at a given alter-nating voltage and taken from

the secondary at a greater or lower

alternating voltage, depending on the relative number of turns in the

primary and secondary coil w

indings, as shown in Figure 37.15.

The relationship betw

een primary and secondary voltages w

ith respect to the relative num

ber of turns is

primary voltage

number of prim

ary turnssecondary voltage

number of secondary turns

Power It m

ight seem that you get som

ething for nothing with a

transformer that steps up the voltage. N

ot so, for energy conservation alw

ays controls what can happen. T

he transformer actually transfers

energy from one coil to the other. T

he rate at which energy is trans-

ferred is the power. T

he power used in the secondary is supplied by

the primary. T

he primary gives no m

ore power than the secondary

uses, in accord with the conservation of energy. If the slight pow

er losses due to heating of the core are neglected, then the pow

er going into the prim

ary equals the power com

ing out of the secondary. Electric pow

er is equal to the product of voltage and current:

(voltagecurrent)

primary

(voltagecurrent)

secondary

You can see that if the secondary has more voltage, it w

ill have less current than the prim

ary. Or vice versa; if the secondary has less

voltage, it will have m

ore current than the primary. T

he ease with

which voltages can be stepped up or dow

n with a transform

er is the principal reason that m

ost electric power is A

C rather than D

C.

Figure 37.16 shows a com

mon household transform

er used today.

CON

CEPT

CHECK

......How

does a transform

er work?

! FIG

URE 37.15

A practical transform

er uses m

any coils. The relative num

bers of turns in the coils determ

ines how

much the voltage

changes.

FIGU

RE 37.16 "

This comm

on transformer

lowers 120 V to 6 V or

9 V. It also converts AC

to D

C by m

eans of a diodeinside—

a tiny electronic device (show

n in Chapter

34, Figure 34.12) that acts as a one-w

ay valve.

0740_CP

09_SE

_CH

37.indd749

11/14/0711:14:20

AM

749

As a student, I rem

ember

being very confused about the seem

ing contradiction with O

hm’s law—

the idea that when voltage in the secondary coil is increased, current in the secondary coil is decreased. M

ake clear that when the voltage in the secondary coil and the circuit it connects to is increased, the current in that circuit also increases. The decrease occurs with respect to the current that powers the prim

ary coil. So P 5 IV does not

contradict Ohm

’s law!

A

transfo

rmer w

orks

by in

du

cing

a ch

ang

ing

mag

netic field

in o

ne

coil, w

hich

ind

uces an

alternatin

g

curren

t in a n

earby seco

nd

coil.

Te

ac

hin

g R

es

ou

rc

es

• Reading and Study W

orkbook• Concept-D

evelopment

Practice Book 37-1• Presentation

EXPR

ESS

• Interactive Textbook

CON

CEPT

CHEC

K

......

CON

CEPT

CHEC

K

......

0740_cp09te_Ch37.indd 749

0740_cp09te_Ch37.indd 749

1/30/08 10:52:21 AM

1/30/08 10:52:21 AM

750

37.6 Power Transm

issionA

lmost all electric energy sold today is in the form

of alternating current because of the ease w

ith which it can be transform

ed from

one voltage to another. Power is transm

itted great distances at high voltages and correspondingly low

currents, a process that otherwise

would result in large energy losses ow

ing to the heating of the wires.

Power m

ay be carried from pow

er plants to cities at about 120,000 volts or m

ore, stepped down to about 2400 volts in the city, and finally

stepped down again by a transform

er such as the one shown in Figure

37.17 to provide the 120 volts used in household circuits.

Energy, then, is transformed from

one system of conducting w

ires to another by electrom

agnetic induction as shown in Figure 37.18.

The sam

e principles account for eliminating w

ires and sending energy from

a radio-transmitter antenna to a radio receiver m

any kilometers

away, and for the transform

ation of energy of vibrating electrons in the sun to life energy on Earth. T

he effects of electromagnetic induc-

tion are very far-reaching.

CON

CEPT

CHECK

......Why is alm

ost all electrical energy sold

today in the

form of alternating

current?

FIGU

RE 37.17 !

A com

mon neighborhood

transformer, w

hich typically steps 2400 V dow

n to 240 V for houses and sm

all businesses. Inside the hom

e or business, the 240 V can be divided to a safer 120 V.

FIGU

RE 37.18 "

Power transm

ission depends on transform

ers. Voltage is increased for long-dis-tance transm

ission and then decreased before it reaches your hom

e.

200 years ago, people got light from

whale

oil. Whales should

be glad that humans

discovered how to

harness electricity!

0740_CP

09_SE

_CH

37.indd750

11/13/0711:15:58

AM

0740_CP

09_SE

_CH

37.indd751

75

0

37.6 Power

Transmission

! Teach

ing

Tip D

iscuss th

e ro

les of b

oth

stepp

ing

up

and

step

pin

g d

ow

n vo

ltages in

po

wer

transm

ission

. Stress that in

no

w

ay is energ

y or p

ow

er stepp

ed

up

or d

ow

n—

a con

servation

of

energ

y no

-no

!

! Teach

ing

Tip Tell stu

den

ts th

at cost is th

e main

reason

fo

r hig

h-vo

ltage p

ow

er lines. If

hig

her cu

rrents w

ere carried in

th

e lines, th

e wires w

ou

ld h

ave to

be th

icker and

therefo

re costlier.

They w

ou

ld also

be h

eavier, w

hich

wo

uld

requ

ire stron

ger

tow

ers.

A

lmo

st all electrical en

ergy so

ld to

day is

in th

e form

of altern

ating

curren

t b

ecause o

f the ease w

ith w

hich

it can

be tran

sform

ed fro

m o

ne

voltag

e to an

oth

er.

Te

ac

hin

g R

es

ou

rc

es

• Reading and Study W

orkbook• Concept-D

evelopment

Practice Book 37-2• Presentation

EXPR

ESS

• Interactive Textbook

CO

NCEP

TCH

ECK

......

CO

NCEP

TCH

ECK

......

