25
For each definition below, write the correct term. eddy current Lenz’s law electric generator electromagnetic induction induced electromotive force mutual inductance self-inductance step-down transformer step-up transformer transformer
1.
_________________________ the statement that an induced current is always produced in a direction such that the resulting magnetic field opposes the change in the magnetic field that is causing the induced current
2.
_________________________ the potential difference that is produced by electromotive induction
3.
_________________________ a transformer with a lower potential difference across the secondary circuit than across the primary circuit
4.
_________________________ the effect that occurs in a transformer when a varying magnetic field produced in the primary coil is carried through the iron core to the secondary coil, where the varying field induces a varying EMF
5.
_________________________ a device that increases or decreases the potential difference with little loss of energy
6.
_________________________ the generation of current through a circuit due to the relative motion of a magnetic field
7.
_________________________ a transformer with higher potential difference across the secondary circuit than across the primary circuit
8.
_________________________ generated by a piece of metal moving through a magnetic field
9.
_________________________ the property of a wire to create an induced EMF that opposes the change in a potential difference across the wire
10.
_________________________ converts mechanical energy to electrical energy
Chapter 25  Electromagnetic Induction
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25
16
In your textbook, read about electric current from charging magnetic fields.
Circle the letter of the choice that best completes the statement.
1.
To generate a current in a wire in a magnetic field, _____. a.
the conductor must move through the magnetic field while the magnetic field remains stationary b.
the magnetic field must move past the conductor while the conductor remains stationary c.
there must be relative movement between the wire and the magnetic field d.
there must be a battery connected to the wire
2.
If a wire moves through a magnetic field at an angle to the field, only the component of the wire’s velocity that is _____ generates EMF . a. negative b. parallel to the direction of the magnetic field c.
perpendicular to the direction of the magnetic field d.
positive
3. When a wire is held stationary or moved _____ to a magnetic field, there is no current in wire. a. parallel c.
tangent b. perpendicular d.
indirectly
4. An electric current is generated in a wire in a constant magnetic field only when _____. a. the wire already has a small current c.
the wire is moved parallel to the field b. the wire moves across magnetic field lines d.
the wire is stationary in the field
5. EMF is measured in _____. a. b. amperes newtons c.
ohms d.
volts
6. Electromotive force is not a force; it is a _____. a. charge c.
potential difference b. current d.
resistance
Chapter 25  Electromagnetic Induction
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25
7. Michael Faraday found that _____ could be produced by moving a wire through a magnetic field. a. a current b. an increase in resistance c.
d.
a net charge
a magnetic force
8. The EMF produced in a wire moving in a magnetic field depends on _____. a.
only the current in the wire b.
only the magnetic field strength c.
only the magnetic field strength, the length of the wire in the field, and the direction the wire moves d.
the magnetic field strength, the length of the wire in the field, and the velocity of the wire
9. Which of the following statements are true for an alternating current electric generator? a.
The average power is half of the maximum power. b.
The effective current is about 71 percent of the maximum current. c.
The effective voltage is about 71 percent of the maximum voltage. d.
all of the above
Answer the following questions. Use complete sentences.
10.
Write a definition for induced EMF .
____________________________________________________________________________________
____________________________________________________________________________________
11.
What is one way to increase the induced EMF produced by a generator?
____________________________________________________________________________________
____________________________________________________________________________________
12.
Describe how an electric generator works.
____________________________________________________________________________________
____________________________________________________________________________________
13.
Explain how an EMF is produced in a microphone.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
Chapter 25  Electromagnetic Induction
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25
In your textbook, read about Lenz’s law.
For each statement below, write true or rewrite the italicized part to make the statement true.
1. __________________________ The opposing force on an armature means that mechanical energy must be supplied to the generator to produce electrical energy; this fact is consistent with the law of conservation of energy .
2. __________________________ If a generator produces a larger current, then the armature will be easier to turn.
3. __________________________ The direction of an induced current is such that the magnetic field resulting from the induced current opposes the field that caused the induced current.
4. __________________________ When a current is produced in a wire by electromagnetic induction, the direction of the current is such that the magnetic field produces a force on the wire that assists the original motion of the wire.
In your textbook, read about self-inductance.
Write the term that correctly completes the statement. Use each term once. work constant energy
EMF current decreases electric field increases larger magnetic field lines self-inductance zero
(5) __________________________ through a coil of wire generates a magnetic field. If the current increases, the magnetic field (6) __________________________. This can be pictured as creation of new (7) __________________________. As they expand, they cut through the coil of wires, generating a(n) (8) __________________________ to oppose the current increase.
This generating process is called (9) __________________________. The faster the current changes, the (10) __________________________ the opposing EMF , and the slower the current change.
If the current reaches a steady value, the magnetic field is (11) __________________________, and the EMF is (12) __________________________. When the current (13) __________________________, the EMF generated helps prevent the reduction in magnetic field and current. Because of self-inductance, (14) __________________________ has to be done to increase the current in the coil. (15) __________________________ is stored in the magnetic field, much like a charged capacitor that stores energy in a(n) (16) __________________________.
Chapter 25  Electromagnetic Induction
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25
In your textbook, read about transformers.
Circle the letter of the choice that best completes the statement.
17. The two coils in a transformer must _____. a. be electrically insulated from each other c.
have the same number of windings b. be wound around different d.
have the same resistance iron cores
18. Transformers can change _____ with relatively little waste of energy. a. magnetic fields c.
resistances b. power d.
voltages
19. An insulated coil in which varying EMF is induced is _____. a. a primary coil c.
found only in step-down transformers b. a secondary coil d.
found only in step-up transformers
20. In an ideal transformer, the electric power delivered to the secondary circuit is _____ the power supplied to the primary circuit. a. double b. equal to c.
d.
greater than
less than
21. In a step-up transformer, the number of loops of wire in the primary coil is _____ the number of loops in the secondary coil. a. double b. equal to c.
d.
greater than
less than
22. In the secondary coil, the varying magnetic field induces a _____. a. charge c.
steady EMF b. resistance d.
varying EMF
23. When the primary coil is connected to a source of AC voltage, the changing current produces a _____. a. potential difference b. resistance c.
d.
steady magnetic field
varying magnetic field
24. _____ in home appliances, adjust potential differences to usable levels. a. Coils c.
Currents b. Magnets d.
Transformers
Chapter 25  Electromagnetic Induction
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Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

