Module 21 continued
Module 21
Electromagnetism
Lesson 1 Inducing
Currents
4.
Practice Problems
p. 537
1.
You move a straight wire that is 0.5 m long
at a speed of 20 m/s vertically through a
0.4-T magnetic field pointed in the
horizontal direction.
CHALLENGE A horseshoe magnet is
mounted so that the magnetic field lines
are vertical. You pass a straight wire
between the poles and pull it toward you.
The current through the wire is from right
to left. Which is the magnet’s north pole?
Explain.
Using a right-hand rule, the north pole
is at the bottom.
Practice Problems
a. What EMF is induced in the wire?
p. 577
5.
A generator develops a maximum potential
difference of 170 V.
a. What is the effective potential
difference?
b. The wire is part of a circuit with a total
resistance of 6.0 . What is the
current?
b. A lamp is placed across the generator
with an Imax of 0.70. Ieff through the
lamp?
c. What is the resistance of the lamp
when it is on?
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
2.
3.
A straight wire that is 25 m long is mounted
on an airplane flying at 125 m/s. The wire
moves in a perpendicular direction through
Earth’s magnetic field (B  5.0105 T).
What EMF is induced in the wire?
6.
A straight wire segment in a circuit is 30.0
m long and moves at 2.0 m/s perpendicular
to a magnetic field.
The RMS potential difference of an AC
household outlet is 117 V. What is the
maximum potential difference across a
lamp connected to the outlet?
a. A 6.0-V EMF is induced. What is the
magnetic field?
b. The total resistance of the circuit is
5.0 . What is the current?
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Module 21 continued
7.
8.
If the average power used over time by an
electric light is 75 W, what is the peak
power?
11.
You (the rider) provide the mechanical
energy that turns the generator’s
armature.
CHALLENGE An AC generator delivers a
peak potential difference of 425 V.
a. What is the Veff in a circuit connected to
the generator?
12.
13.
Frequency What changes to an electric
generator are required to increase
frequency?
You could increase the number of
magnetic pole pairs or make the
armature spin faster.
Lesson 1 Check Your Progress
p. 577
Electric Generator Explain how an
electric generator works.
14.
Output Potential Difference Explain why
the output potential difference of an electric
generator increases when the magnetic
field is made stronger. What is another way
to increase the output potential difference?
The magnitude of the induced EMF is
directly related to the strength of the
magnetic field. A greater potential
difference is induced in the
conductor(s) if the field strength is
increased. Because EMF  BLv(sin  ),
you can also increase output potential
difference by increasing the length of
the wire or the velocity of the wire.
Generator Could you make a generator
by mounting permanent magnets on a
rotating shaft and keeping the coil
stationary? Explain.
15.
Yes; only relative motion between the
coil and the magnetic field is important.
Note, this generator would not have
much power as the relative velocities of
the magnets and coil will be very small.
Critical Thinking A student asks, “Why
does AC dissipate power? The energy
going into a lamp when the current is
positive is removed when the current is
negative. The net current is zero.” Explain
why this reasoning is wrong.
Power is the rate at which energy is
transferred. Power is the product of I
and V. When I is positive, so is V, and
therefore, P is positive. When I is
negative, so is V; thus, P is positive.
Energy is always transferred through
the lamp.
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Inspire Physics
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
An EMF is induced in the armature of
a generator at it is turned—by a
mechanical force—in a magnetic field.
When the generator is in a circuit, the
EMF induces a current. As the
armature rotates through 180°, the
induced EMF—and current—reverse
direction.
10.
Microphone Consider the microphone
shown in Figure 3. What happens when
the diaphragm is pushed in?
A current is induced in the coil.
b. The resistance is 5.0102 . What is
the effective current?
9.
Bike Generator A small generator on
your bike lights the bike’s headlight. What
is the source of the energy for the bulb
when you ride along a flat road?
Module 21 continued
Lesson 2 Check Your Progress
Lesson 2 Applications of
Induced Currents
p. 585
18.
Practice Problems
p. 584
16.