0740_cp09te_Ch37.indd 750

0740_cp09te_Ch37.indd 750

1/30/08 10:52:24 AM

1/30/08 10:52:24 AM

11/13/0711:15:58

AM

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

751

Magnetic Storage C

omputers store data

on plastic disks that have been coated w

ith a magnetic m

aterial. Magnetic

patterns can be applied to the disk by a recording head. C

oded electrical pulses that carry inform

ation are changed into m

agnetic pulses and stored on the disk. W

hen a magnetically stored bit of

information on the disk spins under a reading head that contains

a small coil, the pulses are converted back to electrical signals again.

Link

to TE

CH

NO

LOG

YLin

k to

TEC

HN

OLO

GY

37.7 Induction of Electric and M

agnetic FieldsElectrom

agnetic induction has thus far been discussed in terms of

the production of voltages and currents. Actually, the m

ore fun-dam

ental way to look at it is in term

s of the induction of electric fields. T

he electric fields, in turn, give rise to voltages and currents. Induction takes place w

hether or not a conducting wire or any m

ate-rial m

edium is present. Faraday’s law

states that an electric field is created in any region of space in w

hich a magnetic field is changing

with tim

e. The m

agnitude of the created electric field is proportional to the rate at w

hich the magnetic field changes. T

he direction of the created electric field is at right angles to the changing m

agnetic field.If electric charge happens to be present w

here the electric field is created, this charge w

ill experience a force. For a charge in a wire, the

force could cause it to flow as current, or to push the w

ire to one side. For a charge in an evacuated region, like in the cham

ber of a particle accelerator, the force can accelerate the charge to high speeds.

There is a second effect, w

hich is the counterpart to Faraday’s law

. It is just like Faraday’s law, except that the roles of electric and

magnetic fields are interchanged. T

he symm

etry between electric

and magnetic fields revealed by this pair of law

s is one of the many

beautiful symm

etries in nature. The com

panion to Faraday’s law w

as advanced by the B

ritish physicist James C

lerk Maxw

ell in the 1860s. A

magnetic field is created in any region of space in w

hich an electric field is changing w

ith time. A

ccording to Maxw

ell, the m

agnitude of the created magnetic field is proportional to the rate

at which the electric field changes. T

he direction of the created mag-

netic field is at right angles to the changing electric field.

CON

CEPT

CHECK

......How

can an electric field create a m

agnetic field

?

In making a scientific

discovery, being at the right place at the right tim

e is not enough—curiosity, patience, and hard w

ork are also im

portant.

0740_CP

09_SE

_CH

37.indd751

11/13/0711:16:00

AM

751

37.7 Induction of Electric and M

agnetic Fields

With

the p

ow

er on

, levitate an

alum

inu

m rin

g o

ver the

extend

ed p

ole o

f an Elih

u

Tho

mp

son

device.

With

the p

ow

er o

ff, place th

e rin

g at th

e base

of th

e extend

ed

po

le. Wh

en yo

u

switch

on

the

po

wer th

e cu

rrent

ind

uced

in

the rin

g

via electrom

agn

etic ind

uctio

n

con

verts the rin

g in

to an

AC

electro

mag

net. (B

y Lenz’s law

, th

e po

larity of th

e ind

uced

m

agn

et is always su

ch as to

o

pp

ose th

e mag

netic field

im

po

sed.)

Dem

on

stratio

nD

em

on

stratio

n

Ask H

ow m

uch electrom

agnetic force supports this 1-N

aluminum

ring (assuming

the ring weighs 1 N

)? 1 N.

This can

be an

swered

with

no

kn

ow

ledg

e of electro

mag

netic

forces b

ut fro

m kn

ow

ledg

e abo

ut

forces in

gen

eral that g

oes b

ack to

New

ton

’s laws. Sin

ce the rin

g

is at rest and

no

t accelerating

, th

e up

ward

electrom

agn

etic fo

rce—in

new

ton

s—m

ust b

e eq

ual to

the d

ow

nw

ard fo

rce of

gravity. W

as the electromagnetic

force that lifted the ring more

than, equal to, or less than the m

agnetic force that produced levitation earlier? M

ore th

an,

becau

se it accelerated u

pw

ard,

eviden

ce that th

e up

ward

force

was m

ore th

an th

e weig

ht; th

is is also

un

derstan

dab

le becau

se the

ring

was lo

wer an

d in

terceptin

g

mo

re chan

gin

g m

agn

etic field

lines.

0740_cp09te_Ch37.indd 751

0740_cp09te_Ch37.indd 751

1/30/08 10:52:27 AM

1/30/08 10:52:27 AM

752

0740_CP

09_SE

_CH

37.indd752

11/13/0711:16:01

AM

0740_CP

09_SE

_CH

37.indd753

75

2

Show the classic lighting of

the lamp in a jar of w

ater. Im

pressive! Mount the lam

p

on a waxed w

aterproof coil, w

hich intercepts the changing

magnetic flux of the device,

induces current, and illuminates

the lamp. (The w

ater serves no

purpose other than making

the dem

onstration more

interesting.)

Dem

on

stratio

nD

em

on

stratio

n

Carefully go over the comic

strip “Power Lines.” The physics here is deeper than in the other com

ic strips and may need

elaboration.

! Teach

ing

Tip State th

at u

nd

erlying

all the th

ing

s d

iscussed

and

ob

served is th

e in

du

ction

of b

oth

electric and

m

agn

etic fields. B

ecause o

f this

we can

send

sign

als with

ou

t w

ires—rad

io an

d TV

—an

d

furth

ermo

re, energ

y reaches u

s fro

m th

e sun

thro

ug

h su

nlig

ht.

! Teach

ing

Tip M

entio

n th

at th

e con

cept th

at a chan

ge in

eith

er field in

du

ces the o

ther led

Ein

stein to

develo

p h

is special

theo

ry of relativity. H

e sho

wed

th

at a mag

netic field

results

wh

en an

electric field is seen

by a

mo

ving

ob

server, and

an electric

field resu

lts wh

en a m

agn

etic field

is seen b

y a mo

ving

o

bserver. Th

e fields are relative.

A

mag

netic field

is created

in an

y regio

n

of sp

ace in w

hich

an electric field

is ch

ang

ing

with

time.

