Chapter 14 - Electromagnetic Induction
TRUE/FALSE
1. A steady magnetic field will induce a steady current in a conductor.
ANS: F
2. A current is produced in a conductor when a magnetic field is moved around it, but not when the
conductor is moved through a magnetic field.
ANS: F
3. When a bar magnet is plunged into a coil, a current is induced in the coil.
ANS: T
4. In electromagnetic induction, the induced field opposes the inducing field.
ANS: T
5. A galvanometer connected to the circuit of an AC generator will show a steady flow of electricity.
ANS: F
6. An AC generator can be converted into a DC generator by replacing its slip rings with a split-ring
commutator.
ANS: T
7. The brushes of an AC generator rotate with the armature.
ANS: F
8. The output of a generator can be increased by increasing the speed of the armature's rotation.
ANS: T
9. A 60-Hz AC current changes direction once every 60 s.
ANS: F
10. The coil of a generator moves with the armature.
ANS: T
11. The core of a transformer helps to conduct the current to the coils.
ANS: F
12. In a transformer, the primary coil produces the inducing field.
ANS: T
MULTIPLE CHOICE
1. When a bar magnet is inserted into a coil, a current of 12.0 A is induced in the coil. If four magnets
of the same type are inserted into the coil at the same time, the induced current would be
a. 3.00 A
d. 48.0 A
b. 4.00 A
e. 60.0 A
c. 12.0 A
ANS: D
2. Who first reported the induction of an electric current in the presence of a changing magnetic field?
a. Andre Ampere
d. Michael Faraday
b. Hans Oersted
e. Heinrich Lenz
c. Georg Ohm
ANS: D
3. A conductor be attracted by a magnet when
a. The conductor contains negative electrons.
b. The conductor contains positive protons.
c. The conductor contains moving electrons.
d. all of the above
e. none of the above
ANS: C
4. A current can be induced in a straight conductor when
a. The conductor is held near the N-pole of a bar magnet.
b. The conductor is held near the S-pole of a bar magnet.
c. The conductor is held in a steady magnetic field.
d. The conductor is held in a changing magnetic field.
e. A current cannot be induced in a straight conductor.
ANS: D
5. Which of the following contributions is attributed to Heinrich Lenz?
a. A changing magnetic field will induce an electric current in a conductor.
b. A steady current will produce a steady magnetic field.
c. The direction of an induced current produces a magnetic field that opposes the inducing
field.
d. A magnetic field travels from a N-pole to an S-pole.
e. To operate, a transformer must be provided with AC current.
ANS: C
6. Which of the following factors does not affect the size of the induced current in a coil?
a. number of turns in the coil
d. size of the inducing field
b. rate of change of the inducing field
e. direction of the windings in the coil
c. resistance of the wire in the coil
ANS: E
7. A coil is connected to a galvanometer. When a permanent magnet is held steady in the center of the
coil, the galvanometer will indicate
a. A steady current.
d. A decreasing current.
b. An alternating current.
e. No current.
c. An increasing current.
ANS: E
8. To use the right hand rule to predict the direction of an induced current in a coil, your thumb must
point
a. To the N-pole of the induced magnetic field.
b. To the S-pole of the induced magnetic field.
c. In the direction of the induced current.
d. In the direction of the inducing field.
e. In the direction of the electron flow.
ANS: A
9. The diagram below shows a bar magnet being moved away from a coil. The pole induced at the
side closest to the magnet will be a(n) ____-pole and the current will flow ____ at the front of the
coil.
a. N, up
b. N, down
c. S, up
d. N, down
e. positive, down
ANS: C
10. Which of these factors would increase the output of a generator?
a. increasing the rate of rotation of the armature
b. increasing the strength of the field magnets
c. increasing the number of coils on the armature
d. all of the above
e. none of the above
ANS: D
11. The primary coil of a transformer is supplied with a 45-V current. What is the ratio of the number
of turns in the coils (primary : secondary) if the potential difference in the secondary coil is 450 V?
a. 10 : 1
d. 1 : 45
b. 1 : 10
e. 450 : 1
c. 45 : 1
ANS: B
12. The primary coil of a transformer is supplied with a 45 V, 0.50-A current. If the current in the
secondary coil of the transformer is 5.0 A, what is the ratio (primary : secondary) of the number of
turns in the coils?
a. 1 : 10
d. 1 : 9
b. 10 : 1
e. 1 : 5
c. 9 : 1
ANS: B
13. If the connections to a step-up transformer are reversed and the secondary coil is now the primary
coil, what will be the result?
a. The transformer will not work.
b. The transformer will continue to step up the potential difference.
c. The transformer will step down the potential difference.
d. The transformer will produce DC current.
e. The transformer will become a generator.