A step-down transformer has 7500 turns
on its primary coil and 125 turns on its
secondary coil. The potential difference
across the primary circuit is 7.2 kV. What is
the potential difference across the
secondary circuit? If the current in the
secondary circuit is 36 A, what is the
current in the primary circuit?
Away from the magnet; the changing
magnetic field induces a current in the
coil, producing a magnetic field. This
field opposes the field of the magnet,
and thus, the force between coil and
magnet is repulsive.
19.
CHALLENGE A step-up transformer that
is 95 percent efficient has 300 turns on its
primary coil and 90,000 turns on its
secondary coil. The potential difference of
the generator to which the primary circuit is
attached is 60.0 V. What is the potential
difference across the secondary circuit?
The current in the secondary circuit is 0.50
A. What current is in the primary circuit?
Motors If you unplugged a running
vacuum cleaner from a wall outlet, you
would be much more likely to see a spark
than you would if you unplugged a lighted
lamp from the wall. Why?
The inductance of the motor creates
an EMF that causes the spark. The
bulb has very low self-inductance, so
there is no EMF.
20.
Transformers and Current Explain why a
transformer may be operated only on AC.
Transformers rely on changing
currents to induce changing magnetic
fields. DC always produces the same
magnetic field and thus can’t induce a
current in another wire.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
17.
Lenz’s Law You hang a coil of wire with
its ends joined so that it can swing easily. If
you now plunge a magnet into the coil, the
coil will start to swing. Which way will it
swing relative to the magnet and why?
21.
Transformers Frequently, transformer
coils that have only a few turns are made
of very thick (low-resistance) wire, while
those with many turns are made of thin
wire. Why?
More current can go through the coil
with fewer turns, so thick wires with
capacity for large currents are
needed. Also, resistance must be kept
low to prevent voltage drops and I 2R
power loss and heating.
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Module 21 continued
22.
Step-Up Transformers Refer to the step-up transformer shown in Figure 15. Explain
what would happen to the primary current if the secondary coil were short-circuited.
According to the transformer equations, the ratio of primary to secondary
current is equal to the ratio of turns and doesn’t change. Thus, if the
secondary current increases, so does the primary.
23.
Critical Thinking Would permanent magnets make good transformer cores? Explain.
No; induced EMF depends on a changing magnetic field through the core.
Permanent magnets are “permanent” because they are made of materials that
resist such changes.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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Module 21 continued
26. Protons passing without deflection through a magnetic field of 0.060 T are balanced
by an electric field of 9.0103 V/m. What is the speed of the moving protons?
27.
Challenge What trajectory would a positive ion follow moving in a magnetic field that
increases linearly with time?
Lesson 4 Electric and
Magnetic Fields in Space
39.
An electromagnetic wave has a frequency
of 8.2014 Hz. What is the wavelength of
the wave?
40.
What is the frequency of an
electromagnetic wave that has a
wavelength of 2.2102 m?
41.
CHALLENGE If an electromagnetic wave
is propagating to the right and the electric
field is in and out of the page, in what
direction is the magnetic field?
Practice Problems
p. 595
38.
What is the wavelength of green light that
has a frequency of 5.701014 Hz?
up and down
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Practice Problems
p. 599
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42.
What is the speed of an electromagnetic
wave traveling through air? Use
c  299,792,458 m/s in your calculation.
43.
Water has a dielectric constant of 1.77.
What is the speed of light in water?
44.
The speed of light traveling through a
material is 2.43108 m/s. What is the
dielectric constant of the material?
Solutions Manual
Module 21 continued
45.
Challenge A radio signal is transmitted
from Earth’s surface to the Moon’s surface,
376,290 km away. What is the shortest
time a reply can be expected?
49.
Round-trip distance is 752,580,000 m.
They also must be horizontal.
x
752,580,000 m
 2.51034
t 
2.5109 ss
c 299,792,458 m/s
50.
Lesson 4 Check Your Progress
Electromagnetic Waves Explain how
electromagnetic waves propagate through
space.
The changing electric field induces a
changing magnetic field, and the
changing magnetic field induces a
changing electric field. The waves
propagate as these two fields
regenerate each other.
47.
51.
