Induction
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1. A uniform 4.5-T magnetic field passes perpendicularly through the plane of a wire loop 0.10 m 2 in area. What
flux passes through the loop?
c. 0.25 Tm2
a. 5.0 Tm2
b. 0.45 Tm2
d. 0.135 Tm2
____
2. According to Lenz’s law the direction of an induced current in a conductor will be that which tends to
produce which of the following effects?
a. enhance the effect which produces it
c. produce the greatest voltage
b. produce a greater heating effect
d. oppose the effect which produces it
____
3. The “back emf” of a motor refers to a source of voltage that:
a. occurs when the motor runs backwards.
c. is biggest when the current through the
motor is biggest.
b. occurs when the motor is used as a
d. is biggest when the motor turns fastest.
generator.
____
4. When a voltage is generated by rotating a coil in a magnetic field at a constant rate, the period of the voltage
equals the time that it takes for the coil to rotate through ________ radians.
a. 1
c. 
b. /2
d. 2
____
5. An inductor, battery, resistance, and ammeter and switch are connected in series. If the switch, initially open,
is now closed, what is the current’s final value?
a. zero
c. battery voltage times inductance
b. battery voltage divided by inductance
d. battery voltage divided by resistance
____
6. In a circuit made up of inductor, resistance, ammeter, battery and switch in series, at which of the following
times after the switch is closed is the rate of current increase greatest?
a. zero
c. reciprocal of one time constant
b. one time constant
d. ten time constants
____
7. How is the energy stored in a current-carrying inductor related to its self-inductance, L?
c. directly proportional to L
a. directly proportional to L2
d. inversely proportional to L
b. directly proportional to L1/2
____
8. How is the energy stored in a current-carrying inductor related to the current value, I?
c. directly proportional to I
a. directly proportional to I 2
d. inversely proportional to I
b. directly proportional to I 1/2
____
9. A uniform 4.5-T magnetic field passes through the plane of a wire loop 0.10 m 2 in area. What flux passes
through the loop when the direction of the 4.5-T field is at a 30° angle to the normal of the loop plane?
a. 5.0 Tm2
c. 0.39 Tm2
2
b. 0.52 Tm
d. 0.225 Tm2
1
____ 10. A square coil, enclosing an area with sides 2.0 cm long, is wrapped with 2 500 turns of wire. A uniform
magnetic field perpendicular to its plane is turned on and increases to 0.25 T during an interval of 1.0 s. What
average voltage is induced in the coil?
a. 0.25 V
c. 2.0 V
b. 0.12 V
d. 2.5 V
____ 11. A 10-turn square coil of area 0.036 m 2 and a 20-turn circular coil are both placed perpendicular to the same
changing magnetic field. The voltage induced in each of the coils is the same. What is the area of the circular
coil?
c. 0.018 m2
a. 0.072 m2
2
b. 0.60 m
d. 0.036 m2
____ 12. A flat coil of wire consisting of 20 turns, each with an area of 50 cm 2, is positioned perpendicularly to a
uniform magnetic field that increases its magnitude at a constant rate from 2.0 T to 6.0 T in 2.0 s. If the coil
has a total resistance of 0.40 , what is the magnitude of the induced current?
a. 70 mA
c. 500 mA
b. 140 mA
d. 800 mA
____ 13. A planar loop consisting of four turns of wire, each of which encloses 200 cm 2, is oriented perpendicularly to
a magnetic field that increases uniformly in magnitude from 10 mT to 25 mT in a time of 5.0 ms. What is the
resulting induced current in the coil if the resistance of the coil is 5.0 ?
a. 60 mA
c. 0.24 mA
b. 12 mA
d. 48 mA
____ 14. A 0.200-m wire is moved parallel to a 0.500-T magnetic field at a speed of 1.50 m/s. What emf is induced
across the ends of the wire?
a. 2.25 V
c. 0.600 V
b. 1.00 V
d. zero
____ 15. An airplane with a wingspan of 60.0 m flies parallel to the Earth’s surface at a point where the downward
component of the Earth's magnetic field is 0.400  l04 T. If the induced potential between wingtips is 0.900
V, what is the plane’s speed?
a. 250 m/s
c. 375 m/s
b. 338 m/s
d. 417 m/s
____ 16. A bar magnet is falling through a loop of wire with constant velocity. The north pole enters first. As the south
pole leaves the loop of wire, the induced current (as viewed from above) will be:
a. clockwise.
c. zero.
b. counterclockwise.
d. along the length of the magnet.
____ 17. The wiring in a motor has resistance of 3.0  and produces a back emf of 1.0 V when connected to a 9.0-V
battery of negligible resistance. Find the current flow through the motor.
a. 0.19 A
c. 1.5 A
b. 0.44 A
d. 2.7 A
____ 18. A motor with a coil resistance of 10  is attached to a voltage supply of 90 V. What is the current in the
motor when it is running at its maximum speed with a back emf of 60 V?
a. zero
c. 6.0 A
b. 3.0 A
d. 15 A
2
____ 19. What is the minimum frequency with which a 200-turn, flat coil of cross sectional area 300 cm 2 can be rotated
in a uniform 30-mT magnetic field if the maximum value of the induced emf is to equal 8.0 V?
a. 0.030 Hz
c. 7.1 Hz
b. 2.9 Hz
d. 8.4 Hz
____ 20. A coil is rotated in a magnetic field generating an emf given by emf = NBA sin , where  = t. At which
angle  is the emf half of its maximum value?