CO

NCEP

TCH

ECK

......

CO

NCEP

TCH

ECK

......

0740_cp09te_Ch37.indd 752

0740_cp09te_Ch37.indd 752

1/30/08 10:52:31 AM

1/30/08 10:52:31 AM

11/13/0711:16:01

AM

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

753

37.8 Electromagnetic W

avesShake the end of a stick back and forth in still w

ater and you will

produce waves on the w

ater surface. Similarly shake a charged rod

back and forth in empty space and you w

ill produce electromagnetic

waves in space, as show

n in Figure 37.19. This is because the shak-

ing charge can be considered an electric current. What surrounds an

electric current? The answ

er is a magnetic field. W

hat surrounds a changing electric current? T

he answer is, a changing m

agnetic field. W

hat do we know

about a changing magnetic field? T

he answer is,

it will create a changing electric field, in accord w

ith Faraday’s law.

What do w

e know about a changing electric field? T

he answer is, in

accord with M

axwell’s counterpart to Faraday’s law

, the changing electric field w

ill create a changing magnetic field.

An electrom

agnetic wave is com

posed of oscillating elec-tric and m

agnetic fields that regenerate each other. No m

edium is

required. The oscillating fields em

anate (move outw

ard) from the

vibrating charge. At any point on the w

ave, the electric field is per-pendicular to the m

agnetic field, and both are perpendicular to the direction of m

otion of the wave, as show

n in Figure 37.20.

Speed of Electromagnetic W

aves How

fast does the electro-m

agnetic wave m

ove? This is a very interesting question, and, in the

history of physics, a very important one. If you ask how

fast a baseball m

oves, or a car, or a spacecraft, or a planet, there is no single answer.

It depends on how the m

otion got started, what forces are acting, and

how fast the observer is m

oving. But for electrom

agnetic radiation, there is only one speed—

the speed of light—no m

atter what the fre-

quency or wavelength or intensity of the radiation.

FIGU

RE 37.20 !The electric and m

agnetic fields of an electromagnetic

wave are perpendicular to each other.

FIGU

RE 37.19 !Shake a charged object back and forth and you produce electrom

agnetic waves.

0740_CP

09_SE

_CH

37.indd753

11/13/0711:16:05

AM

753

37.8 Electromagnetic

Waves

" Teach

ing

Tip R

ecall you

r recen

t dem

on

stration

of ch

argin

g

the ru

bb

er rod

with

fur. W

hen

yo

u b

rou

gh

t the ro

d n

ear a ch

arged

pith

ball, yo

u p

rod

uced

actio

n at a d

istance. W

hen

you

m

oved

the ch

arged

rod

, the

charg

ed b

all mo

ved also

. Wh

en

you

gen

tly oscillated

the ro

d, th

e b

all in tu

rn o

scillated. State th

at o

ne can

thin

k of th

is beh

avior as

either actio

n at a d

istance o

r the

interactio

n o

f the b

all and

rod

w

ith th

e surro

un

din

g sp

ace—th

e electric field

. For lo

w freq

uen

cies, th

e ball w

ill swin

g in

rhyth

m w

ith

the sh

aking

rod

.

The inertia of the ball and its pendulum

configuration make

response poor for any vigorous shaking of the rod. That’s why it’s best not to actually show this but to only describe it, and go through the m

otions as if the equipm

ent were present—you’ll

avoid the “that’s the way it should behave” situation.

" Teach

ing

Tip Establish

in your students’ m

inds the reasonableness of the ball shaking

back and forth in response to

the shaking (changing) electric field around the shaking rod. Carry this further by considering

the ball to be a point charge w

ith tiny mass. N

ow it w

ill respond in synchronous rhythm

to the shaking rod. Increase the frequency of the shaking rod

and state that not only is there a shaking electric field about the rod, but because of its changing, there is now

a different kind of field.

0740_cp09te_Ch37.indd 753

0740_cp09te_Ch37.indd 753

1/30/08 10:52:36 AM

1/30/08 10:52:36 AM

754

This rem

arkable constancy of the speed of propagating electric and m

agnetic fields was discovered by M

axwell. T

he key to under-standing it lies in the perfect balance betw

een the two kinds of fields

that must exist if they are to propagate as w

aves. The changing electric

field induces a magnetic field. T

he changing magnetic field acts back

to induce an electric field. The w

ave is continually self-reinforcing. M

axwell’s equations show

ed that only one speed could preserve this harm

onious balance of fields. If, hypothetically, the w

ave traveled at less than the speed of light, the fields w

ould rapidly die out. The electric field w

ould induce a w

eaker magnetic field, w

hich would induce a still w

eaker electric field, and so on. If, still hypothetically, the w

ave traveled at more than the

speed of light, the fields would build up in a crescendo of ever greater

magnitudes—

clearly a no-no with respect to energy conservation. A

t som

e critical speed, however, m

utual induction continues indefinitely, w

ith neither a loss nor a gain in energy.From

his equations of electromagnetic induction, M

axwell cal-

culated the value of this critical speed and found it to be 300,000 kilom

eters per second. To do this calculation, he used only the con-stants in his equations determ

ined by simple laboratory experim

ents w

ith electric and magnetic fields. H

e didn’t use the speed of light. He

found the speed of light!

At only one speed w

ill the linkage betw

een electric and m

agnetic fields be in perfect bal-ance w

ith no gain or loss of energy—

exactly the speed of light!

Cellular Field Technician Wireless com

munication through

such devices as cellular telephones and pagers depends on com

munications tow

ers maintained by cellular field technicians.

Signals carried by electromagnetic w

aves are transferred from one

tower to the next. The Federal Com

munications Com

mission (FCC)

determines the frequency of the w

aves allowed at each tow

er. Cellular field technicians use physics to analyze or alter the electrom

agnetic w

aves coming into a receiver or being sent out by a transm

itter. Cellular field technicians are em

ployed by companies that m

aintain cellular com

munications tow

ers.

Nature of Light M

axwell quickly realized that he had discovered

the solution to one of the greatest mysteries of the universe—

the nature of light. If electric charges are set into vibration w

ith frequen-cies in the range of 4.3 !