Antenna Design Would an FM antenna
designed to be most sensitive to stations
near 88 MHz be shorter or longer than one
designed to receive stations near
108 MHz? Explain your reasoning.
Longer; lower-frequency waves would
have longer wavelengths, so they use
a longer antenna.
Interaction with Matter How do different
wavelengths of electromagnetic radiation
interact with matter? How does this affect
how we use and interact with the radiation?
52.
Dielectric Constant The speed of light
traveling through an unknown material is
1.98108 m/s. Given that the speed of light
in a vacuum is 3.00108 m/s, what is the
dielectric constant of the unknown
material?
Frequency An electromagnetic wave has
a wavelength of 1.5×10−5 m. What is its
frequency?
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Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Longer-wavelength electromagnetic
radiation is generally converted into
thermal energy when it interacts with
matter. Shorter-wavelength
electromagnetic radiation can ionize
atoms and damage living cells. How
we use electromagnetic radiation is
determined in part by how it interacts
with matter. For example, safety
precautions must be taken when
using shorter-wavelength
electromagnetic radiation such as Xrays or gamma rays.
48.
Digital Signals What are the advantages
of storing and transmitting sound, pictures,
and data as digital signals?
Digital signals can be stored reliably
in computer memory and sent over
long distances. They can send more
information in the same amount of
time as AM or FM and are less
affected by noise.
p. 603
46.
Radio Signals Radio antennas normally
have metal rod elements that are oriented
horizontally. From this information, what
can you deduce about the directions of the
electric fields in radio signals?
Module 21 continued
Analyze and Interpret Data
53.
Critical Thinking Most of the UV radiation
from the Sun is blocked by the ozone layer
in Earth’s atmosphere. Scientists have
found that the ozone layer over Antarctica
and the southern hemisphere has thinned.
Use what you have learned about
electromagnetic waves to explain why
some scientists are very concerned about
the thinning ozone layer.
1.
a. In the first novel, how should the aliens’
eyes compare in size to human eyes?
The eyes would be much larger
because the wavelength of
microwave radiation is much larger
than that of visible light.
b. Is Wells correct that the invisible man is
able to see?
If the entire ozone layer is thinning,
the amount of UV radiation from the
Sun that is blocked by the ozone layer
decreases, allowing more UV rays to
reach Earth’s surface. The
wavelengths of UV waves are short
enough and their energies are high
enough to damage skin molecules.
Thus, the resulting increase in UV
rays might increase the prevalence of
skin cancer in humans.
To see, you must detect the light,
which means the light will be
absorbed or scattered. Essentially,
an invisible person would be
completely transparent so light
would just pass through the eye
without ever being absorbed or
scattered.
c. Is the “anti-Lenz’s law” possible?
Go Further
No. It would violate the law of
conservation of energy. It would
allow a changing magnetic field to
grow without limits. The current
would increase without any work
done. A generator would create
energy, not just change it from one
form to another.
Data Analysis Lab
p. 607
Can you evaluate scenarios in science
fiction novels?
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Claim Answer the following questions
You and your friends are reading a collection of
science fiction books. After you read a book, you
meet to discuss whether the science presented
in the book is possible.
2.
Evidence What scientific information or
principles support each of your claims?
Students should use key concepts
from this module to provide support
for their claims. Possible citations in
this text include:
a. In one novel, alien beings have eyes
that are sensitive to microwaves.
b. In H. G. Wells’s novel The Invisible
Man, a man drinks a potion and
becomes invisible, although he retains
all his other faculties including his ability
to see.
a. Text under the heading “Receiving
Electromagnetic Waves” and its
subheads, pp. 601–603 and the
information in Figure 24 on p. 597.
b. Text under the heading “Receiving
Electromagnetic Waves” and its
subheads, pp. 601–603; text under
“Sources of light” on p. 386.
b. Text under the heading “Lenz’s
Law,” pp. 578–579.
c. A scientist in a third novel discovers an
“anti-Lenz’s law” that states an induced
current results in a force that increases
the change in a magnetic field. Thus,
when more energy was demanded, the
force needed to turn a generator was
reduced.
3.
Reasoning Explain your reasoning for
each of the claims that you have made.
Students should use the key concepts
listed above to explain their reasoning
and justify their claims.
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