+
25
Complete the charts below by placing a greater-than sign (

) , less-than sign (

) , or equals sign (

) in each blank.
Assume the transformers are ideal.
Step-Up Transformer Step-Down Transformer
Voltage
Current
25.
V p
______ V s
26.
I p
______ I s
Voltage
Current
29.
V p
______ V s
30.
I p
______ I s
Number of Turns 27.
N p
______ N s
Power 28.
P p
______ P s
Answer the following questions. Show your calculations.
Number of Turns 31.
N p
______ N s
Power 32.
P p
______ P s
33.
A transformer has a primary coil consisting of 150 loops and a secondary coil consisting of
4000 loops. The effective voltage supplied to the primary coil is 60.0 V. a.
What is the induced EMF in the secondary coil? b.
The secondary coil is part of a complete circuit with a resistance of 1220

. What is the
current in the secondary circuit? c.
What is the current in the primary circuit? d.
Is this a step-up transformer or a step-down transformer?
Chapter 25  Electromagnetic Induction
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Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

12.
As the armature of the electric generator turns, the wire loops cut through the magnetic field lines, inducing EMF .
1.
Lenz’s law
2.
induced electromotive force
3.
step-down transformer
13.
In a microphone, a diaphragm is attached to a coil of wire that is free to move in a magnetic field. Sound waves vibrate the diaphragm, which moves the coil in the magnetic field. The motion of the coil induces an EMF across the ends of the coil.
4.
mutual inductance
5.
transformer
6.
electromagnetic induction
1.
true
7.
step-up transformer
2.
harder
8.
eddy currents
3.
the change in the field
9.
self-inductance
4.
opposes
10.
electric generator
5.
Current
6.
increases
1.
c
7.
magnetic field lines
2.
c
8.
EMF
3.
a
9.
self-inductance
4.
b
10.
larger
5.
d
11.
constant
6.
c
12.
zero
7.
a
13.
decreases
8.
d
14.
work
9.
d
15.
energy
10.
Induced electromotive force is the difference in potential that an energy source gives to charges when a wire moves in a magnetic field.
16.
electric field
17.
a
18.
d
11.
The induced EMF can be increased by increasing the number of loops of wire in the armature.
19.
b
20.
b
Chapter 25  Electromagnetic Induction
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21.
d
22.
d
23.
d
33.
a.
V s
V p
 N
S

 N
P


(60.0 V)
4000 loops
150 loops
 3
24.
d
25.

26.

27.
 b.
I
S

V
S
R
S

1600 V
1200


1.33 A
28.

29.

30.

31.
 c.
I p

V p

(1600 V)(1.33 A)
60.0 V

35.5 A
32.
 d.
step-up transformer
Chapter 25  Electromagnetic Induction
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