Induction and Alternating Current with teacher's notes

Chapter 22 

Overview 

Section 22-1 introduces induced 

current, discusses Lenz’s law, and 

applies Faraday’s law of induction 

to calculate induced emf and 

induced current. 

Section 22-2 introduces generators 

and motors as devices that 

convert energy from one form to 

another and shows how to calculate 

the maximum emf for an 

electrical generator, the rms current, 

and the potential difference 

for ac circuits. 

Section 22-3 examines mutual 

induction, discusses transformers, 

shows how to calculate the 

potential difference for a step-up 

or step-down transformer, and 

describes how self-induction 

occurs in a circuit. 

About the Illustration 

The strings of an electric guitar 

are magnetized by a permanent 

magnet located inside coils of 

wire beneath the strings. Two sets 

of coils—one at the base of the 

neck and the other just above the 

bridge—can be seen in each of 

the guitars in the photograph. As 

a string vibrates, the changing 

magnetic field induces in the coil 

an alternating current, whose frequency 

depends on the string’s 

frequency. 

Interactive 

Problem- 

Solving 

Tutor 

See Module 20 

“Induction and Transformers” 

promotes additional development 

of problem-solving skills. 

792 

DTSI Graphics HRW—Holt Physics 2002 Page 792 CMYK 

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Copyright © by Holt, Rinehart and Winston. All rights reserved. 

CHAPTER 22 

Induction and 

Alternating 

Current 

PHYSICS IN ACTION 

In an electric guitar, the vibrations of the 

strings are converted to an electric signal, 

which is then amplified outside the guitar 

and heard as sound from a loudspeaker. 

Yet the electric guitar is not plugged 

directly into an electric source. Instead, it 

generates electric current by a process 

called induction. By changing the magnetic 

field near a coil of wire, an electric current 

can be induced in the coil by the vibrations 

of the guitar strings. 

This chapter explores how induction 

produces and changes alternating currents. 

• Why does the generated current last only as 

long as the string vibrates? 

• Why must the strings be ferromagnetic in 

order for an electric guitar to work? 

CONCEPT REVIEW 

Resistance (Section 19–2) 

emf (Section 20–1) 

Magnetic force (Section 21–3) 

Induction and Alternating Current 793 

Tapping Prior 

Knowledge 

Knowledge to Expect 

✔ “Electric currents and magnets 

can exert a force on 

each other.” (AAAS’s Benchmarks 

for Science Literacy, 

grades 6–8) 

✔ “Energy can be converted 

from one form to another.” 

(AAAS’s Benchmarks for 

Science Literacy, grades 6–8) 

Knowledge to Review 

✔ The definition of resistance 

states that the current in a 

circuit is proportional to the 

applied potential difference 

and inversely proportional to 

the resistance of the circuit. 

(Section 19-2) 

✔ Emf increases the electrical 

potential energy of charges in 

a circuit, causing the charges 

to move. Thus, emf is a 

source of current in a circuit. 


✔ A charged particle moving in 

a magnetic field experiences 

a magnetic force. When the 

particle moves perpendicular 

to the field, F magnetic = qvB. 


Items to Probe 

✔ Resistance: Have students 

calculate current for various 

cases involving changing 

potential differences. 

793

Section 22-1 

Teaching Tip 

Remind students that, as discussed 

in Chapter 21, electric 

charges that move through a 

magnetic field experience a magnetic 

force. The magnitude of this 

force is qvB. When charges are at 

rest with respect to the magnetic 

field (v = 0 m/s), they do not 

experience a magnetic force. 

Visual Strategy 

Figure 22-1 

Be sure students understand the 

conditions required to induce a 

current in a circuit. 

Why is a current generated in 

Q a moving conductor in a 

magnetic field but not in a moving 

insulator in the same magnetic 

field? 

Charges experience a force in 

A both cases. In conductors, the 

electrons are free to move under 

the influence of the magnetic 

force. Electrons in an insulator 

feel the same force but are not 

free to move. 

Q 

What is the condition 

required for current to exist in 

the circuit shown in this figure? 

There must be relative 

A motion between the circuit 

and the magnetic field; when the 

two are at rest relative to one 

another, charges do not experience 

a magnetic force, so they 

remain at rest. 

794 

22-1 SECTION OBJECTIVES 

• Describe how the change in 

the number of magnetic field 

lines through a circuit loop 

affects the magnitude and 

direction of the induced 

current. 

• Apply Lenz’s law to 

determine the direction of an 

induced current. 

• Calculate the induced emf 

and current using Faraday’s 

law of induction. 

electromagnetic induction 

the production of an emf in a 

conducting circuit by a change in 

the strength, position, or orientation 

of an external magnetic field 

Figure 22-1 

When the circuit loop 

crosses the lines of the 

magnetic field, a current 

is induced in the circuit, 

as indicated by the 

movement of the 

galvanometer needle. 

794 

Chapter 22 

100 

200 

300 

400 

500 

600 

700 

800 

900 

0 

22-1 

Induced current 

MAGNETIC FIELDS AND INDUCED EMFS 

Recall that in Chapter 20 you were asked if it was possible to produce an electric 

current using only wires and no battery. So far, all electric circuits that you 

have studied have used a battery or an electrical power supply to create a 

potential difference within a circuit. In both of these cases, an emf increases 

the electrical potential energy of charges in the circuit, causing them to move 

through the circuit and create a current. 

It is also possible to induce a current in a circuit without the use of a battery 

or an electrical power supply. Just as a magnetic field can be formed by a current 

in a circuit, a current can be formed by moving a portion of a closed electric 

circuit through an external magnetic field, as indicated in Figure 22-1. 

The process of inducing a current in a circuit with a changing magnetic field 

is called electromagnetic induction. 

Consider a circuit consisting of only a resistor in the vicinity of a magnet. 

There is no battery to supply a current. If neither the magnet nor the circuit is 

moving with respect to the other, no current will be present in the circuit. But 

if the circuit moves toward or away from the magnet or the magnet moves 

toward or away from the circuit, a current is induced. As long as there is relative 

motion between the two, a current can form in the circuit. 

The separation of charges by the magnetic force induces an emf 

It may seem strange that there can be an induced emf and a corresponding 

induced current without a battery or similar source of electrical energy. Recall 

that in the previous chapter a moving charge can be deflected by a magnetic 

field. This deflection can be used to explain how an emf occurs in a wire that 

moves through a magnetic field. 

100 

200 

300 

400 

500 

600 

700 

800 

900 

1000 

Galvanometer 

S 


N 

Magnetic field 


Consider a conducting wire pulled through a magnetic field, as shown on the 

left in Figure 22-2. You learned in Chapter 21 that positive charges moving with 

a velocity at an angle to the magnetic field will experience a magnetic force. 

According to the right-hand rule, this force will be directed perpendicular to 

both the magnetic field and the motion of the charges. For positive charges in the 

wire, the force is directed downward along the wire. For negative charges, the 

force is upward. This effect is equivalent to replacing the segment of wire and 

the magnetic field with a battery that has a potential difference, or emf, 

between its terminals, as shown on the right in Figure 22-2. As long as the 

conducting wire moves through the magnetic field, the emf will be maintained. 

The polarity of the induced emf—and thus the direction of the induced 

current in the circuit—depends on the direction in which the wire is moved 

through the magnetic field. For instance, if the wire in Figure 22-2 is moved 

to the right, the right-hand rule predicts that the positive charges will be 

pushed downward. If the wire is moved to the left, the positive charges will be 

pushed upward. The magnitude of the induced emf depends on the velocity 

with which the wire is moved through the magnetic field, the length of the 

wire, and the strength of the magnetic field. 

The angle between a magnetic field and a circuit affects induction 

To induce an emf in a wire loop, part of the loop must move through a magnetic 

field or the entire loop must pass into or out of the magnetic field. No 

emf is induced if the loop is static or the magnetic field is constant. 

The magnitude of the induced emf and current depend partly on how the 

loop is oriented to the magnetic field, as shown in Figure 22-3. The induced 

current is largest when the plane of the loop is perpendicular to the magnetic 

field, as in (a); it is less when the plane is tilted into the field, as in (b); and it is 

zero when the plane is parallel to the field, as in (c). 

The role of orientation on induced current can be understood in terms of 

the force exerted by a magnetic field on charges in the moving loop. The smaller 

the component of the magnetic field perpendicular to both the plane and 

the motion of the loop, the smaller the magnetic force on the charges in the 

loop. When the area of the loop is moved parallel to the magnetic field, there is 

no magnetic field component perpendicular to the plane of the loop and therefore 

no force that moves the charges through the circuit. 

v v 

v 

(a) (b) (c) 

Figure 22-3 

The induced emf and current are largest when the plane of the 

loop is perpendicular to the magnetic field (a), smaller when the 

plane of the loop is tilted into the field (b), and zero when the 

plane of the loop and the magnetic field are parallel (c). 


Induction and Alternating Current 

− 

+ 

v 

B (out of page) 

= 

– 

+ 

Figure 22-2 

The separation of positive and negative 

moving charges by the magnetic 

force creates a potential difference 

(emf ) between the ends of the 

conductor. 

NSTA 

TOPIC: Magnetic fields 

GO TO: www.scilinks.org 

sciLINKS CODE: HF2221 

795 

SECTION 22-1 

The Language of Physics 

The term emf originally stood for 

electromotive force. Today, emf is 

considered to be a potential difference 

rather than a force. Specifically, 

emf is a potential difference 

that sets charges in a circuit in 

motion. To avoid misconceptions, 

the term electromotive force is not 

used in this text. 


Figure 22-3 

Point out that any charge with a 

velocity component perpendicular 

to a magnetic field will experience 

a force that is perpendicular 

both to the magnetic field and to 

that velocity component. 

Why does a loop that is paral- 

Q lel to the field, and moving as 

shown in Figure 22-3(c), experience 

no current around the loop 

even though the loop is moving 

in the magnetic field? 

The electrons do experience a 

A magnetic force perpendicular 

to the field, but they cannot 

move in that direction because 

the loop is parallel to the field. 

795

SECTION 22-1 

Demonstration 1 


Purpose Show students an 

example of an induced current. 

Materials flashlight bulb in 

holder, coil of wire, bar magnet 

(two may be required), connecting 

wires 

Procedure Connect the flashlight 

bulb to the coil of wire with the 

connecting wires. Tell students to 

observe the demonstration with 

the intent of explaining the energy 

conversions. Move the bar 

magnet into and out of the coil 

several times in rapid succession. 

Have students explain the energy 

conversions on the board or in 

their notebooks. The following 

energy conversions should be 

discussed: kinetic energy is converted 

to electrical energy (moving 

magnet generates a current) 

and electrical energy is converted 

to light (the current heats the 

light bulb’s filament). 

796 

In 1996, the space shuttle Columbia 

attempted to use a 20.7 km conducting 

tether to study Earth’s magnetic 

field in space. The plan was to 

drag the tether through the magnetic 

field, inducing an emf in the 

tether. The magnitude of the emf 

would directly vary with the 

strength of the magnetic field. 

Unfortunately, the tether broke 

before it was fully extended, so the 

experiment was abandoned. 

Table 22-1 Ways of inducing a current in a circuit 

796 

Chapter 22 

Change in the number of magnetic field lines induces a current 

So far, you have learned that moving a circuit loop into or out of a magnetic 

field can induce an emf and a current in the circuit. Changing the size of the 

loop or the strength of the magnetic field also will induce an emf in the circuit. 

One way to predict whether a current will be induced in a given situation 

involves the concept of changes in magnetic field lines. For example, moving 

the circuit into the magnetic field causes some lines to move into the loop. 

Changing the size of the circuit loop or rotating the loop changes the number 

of field lines passing through the loop, as does changing the magnetic field’s 

strength. Table 22-1 summarizes these three ways of inducing a current. 

CHARACTERISTICS OF INDUCED CURRENT 

Suppose a bar magnet is pushed into a coil of wire. As the magnet moves into 

the coil, the strength of the magnetic field within the coil increases, and a current 

is induced in the circuit. This induced current in turn produces its own 

magnetic field, whose direction can be found by using the right-hand rule. If 

you were to apply this rule for several cases, you would notice that the induced 

magnetic field direction depends on the change in the applied field. 

As the magnet approaches, the magnetic field passing through the coil 

increases in strength. The induced current in the coil must be in a direction 

that produces a magnetic field that opposes the increasing strength of the 

approaching field. The induced magnetic field is therefore in the direction 

opposite that of the approaching magnetic field. 

Description Before After 

Circuit is moved into or out of magnetic 

field (either circuit or magnet moving). 

Circuit is rotated in the magnetic field 

(angle between area of circuit and 

magnetic field changes). 

Intensity of magnetic field is varied. 

v 

B 

B 

I 

v 

B 

I B 

I 


B

Wire 

S 


Wire 


N 

Magnetic field from 

induced current 

Magnetic field from 

induced current 

Approaching 

magnetic field 

Receding 


N S 

The induced magnetic field is like the field of a bar magnet oriented as 

shown in Figure 22-4. Note that the coil and magnet repel each other. 

If the magnet is moved away from the coil, the magnetic field passing 

through the coil decreases in strength. Again, the direction of current induced 

in the coil must be such that it produces a magnetic field that opposes the 

decreasing strength of the receding field. This means that the magnetic field 

set up by the coil must be in the same direction as the receding magnetic field. 

The induced magnetic field is like the field of a bar magnet oriented as 

shown in Figure 22-5. Note that the coil and magnet attract each other. 

N 

S 

v 

v 

N S 

1. Falling magnet A bar magnet is dropped 

toward the floor, on which lies a large ring of conducting 

metal. The magnet’s length—and thus the 

poles of the magnet—is parallel to the direction of 

motion. Disregarding air resistance, does the magnet 

during its fall toward the ring move with the same 

constant acceleration as a freely falling body? Explain 

your answer. 


