Conceptual Physics

Paul G. Hewitt

Chapter 25 Electromagnetic Induction - all with Video Answers

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Chapter Questions

01:25

Problem 1

What is electromagnetic induction?

Jayashree Behera

Numerade Educator

02:21

Problem 2

What kind of energy conversion takes place in electromagnetic induction?

Averell Hause

Carnegie Mellon University

03:34

Problem 3

State Faraday’s law.

Jayashree Behera

Numerade Educator

01:25

Problem 4

What are the three ways in which voltage can be induced
in a loop of wire?

Averell Hause

Carnegie Mellon University

00:44

Problem 5

How does the frequency of induced voltage relate to how frequently a magnet is plunged in and out of a coil of wire?

Jayashree Behera

Numerade Educator

03:14

Problem 6

What are the basic differences and similarities between a generator and an electric motor?

Averell Hause

Carnegie Mellon University

00:11

Problem 7

Is the current that is produced by a common generator ac or dc?

Jayashree Behera

Numerade Educator

00:45

Problem 8

What is the common frequency of ac in homes in the United States?

Averell Hause

Carnegie Mellon University

00:43

Problem 9

Who discovered electromagnetic induction, and who put it to practical use?

Jayashree Behera

Numerade Educator

00:54

Problem 10

What is an armature?

Averell Hause

Carnegie Mellon University

00:35

Problem 11

What commonly supplies the energy input to a turbine?

Jayashree Behera

Numerade Educator

01:40

Problem 12

Is it correct to say that a generator produces energy? Defend your answer.

Averell Hause

Carnegie Mellon University

01:49

Problem 13

What are the principal differences between an MHD generator and a conventional generator?

Jayashree Behera

Numerade Educator

02:25

Problem 14

Does an MHD generator employ Faraday’s law of induction? Explain.

Averell Hause

Carnegie Mellon University

00:12

Problem 15

What name is given to the rate at which energy is transferred?

Jayashree Behera

Numerade Educator

02:27

Problem 16

Is it correct to say that a transformer boosts electric energy? Defend your answer.

Averell Hause

Carnegie Mellon University

00:47

Problem 17

What is a transformer and where do we use it?

Jayashree Behera

Numerade Educator

01:52

Problem 18

How does the power input to an efficient transformer compare with the power output?

Averell Hause

Carnegie Mellon University

02:04

Problem 19

What do you understand by Eddy currents?

Mirza Aslam Beig

Numerade Educator

01:47

Problem 20

In a step-down transformer, how does the input current compare with the output current?

Averell Hause

Carnegie Mellon University

01:21

Problem 21

Why does a transformer require ac?

Jayashree Behera

Numerade Educator

01:13

Problem 22

What is the principal advantage of ac over dc?

Averell Hause

Carnegie Mellon University

00:25

Problem 23

When the magnetic field changes in a coil of wire, voltage in each loop of the coil is induced. Will voltage be induced in a loop if the source of the magnetic field is the coil itself?

Jayashree Behera

Numerade Educator

01:05

Problem 24

What is the purpose of transmitting power at high voltages over long distances?

Averell Hause

Carnegie Mellon University

01:12

Problem 25

Does the transmission of electric energy require electric conductors between the source and receiver? Cite an example to defend your answer.

Jayashree Behera

Numerade Educator

01:28

Problem 26

Who extended Faraday’s law to changing electric fields?

Averell Hause

Carnegie Mellon University

00:38

Problem 27

What is induced by the rapid alternation of a magnetic field?

Jayashree Behera

Numerade Educator

00:27

Problem 28

What is induced by the rapid alternation of an electric field?

Averell Hause

Carnegie Mellon University

00:16

Problem 29

Are wires needed in Maxwell’s view of Faraday’s law?

Jayashree Behera

Numerade Educator

01:26

Problem 30

What do we call electromagnetic waves in the range of frequencies that match what our eyes can see?

Averell Hause

Carnegie Mellon University

01:42

Write a letter to a relative or friend and say that you have discovered what the answer is to what has been a mystery for centuries—the nature of light. State how light is related to electricity and magnetism.

Jayashree Behera

Numerade Educator

01:22

Problem 32

With a length of wire, make a coil, perhaps by wrapping it loosely around the cardboard tube of an empty roll of paper towels. Remove the tube. Attach each end of the wire to a galvanometer. Move a magnet back and forth within the coil and watch the movement of the galvanometer needle. Then increase the number of loops and again watch the galvanometer reading. This is especially interesting if you have magnets of different strengths to play with.