1014 to 7 !

1014 vibrations per second, the

resulting electromagnetic w

ave will activate the “electrical antennae”

in the retina of the eye. Light is simply electrom

agnetic waves in this

range of frequencies! The low

er end of this frequency range appears red, and the higher end appears violet. M

axwell realized that radia-

tion of any frequency would propagate at the sam

e speed as light.

Ph

ysics o

n th

e Jo

b

0740_CP

09_SE

_CH

37.indd754

11/14/0711:14:27

AM

0740_CP

09_SE

_CH

37.indd755

75

4 Ask W

hat kind of field is induced by the shaking rod? W

hat kind of field, in turn, does this induced field induce? A

nd further, in turn, what

kind of field does this further induced field induce? A

nd so on. Th

e shakin

g ch

arge in

du

ces a m

agn

etic field, th

e chan

gin

g

of w

hich

ind

uces an

electric field

, and

so o

n. Th

e result is an

electro

mag

netic w

ave.

0740_cp09te_Ch37.indd 754

0740_cp09te_Ch37.indd 754

1/30/08 10:52:39 AM

1/30/08 10:52:39 AM

11/14/0711:14:27

AM

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

755

FIGU

RE 37.21 !Electrom

agnetic wave em

anation by a sending antenna and reception by a receiving antenna. Successive view

s, (a) through (i), show how

acceleration of the charges up and dow

n the antenna transmits electrom

agnetic waves. O

nly sample electric field

lines of the wave are show

n—the m

agnetic field lines are perpendicular to the electric field lines and extend into and out of the page.

This radiation includes radio w

aves, which can be generated

and received by antennas, as shown in Figure 37.21. A

rotating device in the sending antenna alternately charges the upper and low

er parts of the antenna positively and negatively. T

he charges accelerating up and dow

n the antenna transmit electrom

agnetic waves. W

hen the w

aves hit a receiving antenna, the electric charges inside vibrate in rhythm

with the variations of the field.

On the evening of M

axwell’s discovery of the nature of light, he

had a date with a young w

oman he w

as later to marry. W

hile walking

in a garden, his date remarked about the beauty and w

onder of the stars. M

axwell asked how

she would feel to know

that she was w

alk-ing w

ith the only person in the world w

ho knew w

hat the starlight really w

as. For it was true. A

t that time, Jam

es Clerk M

axwell w

as the only person in the w

orld to know that light of any kind is energy car-

ried in waves of electric and m

agnetic fields that continually regener-ate each other.

CON

CEPT

CHECK

......What m

akes up an electrom

agnetic w

ave?

0740_CP

09_SE

_CH

37.indd755

11/14/075:17:51

PM

755

" Teach

ing

Tip M

ention the idea of the optim

um speed

of field disturbances from

the shaking rod to the ball (consistent w

ith energy conservation). At

only one speed will the linkage

between electric and m

agnetic fields be in perfect balance w

ith

no gain or loss of energy—exactly

at the speed of light!

" Teach

ing

Tip M

ake sure your students know

that the room

they sit in is chock full of waves

of many frequencies. Turn out

the lights and state that the total am

ount of radiation in the room

decreased very slightly as a result—

that the light waves m

ake up a tiny part of the vibrations that engulf us at every m

oment.

" Teach

ing

Tip Explain that

up to the last century, reality w

as what people could see and

touch. Since the discovery of the electrom

agnetic spectrum, people

have learned that what they can

see and touch is less than one-m

illionth of reality.

A

n electro

mag

netic

wave is co

mp

osed

of

oscillatin

g electric an

d m

agn

etic field

s that reg

enerate each

oth

er.

Te

ac

hin

g R

es

ou

rc

es

• Reading and Study W

orkbook• Transparencies 90, 91• Presentation

EXPR

ESS

• Interactive Textbook

CON

CEPT

CHEC

K

......

CON

CEPT

CHEC

K

......

0740_cp09te_Ch37.indd 755

0740_cp09te_Ch37.indd 755

1/30/08 10:52:41 AM

1/30/08 10:52:41 AM

756

Is ELF Radiation Dangerous?

We live in fear of the unsensed. Anything that exists, or is im

agined to exist, yet escapes detection by our five senses, is often a source of fear.

Many people fear radiation. Som

e is hazardous, and some is not. N

o one doubts the hazards of radiation from

some nuclear reactions, and no one

seriously fears the low-frequency radiations of AM

radio. But in recent years a series of books and m

agazine articles have fanned the flames of

public fear by claiming that the extrem

ely low frequency (ELF) radiation

of comm

on 60-Hz electric pow

er causes certain forms of cancer.

Is this claim true? Som

e activists say yes, although the scientific consensus is that this is just one of m

any health scares that has no basis. Careful studies have not substantiated the claim

ed risk. Bioscientists point out that the electric fields due to pow

er lines at the location of a cell in the body are thousands of tim

es smaller than those due to the

normal electrical activity of nearby cells. They also point out that cancer

rates have remained constant or fallen over the last 50 years (w

ith the exception of rising cancer rates due to sm

oking). Yet during this time,

exposure to ELF radiation has increased tremendously. M

ore detailed analysis of the studies that prom

pted the controversy shows no link

between ELF and cancer.

Critical Thinking Suppose you’re a scientist and you find uncertain evidence that som

e comm

on food—tom

atoes, for example—

may be

a serious health risk. What responsibility w

ould you have to make your

findings known to the general public? If you stress the uncertainty

of your findings, perhaps no one will listen. Should you then m

ake sensational claim

s, even unsupported, to get people’s attention?

Scie

nce

, Tech

no

logy, a

nd

So

ciety

For:

–Visit:W

eb Code:

Links on electrom

agnetic induction

w

ww.SciLinks.org

csn 3708

0740_CP

09_SE

_CH

37.indd756

11/13/0711:16:08

AM

0740_CP

09_SE

_CH

37.indd757

75

6 Scie

nce

, Tech

no

logy,

an

d S

ocie

ty

CRITICAL TH

INKIN

G Student opinions and answ

ers will vary.

Allow all reasonable responses,

to generate classroom

discussion.

