Objectives
• Describe how voltage is
induced in a coil of wire. (37.1)
3
• State and explain Faraday’s
law. (37.2)
THE BIG
IDEA
• Describe how a generator
works. (37.3)
• Describe how a magnetic field
affects a moving charge. (37.4)
Magnetism can produce
electricity, and electricity can
produce magnetism.
T
he discovery that an electric current in a
wire produced magnetism was a turning
point in physics and the technology that
followed. The question arose as to whether
magnetism could produce an electric current in
a wire. In 1831, two physicists, Michael Faraday in
England and Joseph Henry in the United States, independently discovered that the answer is yes. Until
their discovery, the only current-producing devices
were voltaic cells, which produced small currents by
dissolving expensive metals in acids. These were the
forerunners of our present-day batteries. The discovery of Faraday and Henry provided a major alternative
to these crude devices. Their discovery was to change
the world by making electricity so commonplace that
it would power industries by day and light up cities
by night.
• Describe how a transformer
works. (37.5)
• Explain why almost all electrical
energy is sold in the form of
alternating current. (37.6)
• Explain how an electric field
creates a magnetic field. (37.7)
• Describe electromagnetic
waves. (37.8)
discover!
MATERIALS
ELECTROMAGNETIC
INDUCTION
..........
ELECTROMAGNETIC
INDUCTION
two galvanometers,
wire
EXPECTED OUTCOME The
needle of the stationary
galvanometer will swing back
and forth.
ANALYZE AND CONCLUDE
1. See Expected Outcome.
2. The needle would swing
back and forth in both
galvanometers.
discover!
3. Moving a magnet through
a wire loop would create an
electric current.
Can You Create an Electric Current
Without a Battery?
A magnet
moved through a wire loop
induces a voltage in the wire.
A galvanometer contains
a magnet, so when one
galvanometer is shaken the
other one registers a small
current.
TEACHING TIP
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1. Use two lengths of wire to connect two
galvanometers.
2. Shake one of the galvanometers while watching the needle of the other meter.
3. Now reverse the roles of the galvanometers
by shaking the galvanometer that was originally stationary.
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Analyze and Conclude
1. Observing Describe the reading on a stationary galvanometer as the other galvanometer
is shaken.
2. Predicting What would happen if you moved
a magnet through the wire loops connecting
the galvanometers?
3. Making Generalizations How could you use
mechanical motion to power an electronic
device?
37.1 Electromagnetic Induction
Faraday and Henry both made the same discovery. Electric
current can be produced in a wire by simply moving a magnet into
or out of a wire coil. No battery or other voltage source was needed—
only the motion of a magnet in a coil or in a single wire loop as shown
in Figure 37.1. They discovered that voltage was induced by the relative motion of a wire with respect to a magnetic field.
The production of voltage depends only on the relative motion
of the conductor with respect to the magnetic field. Voltage is
induced whether the magnetic field of a magnet moves past a stationary conductor, or the conductor moves through a stationary
magnetic field as shown in Figure 37.2. The results are the same for
the same relative motion.
FIGURE 37.1 When the magnet is
plunged into the coil, voltage is induced in the coil
and charges in the coil are
set in motion.
This chapter focuses on
the important features of
electromagnetic induction, and
avoids such complications as
reactance, back emf, Lenz’s law,
and the left- and right-hand
rules, which often overwhelm
students. The important concept
here is the transmitting of
energy from one place to another
without physical contact. The
chapter should be supported
by demonstrations of
electromagnetic induction, such
as those described in the text.
37.1 Electromagnetic
Induction
FIGURE 37.2
Voltage is induced in the wire
loop whether the magnetic
field moves past the wire or
the wire moves through the
magnetic field.
The amount of voltage induced depends on how quickly the magnetic field lines are traversed by the wire. Very slow motion produces
hardly any voltage at all. Quick motion induces a greater voltage.
The greater the number of loops of wire that move in a magnetic
field, the greater are the induced voltage and the current in the wire,
as shown in Figure 37.3. Pushing a magnet into twice as many loops
will induce twice as much voltage; pushing it into ten times as many
loops will induce ten times as much voltage; and so on.37.1.1
CHAPTER 37
Key Term
electromagnetic induction
Common Misconception
Voltage is produced by a magnet.
FACT Voltage is produced by the
work done when a magnet and
a closed loop of wire are moved
relative to each other.
FIGURE 37.3 When a magnet is plunged into a coil of twice as
many loops as another,
twice as much voltage is
induced. If the magnet is
plunged into a coil with
three times as many loops,
then three times as much
voltage is induced.
ELECTROMAGNETIC INDUCTION
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Teaching Tidbit A long
helically-wound coil of insulated
wire is called a solenoid.
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Demonstrations
Does it seem that we get something (energy) for nothing by
simply increasing the number of loops in a coil of wire? We don’t.
Work is done in pushing the magnet into the loop. That’s because the
induced current in the loop creates a magnetic field that repels the
approaching magnet. For example, in Figure 37.4a the north pole of
a bar magnet is pushed toward a single loop. The current induced in
the loop produces a magnetic field that repels the approaching bar
magnet. We see that, in Figure 37.4b, when the magnet is pulled away
from the loop, the induced current produces a magnetic field that
attracts the receding bar magnet. Both cases require work input. If
you try to push a magnet into a coil with more loops, it requires even
more work, as shown in Figure 37.5.
Produce motion of a wire loop
with respect to a magnet as
shown in Figures 37.1 and
37.2. Use a large classroom
demonstration galvanometer
to show the induced voltage
that results from the relative
motion.
If you have a Tesla coil,
demonstrate induction by
lighting up a non-connected
fluorescent lamp a meter or
more away. This is impressive.
FIGURE 37.4 a. Current induced in the loop produces a magnetic field (suggested
by the imaginary yellow bar magnet),
which repels the approaching bar
magnet. b. When the bar magnet
is pulled away from the loop, the
induced current is in the opposite
direction and produces a magnetic
field that attracts the receding bar
magnet.
......
Electric current can
be produced in a
wire by simply moving a magnet
into or out of a wire coil.
CONCEPT
CHECK
Teaching Resources
• Reading and Study
Workbook
• Transparencies 87, 88
FIGURE 37.5 • PresentationEXPRESS
It is more difficult to push
the magnet into the coil
with more loops because
more current flows and the
coil generates a stronger
magnetic field that resists
the motion of the magnet.
• Interactive Textbook
• Next-Time Question 37-1
• Conceptual Physics Alive!
DVDs Magnetism and
Induction
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The law of energy conservation applies here. In Figures 37.3 and
37.4, the force that you exert on the magnet multiplied by the distance
that you move the magnet is your input work. This 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 means more current through the
resistor, and that means more energy expenditure.37.1.2 The amount of
voltage induced depends on how quickly the magnetic field changes.
Very slow movement of the magnet into the coil produces hardly any
voltage at all. Quick motion induces a greater voltage.