Figure 22-4 

The approaching magnetic field is 

opposed by the induced magnetic 

field, which behaves like a bar magnet 

with the orientation shown. 

Figure 22-5 

The receding magnetic field is 

attracted by the induced magnetic 

field, which behaves like a bar magnet 

with the orientation shown. 

2. Induction in a bracelet Suppose you are 

wearing a bracelet that is an unbroken ring of copper. If 

you walk briskly into a strong magnetic field 

while wearing the bracelet, how 

would 

you hold your wrist with 

respect 

to the magnetic field in 

SECTION 22-1 

Teaching Tip 

Remind students that, as seen in 

Chapter 21, a current creates a 

magnetic field whose direction 

can be found with the right-hand 

rule. If the thumb points in the 

direction of conventional current 

(moving positive charges), the 

right hand wraps around the 

wire in the direction of the magnetic 

field. 

Misconception 

STOP 

Alert 

Some students may not distinguish 

between the external magnetic 

field that induces a current 

and the magnetic field that is set 

up by the induced current. Use 

the two examples discussed on 

this page to clarify the difference 

between these two magnetic 

fields. 

ANSWERS TO 

Conceptual Challenge 

1. no; The magnet’s acceleration 

is slightly smaller. The 

magnet induces a current in 

the conducting ring, and the 

magnetic field of this current 

opposes the field of the 

falling magnet. This induced 

field exerts an upward force 

on the magnet, reducing the 

magnet’s net acceleration 

downward. 

2. Upon entering and leaving 

the field, the plane of the 

bracelet must be parallel to 

the direction of the field. 

797

SECTION 22-1 


Lenz’s law 

Purpose Illustrate Lenz’s law 

experimentally. 

Materials flashlight bulb in holder, 

coil of wire, diode, connecting 

wires, bar magnet (or two) 

Procedure Repeat Demonstration 

1, but include a diode in the 

circuit. Tell students that the 

diode allows charges to move in 

only one direction. 

Explain to students that you 

will be testing Lenz’s law experimentally. 

Point out that inserting 

the magnet one way (with the 

north pole first) will generate a 

current in one direction. Reversing 

the magnet will generate a 

current in the opposite direction, 

which the diode will stop. The 

light bulb will serve as an indication 

of the direction of current. 

Thus, the bulb should light up in 

one case but not in the other 

(because the diode blocks current 

in one direction). Perform the 

demonstration and verify these 

conclusions. 

798 

798 

B (cos θ ) 

Loop 

θ 

B 

Chapter 22 

Normal to 

plane of loop 

Figure 22-6 

The angle q is defined as the angle 

between the magnetic field and the 

normal to the plane of the loop. 

B (cos q) equals the strength of the 

magnetic field perpendicular to the 

plane of the loop. 

The rule for finding the direction of the induced current is called Lenz’s law 

and is expressed as follows: 

The magnetic field of the induced current opposes the change in the applied 

magnetic field. 

Note that the field of the induced current does not oppose the applied field 

but rather the change in the applied field. If the applied field changes, the 

induced field attempts to keep the total field strength constant, according to 

the principle of energy conservation. 

Faraday’s law of induction 

Because of the principle of energy conservation, Lenz’s law allows you to determine 

the direction of an induced current in a circuit. Lenz’s law does not provide 

information on the magnitude of the induced current or the induced emf. To 

calculate the magnitude of the induced emf, you must use Faraday’s law of magnetic 

induction. For a single loop of a circuit, this may be expressed as follows: 

emf =−⎯ ∆[AB (cos 

q)] 

⎯ 

∆t 

Certain features of this equation should be noted. First, a change with time 

of any of the three variables—applied magnetic field strength, B, circuit area, 

A, or angle of orientation, q—can give rise to an induced emf. The term 

B (cos q) represents the component of the magnetic field perpendicular to the 

plane of the loop. The angle q is measured between the applied magnetic field 

and the normal to the plane of the loop, as indicated in Figure 22-6. 

The minus sign in front of the equation is included to indicate the polarity 

of the induced emf. It indicates that the induced magnetic field opposes the 

changing applied magnetic field according to Lenz’s law. 

If a circuit contains a number, N, of tightly wound loops, the average 

induced emf is simply N times the induced emf for a single loop. The equation 

thus takes the general form of Faraday’s law of magnetic induction. 

FARADAY’S LAW OF MAGNETIC INDUCTION 

emf =−N⎯ ∆[AB (cos 

q)] 

⎯ 

∆t 

average induced emf =−the number of loops in the circuit × 

the rate of change of (circuit loop area × magnetic field component 

normal to the plane of loop) 

In this chapter, N is always assumed to be a whole number. 

Recall that the SI unit for magnetic field strength is the tesla (T), which equals 

one newton per ampere-meter, or N/(A •m). When calculating induced emf, 

express the tesla in units of one volt-second per meter squared, or (V •s)/m 2 . 


PROBLEM 

SOLUTION 

1. DEFINE 

2. PLAN 

continued on 

next page 

Induced emf and current 

SAMPLE PROBLEM 22A 

A coil with 25 turns of wire is wrapped around a hollow tube with an area 

of 1.8 m 2 . Each turn has the same area as the tube. A uniform magnetic 

field is applied at a right angle to the plane of the coil. If the field increases 

uniformly from 0.00 T to 0.55 T in 0.85 s, find the magnitude of the induced 

emf in the coil. If the resistance in the coil is 2.5 Ω, find the magnitude of 

the induced current in the coil. 

Given: ∆t = 0.85 s A = 1.8 m 2 

Bi = 0.00 T = 0.00 V •s/m 2 

R = 2.5 Ω 

Unknown: emf = ? I = ? 


q = 0.0° N = 25 turns 

Bf = 0.55 T = 0.55 V •s/m 2 

Diagram: Show the coil before and after the change in the magnetic 

field. 

Choose an equation(s) or situation: Use Faraday’s law of magnetic induction 

to find the induced emf in the coil. 

emf =−N ⎯ ∆[AB (cos 

q)] 

⎯ 

∆t 

Substitute the induced emf into the definition of resistance to determine the 

induced current in the coil. 

I = ⎯ emf 

⎯ 

R 

N = 25 turns 

A = 1.8 m2 Rearrange the equation(s) to isolate the unknown(s): In this example, only 

the magnetic field strength changes with time. The other components (the coil 

area and the angle between the magnetic field and the coil) remain constant. 

emf =−NA (cos q) ⎯ ∆B 

⎯ 

∆t 

N = 25 turns 

A = 1.8 m 2 

R = 2.5 Ω 

R = 2.5 Ω 

B = 0.00 T at t = 0.00 s B = 0.55 T at t = 0.85 s 


799 

SECTION 22-1 


Because emf is the potential difference 

across a circuit (∆V), the 

definition of resistance can be 

expressed in terms of emf, as 

follows: 

I = ⎯ ∆V 

emf 

⎯ = ⎯⎯ R 

R 

This form of the definition of 

resistance is used in Sample 

Problem 22A. 

Classroom Practice 

The following may be used 

as a teamwork exercise or for 

demonstration at the board or 

on an overhead projector. 

PROBLEM 


A coil with 25 turns of wire is 

moving in a uniform magnetic 

field of 1.5 T. The magnetic field 

is perpendicular to the plane of 

the coil. The coil has a crosssectional 

area of 0.80 m 2 . The 

coil exits the field in 1.0 s. 

a. Find the induced emf. 

b. Determine the induced current 

if the coil’s resistance is 

1.5 Ω. 

Answers 

a. 3.0 × 10 1 V 

b. 2.0 × 10 1 A 

799

SECTION 22-1 

Teaching Tip 

Point out to students that Lenz’s 

law is used in the fourth step of 

Sample Problem 22A to find the 

direction of the induced current. 

PRACTICE GUIDE 22A 

Solving for: 

emf PE Sample, 1, 3; 

Ch. Rvw. 10, 

12, 42 

PW 4 

PB 4–6 

I PE Sample, 2; 

Ch. Rvw. 11 

PB 7–8 

t PE Ch. Rvw. 37 

PW Sample, 1–3 

PB 9–10 

B PE 4; Ch. Rvw. 39 

PB Sample, 1–3 

N PE Ch. Rvw. 38 

PW 5 

A PW 6 

ANSWERS TO 

Practice 22A 


1. 0.30 V 

2. 14 A 

3. 0.14 V 

4. 4.83 × 10 −5 T 

800 

800 

3. CALCULATE 

4. EVALUATE 

PRACTICE 22A 

Chapter 22 

Substitute the values into the equation(s) and solve: 

emf =−(25)(1.8 m 2 (0.55 − 0.00) ⎯ 

)(cos 0.0°) =−29 V 

V •s 

⎯ 2 

m 

⎯⎯ 

(0.85 s) 

I = ⎯ −29 

V 

⎯ =−12 A 

2.5 

Ω 

emf =−29 V 

I =−12 A 

CALCULATOR SOLUTION 

Because the minimum number of 

significant figures for the data is two, 

the calculator answer, 29.11764706, 

should be rounded to two digits. 

The induced emf, and therefore the induced current, are directed through the 

coil so that the magnetic field produced by the induced current opposes the 

change in the applied magnetic field. For the diagram shown on page 799, the 

induced magnetic field is directed to the right and the current that produces 

it is directed from left to right through the resistor. 


1. A single circular loop with a radius of 22 cm is placed in a uniform external 

magnetic field with a strength of 0.50 T so that the plane of the coil is 

perpendicular to the field. The coil is pulled steadily out of the field in 

0.25 s. Find the average induced emf during this interval. 

2. A coil with 205 turns of wire, a total resistance of 23 Ω, and a crosssectional 

area of 0.25 m 2 is positioned with its plane perpendicular to the 

field of a powerful electromagnet. What average current is induced in the 

coil during the 0.25 s that the magnetic field drops from 1.6 T to 0.0 T? 

3. A circular wire loop with a radius of 0.33 m is located in an external 

magnetic field of strength +0.35 T that is perpendicular to the plane of 

the loop. The field strength changes to −0.25 T in 1.5 s. (The plus and 

minus signs for a magnetic field refer to opposite directions through the 

coil.) Find the magnitude of the average induced emf during this interval. 

4. A 505-turn circular-loop coil with a diameter of 15.5 cm is initially 

aligned so that its plane is perpendicular to the Earth’s magnetic field. In 

2.77 ms the coil is rotated 90.0° so that its plane is parallel to the Earth’s 

magnetic field. If an average emf of 0.166 V is induced in the coil, what is 

the value of the Earth’s magnetic field? 


APPLICATIONS OF INDUCTION 

In electric circuits, the need often arises for a temporary or continuously 

changing current to be produced. This kind of current can be generated 

through electromagnetic induction. 

Door bells 

Certain types of door bells chime when the button is briefly 

pushed. A hint to how these door bells work is the small electric 

light bulb often found behind the doorbell button. When you 

press the button, the light briefly goes out. This indicates that 

the door bell’s circuit has been opened and that the current has 

been temporarily discontinued. 

You can see the effect of opening the circuit in Figure 22-7. 

While the current is constant in the first circuit, the magnetic 

field in the coil of this circuit is steady. As long as this field does 

not change, no current is induced in the coil of the second circuit. 

When the current in the first circuit is interrupted by pressing 

the doorbell button, the magnetic field in the first coil drops 

rapidly, inducing a current in the second circuit and causing a 

magnetic field to quickly rise along the axis of the second coil. 

This induced magnetic field is strong enough to push the iron 

plunger against the chime. 

Tape recorders 

One common use of induced currents and emfs is found in the 

tape recorder. Many different types of tape recorders are made, 

but the same basic principles apply for all. A magnetic tape moves 

past a recording head and a playback head. The tape is a plastic 

ribbon coated with either iron oxide or chromium dioxide. 

A microphone transforms a sound wave into a fluctuating 

electric current. This current is amplified and allowed to pass 

through a wire coiled around an iron ring, as shown in Figure 

22-8, which functions as the recording head. The iron ring 

and wire constitute an electromagnet; the lines of the magnetic 

field are contained completely inside the iron except at the 

point where a gap is cut in the ring. The magnetic tape is 

passed over this gap. 

Because the magnetic field does not pass as easily through 

the air of the gap as it does through the nearby small pieces of 

metal oxide in the tape, the magnetic field passes around the 

gap and magnetizes the metal oxide particles. As the tape moves 

past the slot, it becomes magnetized in a pattern that corresponds 

to both the frequency and the intensity of the sound signal 

entering the microphone. 


emf sources 

Decreasing 


– 

Decreasing 

current 

Increasing 


Coil Coil 

Door bell switch 

+ 

Induced 

current 


Iron 

plunger 

Chime 

Figure 22-7 

The sudden drop in current in the first circuit induces 

a current in the second circuit. The resulting magnetic 

field pushes the plunger against the door-bell chime. 

Changing 

current 

Magnetic tape 

Changing magnetic field 

Figure 22-8 

The signal fluctuations in the current induce a fluctuating 

magnetic field, which is recorded along the length 

of the tape. During playback, the changing magnetic field 

of the tape will induce in the coil a fluctuating current 

that can be converted to reproduce the original 

recorded sound. 

v 

801 

SECTION 22-1 


Magnetic braking 

Purpose Show magnetic braking 

as an application of induction. 

Materials aluminum metal ring 

(approximately 8–10 cm in diameter), 

strong alnico bar magnet, 

thread, ring stand 

Procedure Attach the ring with 

two threads to the ring stand so 

that the ring can swing freely. 

With the magnet far away, set the 

ring swinging and ask the class to 

notice any decrease in the amplitude 

of its swing as you count 

aloud the number of cycles. 