0740_cp09te_Ch37.indd 756

0740_cp09te_Ch37.indd 756

1/30/08 10:52:43 AM

1/30/08 10:52:43 AM

11/13/0711:16:08

AM

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

757

REVIEW

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

757

3For:Visit:W

eb Code: –

Self-Assessment

PHSchool.com

csa 3700

37.2 Sim

ply put, more w

ork is required because m

ore turns mean that m

ore voltage is induced, producing m

ore current in the resistor and m

ore energy transfer. You can also look at it this w

ay: When the m

agnetic fields of tw

o magnets (electro or perm

a-nent) overlap, the tw

o magnets are either

forced together or forced apart. When

one of the fields is induced by motion

of the other, the polarity of the fields is alw

ays such as to force the magnets apart.

This produces the resistive force you feel.

Inducing more current in m

ore coils simply

increases the induced magnetic field and

thus the resistive force.

37.5.1 A

current is only induced in a coil when

there is a change in the magnetic field pass-

ing through it. When the sw

itch remains in

the closed position, there is a steady current in the prim

ary and a steady magnetic field

about the coil. This field extends into the

secondary, but unless there is a change in the field, electrom

agnetic induction does not occur.

37.5.2 O

hm’s law

still holds, and there is no con-tradiction. T

he voltage induced across the secondary circuit, divided by the load (resistance) of the secondary circuit, equals the current in the secondary circuit. T

he current is stepped dow

n in comparison

with the larger current that is draw

n in the prim

ary circuit.

thin

k!

Answ

ers

Conce

pt Su

mm

ary

•••

••

•

•

Electric current can be produced in a w

ire simply by m

oving a magnet into or

out of a wire coil.

•

Faraday’s law states that the induced volt-

age in a coil is proportional to the prod-uct of the num

ber of loops, the cross-sectional area of each loop, and the rate at w

hich the magnetic field changes.

•

A generator converts m

echanical energy into electrical energy.

•

Moving charges experience a force that is

perpendicular to both their motion and

the magnetic field they traverse.

•

A transform

er works by inducing a

changing magnetic field in one coil,

which induces an alternating current in a

nearby second coil.

•

Alm

ost all electric energy sold today is in the form

of alternating current because of the ease w

ith which it can be trans-

formed from

one voltage to another.

•

A m

agnetic field is created anywhere an

electric field changes with tim

e.

•

An electrom

agnetic wave is com

posed of oscillating electric and m

agnetic fields.

Key Te

rms

••

•••

•••

••

•••

electromagnetic

induction (p. 742)

Faraday’s law (p. 743)

generator (p. 743)

transformer (p. 747)

0740_CP

09_SE

_CH

37.indd757

11/14/0711:14:28

AM

757

R

EVIEW

Te

ac

hin

g R

es

ou

rc

es

• TeacherEXPR

ESS

• Conceptual Physics Alive!

DV

Ds M

agn

etism an

d

Ind

uctio

n

0740_cp09te_Ch37.indd 757

0740_cp09te_Ch37.indd 757

1/30/08 10:52:47 AM

1/30/08 10:52:47 AM

758758

3For:Visit:W

eb Code: –

ASSESS

Conce

pt Su

mm

ary

•••

••

•

ASSESS

(continued)

758

3Check

Conce

pts

••

••

••

Section 37.1 1. W

hat did Michael Faraday and Joseph

Henry discover?

2. How

can voltage be induced in a wire w

ith the help of a m

agnet?

3. A m

agnet moved into a coil of w

ire will

induce voltage in the coil. What is the effect

of moving a m

agnet into a coil with m

ore loops?

4. Why is it m

ore difficult to move a m

agnet into a coil of m

ore loops that is connected to a resistor?

Section 37.2 5. C

urrent, as well as voltage, can be induced

in a wire by electrom

agnetic induction. W

hen can voltage be induced but not current?

Section 37.3 6. H

ow does the frequency of a changing m

ag-netic field com

pare with the frequency of

the alternating voltage that is induced?

7. What is a generator, and how

does it differ from

a motor?

8. Why is the voltage induced in an alternator

AC

rather than DC

?

9. The arm

ature of a generator must rotate in

order to induce voltage and current. What

produces the rotation?

Section 37.4 10. A

motor is characterized by three m

ain ingredients: m

agnetic field, moving charges,

and magnetic force. W

hat are the three main

ingredients that characterize a generator?

Section 37.5 11. W

hat does a transformer actually trans-

form—

voltage, current, or energy?

12. What does a step-up transform

er step up—

voltage, current, or energy?

13. How

does the relative number of turns on

the primary and the secondary coil in a

transformer affect the step-up or step-dow

n voltage factor?

14. If the number of secondary turns is 10 tim

es the num

ber of primary turns, and the input

voltage to the primary is 6 volts, how

many

volts will be induced in the secondary coil?

0740_CP

09_SE

_CH

37.indd758

11/13/0711:16:12

AM

0740_CP

09_SE

_CH

37.indd759

75

8

A

SSESS

Check

Conce

pts

1. Electric current can be produced in a w

ire by motion

of a m

agnet.

2. By m

oving the magnet past

the wire or into the coil

3. G

reater induced voltage

4. The coil becom

es a stronger electrom

agnet and repels m

ore.

5. W

hen resistance is large

6. Sam

e

7. A

motor converts electricity

to work, w

hereas a generator converts w

ork to electricity.

8. The m

agnetic field increases and decreases each turn.

9. External source of energy such

as fuel, w

ind, or water

10. Same

11. Voltage and current, but not

energy

12. Voltage

13. More turns on the secondary

coil step up the voltage.

14. 60 V

0740_cp09te_Ch37.indd 758

0740_cp09te_Ch37.indd 758

1/30/08 10:52:49 AM

1/30/08 10:52:49 AM

11/13/0711:16:12

AM

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

759

3For:Visit:W

eb Code: –

ASSESS

Conce

pt Su

mm

ary

••

•••

•

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

759

CH

APTER 37

ELECTRO

MA

GN

ETIC IN

DU

CTIO

N 759

15. a. In a transformer, how

does the power in-

put to the primary coil com

pare with the

power output of the secondary coil?

b. H

ow does the product of voltage and

current in the primary com

pare with

the product of voltage and current in the secondary?