It doesn’t matter which moves—the magnet or the coil. It is the
relative motion of the coil with respect to the magnetic field that
induces voltage. It so happens that any change in the magnetic field
around a conductor induces a voltage. The phenomenon of inducing
voltage by changing the magnetic field around a conductor is
electromagnetic induction.
CONCEPT How can you create a current using a wire and
......
Plunge a bar magnet into a
coil as in Figure 37.3. Show the
twice-as-much deflection for a
coil with twice as many turns,
and so on. Establish the directly
proportional relationship
between induced voltage and
number of turns in the coil.
CHECK
a magnet?
37.2 Faraday’s Law
37.2 Faraday’s Law
......
Faraday’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
number of loops, the cross-sectional area of each loop, and the rate
at which the magnetic field changes within those loops.
Voltage is one thing, and current is another. The amount of current produced by electromagnetic induction depends not only on the
induced voltage but also on the resistance of the coil and the circuit
to which it is connected.37.2 For example, you can plunge a magnet
in and out of a closed rubber loop and in and out of a closed loop
of copper. The voltage induced in each is the same, providing each
intercepts the same number of magnetic field lines. But 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 same electric field as
those in the copper, but their bonding to the fixed atoms prevents the
movement of charge that occurs so freely in the copper.
CONCEPT
CHECK
Key Term
Faraday’s law
think!
If you push a magnet
into a coil connected to
a resistor, as shown in
Figure 37.5, you’ll feel a
resistance to your push.
For the same pushing
speed, why is this resistance greater in a coil
with more loops?
Answer: 37.2
What does Faraday’s law state?
37.3 Generators and
Alternating Current
CHAPTER 37
FIGURE 37.6 A simple generator turns
mechanical energy into
electrical energy. Voltage
is induced in the loop
when it is rotated in the
magnetic field.
ELECTROMAGNETIC INDUCTION
Teaching Tip Discuss the
operation of traffic control
signals that are activated by
the passing of metal vehicles
over wire loops embedded in
the road surface and of the
metal detectors used in airports.
The passage of ferromagnetic
material through or over these
loops alters (changes) the
magnetic field and induces
voltage in the loops.
Faraday’s law states
that the induced
voltage in a coil is proportional to
the product of the number of
loops, the cross-sectional area of
each loop, and the rate at which
the magnetic field changes
within those loops.
......
One way to generate a current is to plunge a magnet into and out of a
coil of wire. As the magnetic field strength inside the coil is increased
(magnet entering), the induced voltage in the coil is directed one
way. When the magnetic field strength diminishes (magnet leaving), the voltage is induced in the opposite direction. The greater the
frequency of field change, the greater the induced voltage. The frequency of the induced alternating voltage equals the frequency of the
changing magnetic field within the loop.
Rather than moving the magnet,
it is more practical to move the coil. This
is best accomplished by rotating the coil
in a stationary magnetic field, as shown
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 motor converts
electrical energy into mechanical energy,
a generator converts mechanical energy
into electrical energy.
Teaching Tip Explain again
that the magnet is not a source
of voltage, but rather the voltage
is induced when work is done to
push the magnet into the coil.
It may seem that one increases
voltage by simply increasing the
number of loops in a coil, but it is
at the expense of added difficulty
in pushing the magnet into more
loops (Figure 37.5). The current
induced is surrounded by its own
magnetic field, which resists
the magnet you are pushing or
pulling. The greater the current
induced by the action, the more
resistance met. This is evident
in cranking a generator, when
additional electrical load is
suddenly introduced.
CONCEPT
CHECK
Teaching Resources
• Problem-Solving Exercises in
Physics 18-2
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37.3 Generators and
FIGURE 37.7 Alternating Current
As the loop rotates, there is a change in the number of
magnetic field lines it encloses.
Key Term
generator
Demonstration
a. The loop starts by
enclosing the maximum
number of field lines.
Return to the motor you
demonstrated in Chapter 36,
and show that when you apply
mechanical energy, the motor
becomes a generator. Use a
large classroom demonstration
galvanometer to show the
effect.
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
more field lines again.
e. After half a turn, the loop
again encloses the maximum number of field lines.
Guitar pickups are tiny
coils with magnets
inside them. The magnets magnetize the
steel strings. When
the strings vibrate,
voltage is induced in
the coils and boosted
by an amplifier, and
sound is produced by
a speaker.
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Simple Generators When the loop of wire is rotated in the magnetic field, there is a change in the number of magnetic field lines
within the loop, as shown in the diagram above. In Figure 37.7a, the
loop has the largest number 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 continues, it encloses more field lines (Figure 37.7d) and reaches a maximum when it has made a half revolution (Figure 37.7e). As rotation
continues, the magnetic field inside the loop changes in cyclic fashion.
The voltage induced by the generator alternates, and the current
produced is alternating current (AC). The current changes magnitude
and direction periodically, as shown in Figure 37.8. The standard
alternating current in North America changes its magnitude and
direction during 60 complete cycles per second—60 hertz.
As the loop rotates, the magnitude and direction of the
induced voltage (and current)
change. One complete rotation of the loop produces
one complete cycle in voltage (and current).
......
Complex Generators The generators used in power plants are
much more complex than the model discussed here. Huge coils made
up of many loops of wire are wrapped on an iron core, to make an
armature much like the armature of a motor. They rotate in the very
strong magnetic fields of powerful electromagnets. The armature is
connected externally to an assembly of paddle wheels called a turbine. Energy from wind or falling water can be used to produce rotation of the turbine, but as shown in Figure 37.9, most commercial
generators are driven by moving steam. At the present time, a fossil
fuel or nuclear fuel is used as the energy source for the steam.
It is important to emphasize that an energy source of some kind
is required to operate a generator. Some fraction of energy from the
source, usually some type of fuel, is converted to mechanical energy
to drive the turbine, and the generator converts most of this to electrical energy. The 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 must have a source.37.3
CONCEPT
CHECK
How is a generator different from a motor?
Link to
TECHNOLOGY
Metal Detectors Walk
through a metal detector
in an airport, and you’re
walking through a coil of
wire that carries a small
electric current. In the
opening in the coil there
is a magnetic field. Any
change in this field is
sensed by the coil. If you
carry iron into the coil,
you change this magnetic
field. A changing
magnetic field induces a
change in the current in
the coil. The change sets
off an alarm.
FIGURE 37.9 Teaching Tip Continue with
a historical theme: Mention that
with the advent of the generator
the task was to design methods
of moving coils of wire past
magnetic fields, or of moving
magnetic fields past coils of wire.
Put turbines beneath waterfalls,
and boil water to make steam
that turns turbine blades—enter
the Industrial Revolution.
Whereas a motor
converts electrical
energy into mechanical energy, a
generator converts mechanical
energy into electrical energy.
......