There should be little change 

during the first 10 to 20 cycles. 

Restart the swinging of the aluminum 

ring, and place the magnet 

near its path. Ask students to 

explain the change in terms of 

Lenz’s law. (The metal ring crosses 

the magnetic field lines, and a 

current is induced in the ring. The 

direction of the current is such that 

its magnetic field opposes the 

changes in the magnetic field that 

induced it.) Repeat the demonstration, 

telling students that magnetic 

braking is silent and does 

not wear out materials. Have students 

suggest some possible uses 

for magnetic braking. (This principle 

is used in many triple-beam 

balances to immediately bring the 

scale to a specific reading.) 

801

SECTION 22-1 

Section Review 

ANSWERS 

1. Answers should include any 

three of the following: moving 

the loop into or out of 

the magnetic field (B through 

loop changing with t); rotating 

the loop within the magnetic 

field (q changing with 

t); changing the strength of 

the magnetic field through 

the static loop (B changing 

with t); altering the loop’s 

shape (A changing with t). 

2. A change in Earth’s magnetic 

field within the spacecraft 

induces an emf, and thus a 

current, in the loop. 

3. from left to right; from right 

to left 

4. 0.21 V 

5. When the magnetized strings 

vibrate, the strength of their 

magnetic field changes periodically 

with respect to the 

wire loops in the coil. An emf 

is thus induced in the coil. 

The fluctuations in the 

induced current match the 

frequencies and amplitudes 

of the sounds produced on 

the vibrating strings. 

6. −0.44 V 

802 


802 

Chapter 22 

In the recording process, the magnetic field is produced by an applied current. 

During the playback process, induction is used to create a current from a 

changing magnetic field. The sound signal is reconstructed by passing the tape 

over the playback head, which is an iron ring with wire wound around it. In 

some recorders, one head is used for both playback and recording. 

When the tape moves past the playback head, the varying magnetic fields 

on the tape produce changing magnetic-field lines through the wire coil, 

inducing a current in the coil. The current corresponds to the current in the 

recording head that originally produced the recording on the tape. This 

changing electric current can be amplified and used to drive a speaker. Playback 

is thus an example of induction of a current by a moving magnet. 

1. A circular current loop made of flexible wire is located in a magnetic 

field. Describe three ways an emf can be induced in the loop. 

2. A spacecraft orbiting Earth has a coil of wire in it. An astronaut measures 

a small current in the coil, even though there is no battery connected to it 

and there are no magnets on the spacecraft. What is causing the current? 

3. A bar magnet is positioned near a coil of 

wire, as shown in Figure 22-9. What is the 

direction of the current through the resistor 

when the magnet is moved to the 

left, as in (a)? to the right, as in (b)? 

S N 

4. A 256-turn coil with a cross-sectional area of 0.0025 m 2 is placed in a 

uniform external magnetic field of strength 0.25 T so that the plane of 

the coil is perpendicular to the field. The coil is pulled steadily out of the 

field in 0.75 s. Find the average induced emf during this interval. 

5. Physics in Action Electric guitar strings are made of ferromagnetic 

materials that can be magnetized. The strings lie closely over and 

perpendicular to a coil of wire. Inside the coil are permanent magnets 

that magnetize the segments of the strings overhead. Using this arrangement, 

explain how the vibrations of a plucked string produce an electric 

signal at the same frequency as the vibration of the string. 

6. Physics in Action The magnetic field strength of a magnetized 

electric guitar string is 9.0 × 10 −4 T. The pickup coil consists of 5200 turns 

of wire and has an effective area for each string of 5.4 × 10 −5 m 2 . If the 

string vibrates with a frequency of 440 Hz, what is the average induced 

emf? (Hint: Assume the magnetic field strength varies from a minimum 

value of 0.0 T to its maximum value during one fourth of the string’s 

vibration cycle.) 

(a) 

(b) 

v 

v 

R 

Figure 22-9 


22-2 

Alternating current, generators, 

and motors 

GENERATORS AND ALTERNATING CURRENT 

In the previous section you learned that a current can be induced in a circuit 

either by changing the magnetic field strength or by moving the circuit loop 

in or out of the magnetic field. Another way to induce a current is to change 

the orientation of the loop with respect to the magnetic field. 

This second approach to inducing a current represents a practical means of generating 

electrical energy. In effect, the mechanical energy used to turn the loop is 

converted to electrical energy. A device that does this is called an electric generator. 

In most commercial power plants, mechanical energy is provided in the form 

of rotational motion. For example, in a hydroelectric plant, falling water directed 

against the blades of a turbine causes the turbine to turn; in a coal or natural-gasburning 

plant, energy produced by burning fuel is used to convert water to 

steam, and this steam is directed against the turbine blades to turn the turbine. 

Basically, a generator uses the turbine’s rotary motion to turn a wire loop in 

a magnetic field. A simple generator is shown in Figure 22-10. As the loop 

rotates, the effective area of the loop changes with time, inducing an emf and a 

current in an external circuit connected to the ends of the loop. 

A generator produces a continuously changing emf 

Consider a single loop of wire that is rotated with a constant angular frequency 

in a uniform magnetic field. The loop can be thought of as four conducting 

wires similar to the conducting wire discussed on page 795 of Section 

22-1. In this example, the loop is rotating counterclockwise within a magnetic 

field directed to the left. 



• Calculate the maximum emf 

for an electric generator. 

• Calculate rms current and 

potential difference for ac 

circuits. 

• Describe how an electric 

motor relates to an electric 

generator. 

generator 

a device that uses induction to 

convert mechanical energy to 

electrical energy 

Figure 22-10 

In a simple generator, the rotation of conducting loops 

(located on the left end of the axis) through a constant 

magnetic field induces an alternating current in the loops. 


803 

Section 22-2 

STOP 

Misconception 

Alert 

Some students may think that no 

work is required to generate electricity. 

Stress that this is not the 

case; work is required to rotate a 

loop in a magnetic field. In a 

hydroelectric power plant, this 

work is generated by falling water. 

The water loses gravitational 

potential energy as it does work. 


Alternating current 

Purpose Illustrate the effects of 

changing cross-sectional area on 

induced emf, and show how generators 

create ac currents. 

Materials overhead projector, 

colored cellophane filter 

mounted in a frame (like a 

35 mm slide) 

Procedure Tell students that the 

light from the overhead projector 

represents a uniform magnetic 

field and the filter represents a 

wire loop. The amount of colored 

light that reaches the screen represents 

the induced emf and thus 

the current. 

Slowly rotate the filter over the 

overhead projector, letting 

changing amounts of colored 

light reach the screen. Have students 

observe the changing 

intensity. Draw a graph of the 

changes in intensity over time on 

the board. (The graph is a sine 

curve.) Point out the analogy 

between the changing intensity 

and a changing current. 

803

SECTION 22-2 

STOP 

804 

Misconception 

Alert 

When trying to determine the 

direction of current in Figure 

22-11, students might try using 

the right-hand rule for a currentcarrying 

wire in a magnetic field 

instead of the right-hand rule for 

charges moving in a magnetic 

field. Explain that the latter rule 

must be used because charges are 

not initially moving along the 

wire; rather, they move with the 

wire, in a direction perpendicular 

to the length of the wire. Thus, 

when applying the right-hand 

rule to find the direction of 

induced current, the thumb 

points along the direction in 

which the wire is moving, not 

along the wire, and the fingers 

point along the direction of the 

magnetic field. The resulting 

force on the charges, and thus the 

current, points out of the palm of 

the hand. Have students apply 

this form of the right-hand rule 

for segments a, b, c, and d in each 

case shown in Figure 22-11 to 

determine the direction of the 

current in each segment. 


Figure 22-11 

Have students compare Figure 

22-11 with the analogy introduced 

in Demonstration 4. After 

asking students the following 

question, plot graphs of y = 

sin x and y = sin x on the chalkboard, 

and use the graphs to 

compare the two cases. 

The amount of colored light 

Q in Demonstration 4 varies 

much like the induced emf in 

Figure 22-11. How are these two 

cases different? 

The emf becomes negative, 

while light does not. 

A 

NSTA 

TOPIC: Alternating current 



Figure 22-11 

For a rotating loop in a magnetic 

field, the induced emf is zero when 

the loop is perpendicular to the 

magnetic field, as in (a) and (c), and 

is at a maximum when the loop is 

parallel to the field, as in (b) and (d). 

804 

Chapter 22 

When the area of the loop is perpendicular to the magnetic field lines, as 

shown in Figure 22-11(a), every segment of wire in the loop is moving parallel 

to the magnetic field lines. At this instant, the magnetic field does not exert 

force on the charges in any part of the wire, so the induced emf in each segment 

is therefore zero. 

As the loop rotates away from this position, segments a and c cross magnetic 

field lines, so the magnetic force on the charges in these segments, and thus 

the induced emf, increases. The magnetic force on the charges in segments b 

and d is directed outside of the wire, so the motion of these segments does not 

contribute to the emf or the current. The greatest magnetic force on the 

charges and the greatest induced emf occur at the instant when segments a 

and c move perpendicularly to the magnetic field lines, as in Figure 22-11(b). 

This occurs when the plane of the loop is parallel to the field lines. 

Because segment a moves downward through the field while segment c moves 

upward, their emfs are in opposite directions, but both produce a counterclockwise 

current. As the loop continues to rotate, segments a and c cross fewer 

lines, and the emf decreases. When the plane of the loop is perpendicular to the 

magnetic field, the motion of segments a and c is again parallel to the magnetic 

lines and the induced emf is again zero, as shown in Figure 22-11(c). Segments a 

and c now move in directions opposite those in which they moved from their positions 

in (a) to those in (b). As a result, the polarity of the induced emf and the 

direction of the current are reversed, as shown in Figure 22-11(d). 

Induced emf 

– 

0 + 

B 

(a) (b) 

Induced emf 

– 

0 + 

B 

(c) (d) 

b 

b 

a 

c 

c 

a 

d 

d 

Induced emf 

– 

– 

0 + 

Induced emf 

0 + 

B 

B 

b 

b 

a d 

c 


a 

c 

d

A graph of the change in emf versus time as the loop rotates is shown in 

Figure 22-12. Note the similarities between this graph and a sine curve. 

The induced emf is the result of the steady change in the angle between the 

magnetic field lines and the normal to the loop. The following equation for 

the emf produced by a generator can be derived from Faraday’s law of induction. 

The derivation is not shown here because it requires the use of calculus. 

In this equation, the angle of orientation, q, has been replaced with the equivalent 

expression wt, where w is the angular frequency of rotation (2pf ). 

PROBLEM 

SOLUTION 

1. DEFINE 

continued on 

next page 

Induction in generators 

Maximum 

emf 

emf 


Time 

Figure 22-12 

The change with time of the induced 

emf is depicted by a sine wave. 

SAMPLE PROBLEM 22B 

A generator consists of exactly eight turns of wire, each with an area 

A = 0.095 m 2 and a total resistance of 12 Ω. The loop rotates in a magnetic 

field of 0.55 T at a constant frequency of 60.0 Hz. Find the maximum 

induced emf and maximum induced current in the loop. 

Given: f = 60.0 Hz A = 0.095 m 2 

B = 0.55 T = 0.55 V •s/m 2 

Unknown: maximum emf = ? I max = ? 

Diagram: 

emf = NABw (sin wt) 

The equation describes the sinusoidal variation of emf with time, as graphed in 

Figure 22-12. 

The emf has a maximum value when the plane of the loop is parallel to the 

magnetic field, that is, when sin (wt) = 1, which occurs when wt = q = 90°.In 

this case, the expression above reduces to the following: 

MAXIMUM EMF FOR A GENERATOR 

maximum emf = NABw 

maximum induced emf = number of loops × cross-sectional area of 

loops × magnetic field strength × angular frequency of rotation of loops 


R = 12 Ω 

R = 12 Ω 

N = 8 

f = 60.0 Hz 

B = 0.55 T 

A = 0.095 m 

N = 8 turns 

2 

805 

SECTION 22-2 

STOP 

Misconception 

Alert 

Some students may think that the 

boxed equation for the maximum 

emf of a generator holds 

for all cases. Be sure they understand 

that this equation only 

holds when the plane of the 

rotating loop and the magnetic 

field vectors are parallel. When 

the plane of the loop and the 

magnetic field vectors are not 

parallel, emf is not at a maximum, 

and the first equation on 

this page must be used. 






PROBLEM 


A 25 loop generator rotates with 

a frequency of 60.0 Hz in a 0.50 T 

field. The maximum emf induced 

is 117 V. 

a. Calculate the cross-sectional 

area of the loops. 

b. The generator is bought by 

another company and moved 

to Europe, where it is run at 

50.0 Hz. Find the new maximum 

emf. 

Answers 

a. 2.5 × 10 −2 m 2 

b. 98 V 

805

SECTION 22-2 

Teaching Tip 

Point out that in Sample Problem 

22B, the emf varies from a maximum 

value of 160 V to a minimum 

value of 0 V during 

1 

one-fourth of a cycle (⎯ 

240 

⎯ s). The 

current changes from a maximum 

value of 13 A to a minimum value 

of 0 A in the same time interval. 

PRACTICE GUIDE 22B 

Solving for: 

emf PE Sample, 1–3; 

Ch. Rvw. 