Manish Kumar

Numerade Educator

02:15

Problem 33

Earth’s magnetic field induces some degree of magnetism in most of the iron objects around you. With a compass you can see that cans of food on your pantry shelf have north and south poles. When you pass the compass from their bottoms to their tops, you can easily identify their poles. Mark the poles N and S. Then turn the cans upside down and note how many days it takes for the poles to reverse themselves. Explain to your friends why the poles reverse.

Averell Hause

Carnegie Mellon University

04:55

Problem 34

Drop a small bar magnet through a vertical plastic pipe, noting its speed of fall. Then do the same with a copper pipe. Whoa! Why the difference?

Averell Hause

Carnegie Mellon University

01:26

Problem 35

Transformer relationship: $\frac{\text { Primary voltage }}{\text { Number of primary turns }}=\frac{\text { secondary voltage }}{\text { number of secondary turns }}$
The primary of a transformer connected to 120 V has 10 turns. The secondary has 100 turns. Show that the output voltage is 1200 V. This is a step-up transformer.

Jayashree Behera

Numerade Educator

01:01

Problem 36

Transformer relationship: $\frac{\text { Primary voltage }}{\text { Number of primary turns }}=\frac{\text { secondary voltage }}{\text { number of secondary turns }}$

The primary of a transformer connected to 120 V has 100 turns. The secondary has 10 turns. Show
that the output voltage is 12 V. This is a step-down transformer.

Averell Hause

Carnegie Mellon University

00:46

Problem 37

A video game console requires 6 V to operate correctly. A transformer allows the device to be powered from a 120-V outlet. If the primary has 500 turns, show that the secondary should have 25 turns.

Jayashree Behera

Numerade Educator

01:02

Problem 38

A model electric train requires 6 V to operate. When it is connected to a 120-V household circuit, a transformer is needed. If the primary coil of the transformer has 360 turns, show that the secondary coil should have 18 turns.

Averell Hause

Carnegie Mellon University

00:53

Problem 39

A transformer for a laptop computer converts a 120-V input to a 24-V output. Show that the primary coil has five times as many turns as the secondary coil.

Jayashree Behera

Numerade Educator

00:43

Problem 40

If the output current for the transformer in the preceding problem is 1.8 A, show that the input current is 0.36 A.

Averell Hause

Carnegie Mellon University

02:18

Problem 41

A transformer has an input of 6 V and an output of 36 V. If the input is changed to 12 V, show that the output would be 72 V.

Jayashree Behera

Numerade Educator

01:47

Problem 42

An ideal transformer has 50 turns in its primary and 250 turns in its secondary. $12-\mathrm{V}$ ac is connected to the primary. Show that (a) 60 $\mathrm{V}$ ac is available at the secondary; $(\mathrm{b}) 6 \mathrm{A}$ of current is in a $10-\Omega$ device connected to the secondary; and (c) the power supplied to the primary is 360 $\mathrm{W}$ .

Averell Hause

Carnegie Mellon University

01:26

Problem 43

Neon signs require about 12,000 V for their operation. Consider a neon-sign transformer that operates off 120-V lines. How many more turns should be on the secondary than on the primary?

Jayashree Behera

Numerade Educator

02:38

Problem 44

A power of 100 $\mathrm{kW}\left(10^{5} \mathrm{W}\right)$ is delivered to the other side of a city by a pair of power lines, between which the voltage is $12,000 \mathrm{V}$ .
(a) Use the formula $P=I V$ to show that the current in the lines is 8.3 $\mathrm{A}$ .
(b) If each of the two lines has a resistance of $10 \Omega,$ show that there is a $83-\mathrm{V}$ change of voltage along each line. (Think carefully.This voltage change is along each line, not between the lines.)
(c) Show that the power expended as heat in both lines together is 1.38 kW (distinct from power delivered to customers).
(d) How do your calculations support the importance of stepping voltages up with transformers for
long-distance transmission?

Averell Hause

Carnegie Mellon University

02:11

Problem 45

Bar magnets are moved into the wire coils in identical quick fashion. Voltage induced in each coil causes a current, as indicated on the galvanometer. Ignore the electrical resistance in the loops in the coil, and rank from greatest to least the readings on the galvanometer.

Prashant Bana

Numerade Educator

03:48

Problem 46

Each of the transformers shown is powered with 100 W, and all have 100 turns on the primary. The number of turns on each secondary varies as indicated.
a. Rank the voltage outputs of the secondaries from greatest to least.
b. Rank the currents in the secondaries from greatest to least.
c. Rank the power outputs in the secondaries from greatest to least.

Averell Hause

Carnegie Mellon University

01:36

Problem 47

Explain two approaches through which the magnetic field produced by a solenoid can be made stronger.