FIGURE 37.8
Teaching Tip In discussing
the operation of a generator via
Figures 37.6, 37.7, and 37.8,
point out that maximum voltage
is induced not when the loop
contains the most magnetic
field lines but when the greatest
number of field lines are
“clipped” (changed) as the loop
is turned. Hence in Figure 37.8
the voltage is at a maximum
when the loop passes through
the zero-number-of-lines point.
Its rate of change of magnetic
field lines is greatest at this point.
CONCEPT
CHECK
Teaching Resources
Steam drives the turbine,
which is connected to the
armature of the generator.
• Reading and Study
Workbook
• PresentationEXPRESS
• Interactive Textbook
CHAPTER 37
ELECTROMAGNETIC INDUCTION
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37.4 Motor and
Generator Comparison
Teaching Tip Compare
motors and generators—in
principle they are the same. A
motor converts electrical energy
into mechanical energy. A
generator converts mechanical
energy into electrical energy.
In fact, a motor acts also as
a generator, which creates a
“back voltage” (back emf) and
an opposing current. The net
current in a motor is the input
current minus the generated
back current. The net current in
a power saw will not cause it to
overheat and damage its motor
windings—so long as it is running
and generating a back current
that keeps the net current low.
But if you should jam the saw
so that it can’t spin, the back
current would cease, causing
the net current to become
dangerously high and possibly
burn out the motor.
FIGURE 37.10 The figure shows the motor effect
and the generator effect. a. When
a current moves to the right,
there is a force on the electrons,
and the wire is tugged upward.
b. When a wire with no current is
moved downward, the electrons in
the wire experience a force, creating current.
37.4 Motor and Generator
Comparison
In Chapter 36 you saw how an electric current is deflected in a magnetic field, which underlies the operation of the motor. This discovery occurred about 10 years before Faraday and Henry discovered
electromagnetic induction, which underlies the operation of a generator. Both of these discoveries, however, stem from the same single
Moving charges experience a force that is perpendicular
fact:
to both their motion and the magnetic field they traverse. We will
call the deflected wire the motor effect and the law of induction the
generator effect. Each of these effects is summarized in Figure 37.10.
Study them. Can you see that the two effects are related?
The motor effect occurs when a current moves through a magnetic field. In the figure, the current is moving to the right, and the
magnetic field creates a perpendicular upward force on the electrons.
Because the electrons can’t leave the wire, the entire wire is tugged
upward along with the electrons. In the generator effect, a wire with
no current is moved downward through a magnetic field. The electrons in this wire experience a force perpendicular to their motion,
which is along the wire. So a current begins to flow.
A striking example of a device functioning as both motor and
generator is found in hybrid automobiles. When extra power for
accelerating or hill climbing is needed, this device draws current from
a battery and acts as a motor to assist the gasoline engine. When
braking or rolling downhill causes the wheels to exert a torque on the
device, it acts as a generator and recharges the battery. The electrical
part of the hybrid engine is both a motor and a generator.
Teaching Tip Mention that
electric motors are used in dieselpowered railroad engines. The
combustion engine alone cannot
move a heavy load from rest, but
when it is coupled to an electric
motor, it can. When the armature
in a motor is not turning, the
current in the windings is huge,
with a correspondingly huge
force. As both the train and
the motor gain speed, the back
current generated by the motor
brings the net current in the
motor down to non-overheating
levels.
......
Moving charges
experience
a force
CHECK
that is perpendicular both to
their motion and the magnetic
field they traverse.
746
......
CONCEPT
CONCEPT
CHECK
746
How does a magnetic field affect a moving charge?
Demonstration
37.5 Transformers
think!
Consider a pair of coils, side by side, as shown 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 power source as
the primary (input), and the other as the secondary (output). As soon
as the switch 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 between the two coils. Only a brief surge of current occurs in the secondary, however. Then when the primary switch
is opened, a surge of current again registers in the secondary but in
the opposite direction.
When the switch of the
primary in Figure 37.11
is opened or closed,
the galvanometer in the
secondary registers a current. But when the switch
remains closed, no current
is registered on the galvanometer of the secondary.
Why?
Answer: 37.5.1
Light a bulb with a handcranked generator and show
how the turning is easier when
the bulb is loosened and the
load removed. Allow students
to try this themselves during
or at the end of class.
TEACHING TIP Stress again
the fact that we don’t get
something for nothing with
electromagnetic induction,
and refer back to Figure 37.5.
Teaching Resources
FIGURE 37.11
Whenever the primary switch
is opened or closed, voltage
is induced in the secondary
circuit.
• Reading and Study
Workbook
• Laboratory Manual 99
• Transparency 89
• PresentationEXPRESS
• Interactive Textbook
The explanation is that the magnetic field that builds up around
the primary extends into the secondary coil. Changes in the magnetic field of the primary 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 we place an iron core inside the primary and secondary coils
of the arrangement shown in Figure 37.11, the magnetic field within
the primary is intensified by the alignment of magnetic domains in
the iron. The magnetic field is also concentrated in the core, which
extends into the secondary, so the secondary intercepts more of the
field change. The galvanometer will show greater surges of current
when the switch of the primary is opened or closed.
Instead of opening and closing a switch to produce the change
of magnetic field, suppose that alternating current is used to power
the primary. Then the rate at which the magnetic field changes in the
primary (and hence in the secondary) is equal to the frequency of the
alternating current. Now we have a transformer, as shown in Figure
37.12. A transformer is a device for increasing or decreasing voltage through electromagnetic induction. A transformer works by
inducing a changing magnetic field in one coil, which induces an
alternating current in a nearby second coil.
CHAPTER 37
37.5 Transformers
Key Term
transformer
Common Misconception
A transformer can step up energy, or
step up power.
FACT As a transformer steps up
(or down) voltage, it transfers
energy from one coil to the
other, always obeying the law of
conservation of energy.
Teaching Tip Explain how
a transformer works, and
demonstrate the setup shown
in Figure 37.11.
FIGURE 37.12 A simple transformer
arrangement using an iron
core creates greater current
in the secondary coil.
ELECTROMAGNETIC INDUCTION
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Teaching Tip Stress that
voltage may be stepped up,
and current may be stepped up,
but the product of current and
voltage cannot be stepped up.
FIGURE 37.13 The iron core guides the changing
magnetic field lines, which makes
a more efficient transformer.
The conservation of energy
reigns!
A more efficient arrangement for a transformer is shown in
Figure 37.13, where the iron core forms a complete loop to guide all
the magnetic field lines through the secondary. All the magnetic field
lines within the primary are intercepted by the secondary.
Demonstration
Caution: Wear goggles
and heat-resistant gloves
when performing this
demonstration. Weld a pair
of nails together with a stepdown transformer. This is a
spectacular demonstration
when you casually place your
fingers between the nail ends
before they make contact.
Then remove your fingers and
bring the points together,
allowing the sparks to fly
while the nails quickly become
red and then white hot.
Teaching Tip Mention
the role of the transformer in
stepping down voltages in toy
electric trains, electric calculators,
mobile phone chargers, and
portable music players, and the
role of stepping up voltages
in television sets and various
electrical devices.