23–24 

PW 7 

PB 4–6 

I max 

806 

PE Sample; 

Ch. Rvw. 24 

PB 7–8 

N PE 4 


PB 9–10 

w PW 5–6 


B PW 3–4 

ANSWERS TO 

Practice 22B 


1. 87 V 

2. 55 V 

3. 1.7 × 10 −2 V 

4. 8 turns 

806 

2. PLAN 

3. CALCULATE 

4. EVALUATE 

PRACTICE 22B 

Chapter 22 

Choose an equation(s) or situation: Use the maximum emf equation for a 

generator. Use the definition of angular frequency to convert f to w. The 

maximum current can be obtained from the definition for resistance. 


maximum emf = NABw w= 2pf 

Imax = ⎯ maximum 

emf 

⎯ 

R 

Rearrange the equation(s) to isolate the unknown(s): Substitute the angular 

frequency expression into the maximum emf equation. 

maximum emf = NAB(2pf ) 


maximum emf = (8)(0.095 m 2 )(0.55 T)(2p)(60.0 Hz) 

maximum emf = 1.6 × 10 2 V 

Imax = ⎯ 1.6 × 10 

12 

Ω 

2 V 

⎯ = 13 A 

I max = 13 A 

By expressing the units used in the calculation in terms of equivalent units 

(1 T = 1 (V •s/m 2 ) and 1 Hz = 1/s), you can see which terms cancel. In this 

case, m 2 and s cancel to leave the answer for emf in units of volts. 

1. In a model generator, a 510-turn rectangular coil 0.082 m by 0.25 m 

rotates with an angular frequency of 12.8 rad/s in a uniform magnetic 

field of 0.65 T. What is the maximum emf induced in the coil? 


Because the minimum number of significant 

figures for the data is two, 

the calculator answer, 157.5822875, 

should be rounded to two digits. 

2. A circular coil with a radius of R = 0.22 m and 17 turns is rotated in a 

uniform magnetic field of 1.7 T. The coil rotates at a constant frequency 

of 2.0 Hz. Determine the maximum value of the emf induced in the coil. 

3. A square coil with an area of 0.045 m 2 consists of 120 turns of wire. The 

coil rotates about a vertical axis at 157 rad/s. The horizontal component 

of Earth’s magnetic field at the location of the loop is 2.0 × 10 –5 T. Calculate 

the maximum emf induced in the coil. 

4. A maximum emf of 90.4 V is induced in a generator coil rotating with a 

frequency of 65 Hz. If the coil has an area of 230 cm 2 and rotates in a 

magnetic field of 1.2 T, how many turns are in the coil? 


Alternating current changes direction at a constant frequency 

The output emf of a typical generator has a sinusoidal pattern, as you can see 

in the graph in Figure 22-12 on page 805. Note that the emf alternates from 

positive to negative. As a result, the output current from the generator 

changes its direction at regular intervals. This variety of current is called alternating 

current, or, more commonly, ac. 

The rate at which the coil in an ac generator rotates determines the maximum 

generated emf. The frequency of the alternating current can differ from 

country to country. In the United States, Canada, and Central America, the 

frequency of rotation for commercial generators is 60 Hz. This means that the 

emf undergoes one full cycle of changing direction 60 times each second. In 

the United Kingdom, Europe, and most of Asia and Africa, 50 Hz is used. 

(Recall that w = 2pf,where f is the frequency in Hz.) 

Resistors can be used in either alternating- or direct-current applications. 

A resistor resists the motion of charges regardless of whether they move in 

one continuous direction or shift direction periodically. Thus, if the definition 

for resistance holds for circuit elements in a dc circuit, it will also hold for the 

same circuit elements with alternating currents and emfs. 

Effective current and potential difference are measured in ac circuits 

An ac circuit consists of combinations of circuit elements and an ac generator 

or an ac power supply, which provides the alternating current. As shown 

earlier, the emf produced by a typical ac generator is sinusoidal and varies 

with time. The induced emf equals the instantaneous ac potential difference, 

which is written as ∆v. The quantity for the maximum emf can be written as 

the maximum potential difference ∆V max, and the emf produced by a generator 

can be expressed as follows: 

∆v =∆V max(sin wt) 

Because all circuits have some resistance, a simple ac circuit can be treated 

as an equivalent resistance and an ac source. In a circuit diagram, the ac 

source is represented by the symbol , as shown in Figure 22-13. 

The instantaneous current that changes with the potential difference can 

be determined using the definition for resistance. The instantaneous current, 

i, is related to maximum current by the following expression: 

i = I max(sin wt) 

The rate at which electrical energy is converted to internal energy in the 

resistor (the power, P) has the same form as in the case of direct current. The 

electrical energy converted to internal energy in a resistor is proportional to 

the square of the current and is independent of the direction—or the change 

of direction—of the current. However, the energy produced by an alternating 

current with a maximum value of I max is not the same as that produced by a 

direct current of the same value. This is because the alternating current is at its 

maximum value for only a very brief instant of time during a cycle. 


alternating current 

an electric current that changes 

direction at regular intervals 

Although the light intensity from a 

60 W incandescent light bulb 

appears to be constant, the current 

in the bulb fluctuates 60 times each 

second between −0.7 1 A and 0.7 1 A. 

The light appears to be steady 

because the fluctuations are too 

rapid for our eyes to perceive. 

R eq 

∆v 

ac source 

Figure 22-13 

An ac circuit represented schematically 

consists of an ac source 

and an equivalent resistance. 


807 

SECTION 22-2 


ac and dc 

Purpose Show the difference 

between alternating current and 

direct current. 

Materials hand-operated generator, 

galvanometer, wires, oscilloscope, 

variable autotransformer, 

dc source 

Procedure Show students that 

the current measured by the galvanometer 

varies from positive to 

negative as the ac generator is 

working. Hook up the dc source 

to the oscilloscope to show students 

that the oscilloscope is a 

type of voltmeter. Explain that 

the sweep allows you to see 

changes in the potential difference 

that may be too quick to see 

with a galvanometer. 

Replace the dc source with the 

autotransformer. Tell students 

what the sweep frequency is, and 

have them determine the frequency 

of the alternating current. 

Repeat the process with a different 

sweep frequency. Show students 

what happens to the trace 

as you increase the potential difference. 

If time permits, you may 

wish to quantify the discussion 

by showing that the heating 

effects of the alternating current 

will be identical to those of the 

direct current if the effective 

(rather than the peak-to-peak) 

ac potential difference is used for 

the comparison. 

807

SECTION 22-2 


Effects of alternating 

current 

Purpose Illustrate the effects of 

alternating current. 

Materials bicolored LED (available 

at many electronics stores), 

step-down transformer, resistor 

(100-250 Ω), 2 m of flexible wire 

Procedure Connect the wire, the 

resistor, and the LED in series 

with the transformer. Explain to 

students that the diode is red and 

green, respectively, for currents 

with opposite directions. Turn 

the lights down, and show the 

class that the LED appears yellow. 

This is because the alternating 

polarity of the current switches 

the color of the LED from red to 

green 60 times each second. 

Hold the wire about halfway 

between the transformer and the 

diode, and twirl the wire in a vertical 

circle so that the diode 

moves in a circular path. Students 

should see red and green bars at 

equally spaced intervals along the 

diode’s path. Have students count 

the number of green bars they see 

in the circle, then have them measure 

the time it takes for the diode 

to travel 10 times around the circular 

path. Ask students how 

much time it takes for the diode 

to change from green to red to 

green ⎯ 1 

⎯ s 60 

. 

808 

rms current 

the amount of direct current that 

dissipates as much energy in a 

resistor as an instantaneous 

alternating current does during a 

complete cycle 

I max 

I rms 


Figure 22-14 

The rms current is a little more 

than two-thirds as large as the 

maximum current. 

808 

Chapter 22 

Time 

The important quantity for current in an ac circuit is the rms current. The 

rms (or root-mean-square) current is the same as the amount of direct current 

that would dissipate the same energy in a resistor as is dissipated by the 

instantaneous alternating current over a complete cycle. 

Figure 22-14 shows a graph in which instantaneous and rms currents are 

compared. Table 22-2 summarizes the notations used in this chapter for these 

and other ac quantities. 

The equation for the power dissipated in an ac circuit has the same form as 

the equation for power dissipated in a dc circuit except that the dc current I is 

replaced by the rms current (I rms). 

P = (I rms) 2 R 

This equation is identical in form to the one for direct current. However, 

the power dissipated in the ac circuit equals half the power dissipated in a dc 

circuit when the dc current equals I max. 

P = (I rms) 2 R = ⎯ 1 

2 ⎯(I max) 2 R 

From this equation you may note that the rms current is related to the 

maximum value of the alternating current by the following equation: 

(I rms) 2 = ⎯ (Imax) 2 

2 

⎯ 

Imax Irms = ⎯⎯ 

= 0.707 Imax 2 

This equation says that an alternating current with a maximum value of 5 A 

produces the same heating effect in a resistor as a direct current of (5/2) A, 

or about 3.5 A. 

Alternating potential differences are also best discussed in terms of rms 

potential differences, with the relationship between rms and maximum values 

being identical to the one for currents. The rms potential difference, ∆Vrms, is 

related to the maximum value of the potential difference, ∆Vmax, as follows: 

∆Vrms = ⎯ ∆Vmax 

⎯ = 0.707 Vmax 2 

Table 22-2 Notation used for ac circuits 

Potential difference Current 

instantaneous values ∆v i 

maximum values ∆Vmax Imax rms values ∆Vrms = ⎯ ∆Vmax 

⎯ Irms = Imax ⎯ 

2 

2 


The ac potential difference of 120 V measured from an electric outlet is 

actually an rms potential difference of 120 V. A quick calculation shows that 

such an ac potential difference has a maximum value of about 170 V. 

A resistor limits the current in an ac circuit just as it does in a dc circuit. If 

the definition of resistance is valid for an ac circuit, the rms potential difference 

across a resistor equals the rms current multiplied by the resistance. 

Thus, all maximum and rms values can be calculated if only one current or 

emf value and the circuit resistance are known. 

Because ac ammeters and voltmeters measure rms values, all values of 

alternating current and alternating potential difference in this chapter will be 

given as rms values unless otherwise noted. The equations for ac circuits have 

the same form as those for dc circuits when rms values are used. 

PROBLEM 

SOLUTION 

1. DEFINE 

2. PLAN 

continued on 

next page 

rms currents and potential differences 

SAMPLE PROBLEM 22C 

A generator with a maximum output emf of 205 V is connected to a 115 Ω 

resistor. Calculate the rms potential difference. Find the rms current 

through the resistor. Find the maximum ac current in the circuit. 

Given: ∆V max = 205 V R = 115 Ω 

Unknown: ∆V rms = ? I rms = ? I max = ? 

Diagram: 


Choose an equation(s) or situation: Use the equation 

for the rms potential difference to find ∆V rms. 

∆Vrms = 0.707 ∆Vmax Rearrange the definition for resistance to calculate Irms. Irms = ⎯ ∆Vrms 

⎯ 

R 

Use the equation for rms current to find I max. 

Irms = 0.707 Imax Rearrange the equation(s) to isolate the unknown(s): 

Rearrange the equation relating rms current to maximum current so that 

maximum current is calculated. 

Irms 

Imax = ⎯⎯ 0. 

707 

R = 115 Ω 

∆Vmax 

= 205 V 

∆Vrms 

= ? 

I rms = ? I max = ? 


809 

SECTION 22-2 


Remind students that because 

emf is a potential difference, 

maximum emf can be abbreviated 

as ∆V max, and rms emf can 

be abbreviated as ∆V rms. 






PROBLEM 

rms currents and potential 

differences 

A generator supplies 110 V rms 

to a 25 Ω circuit. What is the 

maximum current supplied to 

the circuit? 

Answer 

6.2 A 

809

SECTION 22-2 

PRACTICE GUIDE 22C 

Solving for: 

V PE Sample, 1, 4, 5a, 

6; Ch. Rvw. 

25–28 


PB Sample, 1, 4, 

5–6, 9 

I PE Sample, 1–2, 3a, 

4, 5b; Ch. Rvw. 

26–28 

PW Sample, 1–4, 

5, 7 

PB Sample, 1–4, 

5, 7, 8–10 

R PE 3b 

PW 3, 5 

PB 3, 5 

P PE Ch. Rvw. 27 

PW Sample, 1 

PB 8–10 

ANSWERS TO 

Practice 22C 

rms currents and potential 

differences 

1. 4.8 A; 6.8 A, 170 V 

2. 7.8 A 

3. a. 7.42 A 

b. 14.8 Ω 

4. 1.44 A; 2.04 A, 21.2 V 

5. a. 1.10 × 10 2 V 

b. 2.1 A 

6. 319 V 

810 

810 

3. CALCULATE 

4. EVALUATE 

PRACTICE 22C 

Chapter 22 


∆Vrms = (0.707)(205 V) = 145 V 

145 

V 

Irms = ⎯⎯ = 1.26 A 

115 

Ω 

Imax = ⎯ 1. 

26 

A 

⎯ = 1.78 A 

0. 

707 

∆Vrms = 145 V 

Irms = 1.26 A 

Imax = 1.78 A 

The rms values for potential difference and current are a little more than 

two-thirds the maximum values, as expected. 

rms currents and potential differences 


Because the minimum number of 

significant figures for the data is three, 

the calculator solution for the rms 

potential difference, 144.935, should 

be rounded to three digits. 

1. What is the rms current in a light bulb that has a resistance of 25 Ω and 

an rms potential difference of 120 V? What are the maximum values for 

current and potential difference? 

2. The current in an ac circuit is measured with an ammeter. The meter 

gives a reading of 5.5 A. Calculate the maximum ac current. 

3. A toaster is plugged into a source of alternating potential difference with 

an rms value of 110 V. The heating element is designed to convey a current 

with a maximum value of 10.5 A. Find the following: 

a. the rms current in the heating element 

b. the resistance of the heating element 

4. An audio amplifier provides an alternating rms potential difference of 

15.0 V. A loudspeaker connected to the amplifier has a resistance of 

10.4 Ω. What is the rms current in the speaker? What are the maximum 

values of the current and the potential difference? 