ANS: C
14. A transformer has 200 turns in its primary coil and 600 turns in its secondary coil. If the current in
the primary coil is 12.0 A, what is the current in the secondary coil?
a. 3.0 A
d. 20.0 A
b. 4.0 A
e. 36.0 A
c. 12.0 A
ANS: B
15. A step-down transformer changes
a. High potential difference to low potential difference.
b. AC current to DC current.
c. High current to low current.
d. High resistance to low resistance.
e. DC current to AC current.
ANS: A
16. A transformer has 100 turns in its primary coil and 200 turns in its secondary coil. If the primary
coil is supplied with a 1.20 A, 270-V current, what is the power output of the secondary coil?
a. 1.62 W
d. 324 W
b. 32.4 W
e. 648 W
c. 162 W
ANS: D
17. A good transformer design includes
a. Wires with low resistance.
b. A solid core.
c. A core that is non-magnetic.
d. All of the above.
e. None of the above.
ANS: A
18. If a 15.0-A current is transmitted through a wire with a resistance of 20.0 Ω, what is the power loss
in the wire?
a. 11.2 W
d. 6.00 × 102 W
b. 26.7 W
e. 4.50 × 103 W
c. 3.00 × 102 W
ANS: E
19. As a videotape moves past the playback head of a VCR,
a. It induces a magnetic field that is converted to electricity.
b. It produces an electric current that is converted into a magnetic field.
c. Its dipoles become aligned with the magnetic field of the VCR.
d. Its dipoles realign the magnetic field of the VCR.
e. none of the above
ANS: A
20. The advantage of AC power over DC power is that
a. AC power allows the operation of transformers.
b. AC power travels though wires with less energy loss.
c. AC power can be transmitted at higher voltages.
d. AC power can be transmitted with a higher current.
e. none of the above
ANS: A
COMPLETION
1. No current is induced in a conductor if the magnetic field around it is ____________________ or
____________________.
ANS: steady, zero
2. The direction of an induced magnetic field ____________________ the inducing magnetic field.
ANS: opposes
3. In any generator, the type of current in the armature is ____________________.
ANS: alternating
4. In a step-down transformer, the potential in the secondary coil is ____________________ than the
potential in the primary coil.
ANS: lower
5. If the potential difference of the primary coil in a transformer is greater than the potential difference
in the secondary coil, the current in the secondary coil will be ____________________ than in the
primary coil.
ANS: greater
6. An induced current that forms closed loops inside a conductor is called a(n)
____________________ current.
ANS: eddy
7. As the temperature of a wire increases, the rate at which it loses energy ____________________.
ANS: increases
8. Electricity is transmitted to homes with a low ____________________ and a high
____________________.
ANS: current, voltage
MATCHING
Match each item with the letter of the term it best describes. A letter may used more than once, or
not at all.
a. brush
d. armature
b. split-ring commutator
e. coil
c. slip ring
f. field magnet
1.
2.
3.
4.
5.
6.
transfers the current from the armature coil to the circuit
used only in AC generators
induces the magnetic field in the coil
rotates the coil in the magnetic field
provides a constant magnetic field
used only in DC generators
1.
2.
3.
4.
5.
6.
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
A
C
F
D
F
B
SHORT ANSWER
1. The diagram shows a bar magnet moving down into a coil. Label all N- and S-poles on the inducing
and induced magnet.
ANS:
The top of the bar magnet is an S-pole, the top of the coil is a N-pole, and the bottom of the coil is
an S-pole.
2. In the diagram, a bar magnet is pulled away from a coil. The poles of the induced field in the coil
are shown. Which pole is at the bottom of the bar magnet?