Section 37.6 16. W

hy is it advantageous to transmit electric

power long distances at high voltages?

Section 37.7 17. W

hat fundamental quantity underlies the

concepts of voltages and currents?

18. Distinguish betw

een Faraday’s law expressed

in terms of fields and M

axwell’s counterpart

to Faraday’s law. H

ow are the tw

o laws

symm

etrical?

Section 37.8 19. H

ow do the w

ave speeds compare for high-

frequency and low-frequency electrom

ag-netic w

aves?

20. What is light?

Thin

k a

nd R

ank •••

••

•

Rank each of the follow

ing sets of scenarios in order of the quantity or property involved. List them

from left to right. If scenarios have equal

rankings, then separate them w

ith an equal sign. (e.g., A

! B

)

21. The m

agnets are moved into the w

ire coilsin identical quick fashion. Voltage induced in each coil causes a current to flow

, as indicated on the galvanom

eter. Neglect the

electrical resistance of the loops in the coil.

a. R

ank from greatest to least the reading on

the galvanometer.

b. M

ake the same ranking, only this tim

e for each coil having tw

ice as many loops as in

part (a).

0740_CP

09_SE

_CH

37.indd759

11/13/0711:16:16

AM

759

15. a. Same

b. Same, if pow

er loss is negligible

16. Lower current results in less

energy loss through heating

of the wires.

17. Electric field 18. Faraday’s law

—induced

electricity; M

axwell’s—

induced

magnetism

; they are inverses.

19. The speeds are the same, c.

20. Electromagnetic w

aves, l range 5

400–700 nm

Thin

k a

nd R

ank

21. a. B, C, Ab. B, C, A

0740_cp09te_Ch37.indd 759

0740_cp09te_Ch37.indd 759

1/30/08 10:52:53 AM

1/30/08 10:52:53 AM

760760

3For:Visit:W

eb Code: –

ASSESS

Conce

pt Su

mm

ary

•••

••

•

ASSESS

(continued)

760

3 22. The transform

ers are all powered w

ith 100 W

, and all have 100 turns on the prim

ary. The num

ber of turns on each secondary varies as show

n.

a. Rank the voltage output of the secondar-

ies from greatest to least.

b. R

ank the current in the secondaries from

greatest to least.

c. Rank the pow

er output in the secondaries from

greatest to least.

Thin

k a

nd Ex

pla

in •

•••

••

23. When T

im pushes the w

ire down betw

een the poles of the m

agnet, the galvanometer

registers a pulse. When he lifts the w

ire, an-other pulse is registered. H

ow do the pulses

differ?

24. A com

mon pickup for an electric guitar

consists of a coil of wire around a per-

manent m

agnet. The perm

anent magnet

magnetizes the nearby guitar string. W

hen the string is plucked, it oscillates above the coil, thereby changing the m

agnetic field that passes through the coil. T

he rhythmic

oscillations of the string produce the same

rhythmic changes in the m

agnetic field in the coil, w

hich in turn induce the same

rhythmic voltages in the coil, w

hich when

amplified and sent to a speaker produce

music! W

hy will this type of pickup not

work w

ith nylon strings?

25. Two separate but sim

ilar coils of wire are

mounted close to each other, as show

n be-low

. The first coil is connected to a battery

and has a direct current flowing through it.

The second coil is connected to a galvanom

-eter. H

ow does the galvanom

eter respond w

hen the switch in the first circuit is closed?

When the current is steady after the sw

itch is closed? W

hen the switch is opened?

26. If you place a metal ring in a region in

where a m

agnetic field is rapidly alternating, the ring m

ay become hot to your

touch. Why?

0740_CP

09_SE

_CH

37.indd760

11/13/0711:16:18

AM

0740_CP

09_SE

_CH

37.indd761

76

0

22. a. B, C, Ab. A

, C, Bc. A

5 B 5

C

Thin

k a

nd Ex

pla

in23. The tw

o pulses are opposite in

direction.

24. Nylon is nonm

agnetic, has no m

agnetic domains, and

is not m

agnetized by the perm

anent magnet.

25. Induction occurs only for a change in intercepted

m

agnetic field. Pulse occurs w

hen switch in the first circuit

is closed and current in the coil increases from

zero. W

hen current in first coil is steady, no current induced

in secondary; galvanom

eter reads zero. The needle sw

ings in opposite direction w

hen

switch is opened and current

falls to zero.

26

. In accord with electrom

agnetic induction, if the m

agnetic field alternates in the hole of the ring, an alternating

voltage w

ill be induced

in the ring. Because the ring is m

etal, its relatively low

resistance will result

in a correspondingly high

alternating current. This current is evident in the heating of the ring.

27. The ch

ang

ing

mag

netic field

p

rod

uced

wh

en cu

rrent

flow

s ind

uces cu

rrent in

the

alum

inu

m rin

g, w

hich

in

turn

gen

erates a mag

netic

field th

at op

po

ses the field

p

rod

uced

by th

e mag

net

ben

eath tab

le. The rin

g

beco

mes, m

om

entarily, a

mag

net th

at is repelled

by

the h

idd

en m

agn

et.

28. Co

nn

ect bu

lb to

a wire lo

op

th

at intercep

ts chan

gin

g

mag

netic field

. Ch

ang

e is the

key, so p

ow

er mu

st be A

C.

29. No

ne; th

ey simp

ly op

erate in

op

po

site man

ners.

0740_cp09te_Ch37.indd 760

0740_cp09te_Ch37.indd 760

1/30/08 10:52:55 AM

1/30/08 10:52:55 AM

11/13/0711:16:18

AM

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

761

3For:Visit:W

eb Code: –

ASSESS

Conce

pt Su

mm

ary

••

•••

•

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

761

CH

APTER 37

ELECTRO

MA

GN

ETIC IN

DU

CTIO

N 761

27. A m

agician places an aluminum

ring on a table, underneath w

hich is hidden an electrom

agnet. When the m

agician says “abracadabra” (and pushes a sw

itch that starts current flow

ing through the coil under the table), the ring jum

ps into the air. Explain his trick.