5. An ac generator has a maximum potential difference output of 155 V. 

a. Find the rms potential difference output. 

b. Find the rms current in the circuit when the generator is connected to 

a 53 Ω resistor. 

6. The largest potential difference that can be placed across a certain capacitor 

at any instant is 451 V. What is the largest rms potential difference 

that can be placed across the capacitor without damaging it? 


Slip rings 

Brush 

N 

Brush 

AC Generator DC Generator 

Figure 22-15 

A simple dc generator (shown on the right) employs the same 

design as an ac generator (shown on the left). A split slip ring 

converts alternating current to direct current. 

Alternating current can be converted to direct current 

The conducting loop in an ac generator must be free to rotate through the 

magnetic field. Yet it must also be part of an electric circuit at all times. To 

accomplish this, the ends of the loop are connected to conducting rings, called 

slip rings, that rotate with the loop. Connections to the external circuit are 

made by stationary graphite strips, called brushes, that make continuous contact 

with the slip rings. Because the current changes direction in the loop, the 

output current through the brushes alternates direction as well. 

By varying this arrangement slightly, an ac generator can be converted to a 

dc generator. Note in Figure 22-15 that the components of a dc generator are 

essentially the same as those of the ac generator except that the contacts to the 

rotating loop are made by a single split slip ring, called a commutator. 

At the point in the loop’s rotation when the current has dropped to zero 

and is about to change direction, each half of the commutator comes into 

contact with the brush that was previously in contact with the other half of 

the commutator. The reversed current in the loop changes directions again so 

that the output current has the same direction as it originally had, although it 

still changes from a maximum value to zero. A plot of this pulsating direct 

current is shown in Figure 22-16. 

A steady direct current can be produced by using many loops and commutators 

distributed around the rotation axis of the dc generator. The sinusoidal 

currents from each loop overlap. The superposition of the currents produces a 

direct current output that is almost entirely free of fluctuations. 


S 

Brush 

N 

Commutator 

Brush 



S 

NSTA 

TOPIC: Electrical safety 



Time 

Figure 22-16 

The output current for a dc generator 

is a sine wave with the negative 

parts of the curve made positive. 

811 

SECTION 22-2 

Teaching Tip 

Inform students that converting 

from ac to dc may also be accomplished 

electronically. This is 

done, for example, by ac-to-dc 

power converters, such as those 

used for portable electronic 

games or CD players. 

Teaching Tip 

Point out that, as shown by the 

graph of direct current in Figure 

22-16, the direct current produced 

by a generator alternates 

over time but does not change 

polarity, as does alternating current. 

The direct current generated 

by a battery, on the other hand, is 

steady and does not fluctuate. 

(See Figure 19-5 on page 699.) 

Generators can produce a steady 

direct current similar to that produced 

by batteries if many loops 

and commutators are used. 

811

SECTION 22-2 

Teaching Tip 

Point out that generators and 

motors have opposite functions. 

Generators convert mechanical 

energy to electrical energy, while 

motors convert electrical energy 

to mechanical energy. 


Electric motor 

Purpose Illustrate the similarity 

between generators and motors. 

Materials hand-operated generator, 

capacitor (1 F), 100 Ω resistor, 

connecting wires 

Procedure Connect the capacitor 

to the hand-operated generator. 

Charge the capacitor with the 

generator. Do not apply a large 

potential difference. Release the 

handle, and watch the generator 

become a motor. 

Optional: An interesting comparison 

can be made between the 

responses of the generator to different 

loads. Repeat the demonstration 

with and without the 

resistor attached. 

812 

back emf 

the emf induced in a motor’s coil 

that tends to reduce the current 

powering the motor 

812 

Chapter 22 

MOTORS 

Figure 22-17 

In a motor, the current in the coil interacts 

with the magnetic field, causing the coil and the 

shaft on which the coil is mounted to turn. 

Motors are devices that convert electrical energy to mechanical energy. 

Instead of a current being generated by a rotating loop in a magnetic field, a 

current is supplied to the loop by an emf source and the magnetic force on the 

current loop causes it to rotate (see Figure 22-17). 

A motor is almost identical in construction to a dc generator. The coil of 

wire is mounted on a rotating shaft and is positioned between the poles of a 

magnet. Brushes make contact with a commutator, which alternates the current 

in the coil. This alternation of the current causes the magnetic field produced 

by the current to regularly reverse and thus always be repelled by the 

fixed magnetic field. Thus, the coil and the shaft are kept in continuous rotational 

motion. 

A motor can perform useful mechanical work when a shaft connected to 

its rotating coil is attached to some external device. As the coil in the motor 

rotates, however, the changing normal component of the magnetic field 

through it induces an emf that acts to reduce the current in the coil. If this 

were not the case, Lenz’s law would be violated. This induced emf is called 

the back emf. 

The back emf increases in magnitude as the magnetic field changes at a 

higher rate. In other words, the faster the coil rotates, the greater the back 

emf becomes. The potential difference available to supply current to the 

motor equals the difference between the applied potential difference and the 

back emf. Consequently, the current in the coil is also reduced because of 

the presence of back emf. The faster the motor turns, the smaller the net 

potential difference across the motor, and the smaller the net current in the 

coil, becomes. 

Commutator 

Brush 

N 

DC Motor 

emf 

Brush 

S 

+ 


A person can receive an electric shock by touching 

a conducting or “live” wire while in contact with a 

lower electric potential, or ground. The ground 

contact might be made by touching a water 

pipe (which is normally at zero potential) 

or by standing on the floor with wet feet 

(because impure water is a good conductor). 

Electric shock can result in fatal burns or can 

cause the muscles of vital organs, such as the 

heart, to malfunction. The degree of damage to 

the body depends on the magnitude of the current, 

the length of time it acts, and the part of the body 

through which it passes. A current of 

100 milliamps (mA) can be fatal. If the 

current is larger than about 10 mA, 

the hand muscles contract and 

the person may be unable to let 

go of the wire. 


To prevent electrocution, any wires 

designed to have such currents in them 

are wrapped in insulation, usually plastic or 

rubber. However, with frequent use, electric 


Avoiding Electrocution 

cords can fray, exposing some of the conductors. To 

further prevent electrocution in these and other situations 

in which electrical contact can be made, devices 

called a Ground Fault Circuit Interrupter (GFCI) and 

a Ground Fault Interrupter (GFI) are mounted in electrical 

outlets and individual appliances. 

GFCIs and GFIs provide protection by comparing 

the current in one side of the electrical outlet socket 

to the current in the other socket. If there is even 

a 5 mA difference, the interrupter opens the circuit 

in a few milliseconds (thousandths of a second). If 

you accidentally touch a bare wire and create an 

alternate conducting path through you to ground, 

the device detects this redirection of current and 

you get only a small shock or tingle. 

Despite these safety devices, you can still be electrocuted. 

Never use electrical appliances near 

water or with wet hands. Use a battery-powered 

radio near water because batteries cannot supply 

enough current to harm you. It is also a good idea 

to replace old outlets with GFCI-equipped units or 

to install GFI-equipped circuit breakers. 

1. A loop with an area of 0.33 m 2 is rotating at 281 rad/s with its axis of 

rotation perpendicular to a uniform magnetic field. The magnetic field 

has a strength of 0.035 T. If the loop contains 37 turns of wire, what is 

the maximum potential difference induced in the loop? 

2. A generator develops a maximum emf of 2.8 V. If the generator coil has 

25 turns of wire, a cross-sectional area of 36 cm 2 , and rotates with a constant 

frequency of 60.0 Hz, what is the strength of the magnetic field in 

which the coil rotates? 

3. What is the purpose of a commutator in a motor? Explain what would 

happen if a commutator were not used in a motor. 

4. Physics in Action The rms current produced by a single coil of an 

electric guitar is 0.025 mA. How large is the maximum instantaneous 

current? If the coil’s resistance is 4.3 kΩ, what are the rms and maximum 

potential differences produced by the coil? 


813 

SECTION 22-2 

BACKGROUND 

Ground Fault Interrupters make 

use of Faraday’s law. Both the 

wire that leads from the wall outlet 

to the appliance and the wire 

that leads from the appliance 

back to the wall pass through an 

iron ring. Part of the iron ring is 

wrapped in a coil called a sensing 

coil. When the current to the 

appliance equals the current 

from the appliance, the net magnetic 

field through the sensing 

coil is zero. 

If a short circuit occurs, the 

net magnetic field through the 

coil is no longer zero. Because the 

current in the wire is alternating, 

an ac potential difference is 

induced in the sensing coil. This 

induced potential difference in 

the coil is used to trigger a circuit 

breaker, stopping the current 

before it reaches a level that 

might be harmful to the person 

using the appliance. 


ANSWERS 

1. 120 V 

2. 8.3 × 10 −2 T 

3. The commutator reverses the 

direction of the current in 

the coil so that the magnetic 

field produced by the current 

always opposes the external 

field and the coil turns continuously. 

Without the commutator, 

the coil would turn 

until the coil’s magnetic field 

was aligned with the external 

field. The coil would then 

cease to turn. 

4. 0.035 mA; 0.11 V, 0.15 V 

813

Section 22-3 


Transformers 

Purpose Show students how a 

transformer works, and reinforce 

the idea that a changing current 

is required. 

Materials iron bar, 9 V battery, 

knife switch, flashlight bulb in 

holder, two wires (each about 

1 m long) 

Procedure Set up the primary side 

of the transformer before class by 

connecting the battery and switch 

in series with one of the wires 

coiled around the iron bar. The 

primary coil should have 50 turns. 

Set up the secondary coil with 25 

turns, and connect the flashlight 

bulb to the secondary coil. 

In class, demonstrate this stepdown 

transformer by momentarily 

closing the switch and then 

opening it again. Have students 

discuss the transfer of energy that 

occurs in this situation. 

Close the switch and keep it 

closed. Have students note the 

behavior of the bulb. Discuss the 

brief illumination and fading of 

the bulb with students. Lead students 

to consider the concept of 

changing current. Open the 

switch, and note the illumination. 

Discuss this effect. 

814 


• Describe how mutual 

induction occurs in circuits. 

• Calculate the potential 

difference from a step-up or 

step-down transformer. 

• Describe how self-induction 

occurs in an electric circuit. 

mutual inductance 

a measure of the ability of one circuit 

carrying a changing current to 

induce an emf in a nearby circuit 

Figure 22-18 

Faraday’s electromagnetic-induction 

experiment used a changing current 

in one circuit to induce a current in 

another circuit. 

814 

Chapter 22 

22-3 

Inductance 

MUTUAL INDUCTANCE 

The basic principle of electromagnetic induction was first demonstrated by 

Michael Faraday. His experimental apparatus, which resembled the arrangement 

shown in Figure 22-18, used a coil connected to a switch and a battery 

instead of a magnet to produce a magnetic field. This coil is called the primary 

coil, and its circuit is called the primary circuit. The magnetic field is strengthened 

by the magnetic properties of the iron ring around which the primary 

coil is wrapped. 

A second coil is wrapped around another part of the iron ring and is connected 

to a galvanometer. An emf is induced in this coil, called the secondary 

coil, when the magnetic field of the primary coil is changed. When the switch in 

the primary circuit is closed, the galvanometer in the secondary circuit deflects 

in one direction and then returns to zero. When the switch is opened, the galvanometer 

deflects in the opposite direction and again returns to zero. When 

there is a steady current in the primary circuit, the galvanometer reads zero. 

The magnitude of this emf is predicted by Faraday’s law of induction. 

However, Faraday’s law can be rewritten so that the induced emf is proportional 

to the changing current in the primary coil. This can be done because of 

the direct proportionality between the magnetic field produced by a current 

in a coil, or solenoid, and the current itself. The form of Faraday’s law in terms 

of changing primary current is as follows: 


q)] 

⎯ =−M ⎯ 

∆t 

∆I 

⎯ 

∆t 

The constant, M, is called the mutual inductance of the two-coil system. The 

mutual inductance depends on the geometrical properties of the coils and 

+ 

Primary 

coil 

Iron 

ring 

Secondary 

coil 

Galvanometer 

100 

200 

300 

400 

500 

600 

700 

800 

900 

0 + 

100 

200 

300 

400 

500 

600 

700 

800 

900 

1000 


their orientation to each other. Because these properties are kept constant, it 

follows that a changing current in the secondary coil can also induce an emf in 

the primary circuit. The equation holds as long as the coils remain unchanged 

with respect to each other, so that the mutual inductance is constant. 

By changing the number of turns of wire in the secondary coil, the induced 

emf in the secondary circuit can be changed. This arrangement is the basis of 

an extremely useful electrical device: the transformer. 

TRANSFORMERS 

It is often desirable or necessary to change a small ac potential difference to a 

larger one or to change a large potential difference to a smaller one. The 

device that makes these conversions possible is the transformer. 

In its simplest form, an ac transformer consists of two coils of wire wound 

around a core of soft iron, like the apparatus for the Faraday experiment. The 

coil on the left in Figure 22-19 has N 1 turns and is connected to the input ac 

potential difference source. This coil is called the primary winding, or the primary. 

The coil on the right, which is connected to a resistor R and consists of 

N 2 turns, is the secondary. As in Faraday’s experiment, the iron core provides a 

medium for nearly all magnetic field lines passing through the two coils. 

Because the strength of the magnetic field in the iron core and the crosssectional 

area of the core are the same for both the primary and secondary 

windings, the potential differences across the two windings differ only 

because of the different number of turns of wire for each. The ac potential difference 

that gives rise to the changing magnetic field in the primary is related 

to that changing field by Faraday’s law of induction. 