ANS:
N-pole
3. A galvanometer is connected to a coil. Describe the readings on the galvanometer when a bar
magnet is plunged into the coil, held steady, and then removed.
ANS:
-The galvanometer indicates a current.
-The reading goes to zero.
-The galvanometer shows a current, but in the opposite direction to the first current.
4. Why do commercial generators have several sets of coils, rather than just one?
ANS:
The current produced by a single coil fluctuates in strength. With more than one coil, the outputs
can be overlapped, allowing a more even output.
5. What is the difference between alternating current and direct current?
ANS:
Direct current always flows in the same direction. Alternating current reverses direction at specific
intervals.
6. Why doesn't the armature of a DC generator stop when the split in the commutator ring allows no
contact with the brushes?
ANS:
The momentum of the armature keeps it moving past this point until the next brush connects with
the commutator segment.
7. What happens when the connections to a step-down transformer are reversed?
ANS:
It becomes a step-up transformer.
8. If you are building a transformer and you want a lower potential difference in the secondary coil
than in the primary, should the secondary coil have fewer loops or more loops?
ANS:
fewer
9. Describe three devices or methods that could be used to turn the turbine of a generator, allowing it
to produce electricity.
ANS:
-moving water turning a water wheel
-wind turning a windmill
-steam turning a turbine (steam produced from water heated by solar energy, fossil fuel or nuclear
reaction, etc.)
-bicycle wheel turning
-hand turning a crank
10. The electrons in the current produced at a generating station never actually reach your home. How
is it possible for electricity to be transmitted?
ANS:
-energy is transferred, not electrons
-the electrons in an AC current reverse direction continuously—they do not leave the circuit
-transformers along the way induce currents in separate circuit—electrons do not move from
one circuit to another
PROBLEM
1. An appliance has a resistance of 20.0 Ω. It uses 30.0 V, supplied by the secondary coil of a
transformer. The transformer has 200 turns in its primary coil and is plugged into a 120-V source.
Calculate
(a) the number of turns in the secondary coil.
(b) the current in the secondary coil.
ANS:
Rs = 20.0 Ω
Vs = 30.0 V
Vp = 120 V
Np = 200
Ns = ?
Is = ?
(a)
There are 50 turns in the secondary coil.
(b)
The current in the secondary coil is 1.5 A.
2. An appliance has a resistance of 15.0 Ω. It uses 30.0 V, supplied by the secondary coil of a
transformer. The transformer has 200 turns in its primary coil and is plugged into a 120-V source.
Calculate
(a) the current in the secondary coil
(b) the current in the primary coil
(c) the power rating of the transformer
(d) the amount of energy the appliance uses in one minute
ANS:
Rs = 15.0 Ω
Vs = 30.0 V
Vp = 120 V
Np = 200
∆t = 1 min = 60.0 s
Is = ?
Ip = ?
P=?
∆E = ?
(a)
The current in the secondary coil is 2.00 A.
(b)
The current in the secondary coil is 0.50 A
(c)
The power rating is 60.0 W.
(d)
It used 3.60 × 103 J of energy.
3. A transformer has 3.0 × 103 turns in its primary coil and 5.0 × 103 turns in its secondary coil. If the
potential difference in the primary coil is 15.0 V, what is the potential difference in the secondary
coil?
ANS:
Np = 3.0 × 103
Ns = 5.0 × 103
Vp = 15.0 V
Vs = ?
The potential difference in the secondary coil is 25 V.
4. A welding machine uses a current of 75.0 A with a potential difference of 60.0 V. If it is supplied
by a transformer connected to a 240 V source, calculate the power rating of the welding machine.
ANS:
Is = 75.0 A
Vs = 60.0 V
Vp = 240 V
P=?
The power rating is 4.5 × 103 W.
5. Calculate the power loss in a wire that has a resistance of 3.0 Ω and transmits 23.0 kW of power
with a potential difference of 3.45 × 104 V.
ANS:
R = 3.0 Ω
P = 23.0 kW
V = 3.45 × 10 4 V
Ploss = ?
The power loss in the wire is 1.33 W.