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).
Voltage Voltages may be stepped up or stepped down with a transformer. To see how, consider the simple case shown in Figure 37.14a.
Suppose the primary consists of one loop connected to a 1-V alternating source. Consider the symmetrical arrangement of a secondary
of one loop that intercepts all the changing magnetic field lines of the
primary. Then a voltage of 1 V is induced in the secondary.
If another loop is wrapped around the core, as shown in Figure
37.14c, the induced voltage will be twice as much, in accord with
Faraday’s law. If the secondary is wound with three times as many
loops, or turns as they are called, then three times as much voltage will be induced. If the secondary has a hundred times as many
turns as the primary, then a hundred times as much voltage will be
induced, and so on. This arrangement of a greater number of turns
on the secondary than on the primary makes up a step-up transformer. Stepped-up voltage may light a neon sign or operate the picture tube in a television receiver.
FIGURE 37.14 a. The voltage of 1 V induced in
the secondary equals the voltage
of the primary. b. A voltage of
1 V is induced in the added secondary also because it intercepts
the same magnetic field change
from the primary. c. The voltages
of 1 V induced in each of the
two one-turn secondaries
are equivalent to a voltage of
2 V induced in a single two-turn
secondary.
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If the secondary has fewer turns than the primary, the alternating voltage produced in the secondary will be lower than that in the
primary. The voltage is said to be stepped down. If the secondary has
half as many turns as the primary, then only half as much voltage is
induced in the secondary.
So electrical energy can be fed into the primary at a given alternating 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 windings, as shown in Figure 37.15.
The relationship between primary and secondary voltages with
respect to the relative number of turns is
think!
If the voltage in a
transformer is stepped
up, then the current is
stepped down. Ohm’s
law says that increased
voltage will produce
increased current. Is there
a contradiction here,
or does Ohm’s Law not
apply to transformers?
Answer: 37.5.2
As a student, I remember
being very confused about the
seeming contradiction with
Ohm’s law—the idea that when
voltage in the secondary coil
is increased, current in the
secondary coil is decreased.
Make 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
primary coil. So P 5 IV does not
contradict Ohm’s law!
primary voltage
secondary voltage
number of primary turns
number of secondary turns
FIGURE 37.15
A practical transformer
uses many coils. The
relative numbers of turns
in the coils determines
how much the voltage
changes.
Power It might seem that you get something for nothing with a
transformer that steps up the voltage. Not so, for energy conservation
always controls what can happen. The transformer actually transfers
energy from one coil to the other. The rate at which energy is transferred is the power. The power used in the secondary is supplied by
the primary. The primary gives no more power than the secondary
uses, in accord with the conservation of energy. If the slight power
losses due to heating of the core are neglected, then the power going
into the primary equals the power coming out of the secondary.
Electric power is equal to the product of voltage and current:
......
You can see that if the secondary has more voltage, it will have
less current than the primary. Or vice versa; if the secondary has less
voltage, it will have more current than the primary. The ease with
which voltages can be stepped up or down with a transformer is
the principal reason that most electric power is AC rather than DC.
Figure 37.16 shows a common household transformer used today.
CHECK
......
FIGURE 37.16 (voltage current)primary (voltage current)secondary
CONCEPT
A transformer works
CHECK by inducing a
changing magnetic field in one
coil, which induces an alternating
current in a nearby second coil.
CONCEPT
This common transformer
lowers 120 V to 6 V or
9 V. It also converts AC to
DC by means of a diode
inside—a tiny electronic
device (shown in Chapter
34, Figure 34.12) that acts
as a one-way valve.
Teaching Resources
• Reading and Study
Workbook
• Concept-Development
Practice Book 37-1
• PresentationEXPRESS
• Interactive Textbook
How does a transformer work?
CHAPTER 37
ELECTROMAGNETIC INDUCTION
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37.6 Power
37.6 Power Transmission
Transmission
Teaching Tip Discuss the
roles of both stepping up and
stepping down voltages in power
transmission. Stress that in no
way is energy or power stepped
up or down—a conservation of
energy no-no!
Teaching Tip Tell students
that cost is the main reason
for high-voltage power lines. If
higher currents were carried in
the lines, the wires would have to
be thicker and therefore costlier.
They would also be heavier,
which would require stronger
towers.
200 years ago, people
got light from whale
oil. Whales should
be glad that humans
discovered how to
harness electricity!
Almost all electric energy sold today is in the form of alternating
current because of the ease with which it can be transformed from
one voltage to another. Power is transmitted great distances at high
voltages and correspondingly low currents, a process that otherwise
would result in large energy losses owing to the heating of the wires.
Power may be carried from power plants to cities at about 120,000
volts or more, stepped down to about 2400 volts in the city, and finally
stepped down again by a transformer such as the one shown in Figure
37.17 to provide the 120 volts used in household circuits.
FIGURE 37.17 A common neighborhood
transformer, which typically
steps 2400 V down to
240 V for houses and small
businesses. Inside the home
or business, the 240 V can
be divided to a safer 120 V.
......
Almost all electrical
energy sold today is
in the form of alternating current
because of the ease with which it
can be transformed from one
voltage to another.
CONCEPT
CHECK
Power transmission depends
on transformers. Voltage
is increased for long-distance transmission and then
decreased before it reaches
your home.
Teaching Resources
• Reading and Study
Workbook
• Concept-Development
Practice Book 37-2
• PresentationEXPRESS
• Interactive Textbook
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CONCEPT Why is almost all electrical energy sold today in the
......
FIGURE 37.18 Energy, then, is transformed from one system of conducting wires
to another by electromagnetic induction as shown in Figure 37.18.
The same principles account for eliminating wires and sending energy
from a radio-transmitter antenna to a radio receiver many kilometers
away, and for the transformation of energy of vibrating electrons in
the sun to life energy on Earth. The effects of electromagnetic induction are very far-reaching.
CHECK
form of alternating current?
37.7 Induction of
37.7 Induction of Electric and
Magnetic Fields
......
Electromagnetic induction has thus far been discussed in terms of
the production of voltages and currents. Actually, the more fundamental way to look at it is in terms of the induction of electric
fields. The electric fields, in turn, give rise to voltages and currents.
Induction takes place whether or not a conducting wire or any material medium is present. Faraday’s law states that an electric field is
created in any region of space in which a magnetic field is changing
with time. The magnitude of the created electric field is proportional
to the rate at which the magnetic field changes. The direction of the
created electric field is at right angles to the changing magnetic field.
If electric charge happens to be present where the electric field is
created, this charge will experience a force. For a charge in a wire, the
force could cause it to flow as current, or to push the wire to one side.
For a charge in an evacuated region, like in the chamber of a particle
accelerator, the force can accelerate the charge to high speeds.
There is a second effect, which 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. The symmetry between electric
and magnetic fields revealed by this pair of laws is one of the many
beautiful symmetries in nature. The companion to Faraday’s law was
advanced by the British physicist James Clerk Maxwell in the 1860s.