∆V1 =−N1⎯ ∆(AB cos 

q) 

⎯ 

∆t 

Similarly, the induced potential difference (emf) across the secondary coil is 

∆V2 =−N2⎯ ∆(AB cos 

q) 

⎯ 

∆t 

Taking the ratio of ∆V 1 to ∆V 2 causes all terms on the right side of both equations 

except for N 1 and N 2 to cancel. This result is the transformer equation. 

TRANSFORMER EQUATION 

∆V2 = ⎯ N2 

⎯ ∆V1 N1 

potential difference in secondary = 

number of turns in secondary 

⎯⎯⎯ potential difference in primary 

number of turns in primary 


transformer 

a device that changes one ac 

potential difference to a different 

ac potential difference 

∆V 1 

Primary 

(input) 

Soft iron core 

N 1 N 2 R 


∆V 2 

Secondary 

(output) 

Figure 22-19 

A transformer uses the alternating 

current in the primary circuit to 

induce an alternating current in the 

secondary circuit. 

NSTA 

TOPIC: Transformers 



815 

SECTION 22-3 


Induced potential 

difference 

Purpose Show that induced 

potential difference depends on 

the number of turns of wire. 

Materials 9 V battery, knife 

switch, 30 cm (or longer) iron 

bar, 2 flashlight bulbs (or more) 

in holders, three connecting 

wires (each about 1 m long) 

Procedure Set up the primary 

coil as in Demonstration 8. Set 

up two secondary coils, one with 

25 turns of wire and one with 50 

turns of wire. Close the switch, 

and have students compare the 

brightness of the two bulbs. You 

may wish to open and close the 

switch several times for repeated 

observations. 

815

SECTION 22-3 

Teaching Tip 

Some students may wonder 

whether dc transformers are possible. 

The dc produced by a battery 

would not work with a 

transformer. This is because a 

changing current is required, and 

a battery generates a steady direct 

current. However, as seen on page 

811, a generator can produce a 

fluctuating direct current. Such a 

current could be used by a 

transformer. 

Teaching Tip 

Point out to students that spark 

plugs create a spark because the 

potential difference across the gap 

increases to 20 000 V. At such 

high potential differences, air is 

ionized and becomes a conductor. 

816 

816 

Step-up transfomer 

(ignition coil) 

Ignition 

switch 

+ 

Chapter 22 

– 

12 V battery 

Another way to express this equation is to equate the ratio of the potential differences 

to the ratio of the number of turns. 

∆V2 

⎯⎯ 

= ⎯ 

∆V1 

N2 

⎯ 

N1 

When N2 is greater than N1, the secondary potential difference is greater 

than that of the primary, and the transformer is called a step-up transformer. 

When N2 is less than N1, the secondary potential difference is less than that 

of the primary, and the transformer is called a step-down transformer. 

It may seem that a transformer provides something for nothing. For 

example, a step-up transformer can change an input potential difference 

from 10 V to 100 V. However, the power input at the primary must equal the 

power output at the secondary. An increase in potential difference at the secondary 

means that there must be a proportional decrease in current. If the 

potential difference at the secondary is 10 times that at the primary, then the 

current at the secondary is reduced by a factor of 10. 

Computer 

Crank angle 

sensor 

Spark plug 

Figure 22-20 

The transformer in an automobile engine raises the 

potential difference across the gap in a spark plug so that 

sparking occurs. 

Real transformers are not perfectly efficient 

The transformer equation assumes that there are no 

power losses between the transformer’s primary and its 

secondary. Real transformers typically have efficiencies 

ranging from 90 percent to 99 percent. Power losses 

occur because of the small currents induced by changing 

magnetic fields in the iron core of the transformer and 

because of resistance in the wires of the windings. 

When electric power is transmitted over large distances, 

it is economical to use a high potential difference 

and a low current. This is because the power lost to resistive 

heating in the transmission lines varies as I 2 R. By 

reducing the current by a factor of 10, the power loss is 

reduced by a factor of 100. In practice, potential difference 

is stepped up to around 230 000 V at the generating 

station, then stepped down to 20 000 V at a regional distribution 

station, and finally stepped down to 120 V at the 

customer’s utility pole. The high potential difference in 

long-distance transmission lines makes them especially 

dangerous when they are knocked down by high winds. 

Coils in gasoline engines are transformers 

An automobile ignition system uses a transformer, or 

ignition coil, to convert the car battery’s 12 dc volts to a 

potential difference that is large enough to cause sparking 

between the gaps of the spark plugs. The diagram in Figure 

22-20 shows the type of ignition system that has been 


used in automobiles since 1990. In this arrangement, each cylinder has its own 

transformer, and a photoelectric detector called a crank angle sensor determines 

from the crankshaft’s position which cylinder’s contents are at maximum compression. 

Upon receiving this signal, the computer closes the primary circuit to 

the cylinder’s coil, causing the current in the primary to rapidly increase and 

the magnetic field in the transformer to change. This, in turn, induces a potential 

difference of about 20 000 V across the secondary. 

PROBLEM 

SOLUTION 

1. DEFINE 

2. PLAN 

3. CALCULATE 

4. EVALUATE 

Transformers 

SAMPLE PROBLEM 22D 

A step-up transformer is used on a 120 V line to provide a potential difference 

of 2400 V. If the primary has 75 turns, how many turns must the secondary 

have? 

Given: ∆V 1 = 120 V ∆V 2 = 2400 V N 1 = 75 turns 

Unknown: N 2 = ? 

Diagram: 


∆V 1 = 120 V 

N 1 = 

75 turns 

Choose an equation(s) or situation: Use the transformer equation. 

∆V2 = ⎯ N2 

⎯ ∆V1 N1 

Rearrange the equation(s) to isolate the unknown(s): 

N2 = ⎯ ∆V2 

⎯ N1 ∆V1 


⎯ 

N2 = ⎯2 400 

V 

75 turns = 1500 turns 

120 

V 

N 2 = 1500 turns 

N 2 = ? 

∆V 2 = 2400 V 

The greater number of turns in the secondary accounts for the increase in 

the potential difference in the secondary. The step-up factor for the transformer 

is 20:1. 

PHYSICS PHYSICS 

Module 20 

“Induction and 

Transformers” 

provides an interactive lesson 

with guided problem-solving 

practice to teach you about 

induction and transformers. 


817 

SECTION 22-3 




demonstration at the chalkboard 

or on an overhead projector. 

PROBLEM 

Transformers 

A transformer has 75 turns on 

the primary and 1500 turns on 

the secondary. 

a. If the potential difference 

across the primary is 120 V, 

what is the potential difference 

across the secondary? 

b. If the transformer has 1625 

turns on the secondary instead 

of 1500, what is the potential 

difference across the secondary? 

Answers 

a. 2400 V 

b. 2600 V 

Interactive 

Problem- 

Solving 

Tutor 

See Module 20 

“Induction and Transformers” 

provides additional development 

of problem-solving skills. 

817

SECTION 22-3 

PRACTICE GUIDE 22D 

Solving for: 

N PE Sample, 1–3; 

Ch. Rvw. 35–36 

PW 4–5, 7 

PB 6, 8–10 

N 1 

⎯ N2 

ANSWERS TO 

Practice 22D 

Transformers 

1. 55 turns 

2. 3.5 × 10 4 turns 

3. 25 turns 

4. 156:1 

5. 2.6 × 10 4 V 

6. 147 V 

818 

PE 4; Ch. Rvw. 41 

PB 7 

V PE 5–6; Ch. Rvw. 

40 

PW Sample, 1–3, 6b 


I PW 6a, 7 

PB 10 

818 

PRACTICE 22D 

Switch 

R 

B 

Chapter 22 

Transformers 

1. A step-down transformer providing electricity for a residential neighborhood 

has exactly 2680 turns in its primary. When the potential difference 

across the primary is 5850 V, the potential difference at the secondary is 

120 V. How many turns are in the secondary? 

2. A step-up transformer used in an automobile has a potential difference 

across the primary of 12 V and a potential difference across the secondary 

of 2.0 × 10 4 V. If the number of turns in the primary is 21, what is the 

number of turns in the secondary? 

3. A step-up transformer for long-range transmission of electric power is used 

to create a potential difference of 119 340 V across the secondary. If the 

potential difference across the primary is 117 V and the number of turns in 

the secondary is 25 500, what is the number of turns in the primary? 

4. A potential difference of 0.750 V is needed to provide a large current for 

arc welding. If the potential difference across the primary of a step-down 

transformer is 117 V, what is the ratio of the number of turns of wire on 

the primary to the number of turns on the secondary? 

5. A television picture tube requires a high potential difference, which in 

older models is provided by a step-up transformer. The transformer has 

12 turns in its primary and 2550 turns in its secondary. If 120 V is placed 

across the primary, what is the output potential difference? 

6. A step-down transformer has 525 turns in its secondary and 12 500 turns 

in its primary. If the potential difference across the primary is 3510 V, 

what is the potential difference across the secondary? 

Induced 

emf 

emf 

Figure 22-21 

The changing magnetic field that is 

produced by changing current 

induces an emf that opposes the 

applied emf. 

I 

SELF-INDUCTION 

Consider a circuit consisting of a switch, a resistor, and a source of emf, as 

shown in Figure 22-21. When the switch is closed, the current does not immediately 

change from zero to its maximum value, I max. Instead, the current 

increases with time, and the magnetic field through the loop due to this current 

also increases. The increasing field induces an emf in the circuit to oppose 

the change in magnetic field, according to Lenz’s law, and so the polarity of the 

induced emf must oppose the direction of the original current. The induced 

emf is in the direction indicated by the dashed battery. The net potential difference 

across the resistor is the emf of the battery minus the induced emf. 


As the current increases, the rate of increase lessens and the induced emf 

decreases, as shown in Figure 22-22. The decrease in the induced emf results 

in a gradual increase in the current. Similarly, when the switch is opened, 

the current gradually decreases to zero. This effect is called self-induction 

because the changing magnetic field through the circuit arises from the current 

in the circuit itself. The induced emf in this case is called self-induced emf. 

An example of self-induction is seen in a coil wound on a cylindrical iron 

core. (A practical device would have several hundred turns.) When the current 

is in the direction shown in Figure 22-23(a), a magnetic field forms 

inside the coil. As the current changes with time, the field through the coil 

changes and induces an emf in the coil. 

Lenz’s law indicates that this induced emf opposes the change in the current. 

For increasing current, the induced emf is as pictured in Figure 22-23(b), and 

for decreasing current, the induced emf is as shown in Figure 22-23(c). 

Faraday’s law of induction takes the same form for self-induction as it does 

for mutual induction except that the symbol for mutual inductance, M, is 

replaced with the symbol for self-inductance, L. 


q)] 

⎯ =−L ⎯ 

∆t 

∆I 

⎯ 

∆t 

The self-inductance of a coil depends on the number of turns of wire in the 

coil, the coil’s cross-sectional area, and other geometric factors. The SI unit of 

inductance is the henry (H), which is equal to 1 volt-second per ampere. 

B 

I 

Lenz’s law emf 

for increasing I 

− + 

(a) (b) (c) 



Lenz’s law emf 

for decreasing I 

+ − 

self-induction 

1. The centers of two circular loops are separated by a fixed distance. For 

what relative orientation of the loops will their mutual inductance be a 

maximum? For what orientation will it be a minimum? 

2. A step-up transformer has exactly 50 turns in its primary and exactly 

7000 turns in its secondary. If the potential difference across the primary 

is 120 V, what is the potential difference across the secondary? 

3. Does the self-inductance (L) of a coil depend on the current in the coil? 

the process by which a changing 

current in a circuit induces an 

emf in that same circuit 


Figure 22-22 

The self-induced emf reduces the 

rate of increase of the current in 

the circuit immediately after the 

circuit is closed. 


Time 

Figure 22-23 

The polarity of the self-induced emf 

is such that it can produce a current 

whose direction is opposite the 

change in the current in the coil. 

819 

SECTION 22-3 


The SI unit of inductance, the 

henry (H), was named after the 

American scientist Joseph Henry. 

Henry was a professor of mathematics 

and philosophy in Albany, 

New York. He discovered electromagnetic 

induction while on a 

one-month vacation, but he then 

returned to work and was not 

able to complete his research for 

publication. By the time Henry 

published his discovery, Faraday 

had already discovered the same 

phenomenon and published his 

results. 


ANSWERS 

1. The planes of the two loops 

must be parallel for maximum 

mutual inductance. The planes 

of the loops must be perpendicular 

to each other for minimum 

mutual inductance. 

2. 1.7 × 10 4 V 

3. No, self-inductance (L) only 

depends on the number of 

turns of wire in the coil, the 

coil’s cross-sectional area, and 

geometric factors. 

819

Chapter 22 

Summary 

Teaching Tip 

Ask students to prepare a concept 

map for the chapter. The concept 

map should include most of the 

vocabulary terms, along with 

other integral terms or concepts. 

820 

KEY TERMS 

alternating current (p. 807) 

back emf (p. 812) 

electromagnetic induction 

(p. 794) 

generator (p. 803) 

mutual inductance (p. 814) 

rms current (p. 808) 

self-induction (p. 819) 

transformer (p. 815) 

Diagram symbols 

Induced emf 

ac generator/ 

alternating current 

emf source 

820 

Chapter 22 

− 

+ 

CHAPTER 22 

Summary 

KEY IDEAS 

Section 22-1 Induced current 

• Changing the magnetic field strength near a conductor induces an emf. 

• The direction of an induced current in a circuit is such that its magnetic 

field opposes the change in the applied magnetic field. 

• Induced emf can be calculated using 

Faraday’s law of magnetic induction. emf =−N⎯ ∆[AB (cos 

q)] 

⎯ 

∆t 

Section 22-2 Alternating current, generators, and motors 

• Generators use induction to convert mechanical energy into electrical energy. 

• Alternating current is measured in terms of rms current. 