6. In Spain, the standard electric current has a potential difference of 220 V. Canadian appliances,
designed for a 120-V current, can be made to work safely if they are first connected to a
transformer. If a 1750-W Canadian hair dryer is brought to Spain,
(a) What ratio of loops is needed in the primary and secondary coils of the transformer for the hair
dryer to work properly?
(b) How much current will the hair dryer use if it is connected to the transformer?
(c) How much current would the dryer use if it is plugged directly into the wall outlet in Spain?
ANS:
Vp = 220 V
Vs = 120 V
Ps = 1750 W
Is = ?
I=?
(a)
The ratio must be 1.8 : 1 (Np : Ns).
(b)
The hair dryer will use 14.6 A of current if connected to the transformer.
(c)
The hair dryer would initially operate at 27 A, but would quickly overheat and
stop.
7. An outdoor heater has a resistance of 50.0 Ω. It operates at 275 V. A transformer connected to a
120-V source supplies electricity to the heater. What is the power used by the transformer?
ANS:
R = 50.0 Ω
Vs = 275 V
Vp = 120 V
P=?
The power used by the transformer is 1.51 × 103 W.
8. A transformer has three different secondary coils that can be adjusted to any one at a time. When a
voltmeter is connected to a circuit supplied by the transformer, the first setting produces a reading
of 5.6 V, the second setting a reading of 4.0 V, and the third setting a reading of 2.5 V. If the
secondary coil with the fewest loops has 100 turns, how many turns are on each of the other coils?
ANS:
V1 = 5.6 V
V2 = 4.0 V
V3 = 2.5 V
N1 : N2 : N3 = ?
N1 = 2.24 × 100 = 224 loops
N2 = 1.6 × 100 = 160 loops
The first coil has 224 loops and the second coil has 160 loops.
9. How long will it take, in minutes, for a transformer to transfer 2.3 × 106 J of energy from a 120-V
circuit to a 345-V circuit with a current of 1.5 A?
ANS:
Vp = 120 V
Vs = 345 V
∆E = 2.3 × 106 J
∆t = ?
It will take 74 min to transfer the energy from one circuit to the other.
ESSAY
1. Explain how Lenz reasoned that an induced magnetic field in a coil opposes the inducing field of a
bar magnet.
ANS:
-If the induced field had the same polarity as the bar magnet, the magnet would be attracted to it,
and pulled inside the coil.
-Since the magnet would be moving faster, a greater current would then be induced in the coil.
-The induced current would produce an even greater induced field in the coil and the current would
continue to increase without any outside energy input.
-This is a violation of the law of conservation of energy.
-The induced pole must oppose the inducing magnet.
2. Explain how the structure and arrangement of slip rings on the armature assists in the production of
an AC current from a generator.
ANS:
The slip rings are two separate conducting rings of metal, attached around the pivot point of the
armature. One ring is attached to each end of the coil on the armature. Current cannot transfer from
one ring to the other without first passing through the coil. One brush is always in contact with the
same ring. As the armature turns, the induced current changes direction every half-cycle. The
current alternates between the slip rings, leaving the coil through the brush connected to it.
3. Why must a transformer be supplied with an alternating current before it will work?
ANS:
A transformer contains two coils. The current in one coil produces a magnetic field to induce the
current to flow in the second coil. However, the magnetic field must keep changing or the current
flow will stop. A direct current, which always flows in the same direction, cannot produce a
changing magnetic field. An alternating current changes direction every half cycle, so the magnetic
field it produces in a coil is always changing
4. Describe why transformers are important to the safe and efficient distribution of electrical energy.
ANS:
Electricity is most efficiently transmitted (with the least amount of energy lost as heat) at high
potential difference and low current. Whatever the initial potential difference of a current, a
transformer can be used to convert it to a higher potential before it is transmitted. When the
electricity arrives at a home, another transformer can convert the high voltage to a safer, lower one.
5. Why is electrical energy transmitted in an alternating current rather than a direct current?
ANS:
Electricity must be transmitted at high voltages and low current for minimum energy loss during
transmission. However, the current must be provided to homes with a safer, lower potential
difference. To convert the potential, a transformer is used. However, it can operate only with an
alternating current—the magnetic field must constantly be changing in order for the current to be
induced in the secondary coil. Direct current always travels in the same direction and, therefore,
cannot produce a changing magnetic field.