A magnetic field is created in any region of space in which
an electric field is changing with time. According to Maxwell, the
magnitude of the created magnetic field is proportional to the rate
at which the electric field changes. The direction of the created magnetic field is at right angles to the changing electric field.
CONCEPT
CHECK
In making a scientific
discovery, being at the
right place at the right
time is not enough—
curiosity, patience,
and hard work are also
important.
With the power on, levitate
an aluminum ring over the
extended pole of an Elihu
Thompson device.
Ask How much
electromagnetic force supports
this 1-N aluminum ring (assuming
the ring weighs 1 N)? 1 N.
This can be answered with no
knowledge of electromagnetic
forces but from knowledge about
forces in general that goes back
to Newton’s laws. Since the ring
is at rest and not accelerating,
the upward electromagnetic
force—in newtons—must be
equal to the downward force of
gravity. Was the electromagnetic
force that lifted the ring more
than, equal to, or less than the
magnetic force that produced
levitation earlier? More than,
because it accelerated upward,
evidence that the upward force
was more than the weight; this is
also understandable because the
ring was lower and intercepting
more changing magnetic field
lines.
Link to TECHNOLOGY
Magnetic Storage Computers store data
on plastic disks that have been coated
with a magnetic material. Magnetic
patterns can be applied to the disk by a
recording head. Coded electrical pulses
that carry information are changed
into magnetic pulses and stored on the
disk. When 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.
ELECTROMAGNETIC INDUCTION
Demonstration
With the power
off, place the
ring at the base
of the extended
pole. When you
switch on the
power the
current
induced
in the ring
via electromagnetic induction
converts the ring into an AC
electromagnet. (By Lenz’s law,
the polarity of the induced
magnet is always such as to
oppose the magnetic field
imposed.)
How can an electric field create a magnetic field?
CHAPTER 37
Electric and Magnetic
Fields
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Demonstration
Show the classic lighting of
the lamp in a jar of water.
Impressive! Mount the lamp
on a waxed waterproof coil,
which intercepts the changing
magnetic flux of the device,
induces current, and illuminates
the lamp. (The water serves no
purpose other than making
the demonstration more
interesting.)
Carefully go over the comic
strip “Power Lines.” The physics
here is deeper than in the other
comic strips and may need
elaboration.
Teaching Tip State that
underlying all the things
discussed and observed is the
induction of both electric and
magnetic fields. Because of this
we can send signals without
wires—radio and TV—and
furthermore, energy reaches us
from the sun through sunlight.
Teaching Tip Mention that
the concept that a change in
either field induces the other led
Einstein to develop his special
theory of relativity. He showed
that a magnetic field results
when an electric field is seen by a
moving observer, and an electric
field results when a magnetic
field is seen by a moving
observer. The fields are relative.
......
A magnetic field is
created in any region
of space in which an electric field
is changing with time.
CONCEPT
CHECK
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37.8 Electromagnetic
37.8 Electromagnetic Waves
Waves
Shake the end of a stick back and forth in still water and you will
produce waves on the water surface. Similarly shake a charged rod
back and forth in empty space and you will produce electromagnetic
waves in space, as shown in Figure 37.19. This is because the shaking charge can be considered an electric current. What surrounds an
electric current? The answer is a magnetic field. What surrounds a
changing electric current? The answer is, a changing magnetic field.
What do we know about a changing magnetic field? The answer is,
it will create a changing electric field, in accord with Faraday’s law.
What do we know about a changing electric field? The answer is, in
accord with Maxwell’s counterpart to Faraday’s law, the changing
electric field will create a changing magnetic field.
An electromagnetic wave is composed of oscillating electric and magnetic fields that regenerate each other. No medium is
required. The oscillating fields emanate (move outward) from the
vibrating charge. At any point on the wave, the electric field is perpendicular to the magnetic field, and both are perpendicular to the
direction of motion of the wave, as shown in Figure 37.20.
Speed of Electromagnetic Waves How fast does the electromagnetic wave move? This is a very interesting question, and, in the
history of physics, a very important one. If you ask how fast a baseball
moves, or a car, or a spacecraft, or a planet, there is no single answer.
It depends on how the motion got started, what forces are acting, and
how fast the observer is moving. But for electromagnetic radiation,
there is only one speed—the speed of light—no matter what the frequency or wavelength or intensity of the radiation.
Teaching Tip Recall your
recent demonstration of charging
the rubber rod with fur. When
you brought the rod near a
charged pith ball, you produced
action at a distance. When you
moved the charged rod, the
charged ball moved also. When
you gently oscillated the rod, the
ball in turn oscillated. State that
one can think of this behavior as
either action at a distance or the
interaction of the ball and rod
with the surrounding space—the
electric field. For low frequencies,
the ball will swing in rhythm with
the shaking rod.
FIGURE 37.19 Shake a charged object back
and forth and you produce
electromagnetic waves.
Teaching Tip Establish
in your students’ minds 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
with tiny mass. Now it will
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.
FIGURE 37.20 The electric and magnetic fields of an electromagnetic
wave are perpendicular to each other.
CHAPTER 37
ELECTROMAGNETIC INDUCTION
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 motions as if the
equipment were present—you’ll
avoid the “that’s the way it
should behave” situation.
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Ask What kind of field is
induced by the shaking rod?
What kind of field, in turn,
does this induced field induce?
And further, in turn, what
kind of field does this further
induced field induce? And so
on. The shaking charge induces
a magnetic field, the changing
of which induces an electric
field, and so on. The result is an
electromagnetic wave.
At only one speed will
the linkage between
electric and magnetic
fields be in perfect balance with no gain or
loss of energy—exactly
the speed of light!
This remarkable constancy of the speed of propagating electric
and magnetic fields was discovered by Maxwell. The key to understanding it lies in the perfect balance between the two kinds of fields
that must exist if they are to propagate as waves. The changing electric
field induces a magnetic field. The changing magnetic field acts back
to induce an electric field. The wave is continually self-reinforcing.
Maxwell’s equations showed that only one speed could preserve this
harmonious balance of fields.
If, hypothetically, the wave traveled at less than the speed of light,
the fields would rapidly die out. The electric field would induce a
weaker magnetic field, which would induce a still weaker electric field,
and so on. If, still hypothetically, the wave 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. At
some critical speed, however, mutual induction continues indefinitely,
with neither a loss nor a gain in energy.
From his equations of electromagnetic induction, Maxwell calculated the value of this critical speed and found it to be 300,000
kilometers per second. To do this calculation, he used only the constants in his equations determined by simple laboratory experiments
with electric and magnetic fields. He didn’t use the speed of light. He
found the speed of light!
Physics on the Job
Cellular Field Technician Wireless communication through
such devices as cellular telephones and pagers depends on
communications towers maintained by cellular field technicians.