28. How

could you light a lightbulb that is near, yet not touching, an electrom

agnet? Is A

C or D

C required? D

efend your answer.

29. What is the basic difference betw

een an electric generator and an electric m

otor?

30. With no m

agnets around, why w

ill current flow

in a coil of wire w

aved around in the air?

31. What is the source of all electrom

agnetic w

aves?

32. Why is a generator arm

ature more difficult

to rotate when it is connected to and sup-

plying electric current to a circuit?

33. Your classmate says that, if you crank the

shaft of a conventional motor m

anually, the m

otor becomes a generator. D

o you agree or disagree, and w

hy?

34. Some bicycles have electric generators that

are made to turn w

hen the bike wheel turns.

These generators provide energy for the

bike’s lamp. W

ill a cyclist coast farther if the lam

p connected to the generator is turned off? Explain.

35. An electric hair drier running at norm

al speed draw

s a relatively small current. B

ut if som

ehow the m

otor shaft is prevented from

turning, the current dramatically increases

and the motor overheats. W

hy?

36. When a piece of plastic tape coated w

ith iron oxide that is m

agnetized more in som

e parts than others is m

oved past a small coil

of wire, w

hat happens in the coil? What is

a practical application of this?

37. Why is it im

portant that the core of a trans-form

er pass through both coils?

38. Why can a hum

often be heard when a

transformer is operating?

0740_CP

09_SE

_CH

37.indd761

11/13/0711:16:20

AM

761

30. Wavin

g th

e coil m

oves it

thro

ug

h Earth

’s mag

netic

field, in

du

cing

voltag

e and

, h

ence, cu

rrent.

31. Acceleratin

g electric ch

arges

32. The rep

ulsio

n o

f the

electrom

agn

ets op

po

ses the

rotatio

n o

f the arm

ature.

The g

reater the cu

rrent, th

e g

reater the rep

ulsio

n, an

d

the m

ore w

ork th

at mu

st be

do

ne to

spin

the arm

ature.

The an

swer is im

plied

by

energ

y con

servation

. Wo

rk d

on

e in tu

rnin

g th

e armatu

re g

oes in

to th

e electrical en

ergy su

pp

lied to

the

external circu

it.

33. Ag

ree; any co

il of w

ire spu

n

in a m

agn

etic field th

at cuts

thro

ug

h m

agn

etic field lin

es is a g

enerato

r.

34. When the lam

p is on, the energy that goes into

lighting the lam

p comes at

the expense of the KE of the

moving bicycle. The extra K

E saved by not lighting the lam

p

makes the bicycle go farther.

35. A running m

otor always

draws less net current than

a stalled m

otor. If the motor

jams or is som

ehow prevented

from

turning, then the back current is no longer generated

and the net current in the m

otor windings is greater.

This overheats the motor.

36. Variations in voltage, the

principle that underlies the operation of a tape recorder

37. To ensure the maxim

um

number of m

agnetic field

lines produced in the primary

coil are intercepted by the secondary coil

38. The h

um

is a same-frequency

forced vibration of the iron

slabs in the transformer core

as their magnetic polarities

alternate.

0740_cp09te_Ch37.indd 761

0740_cp09te_Ch37.indd 761

1/30/08 10:52:59 AM

1/30/08 10:52:59 AM

762762

3For:Visit:W

eb Code: –

ASSESS

Conce

pt Su

mm

ary

•••

••

•

ASSESS

(continued)

762

3 39. When a strip of m

agnetic material, variably

magnetized, is em

bedded in a plastic card that is m

oved past a small coil of w

ire, what

happens in the coil? What is a practical ap-

plication of this?

40. If a car made of iron and steel m

oves over a w

ide closed loop of wire em

bedded in a road surface, w

ill the magnetic field of Earth

in the loop be altered? Will this produce a

current pulse? (Can you think of a practical

application of this?)

41. At the security area of an airport, you w

alk through a m

etal detector that uses a weak

AC

magnetic field inside a large coil of w

ire. You are surprised that a piece of alum

inum

(nonmagnetic) in your pocket sounds the

alarm. T

he security officer explains that loops of current (eddy currents) w

ere in-duced in the m

etal. Why w

ould eddy cur-rents affect the net field in the detector?

42. How

could you move a conducting loop

of wire through a m

agnetic field without

inducing a voltage in the loop?

43. Why does a transform

er require alternating voltage?

44. How

does the current in the secondary of a transform

er compare w

ith the current in the prim

ary when the secondary voltage is

twice the prim

ary voltage?

45. In what sense can a transform

er be thought of as an electrical lever? W

hat does it m

ultiply? What does it not m

ultiply?

46. Can an efficient transform

er step up en-ergy? D

efend your answer.

47. A friend says that changing electric and

magnetic fields generate one another, and

this gives rise to visible light when the fre-

quency of change matches the frequencies

of light. Do you agree? Explain.

48. Would electrom

agnetic waves exist if

changing magnetic fields could produce

electric fields but changing electric fields could not in turn produce m

agnetic fields? Explain.

49. When a bar m

agnet is dropped through a vertical length of copper pipe, it falls no-ticeably m

ore slowly than it does w

hen it is dropped through a vertical length of plas-tic pipe. If the copper pipe is long enough, the dropped m

agnet will reach a term

inal falling speed. Propose an explanation.

50. What is w

rong with this schem

e? To generate electricity w

ithout fuel, arrange a m

otor to run a generator that will produce

electricity that is stepped up with trans-

formers so that the generator can run

the motor and sim

ultaneously furnish electricity for other uses.

0740_CP

09_SE

_CH

37.indd762

11/13/0711:16:22

AM

0740_CP

09_SE

_CH

37.indd763

76

2

39. Voltage is induced in the coil;

a credit card reader

40. Yes; yes; triggering

autom

obile traffic lights

41. Like all currents, the eddy currents produce their ow

n

magnetic fields, w

hich alter the net field in the detector.

42. Move it so that the num

ber of field lines doesn’t change.

43. AC provides the change

needed for induction.