Mayukh Banik

Numerade Educator

00:54

Problem 48

A common pickup for an electric guitar consists of a coil of wire around a small permanent magnet, as described in Figure 25.5. Why will this type of pickup fail with nylon strings?

Averell Hause

Carnegie Mellon University

03:50

Problem 49

How does the number of turns in the primary and secondary coils of an electromagnet affect its performance?

Jayashree Behera

Numerade Educator

00:43

Problem 50

Why are the armature and field windings of an electric motor usually wound on an iron core?

Averell Hause

Carnegie Mellon University

00:47

Problem 51

Why is a generator armature harder to rotate when it is connected to a circuit and supplying electric current?

Jayashree Behera

Numerade Educator

02:08

Problem 52

Why does a motor also tend to act as a generator?

Averell Hause

Carnegie Mellon University

02:19

Problem 53

Will a cyclist coast farther if the lamp connected to the generator on his bicycle is turned off? Explain.

Jayashree Behera

Numerade Educator

05:42

Problem 54

When an automobile moves over a wide, closed loop of wire embedded in a road surface, is the magnetic field of Earth within the loop altered? Is a pulse of current produced? Can you cite a practical application for this at a traffic intersection?

Averell Hause

Carnegie Mellon University

01:00

Problem 55

At the security area, people walk through a large coil of wire and through a weak ac magnetic field. What is the result of a small piece of metal on a person that slightly alters the magnetic field in the coil?

Jayashree Behera

Numerade Educator

01:27

Problem 56

A piece of plastic tape coated with iron oxide is magnetized more in some parts than in others. When the tape is moved past a small coil of wire, what happens in the coil? What is a practical application of this?

Averell Hause

Carnegie Mellon University

02:43

Problem 57

How do the direction of the magnetic force and its effects differ between the motor effect and the generator effect, as shown in Figure 25.7?

Jayashree Behera

Numerade Educator

01:01

Problem 58

When you turn the shaft of an electric motor by hand, what occurs in the interior coils of wire?

Averell Hause

Carnegie Mellon University

03:30

Problem 59

Your friend says that if you crank the shaft of a dc motor manually, the motor becomes a dc generator. Do you agree or disagree?

Jayashree Behera

Numerade Educator

01:11

Problem 60

Does the voltage output increase when a generator is made to spin faster? Defend your answer.

Averell Hause

Carnegie Mellon University

01:02

Problem 61

If you place a metal ring in a region in which a magnetic field is rapidly alternating, the ring may become hot to your touch. Why?

Jayashree Behera

Numerade Educator

01:57

Problem 62

A magician places an aluminum ring on a table with a hidden electromagnet underneath. 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.”

Averell Hause

Carnegie Mellon University

01:15

Problem 63

In one of the chapter-opening photographs, Jean Curtis asks her class why the copper ring levitates about the iron core of the electromagnet. What is the explanation, and does it involve ac or dc?

Jayashree Behera

Numerade Educator

01:17

Problem 64

How could a lightbulb near an electromagnet, but not touching it, be lit? Is ac or dc required? Defend your answer.

Averell Hause

Carnegie Mellon University

02:14

Problem 65

A length of wire is bent into a closed loop and a magnet is plunged into it, inducing a voltage and, consequently, a current in the wire. A second length of wire, twice as long, is bent into two loops of wire, and a magnet is similarly plunged into it. Twice the voltage is induced, but the current is the same as that produced in the single loop. Why?

Jayashree Behera

Numerade Educator

01:52

Problem 66

Two separate but similar coils of wire are mounted close to each other, as shown. The first coil is connected to a battery. The second coil is connected to a galvanometer.
(a) How does the galvanometer respond when the switch in the first circuit is closed?
(b) After being closed, how does the meter respond when the current is steady?
(c) How does the meter respond when the switch is opened?

Dading Chen

Numerade Educator

00:58

Problem 67

Why will more voltage be induced with the apparatus shown above if an iron core is inserted in the coils?

Jayashree Behera

Numerade Educator

01:13

Problem 68

Why won’t a transformer work if you are using dc?

Averell Hause

Carnegie Mellon University

02:05

Problem 69

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?

Jayashree Behera

Numerade Educator

01:30

Problem 70

In what sense can a transformer be considered an electrical lever? What does it multiply? What does it not multiply?

Averell Hause

Carnegie Mellon University

01:26

Problem 71

What is the principal difference between a step-up transformer and a step-down transformer?

Jayashree Behera

Numerade Educator

01:09

Problem 72

Why can a hum usually be heard when a transformer is operating?

Averell Hause

Carnegie Mellon University

View

Problem 73

Why doesn’t a transformer work with direct current? Why is ac required?