Signals carried by electromagnetic waves are transferred from one
tower to the next. The Federal Communications Commission (FCC)
determines the frequency of the waves allowed at each tower. Cellular
field technicians use physics to analyze or alter the electromagnetic
waves coming into a receiver or being sent out by a transmitter.
Cellular field technicians are employed by companies that maintain
cellular communications towers.
Nature of Light Maxwell 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 with frequencies in the range of 4.3 ⫻ 1014 to 7 ⫻ 1014 vibrations per second, the
resulting electromagnetic wave will activate the “electrical antennae”
in the retina of the eye. Light is simply electromagnetic waves in this
range of frequencies! The lower end of this frequency range appears
red, and the higher end appears violet. Maxwell realized that radiation of any frequency would propagate at the same speed as light.
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Teaching Tip Mention the
idea of the optimum speed
of field disturbances from the
shaking rod to the ball (consistent
with energy conservation). At
only one speed will the linkage
between electric and magnetic
fields be in perfect balance with
no gain or loss of energy—exactly
at the speed of light!
......
This radiation includes radio waves, 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 lower parts
of the antenna positively and negatively. The charges accelerating
up and down the antenna transmit electromagnetic waves. When
the waves hit a receiving antenna, the electric charges inside vibrate
in rhythm with the variations of the field.
On the evening of Maxwell’s discovery of the nature of light, he
had a date with a young woman he was later to marry. While walking
in a garden, his date remarked about the beauty and wonder of the
stars. Maxwell asked how she would feel to know that she was walking with the only person in the world who knew what the starlight
really was. For it was true. At that time, James Clerk Maxwell was the
only person in the world to know that light of any kind is energy carried in waves of electric and magnetic fields that continually regenerate each other.
CHECK
Teaching Tip Explain that
up to the last century, reality
was what people could see and
touch. Since the discovery of the
electromagnetic spectrum, people
have learned that what they can
see and touch is less than onemillionth of reality.
What makes up an electromagnetic wave?
An electromagnetic
wave is composed of
oscillating electric and magnetic
fields that regenerate each other.
......
CONCEPT
Teaching Tip Make 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 amount of radiation in the
room decreased very slightly as a
result—that the light waves make
up a tiny part of the vibrations
that engulf us at every moment.
CONCEPT
CHECK
Teaching Resources
FIGURE 37.21 • Reading and Study
Workbook
Electromagnetic wave emanation by a sending antenna and reception by a receiving
antenna. Successive views, (a) through (i), show how acceleration of the charges up
and down the antenna transmits electromagnetic waves. Only sample electric field
lines of the wave are shown—the magnetic field lines are perpendicular to the electric
field lines and extend into and out of the page.
CHAPTER 37
ELECTROMAGNETIC INDUCTION
• Transparencies 90, 91
• PresentationEXPRESS
• Interactive Textbook
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Science, Technology,
and Society
Student
opinions and answers will vary.
Allow all reasonable responses,
to generate classroom
discussion.
CRITICAL THINKING
Science, Technology, and Society
Is ELF Radiation Dangerous?
We live in fear of the unsensed. Anything that exists, or is imagined
to exist, yet escapes detection by our five senses, is often a source
of fear.
Many people fear radiation. Some is hazardous, and some is not. No 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 magazine articles have fanned the flames of
public fear by claiming that the extremely low frequency (ELF) radiation
of common 60-Hz electric power causes certain forms of cancer.
Is this claim true? Some activists say yes, although the scientific
consensus is that this is just one of many health scares that has no basis.
Careful studies have not substantiated the claimed risk. Bioscientists
point out that the electric fields due to power lines at the location of a
cell in the body are thousands of times 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 (with the
exception of rising cancer rates due to smoking). Yet during this time,
exposure to ELF radiation has increased tremendously. More detailed
analysis of the studies that prompted the controversy shows no link
between ELF and cancer.
Critical Thinking Suppose you’re a scientist and you find uncertain
evidence that some common food—tomatoes, for example—may be
a serious health risk. What responsibility would 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 make
sensational claims, even unsupported, to get people’s attention?
For: Links on
electromagnetic induction
Visit: www.SciLinks.org
Web Code: csn – 3708
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Teaching Resources
• TeacherEXPRESS
• Conceptual Physics Alive!
DVDs Magnetism and
Induction
Concept Summary
•
•
•
•
•
•
•
•
••••••
Electric current can be produced in a
wire simply by moving a magnet into or
out of a wire coil.
Faraday’s law states that the induced voltage in a coil is proportional to the product of the number of loops, the crosssectional area of each loop, and the rate
at which the magnetic field changes.
A generator converts mechanical energy
into electrical energy.
Moving charges experience a force that is
perpendicular to both their motion and
the magnetic field they traverse.
A transformer works by inducing a
changing magnetic field in one coil,
which induces an alternating current in a
nearby second coil.
Almost all electric energy sold today is in
the form of alternating current because
of the ease with which it can be transformed from one voltage to another.
A magnetic field is created anywhere an
electric field changes with time.
An electromagnetic wave is composed of
oscillating electric and magnetic fields.
Key Terms
•••••••••••••
electromagnetic
induction (p. 742)
Faraday’s law (p. 743)
generator (p. 743)
transformer (p. 747)
think! Answers
37.2
Simply put, more work is required because
more turns mean that more voltage is
induced, producing more current in the
resistor and more energy transfer. You can
also look at it this way: When the magnetic
fields of two magnets (electro or permanent) overlap, the two 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
always such as to force the magnets apart.
This produces the resistive force you feel.
Inducing more current in more 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 passing through it. When the switch remains in
the closed position, there is a steady current
in the primary and a steady magnetic field
about the coil. This field extends into the
secondary, but unless there is a change in
the field, electromagnetic induction does
not occur.
37.5.2 Ohm’s law still holds, and there is no contradiction. The voltage induced across
the secondary circuit, divided by the load
(resistance) of the secondary circuit, equals
the current in the secondary circuit. The
current is stepped down in comparison
with the larger current that is drawn in the
primary circuit.
CHAPTER 37
ELECTROMAGNETIC INDUCTION
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Check Concepts
1. Electric current can be
produced in a wire by motion
of a magnet.
2. By moving the magnet past
the wire or into the coil
3. Greater induced voltage
4. The coil becomes a stronger
electromagnet and repels
more.
5. When resistance is large
6. Same
7. A motor converts electricity
to work, whereas a generator
converts work to electricity.
8. The magnetic field increases
and decreases each turn.
9. External source of energy such
as fuel, wind, or water
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(continued)
Concept
Check
Concepts
Summary
••••••
••••••
Section 37.1
1. What did Michael Faraday and Joseph
Henry discover?
2. How can voltage be induced in a wire with
the help of a magnet?
3. A magnet moved into a coil of wire will
induce voltage in the coil. What is the effect
of moving a magnet into a coil with more
loops?
–
8. Why is the voltage induced in an alternator
AC rather than DC?
9. The armature 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 main
ingredients: magnetic field, moving charges,
and magnetic force. What are the three main
ingredients that characterize a generator?
