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
..........
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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.
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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
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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.
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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.
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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
......
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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
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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.
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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.
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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
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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
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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
......
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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.
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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
......
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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.
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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
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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).
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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.
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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.
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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
......
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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!
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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
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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.
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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.
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752
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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
......
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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.
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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.
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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
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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.
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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?
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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
......
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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
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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.
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REVIEW
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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)
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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
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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?
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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
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pt Su
mm
ary
••
•••
•
C
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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).
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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
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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?
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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.
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AG
NETIC
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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?
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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.
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(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.
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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.
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C
HA
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AG
NETIC
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UC
TION
763
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eb Code: –
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••
•••
•
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
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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
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