44. Secondary current is half the current in the prim

ary.45. A

nalogous to a mechanical

lever in that work (or energy)

is transferred from one part

to another; mechanical lever

multiplies fo

rce; electrical lever m

ultip

lies voltag

e. In

bo

th, en

ergy an

d p

ow

er are co

nserved

, so w

hat is n

ot

mu

ltiplied

is energ

y.

46. No

; it wo

uld

violate law

of

energ

y con

servation

.

47. Ag

ree; ligh

t is an

electrom

agn

etic wave w

ith

a frequ

ency m

atchin

g th

e freq

uen

cy of o

scillating

ch

arges p

rod

ucin

g it.

48. No

; electrom

agn

etic waves

dep

end

on

mu

tual field

reg

eneratio

n. If th

e ind

uced

electric field

s did

no

t in tu

rn

ind

uce m

agn

etic fields an

d

pass en

ergy to

them

, the

energ

y wo

uld

be lo

calized

rather th

an “w

aved” in

to

space.

49. The m

agn

et ind

uces cu

rrent

loo

ps in

the co

pp

er as it falls, w

hich

pro

du

ce mag

netic

fields th

at repel th

e mag

net

as it app

roach

es and

attract it as it leaves, exertin

g an

u

pw

ard fo

rce on

it. This

force in

creases with

speed

. A

t som

e speed

it match

es g

ravity and

reaches term

inal

speed

. Since p

lastic is an

insu

lator, th

ere is no

curren

t an

d n

o in

du

ced m

agn

etic field

to o

pp

ose fallin

g.

0740_cp09te_Ch37.indd 762

0740_cp09te_Ch37.indd 762

1/30/08 10:53:01 AM

1/30/08 10:53:01 AM

11/13/0711:16:22

AM

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

763

3For:Visit:W

eb Code: –

ASSESS

Conce

pt Su

mm

ary

••

•••

•

C

HA

PTER 37 ELEC

TROM

AG

NETIC

IND

UC

TION

763

CH

APTER 37

ELECTRO

MA

GN

ETIC IN

DU

CTIO

N 763

51. An electrom

agnet A w

ith a coil of 10 turns carrying 1 A

interacts with electrom

agnet B,

which has 100 turns carrying 2 A

. Which

electromagnet exerts the greater force on

the other?

Thin

k a

nd So

lve •

••

•••

52. An electric doorbell requires 12 volts to

operate correctly. A transform

er nicely allow

s it to be powered from

a 120-volt outlet. If the prim

ary has 500 turns, show

that the secondary should have 50 turns.

53. A m

odel electric train requires 6 V to

operate. When connected to a 120-V

house-hold circuit, a transform

er is needed. If the prim

ary coil of the transformer has 240

windings, show

that there should be 12 turns in the secondary coil.

54. If the output current for the above trans-form

er is 1.8 amps, show

that the input current is 0.09 A

.

55. A transform

er has an input of 9 volts and an output of 36 volts. If the input is changed to 12 volts, show

that the output would be

48 volts.

56. A m

odel electric train requires a low voltage

to operate. If the primary coil of its trans-

former has 400 turns, and the secondary

has 40 turns, how m

any volts will pow

er the train w

hen the primary is connected to a

120-volt household circuit?

57. The prim

ary coil of a step-up transformer

draws 100 W

. Find the power provided to

the secondary circuit.

58. An ideal transform

er has 50 turns in its prim

ary coil and 250 turns in its secondary coil. A

12-V A

C source is connected to the

primary. Find

a. the A

C voltage available at the secondary

b. the current in a 10- !

device connectedto the secondary

c. the pow

er supplied to the primary

59. Neon signs require about 12,000 V

for their operation. W

hat should be the ratio of the num

ber of loops in the secondary to the num

ber of loops in the primary for a

neon-sign transformer that operates off

120-V lines?

More Problem

-Solving PracticeA

ppendix F

0740_CP

09_SE

_CH

37.indd763

11/13/0711:16:23

AM

763

50. Schem

e violates b

oth

the

first and

secon

d law

s of

therm

od

ynam

ics. The

gen

erator p

rod

uces less

electricity than

is used

by th

e ad

join

ing

mo

tor to

po

wer it.

A tran

sform

er cann

ot step

u

p en

ergy o

r po

wer. So

there

is mo

re inp

ut en

ergy th

an

ou

tpu

t energ

y.

51. Back to New

ton’s third law!

A and B exert equal and

opposite forces on each other.

Thin

k a

nd So

lve

52. (120 V)/(500 tu

rns) 5

(12 V)/x,

so x 5

50 turn

s. 53. (120 V

)/(240 turn

s) 5 6 V

/x tu

rns, so

x 5 12 tu

rns.

54. Pow

er same in

bo

th:

IVp

rim 5

IVsec , so

20 times

greater vo

ltage in

prim

ary m

eans 1/20 as m

uch

curren

t as in

the seco

nd

ary. That’s

1/20 3 1.8 A

5 0.09 A

.

55. Steps u

p vo

ltage b

y a factor

36/9 5 4; th

erefore a 12-V

in

pu

t will b

e stepp

ed u

p to

4 3

12 V 5

48 V.

56. (120 V)/(400 tu

rns) 5

x/40, so

x 5 12 V

.

57. Pp 5

Ps 5

100 W

58. a. Vp /(# prim

ary turns) 5

Vs /(# secondary turns), so V

s 5

250 3 (12 V

/50) 5 60 V

.

b. I 5

V/R 5

(60 V)/(10 V

) 5

6 A

c. P

p 5 P

s 5 (V

I) 5

(60 V)(6 A

) 5 360 W

59. Vp /(# prim

ary turns) 5

Vs /(# secondary turns),

so (# secondary turns) "(# prim

ary turns) 5 V

s /Vp 5

(12,000 V

)/(120 V) 5

100.

Te

ac

hin

g R

es

ou

rc

es

• Computer Test Bank

• Chapter and Unit Tests

0740_cp09te_Ch37.indd 763

0740_cp09te_Ch37.indd 763

1/30/08 11:21:39 AM

1/30/08 11:21:39 AM