Section 37.5
10. Same
11. What does a transformer actually transform—voltage, current, or energy?
11. Voltage and current, but not
energy
12. Voltage
13. More turns on the secondary
coil step up the voltage.
14. 60 V
4. Why is it more difficult to move a magnet
into a coil of more loops that is connected
to a resistor?
Section 37.2
5. Current, as well as voltage, can be induced
in a wire by electromagnetic induction.
When can voltage be induced but not
current?
Section 37.3
6. How does the frequency of a changing magnetic field compare with the frequency of
the alternating voltage that is induced?
7. What is a generator, and how does it differ
from a motor?
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12. What does a step-up transformer 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-down
voltage factor?
14. If the number of secondary turns is 10 times
the number 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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15. a. In a transformer,
how does ••••••
the power inConcept
Summary
put to the primary coil compare with the
power output of the secondary coil?
b. How does the product of voltage and
current in the primary compare with
the product of voltage and current in the
secondary?
Section 37.6
16. Why is it advantageous to transmit electric
power long distances at high voltages?
15. a. Same
b. Same, if power loss is
negligible
16. Lower current results in less
energy loss through heating
of the wires.
–
17. Electric field
Think and Rank
18. Faraday’s law—induced
electricity; Maxwell’s—induced
magnetism; they are inverses.
••••••
19. The speeds are the same, c.
Rank each of the following 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 with an equal sign.
(e.g., A ⫽ B)
21. The magnets are moved into the wire coils
in identical quick fashion. Voltage induced
in each coil causes a current to flow, as
indicated on the galvanometer. Neglect the
electrical resistance of the loops in the coil.
20. Electromagnetic waves,
l range 5 400–700 nm
Think and Rank
21. a. B, C, A
b. B, C, A
Section 37.7
17. What fundamental quantity underlies the
concepts of voltages and currents?
18. Distinguish between Faraday’s law expressed
in terms of fields and Maxwell’s counterpart
to Faraday’s law. How are the two laws
symmetrical?
Section 37.8
19. How do the wave speeds compare for highfrequency and low-frequency electromagnetic waves?
a. Rank from greatest to least the reading on
the galvanometer.
b. Make the same ranking, only this time for
each coil having twice as many loops as in
part (a).
20. What is light?
CHAPTER 37
ELECTROMAGNETIC INDUCTION
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22. a. B, C, A
b. A, C, B
c. A 5 B 5 C
Think and Explain
23. The two pulses are opposite in
direction.
24. Nylon is nonmagnetic, has
no magnetic domains, and
is not magnetized by the
permanent magnet.
25. Induction occurs only for
a change in intercepted
magnetic field. Pulse occurs
when switch in the first circuit
is closed and current in the
coil increases from zero.
When current in first coil is
steady, no current induced
in secondary; galvanometer
reads zero. The needle swings
in opposite direction when
switch is opened and current
falls to zero.
26. In accord with electromagnetic
induction, if the magnetic
field alternates in the hole
of the ring, an alternating
voltage will be induced
in the ring. Because the
ring is metal, its relatively
low resistance will result
in a correspondingly high
alternating current. This
current is evident in the
heating of the ring.
3
(continued)
–
22. The transformers
are all powered
with
Concept
Summary
••••••
24. A common pickup for an electric guitar
consists of a coil of wire around a permanent magnet. The permanent magnet
magnetizes the nearby guitar string. When
the string is plucked, it oscillates above the
coil, thereby changing the magnetic field
that passes through the coil. The rhythmic
oscillations of the string produce the same
rhythmic changes in the magnetic field
in the coil, which in turn induce the same
rhythmic voltages in the coil, which when
amplified and sent to a speaker produce
music! Why will this type of pickup not
work with nylon strings?
Think and Explain
25. Two separate but similar coils of wire are
mounted close to each other, as shown below. The first coil is connected to a battery
and has a direct current flowing through it.
The second coil is connected to a galvanometer. How does the galvanometer respond
when the switch in the first circuit is closed?
When the current is steady after the switch
is closed? When the switch is opened?
100 W, and all have 100 turns on the
primary. The number of turns on each
secondary varies as shown.
a. Rank the voltage output of the secondaries from greatest to least.
b. Rank the current in the secondaries from
greatest to least.
c. Rank the power output in the secondaries
from greatest to least.
••••••
23. When Tim pushes the wire down between
the poles of the magnet, the galvanometer
registers a pulse. When he lifts the wire, another pulse is registered. How do the pulses
differ?
27. The changing magnetic field
produced when current
flows induces current in the
aluminum ring, which in
turn generates a magnetic
field that opposes the field
produced by the magnet
beneath table. The ring
becomes, momentarily, a
magnet that is repelled by
the hidden magnet.
26. If you place a metal ring in a region in
where a magnetic field is rapidly alternating,
the ring may become hot to your
touch. Why?
28. Connect bulb to a wire loop
that intercepts changing
magnetic field. Change is the
key, so power must be AC.
29. None; they simply operate in
opposite manners.
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Concept
••••••
27. A magician Summary
places an aluminum
ring on
a table, underneath which is hidden an
electromagnet. When the magician says
“abracadabra” (and pushes a switch that
starts current flowing through the coil
under the table), the ring jumps into the air.
Explain his trick.
28. How could you light a lightbulb that is near,
yet not touching, an electromagnet?
Is AC or DC required? Defend your answer.
30. Waving the coil moves it
through Earth’s magnetic
field, inducing voltage and,
hence, current.
31. Accelerating electric charges
–
34. Some bicycles have electric generators that
are made to turn when the bike wheel turns.
These generators provide energy for the
bike’s lamp. Will a cyclist coast farther if the
lamp connected to the generator is turned
off? Explain.
33. Agree; any coil of wire spun
in a magnetic field that cuts
through magnetic field lines
is a generator.
35. An electric hair drier running at normal
speed draws a relatively small current. But if
somehow the motor shaft is prevented from
turning, the current dramatically increases
and the motor overheats. Why?
29. What is the basic difference between an
electric generator and an electric motor?
30. With no magnets around, why will current
flow in a coil of wire waved around in the
air?
31. What is the source of all electromagnetic
waves?
32. Why is a generator armature more difficult
to rotate when it is connected to and supplying electric current to a circuit?
33. Your classmate says that, if you crank the
shaft of a conventional motor manually, the
motor becomes a generator. Do you agree or
disagree, and why?
32. The repulsion of the
electromagnets opposes the
rotation of the armature.
The greater the current, the
greater the repulsion, and
the more work that must be
done to spin the armature.
The answer is implied by
energy conservation. Work
done in turning the armature
goes into the electrical
energy supplied to the
external circuit.
36. When a piece of plastic tape coated with
iron oxide that is magnetized more in some
parts than others is moved past a small coil
of wire, what happens in the coil? What is
a practical application of this?