Jayashree Behera

Numerade Educator

00:58

Problem 74

Why is it important that the core of a transformer pass through both coils?

Averell Hause

Carnegie Mellon University

00:32

Problem 75

Are the primary and secondary coils in a transformer physically linked, or is there space between the two? Explain.

Jayashree Behera

Numerade Educator

03:45

Problem 76

A 120-volt ac source powers the circuit shown. How many volts are impressed across the lightbulb, and how many amps flow through it?

Vishal Gupta

Numerade Educator

01:38

Problem 77

In the circuit shown, how many volts are impressed across the meter, and how many amps flow through it?

Jayashree Behera

Numerade Educator

01:06

Problem 78

How would you answer the preceding question if the input were 12 V ac?

Averell Hause

Carnegie Mellon University

00:43

Problem 79

Can an efficient transformer step up energy? Defend your answer.

Jayashree Behera

Numerade Educator

01:10

Problem 80

If a bar magnet is thrown into a coil of high-resistance wire, it will slow down. Why?

Averell Hause

Carnegie Mellon University

View

Problem 81

We know that the source of a sound wave is a vibrating object. What is the source of an electromagnetic wave?

Jayashree Behera

Numerade Educator

02:16

Problem 82

With no magnets in the vicinity, why will current flow in a large coil of wire waved around in the air?

Averell Hause

Carnegie Mellon University

01:10

Problem 83

What does an incident radio wave do to the electrons in a receiving antenna?

Jayashree Behera

Numerade Educator

01:13

Problem 84

How do you suppose the frequency of an electromagnetic wave compares with the frequency of the electrons it sets into oscillation in a receiving antenna?

Averell Hause

Carnegie Mellon University

00:34

Problem 85

A friend says that changing electric and magnetic fields generate one another and that this gives rise to visible light when the frequency of change matches the frequencies of light. Do you agree? Explain.

Jayashree Behera

Numerade Educator

00:41

Problem 86

Would electromagnetic waves exist if changing magnetic fields could produce electric fields but changing electric fields could not, in turn, produce magnetic fields? Explain.

Averell Hause

Carnegie Mellon University

00:22

Problem 87

Joseph Henry’s wife donated part of her silk wedding gown to cover the wires of Joseph’s electromagnets. What was the purpose of the silk covering?

Jayashree Behera

Numerade Educator

01:37

Problem 88

A certain simple earthquake detector consists of a little box firmly anchored to Earth. Suspended inside the box is a massive magnet that is surrounded by stationary coils of wire fastened to the box. Explain how this device works by applying two important principles of physics—one from Chapter 2 and the other in this chapter.

Averell Hause

Carnegie Mellon University

01:50

Problem 89

An electric saw operating at normal speed draws a relatively small current. But if a piece of wood being sawed jams and the motor shaft is prevented from turning, the current dramatically increases and the motor overheats. Why?

Ajay Singhal

Numerade Educator

01:36

Problem 90

Your friend says that according to Ohm’s law, high voltage produces high current. Then your friend asks, “So how can power be transmitted at high voltage and low current in a power line?” What is your illuminating response?

Averell Hause

Carnegie Mellon University

00:55

Problem 91

Your physics instructor drops a magnet through a long vertical copper pipe and it moves slowly compared with the drop of a nonmagnetized object. Provide an explanation.

Keshav Singh

Numerade Educator

01:40

Problem 92

This exercise is similar to the preceding one. Why will a bar magnet fall slower and reach terminal velocity in a vertical copper or aluminum tube but not in a cardboard tube?

Averell Hause

Carnegie Mellon University

00:43

Problem 93

Although copper and aluminum are not magnetic, why is a sheet of either metal more difficult to pass between the pole pieces of a magnet than a sheet of cardboard?

Jayashree Behera

Numerade Educator

01:29

Problem 94

A metal bar, pivoted at one end, oscillates freely in the absence of a magnetic field. But when it oscillates
between the poles of a magnet, its oscillations are quickly damped. Why? (Such magnetic damping is used in a number of practical devices.)

Averell Hause

Carnegie Mellon University

01:39

Problem 95

The metal wing of an airplane acts like a “wire” flying through Earth’s magnetic field. A voltage is induced between the wing tips, and a current flows along the wing, but only for a short time. Why does the current stop even though the airplane continues flying through Earth’s magnetic field?

Jayashree Behera

Numerade Educator

02:10

Problem 96

What is wrong with this scheme? To generate electricity without fuel, arrange a motor to operate a generator that will produce electricity that is stepped up with transformers so that the generator can operate the motor and simultaneously furnish electricity for other uses.

Averell Hause

Carnegie Mellon University

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