• Motors use an arrangement similar to that of generators to convert electrical 

energy into mechanical energy. 

Section 22-3 Inductance 

• Mutual inductance involves the induction of a current in one circuit by 

means of a changing current in a nearby circuit. 

• Transformers change the potential difference of an alternating current. 

• Self-induction occurs when the changing current in a circuit induces an 

emf in the same circuit. 

Variable symbols 

Quantities Units 

N number of turns (unitless) 

∆Vmax maximum potential difference V volt 

∆Vrms rms potential difference V volt 

Imax maximum current A ampere 

Irms rms current A ampere 

M mutual inductance H henry = ⎯ volt•second 

⎯ 

ampere 

L self-inductance H henry = ⎯ volt•second 

⎯ 

ampere 


INDUCED CURRENT 

Review questions 

1. Suppose you have two circuits. One consists of an 

electromagnet, a dc emf source, and a variable resistor 

that permits you to control the strength of the 

magnetic field. In the second circuit, you have a coil 

of wire and a galvanometer. List three ways that you 

can induce a current in the second circuit. 

2. Explain how Lenz’s law allows you to determine the 

direction of an induced current. 

3. What four factors affect the magnitude of the induced 

emf in a coil of wire? 

4. If you have a fixed magnetic field and a length of 

wire, how can you increase the induced emf across 

the ends of the wire? 

Conceptual questions 

5. Rapidly inserting the north pole of a bar magnet 

into a coil of wire connected to a galvanometer 

causes the needle of the galvanometer to deflect to 

the right. What will happen to the needle if you do 

the following? 

a. pull the magnet out of the coil 

b. let the magnet sit at rest in the coil 

c. thrust the south end of the magnet into the coil 

6. Explain how Lenz’s law illustrates the principle of 

energy conservation. 

7. Does dropping a bar magnet down a long copper 

tube induce a current in the tube? If so, how must 

the magnet be oriented with respect to the tube? 

8. Two bar magnets are placed side by side so that 

the north pole of one magnet is next to the south 

pole of the other magnet. If these magnets are 

then pushed toward a coil of wire, would you 

expect an emf to be induced in the coil? Explain 

your answer. 


CHAPTER 22 

Review and Assess 

9. An electromagnet is placed next to a coil of wire in the 

arrangement shown in Figure 22-24. According to 

Lenz’s law, what will be the direction of the induced 

current in the resistor R in the following cases? 

a. the magnetic field suddenly decreases after 

the switch is opened 

b. the coil is moved closer to the electromagnet 

Switch 

Electromagnet 

− + 

emf 

Practice problems 

10. A flexible loop of conducting wire has a radius of 

0.12 m and is perpendicular to a uniform magnetic field 

with a strength of 0.15 T, as in Figure 22-25(a). The 

loop is grasped at opposite ends and stretched until it 

closes to an area of 3 × 10 −3 m 2 , as in Figure 22-25(b). 

If it takes 0.20 s to close the loop, find the magnitude of 

the average emf induced in the loop during this time. 

(See Sample Problem 22A.) 

(a) (b) 

11. A rectangular coil 0.055 m by 0.085 m is positioned 

so that its cross-sectional area is perpendicular to 

the direction of a magnetic field, B. If the coil has 75 

turns and a total resistance of 8.7 Ω and the field 

decreases at a rate of 3.0 T/s, what is the magnitude 

of the induced current in the coil? 


B 

Coil 


R 

Figure 22-24 

Figure 22-25 

821 

Chapter 22 


ANSWERS TO 

Chapter 22 


1. Place plane of coil perpendicular 

to magnetic field and move 

it into or out of field, rotate 

coil about axis perpendicular 

to magnetic field, and change 

strength of magnetic field with 

variable resistor. 

2. The direction of the induced 

current is such that its magnetic 

field will oppose the 

change in the external magnetic 

field. 

3. magnetic field component 

perpendicular to plane of coil, 

area of coil, time in which 

changes occur, number of 

turns of wire in coil 

4. Wrap the wire into a coil that 

has many turns (large N) and 

that can be moved in and out 

of the B field quickly (small 

∆t). 

5. a. needle deflects to the left 

b. no deflection of needle 

c. needle deflects to the left 

6. By opposing changes in the 

external field, the induced 

B field prevents the system’s 

energy from increasing or 

decreasing. 

7. yes; The magnet’s poles must 

be perpendicular to the tube’s 

cross-sectional area. 

8. no; Effects of one magnet cancel 

those of other magnet. 

9. a. from left to right 

b. from right to left 

10. 3.2 × 10 −2 V 

11. 0.12 A 

821

22 REVIEW & ASSESS 

12. −0.63 V 

13. B field (induces emf in 

turning coil), wire coil (conducts 

induced current), slip 

rings (maintain contact with 

rest of circuit by means of 

conducting brushes) 

14. turn the handle faster 

15. Frequency indicates how often 

each second the current goes 

from a maximum value in one 

direction to a maximum value 

in the other direction and back. 

16. Replace the slip rings with a 

commutator, which prevents 

the reversal of the current 

every half-cycle. 

17. Battery current is constant, 

while dc generator current 

fluctuates. 

18. an emf with polarity opposite 

that of the emf powering the 

motor; The coil’s rotation in 

the B field induces a back emf 

that reduces the net potential 

difference across the motor. 

19. The magnetic forces are greatest 

on charges in the sides of a 

loop that move perpendicular 

to the B field (that is, when the 

plane of the loop is parallel to 

the field lines). 

20. The B field of an induced current 

opposes the change (due to 

coil rotation) in the external B 

field. Faster rotation of the coil 

increases this induced current 

and thus the opposing field. 

21. Maximum values are maintained 

only for an instant, whereas 

rms values remain steady 

and thus are easier to measure. 

22. a, b; The B field lines in these 

cases are in a plane perpendicular 

to the plane of the loop, so 

the loop crosses the field lines. 

23. 3.88 × 10 −2 V 

24. a. 2.4 × 10 2 V 

b. 6.9 A 

822 

12. A 52-turn coil with an area of 5.5 × 10 −3 m 2 is 

dropped from a position where B = 0.00 T to a new 

position where B = 0.55 T. If the displacement 

occurs in 0.25 s and the area of the coil is perpendicular 

to the magnetic field lines, what is the 

resulting average emf induced in the coil? 


ALTERNATING CURRENT, 

GENERATORS, AND MOTORS 


13. List the essential components of an electric generator, 

and explain the role of each component in generating 

an alternating emf. 

14. A student turns the handle of a small generator 

attached to a lamp socket containing a 15 W bulb. 

The bulb barely glows. What should the student do 

to make the bulb glow more brightly? 

15. What is meant by the term frequency in reference to 

an alternating current? 

16. How can an ac generator be converted to a dc generator? 

Explain your answer. 

17. In what way is the output of a dc generator different 

from the output of a battery? 

18. What is meant by back emf? How is it induced in an 

electric motor? 


19. When the plane of a rotating loop of wire is parallel 

to the magnetic field lines, the number of lines passing 

through the loop is zero. Why is the current at a 

maximum at this point in the loop’s rotation? 

20. The faster the coil of loops, or armature, of an ac 

generator rotates, the harder it is to turn the armature. 

Use Lenz’s law to explain why this happens. 

21. Voltmeters and ammeters that measure ac quantities 

measure the rms values of emf and current, 

respectively. Why would this be preferred to measuring 

the maximum emf or current? (Hint: Think 

about what a meter reading would look like if ac 

quantities other than rms values were measured.) 

822 

Chapter 22 

22. A bar magnet is attached perpendicular to a rotating 

shaft. It is then placed in the center of a coil of 

wire. In which of the arrangements shown in 

Figure 22-26 could this device be used as an electric 

generator? Explain your choice. 


(a) 

(b) (c) 

R S N 

S N 

Figure 22-26 

23. A generator can be made using the component of 

Earth’s magnetic field that is parallel to Earth’s surface. 

A 112-turn square wire coil with an area of 

4.41 × 10 −2 m 2 is mounted on a shaft so that the 

cross-sectional area of the coil is perpendicular 

to the ground. The shaft then rotates with a frequency 

of 25.0 Hz. The horizontal component of 

Earth’s magnetic field at the location of the loop is 

5.00 × 10 −5 T. Calculate the maximum emf induced 

in the coil by Earth’s magnetic field. 

(See Sample Problem 22B.) 

24. An ac generator consists of 45 turns of wire with 

an area of 0.12 m 2 . The loop rotates in a magnetic 

field of 0.118 T at a constant frequency of 

60.0 Hz. The generator is connected across a circuit 

load with a total resistance of 35 Ω. Find the 

following: 

a. the maximum induced emf 

b. the maximum induced current 

(See Sample Problem 22B.) 

N 

S 

R 


R

25. The rms potential difference across high-voltage 

transmission lines in Great Britain is 220 000 V. 

What is the maximum potential difference? 

(See Sample Problem 22C.) 

26. The maximum potential difference across certain 

heavy-duty appliances is 340 V. If the total resistance 

of an appliance is 120 Ω, calculate the following: 

a. the rms potential difference 

b. the rms current 


27. The maximum current that can pass through a light 

bulb filament is 0.909 A when its resistance is 182 Ω. 

a. What is the rms current conducted by the filament 

of the bulb? 

b. What is the rms potential difference across the 

bulb’s filament? 

c. How much power does the light bulb use? 


28. A 996 W hair dryer is designed to carry a maximum 

current of 11.8 A. 

a. How large is the rms current in the hair dryer? 

b. What is the rms potential difference across the 

hair dryer? 


INDUCTANCE 


29. Describe how mutual induction occurs. 

30. What is the difference between a step-up transformer 

and a step-down transformer? 

31. Does a step-up transformer increase power? Explain 

your answer. 

32. Describe how self-induction occurs. 


33. In many transformers, the wire around one winding 

is thicker, and therefore has lower resistance, than the 

wire around the other winding. If the thicker wire is 

wrapped around the secondary winding, is the device 

a step-up or a step-down transformer? Explain. 

34. Would a transformer work with pulsating direct 

current? Explain your answer. 



35. A transformer is used to convert 120 V to 9.0 V for 

use in a portable CD player. If the primary, which is 

connected to the outlet, has 640 turns, how many 

turns does the secondary have? 

(See Sample Problem 22D.) 

36. A transformer is used to convert 120 V to 6.3 V in 

order to power a toy electric train. If there are 210 

turns in the primary, how many turns should there 

be in the secondary? 

(See Sample Problem 22D.) 

MIXED REVIEW PROBLEMS 

37. A student attempts to make a simple generator by 

passing a single loop of wire between the poles of a 

horseshoe magnet with a 2.5 × 10 −2 T field. The 

area of the loop is 7.54 × 10 −3 m 2 and is moved perpendicular 

to the magnetic field lines. In what 

time interval will the student have to move the 

loop out of the magnetic field in order to induce 

an emf of 1.5 V? Is this a practical generator? 

38. The same student in item 37 modifies the simple 

generator by wrapping a much longer piece of 

wire around a cylinder with about one-fourth the 

area of the original loop (1.886 × 10 −3 m 2 ). Again 

using a uniform magnetic field with a strength of 

2.5 × 10 −2 T, the student finds that by removing the 

coil perpendicular to the magnetic field lines during 

0.25 s, an emf of 149 mV can be induced. How 

many turns of wire are wrapped around the coil? 

39. A coil of 325 turns and an area of 19.5 × 10 −4 m 2 is 

removed from a uniform magnetic field at an angle 

of 45° in 1.25 s. If the induced emf is 15 mV, what is 

the magnetic field’s strength? 

40. A transformer has 22 turns of wire in its primary 

and 88 turns in its secondary. 

a. Is this a step-up or step-down transformer? 

b. If 110 V ac is applied to the primary, what is 

the output potential difference? 

41. The potential difference in the lines that carry electric 

power to homes is typically 20.0 kV. What is the 

ratio of the turns in the primary to the turns in the 

secondary of the transformer if the output potential 

difference is 117 V? 


823 


25. 3.1 × 10 5 V 

26. a. 2.4 × 10 2 V 

b. 2.0 A 

27. a. 0.643 A 

b. 117 V 

c. 75.2 W 

28. a. 8.34 A 

b. 119 V 

29. The changing B field produced 

by a changing current in one 

circuit induces an emf and 

current in a nearby circuit. 

30. A step-up transformer uses 

the B field of an alternating 

current to induce an increased 

emf in the secondary. A stepdown 

transformer uses the 

same principle to induce a 

smaller emf in the secondary. 

31. no; The change in potential 

difference in a transformer is 

accompanied by an inverse 

change in the current. In an 

ideal transformer, power is 

unchanged, as expected from 

energy conservation. 

32. Changing current in a circuit 

produces a changing magnetic 

field, which induces an opposing 

emf and current in the 

same circuit. 

33. a step-down transformer; I is 

larger in the secondary, so wire 

with a lower R is needed to 

reduce energy dissipaton. 

34. yes; Current changes continually, 

so its changing B field can 

induce an emf in a transformer’s 

secondary. 

35. 48 turns 

36. 11 turns 

37. 1.3 × 10 −4 s; no 

38. 790 turns 

39. 4.2 × 10 −2 T 

40. a. a step-up transformer 

b. 440 V 

41. 171:1 

823


42. 1.03 × 10 5 V 

43. a. 28 kW 

b. 3.6 × 10 5 kW; The power 

dissipated by the alternating 

current whose potential 

difference has not been 

stepped up is more than the 

power generated. This indicates 

that without stepping 

up its potential difference, 

electricity cannot be conveyed 

very far along a 

transmission line. 