37. Why is it important that the core of a transformer pass through both coils?
ELECTROMAGNETIC INDUCTION
35. A running motor always
draws less net current than
a stalled motor. If the motor
jams or is somehow prevented
from turning, then the back
current is no longer generated
and the net current in the
motor 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 maximum
number of magnetic field
lines produced in the primary
coil are intercepted by the
secondary coil
38. The hum is a same-frequency
forced vibration of the iron
slabs in the transformer core
as their magnetic polarities
alternate.
38. Why can a hum often be heard when a
transformer is operating?
CHAPTER 37
34. When the lamp is on, the
energy that goes into
lighting the lamp comes at
the expense of the KE of the
moving bicycle. The extra KE
saved by not lighting the lamp
makes the bicycle go farther.
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39. Voltage is induced in the coil;
a credit card reader
40. Yes; yes; triggering
automobile traffic lights
41. Like all currents, the eddy
currents produce their own
magnetic fields, which alter
the net field in the detector.
42. Move it so that the number of
field lines doesn’t change.
43. AC provides the change
needed for induction.
44. Secondary current is half the
current in the primary.
45. Analogous to a mechanical
lever in that work (or energy)
is transferred from one part
to another; mechanical lever
multiplies force; electrical
lever multiplies voltage. In
both, energy and power are
conserved, so what is not
multiplied is energy.
46. No; it would violate law of
energy conservation.
47. Agree; light is an
electromagnetic wave with
a frequency matching the
frequency of oscillating
charges producing it.
48. No; electromagnetic waves
depend on mutual field
regeneration. If the induced
electric fields did not in turn
induce magnetic fields and
pass energy to them, the
energy would be localized
rather than “waved” into
space.
49. The magnet induces current
loops in the copper as it falls,
which produce magnetic
fields that repel the magnet
as it approaches and attract
it as it leaves, exerting an
upward force on it. This
force increases with speed.
At some speed it matches
gravity and reaches terminal
speed. Since plastic is an
insulator, there is no current
and no induced magnetic
field to oppose falling.
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ASSESS
(continued)
39. When a strip
of magnetic material,
variably
Concept
Summary
••••••
magnetized, is embedded in a plastic card
that is moved past a small coil of wire, what
happens in the coil? What is a practical application of this?
40. If a car made of iron and steel moves over
a wide closed loop of wire embedded in a
road surface, will 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 walk
through a metal detector that uses a weak
AC magnetic field inside a large coil of wire.
You are surprised that a piece of aluminum
(nonmagnetic) in your pocket sounds the
alarm. The security officer explains that
loops of current (eddy currents) were induced in the metal. Why would eddy currents affect the net field in the detector?
42. How could you move a conducting loop
of wire through a magnetic field without
inducing a voltage in the loop?
43. Why does a transformer require alternating
voltage?
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44. How does the current in the secondary of
a transformer compare with the current in
the primary when the secondary voltage is
twice the primary voltage?
45. In what sense can a transformer be
thought of as an electrical lever? What does
it multiply? What does it not multiply?
46. Can an efficient transformer step up energy? Defend your answer.
47. A friend says that changing electric and
magnetic fields generate one another, and
this gives rise to visible light when the frequency of change matches the frequencies
of light. Do you agree? Explain.
48. Would electromagnetic waves exist if
changing magnetic fields could produce
electric fields but changing electric fields
could not in turn produce magnetic fields?
Explain.
49. When a bar magnet is dropped through a
vertical length of copper pipe, it falls noticeably more slowly than it does when it is
dropped through a vertical length of plastic pipe. If the copper pipe is long enough,
the dropped magnet will reach a terminal
falling speed. Propose an explanation.
50. What is wrong with this scheme? To
generate electricity without fuel, arrange a
motor to run a generator that will produce
electricity that is stepped up with transformers so that the generator can run
the motor and simultaneously furnish
electricity for other uses.
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ASSESS
51. An electromagnet
A with a coil
of 10 turns
Concept
Summary
••••••
carrying 1 A interacts with electromagnet B,
which has 100 turns carrying 2 A. Which
electromagnet exerts the greater force on
the other?
50. Scheme violates both the
first and second laws of
thermodynamics. The
generator produces less
electricity than is used by the
adjoining motor to power it.
A transformer cannot step
up energy or power. So there
is more input energy than
output energy.
–
54. If the output current for the above transformer is 1.8 amps, show that the input
current is 0.09 A.
51. Back to Newton’s third law!
A and B exert equal and
opposite forces on each other.
55. A transformer 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.
Think and Solve
56. A model electric train requires a low voltage
to operate. If the primary coil of its transformer has 400 turns, and the secondary
has 40 turns, how many volts will power the
train when the primary is connected to a
120-volt household circuit?
57. The primary coil of a step-up transformer
draws 100 W. Find the power provided to
the secondary circuit.
Think and Solve
••••••
52. An electric doorbell requires 12 volts to
operate correctly. A transformer nicely
allows it to be powered from a 120-volt
outlet. If the primary has 500 turns, show
that the secondary should have 50 turns.
53. A model electric train requires 6 V to
operate. When connected to a 120-V household circuit, a transformer is needed. If the
primary coil of the transformer has 240
windings, show that there should be 12
turns in the secondary coil.
58. An ideal transformer has 50 turns in its
primary coil and 250 turns in its secondary
coil. A 12-V AC source is connected to the
primary. Find
a. the AC voltage available at the secondary
b. the current in a 10-⍀ device connected
to the secondary
c. the power supplied to the primary
59. Neon signs require about 12,000 V for their
operation. What should be the ratio of the
number of loops in the secondary to the
number of loops in the primary for a
neon-sign transformer that operates off
120-V lines?
52. (120 V)/(500 turns) 5 (12 V)/x,
so x 5 50 turns.
53. (120 V)/(240 turns) 5 6 V/x
turns, so x 5 12 turns.
54. Power same in both:
IVprim 5 IVsec, so 20 times
greater voltage in primary
means 1/20 as much current
as in the secondary. That’s
1/20 3 1.8 A 5 0.09 A.
55. Steps up voltage by a factor
36/9 5 4; therefore a 12-V
input will be stepped up to
4 3 12 V 5 48 V.
56. (120 V)/(400 turns) 5 x/40, so
x 5 12 V.
57. Pp 5 Ps 5 100 W
58. a. Vp/(# primary turns) 5
Vs/(# secondary turns), so Vs 5
250 3 (12 V/50) 5 60 V.
b. I 5 V/R 5 (60 V)/(10 V)
56A
c. Pp 5 Ps 5 (VI) 5
(60 V)(6 A) 5 360 W
59. Vp/(# primary turns) 5
Vs/(# secondary turns),
so (# secondary turns) ⫼
(# primary turns) 5 Vs/Vp 5
(12,000 V)/(120 V) 5 100.
Teaching Resources
More Problem-Solving Practice
Appendix F
CHAPTER 37
ELECTROMAGNETIC INDUCTION
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• Computer Test Bank
• Chapter and Unit Tests
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