a. 0°
c. 45°
b. 30°
d. 60°
____ 21. A motor has an internal resistance of 12 . When running, the motor has a back emf of 30 V and draws a
current of 4.0 A. What is the supply voltage in this case?
a. 30 V
c. 78 V
b. 48 V
d. 120 V
____ 22. The current in a coil with a self-inductance of 1.5 mH increases from 0 to 1.0 A in a tenth of a second. What
is the induced emf in the coil?
a. 15 mV
c. 0.10 V
b. 30 mV
d. 0.30 V
____ 23. In a circuit made up of inductor L, resistance R, ammeter, battery and switch in series, the current is greatest at
which of the following times, as measured after the switch is closed?
a. zero
c. at a time t = L/R
b. one time constant
d. ten time constants
____ 24. A coil with a self-inductance of 0.75 mH experiences a constant current buildup from zero to 10 A in 0.25 s.
What is the induced emf during this interval?
a. 0.045 V
c. 0.47 V
b. 0.030 V
d. 0.019 V
____ 25. What is the self-inductance in a coil that experiences a 3.0-V induced emf when the current is changing at a
rate of 110 A/s?
a. 83 mH
c. 37 mH
b. 45 mH
d. 27 mH
____ 26. By what factor is the self-inductance of an air solenoid changed if only its number of coil turns, N, is tripled?
a. 1/3
c. 6
b. 3
d. 9
____ 27. By what factor is the self-inductance of an air solenoid changed if its length and number of coil turns are both
tripled?
a. 1/3
c. 6
b. 3
d. 9
____ 28. Two loops of wire are arranged so that a changing current in one, the primary, will induce a current in the
other, the secondary. The secondary loop has twice as many turns as the primary loop. As long as the current
in the primary is steady at 3 A, the current in the secondary will be:
a. 3 A.
c. 1.5 A.
b. 6 A.
d. zero.
3
____ 29. Two loops of wire are arranged so that a changing current in one, the primary, will induce a current in the
other, the secondary. The secondary loop has twice as many turns as the primary loop. The current in the
primary at this moment is 3 A and increasing. The current in the secondary must be:
a. 3 A.
c. zero.
b. 6 A.
d. There is insufficient information to work
this problem.
____ 30. An air-core inductor has 1 000 turns/m and an internal volume of 3.0 cm 3. What is its inductance?
a. 3.8 H
c. 0.38 H
b. 38 mH
d. Insufficient information is given.
____ 31. An RL series circuit has the following components: 5.0-mH coil, 1.0- resistor, 12-V battery, ammeter and
switch. What is the time constant of this circuit?
c. 2.5  102 s
a. 12  103 s
3
b. 5.0  10 s
d. 200 s
____ 32. An RL series circuit has: 5.0-mH coil, 1.0- resistor, 12-V battery, ammeter and switch. After the switch is
closed for a long time, find the final value of the current.
a. 2.5 A
c. 0.015 A
b. 12 A
d. 2 400 A
____ 33. A 12-V battery is connected in series with a switch, resistor and coil. If the circuit’s time constant is 2.0 
104 s and the final steady current after the switch is closed becomes 1.0 A, what is the value of the
inductance?
a. 1.2 mH
c. 9.6 mH
b. 2.4 mH
d. 48 mH
____ 34. A series circuit contains a 12-V battery, a 2.0- resistor, and a 3.0-mH inductor. If the switch to the battery is
closed at t = 0, find the time required for the current in the circuit to reach 63% of its final value.
a. 1.5 ms
c. 4.0 ms
b. 3.0 ms
d. 5.0 ms
____ 35. A circuit consists of a 10-mH coil, a 12- resistor, a 6.0- resistor, a 9.0-V battery and a switch, all in series.
What is the time constant of this circuit?
c. 1.4  10-4 s
a. 9.0  10-2 s
b. 2.5  10-3 s
d. 5.6  10-4 s
____ 36. An emf of 0.32 V is induced in a 0.40-H inductor. What is the rate of change of current through the inductor?
a. 0.80 A/s
c. 1.3 A/s
b. 0.13 A/s
d. 0.64 A/s
____ 37. What is the stored energy in a 0.50-mH coil carrying a current of 4.0 A?
c. 8.0  103 J
a. 2.0  103 J
b. 4.0  103 J
d. 12  103 J
____ 38. A 12-V battery is connected in series with a switch, 6.0- resistor and coil. What energy is stored in the coil
when the current is 2.0 A? The time constant is 4.0  104 s.
a. 4.8  103 J
c. 14  103 J
b. 9.6  103 J
d. 29  103 J
4
____ 39. Superconductors have been discussed as a means for electrical energy storage. Because they are
resistanceless, a current once started in a loop would continue without loss. If a current of 1.0  104 A were
started in a huge toroidal coil of radius 1.0 km and inductance 50 H, how much electrical energy (in kWh)
could be stored?
a. 300 kWh
c. 690 kWh
b. 480 kWh
d. 840 kWh
____ 40. An RL circuit has L = 0.40 H and R = 5.0 . It is connected to a battery with  = 22 V at time t = 0. Find the
energy stored in the inductor when the current in the circuit is 0.50 A.
a. 50 mJ
c. 2.0 J
b. 1.0 J
d. 5.0 J
5
ID: A
Induction
Answer Section
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