ANSWERS TO 

Technology & Learning 

Answers may vary slightly, 

depending on viewing window 

settings. 

a. Y 2 

b. i = 0.238 A; Irms = 0.177 A 

c. i = 0.147 A; Irms = 0.177 A 

d. i = 0.00 A; Irms = 0.0778 A 

e. i = 0.0647 A; Irms = 0.0778 A 

f. no 

824 

42. A bolt of lightning, such as the one shown on the left in 

Figure 22-27, behaves like a vertical wire conducting 

electric current. As a result, it produces a magnetic field 

whose strength varies with the distance from the lightning. 

A 105-turn circular coil is oriented perpendicular 

to the magnetic field, as shown on the right in Figure 

22-27. The coil has a radius of 0.833 m. If the magnetic 

field at the coil drops from 4.72 × 10 –3 T to 0.00 T in 

10.5 ms, what is the average emf induced in the coil? 

824 

Graphing calculators 

Refer to Appendix B for instructions on downloading 

programs for your calculator. The program 

“Chap22” allows you to analyze graphs of instantaneous 

and root-mean-square currents versus time 

in various ac circuits. 

Instantaneous current and rms current, as you 

learned earlier in this chapter, are related to the 

maximum current in an ac circuit by the following 

equations: 

i = I max (sin wt) and I rms = 

The program “Chap22” stored on your graphing 

calculator makes use of the equations for instantaneous 

and rms currents. Once the “Chap22” program 

is executed, your calculator will ask for the 

frequency and maximum current. The graphing calculator 

will use the following equations to create 

graphs of the instantaneous current (Y1) versus the 

time (X) and the rms current (Y2) versus the time 

(X). The relationships in these equations are the 

same as those in the equations shown above. 

Y1 = I sin (2πFX) and Y2 = I/ 

2 

a. What value in the calculator equations equals the 

amount of direct current that would dissipate the 

same energy in a resistor as is dissipated by the 

instantaneous ac current during a full cycle? 

Chapter 22 

I max 

⎯ 

2 

0.833 m 

Figure 22-27 

First make sure your calculator is in radian mode 

by pressing m ∂ ∂ ¬. 

Execute “Chap22” on the p menu and press 

e to begin the program. Enter the values for the 

frequency and maximum current (shown below), 

pressing e after each value. 

The calculator will provide graphs of the instantaneous 

current and root-mean-square current versus 

time in various ac circuits. (If the graphs are not 

visible, press w and change the settings for the 

graph window, then press g.) 

Press ◊ and use the arrow keys to trace along 

the curve. The x value corresponds to the time in 

seconds, and the y value of the sine curve corresponds 

to the instantaneous current in amperes. 

The y value of the straight line gives the root-meansquare 

value for the current in amperes. Use the ¨ 

and ∂ keys to toggle between the two graphs. 

Determine the instantaneous and root-meansquare 

values for the current in the following 

ac circuits: 

b. an ac circuit with a maximum current of 0.250 A 

and a frequency of 60.0 Hz at t = 3.27 s 

c. the same circuit at t = 5.14 s 

d. an ac circuit with a maximum current of 0.110 A 

and a frequency of 110.0 Hz at t = 2.50 s 

e. the same circuit at t = 4.24 s 

f. Would you expect the graph of Y 2 to be above the 

maximum instantaneous current (Y 1)? 

Press @ q to stop graphing. Press e to 

input new values or ı to end the program. 


43. A generator supplies 5.0 × 10 3 kW of power. The output 

potential difference is 4500 V before it is stepped 

up to 510 kV. The electricity travels 410 mi (6.44 × 

10 5 m) through a transmission line that has a resistance 

per unit length of 4.5 × 10 −4 Ω/m. 

a. How much power is lost through transmission 

of the electrical energy along the line? 

b. How much power would be lost through 

transmission if the generator’s output potential 

difference were not stepped up? What does 

this answer tell you about the role of large 

potential differences in power transmission? 

Alternative Assessment 

Performance assessment 

1. Identify the chain of electromagnetic energy transformations 

involved in making the blades of a ceiling 

fan spin. Include the fan’s motor, the transformers 

bringing electricity to the house, and the turbines 

generating the electricity. 

2. Two identical magnets are dropped simultaneously 

from the same point. One of them passes through a 

coil of wire in a closed circuit. Predict whether the 

two magnets will hit the ground at the same time. 

Explain your reasoning, then plan an experiment to 

test which of the following variables measurably 

affect how long each magnet takes to fall: magnetic 

strength, coil cross-sectional area, and the number 

of loops the coil has. What measurements will you 

make? What are the limits of precision in your measurements? 

If your teacher approves your plan, 

obtain the necessary materials and perform the 

experiments. Report your results to the class, 

describing how you made your measurements, what 

you concluded, and what additional questions need 

to be investigated. 

3. What do adapters do to potential difference, current, 

frequency, and power? Examine the input/output 

information on several adapters to find out. Do they 

contain step-up or step-down transformers? How 


44. The alternating potential difference of a generator is 

represented by the equation emf = (245 V) sin 560t, 

where emf is in volts and t is in seconds. Use these 

values to find the frequency of the potential difference 

and the maximum potential difference output 

of the source. 

45. A pair of adjacent coils has a mutual inductance of 

1.06 H. Determine the average emf induced in the 

secondary circuit when the current in the primary 

circuit changes from 0 A to 9.50 A in a time interval 

of 0.0336 s. 

does the output current compare to the input? What 

happens to the frequency? What percentage of the 

energy do they transfer? What are they used for? 

Portfolio projects 

4. Research the debate between the proponents of 

alternating current and those who favored direct 

current in the 1880–1890s. How were Thomas 

Edison and George Westinghouse involved in the 

controversy? What advantages and disadvantages 

did each side claim? What uses of electricity were 

anticipated? What kind of current was finally generated 

in the Niagara Falls hydroelectric plant? Had 

you been in a position to fund these projects at that 

time, which projects would you have funded? Prepare 

your arguments to re-enact a meeting of businesspeople 

in Buffalo in 1887. 

5. Research the history of telecommunication. Who 

invented the telegraph? Who patented it in England? 

Who patented it in the United States? 

Research the contributions of Charles Wheatstone, 

Joseph Henry, and Samuel Morse. How did each of 

these men deal with issues of fame, wealth, and 

credit to other people’s ideas? Write a summary of 

your findings, and prepare a class discussion about 

the effect patents and copyrights have had on modern 

technology. 

Induction and Alternating Current 825 


44. f = 89 Hz, 245 V 

45. 300 V 

Alternative Assessment 

ANSWERS 

Performance assessment 

1. Students’ answers will vary. 

Transformations include 

turning a generator coil in a 

magnetic field to induce an 

emf, step-up transformers, 

step-down transformers, and 

torque exerted by a B field on 

the coil in the fan’s motor. 

2. Students’ plans will vary. Be 

sure proposed tests are safe, 

measure time accurately, and 

adjust one variable at a time. 

3. Most adapters use step-down 

transformers. Students 

should note that power output 

is always less than input. 

4. Students’ answers 

Portfolio will vary. Edison 

projects 

claimed that ac was 

dangerous and unreliable. 

Westinghouse noted that dc 

could not be stepped up and 

that long-distance transmission 

was inefficient. Batteries 

can store dc for peak use, but 

new industries that used electricity 

at all hours favored ac. 

5. Wheatstone (1802–1875) 

took out the first telegraph 

patent in England in 1837. In 

the United States, Henry 

(1797–1878) invented the 

telegraph 10 years before 

Morse (1791–1872), who 

claimed in court that he 

invented it by himself. 

825

Chapter 22 

Laboratory Exercise 

NOTE 

Materials Preparation is given on 

pp. 792A–792B. Blank data table 

and sample data table are on the 

One-Stop Planner CD-ROM. All 

calculations shown use sample 

data. 

Planning 

Recommended time: 

1 lab period 

Classroom organization: 

Each lab group should have 

two or more students. 

Safety warnings: Emphasize 

the dangers of working with 

electricity. For the safety of the 

students and of the equipment, 

remind students to have 

you check their circuits before 

turning on the power supply 

or closing the switch. 

Techniques to 

Demonstrate 

Show students how to set up the 

circuit for the second part of the 

lab. Make sure they know how to 

use the rheostat to adjust the current 

in the circuit. 

Checkpoints 

Step 3: Because this lab requires 

no measurements, students may 

be unsure what to record in their 

notebooks. Guide students to 

come up with a format for 

recording their observations. 

826 

OBJECTIVES 

•Use a galvanometer 

to detect an induced 

current. 

•Determine the relationship 

between the magnetic 

field of a magnet 

and the current induced 

in a conductor. 

•Determine what factors 

affect the direction and 

magnitude of an induced 

current. 

MATERIALS LIST 

✔ galvanometer 

✔ insulated connecting wires 

✔ momentary contact switch 

✔ pair of bar magnets or a large 

horseshoe magnet 

✔ power supply 

✔ rheostat (10 Ω) or 

potentiometer 

✔ student primary and secondary 

coil set with iron core 

826 

Chapter 22 

CHAPTER 22 

Laboratory Exercise 

ELECTROMAGNETIC INDUCTION 

In this laboratory, you will use a magnet, a conductor, and a galvanometer to 

explore electromagnets and the principle of self-induction. 

SAFETY 

• Never close a circuit until it has been approved by your teacher. Never 

rewire or adjust any element of a closed circuit. Never work with electricity 

near water; be sure the floor and all work surfaces are dry. 

• If the pointer on any kind of meter moves off scale, open the circuit 

immediately by opening the switch. 

• Do not attempt this exercise with any batteries, electrical devices, or 

magnets other than those provided by your teacher for this purpose. 

PREPARATION 

1. Read the entire lab, and plan what measurements you will take. 

2. In your lab notebook, prepare an observation table with three wide 

columns. Label the columns Sketch of setup, Experiment, and Observation. 

For each part of the lab, you will sketch the apparatus and label the poles 

of the magnet, write a brief description, and record your observations. 

PROCEDURE 

Induction with a permanent magnet 

3. Connect the ends of the smaller coil to the galvanometer. Hold the magnet 

still and move the coil quickly over the north pole of the magnet, as shown 

in Figure 22-28. Remove the coil quickly. Observe the galvanometer. 

4. Repeat, moving the coil more slowly. Observe the galvanometer. 

5. Turn the magnet over, and repeat steps 3 and 4, moving the coil over 

the south pole of the magnet. Observe the galvanometer. 

6. Hold the coil stationary and quickly move the north pole of the magnet 

in and out of the coil. Repeat slowly. Turn the magnet, and repeat both 

quickly and slowly with the south pole. Observe the galvanometer. 


DTSI Graphics HRW—Holt Physics 2002 Page 827 CMYK 

Induction with an electromagnet 

7. Connect the larger coil to the galvanometer. Connect the small coil in 

series with a switch, battery, and rheostat. Slip the smaller coil inside the 

larger coil, so that the arrangement resembles that shown in Figure 22-29. 

Close the switch. Adjust the rheostat so that the galvanometer reading registers 

on the scale. Observe the galvanometer. 

8. Open the switch to interrupt the current in the small coil. Observe the 

galvanometer. 

9. Close the switch again, and open it after a few seconds. Observe the 

galvanometer. 

10. Adjust the rheostat to increase the current in the small coil. Close the 

switch, and observe the galvanometer. 

11. Decrease the current in the circuit, and observe the galvanometer. Open 

the switch. 

12. Reverse the direction of the current by reversing the battery connections. 

Close the switch, and observe the galvanometer. 

13. Place an iron rod inside the small coil. Open and close the switch while 

observing the galvanometer. Record all observations in your notebook. 

14. Clean up your work area. Put equipment away safely so that it is ready to 

be used again. 

ANALYSIS AND INTERPRETATION 

Calculations and data analysis 

1. Organizing information Based on your observations from the first 

part of the lab, did the speed of the motion have any effect on the 

galvanometer? 

2. Organizing results In the first part of the lab, did it make any difference 

whether the coil or the magnet moved? Explain why or why not. 

Conclusions 

3. Applying ideas Explain what the galvanometer readings revealed to 

you about the magnet and the wire coil. 

4. Evaluating results Based on your observations, what conditions are 

required to induce a current in a wire? 

5. Evaluating results Based on your observations, what factors influence 

the direction and magnitude of the induced current? 


Figure 22-28 

Step 3: Connect the coil to the 

galvanometer. Holding the magnet 

still, move the coil over the magnet 

quickly. 

Step 4: Holding the magnet still, 

move the coil over the magnet 

slowly. 

Step 6: Repeat the procedure, but 

hold the coil still while moving the 

magnet. 

Figure 22-29 

Step 7: Connect the larger coil to 

the galvanometer, and connect the 

smaller coil in series with the battery, 

switch, and rheostat. Place the 

smaller coil inside the larger coil. 


827 

CHAPTER XX 22 LAB 

Step 7: Make sure the power 

supply or battery, rheostat, galvanometer, 

coil, and switch are 

connected properly and set at the 

proper settings. Students should 

be able to demonstrate that all 

connections are properly made. 

Step 12: Students should be able 

to demonstrate that they reversed 

the current in the circuit. 

ANSWERS TO 

Analysis and 

Interpretation 

CALCULATIONS AND 

DATA ANALYSIS 

1. Yes, the faster the motion, 

the greater the deflection of 

the galvanometer needle. 

2. It did not matter which one 

moved. The important thing 

was the motion between 

them, or relative motion. 

CONCLUSIONS 

3. The galvanometer showed 

that there was a current in 

the wire coil. The direction of 

the deflection indicated the 

direction of the current. 

4. A changing magnetic field is 

needed to induce a current in 

a circuit. 

5. The direction of the current 

is influenced by the direction 

the magnet is moved and by 

whether the field is increasing 

or decreasing. The magnitude 

is influenced by the 

speed of the change. 

827

Induction and Alternating Current with teacher's notes

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