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Chapter 20 (Electromagnetic Induction)
1. A sensitive galvanometer is connected to a wire loop and placed within the magnetic field of a strong horseshoe magnet. The galvanometer shows a deflection when:
a. the wire is moved parallel to the field b. the wire is moved perpendicularly to the field c. neither wire nor magnet are moving d. the wire's axis is parallel to the field
2. The operation of a tape player to play music depends on which of the following?
a. the Doppler effect b. the photoelectric effect c. the force acting on a current‑carrying wire in a magnetic field d. induced current from the motion of a magnet past a wire
3. The operation of an electric motor depends on which of the following effects?
a. the Doppler effect b. the photoelectric effect c. the force acting on a current‑carrying wire in a magnetic field d. current from the motion of a wire in a magnetic field
4. 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 b. produce a greater heating effect c. produce the greatest voltage d. oppose the effect which produces it
5. 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 mA b. 30 mA c. 0.10 A d. 0.30 A
6. The self‑inductance of a solenoid increases under which of the following conditions?
a. solenoid length is increased b. cross sectional area is decreased c. number of coils per unit length is decreased d. number of coils is increased
7. The wiring in a motor has resistance of 3.0 Ohm and produces back emf of 1.0 V when connected to a 9 V battery of negligible resistance.Find current flow through the motor.
a. 0.19 A b. 0.44 A c. 1.5 A d. 2.7 A
8. 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 b. battery voltage divided by inductance c. battery voltage times inductance d. battery voltage divided by resistance
9. What is the stored energy in a 0.50 mH coil carrying a current of 4.0 A?
a. 2.0 ´ 10‑3 J b. 4.0 ´ 10‑3 J c. 8.0 ´ 10‑3 J d. 12.0 ´ 10‑3 J
10. In a primary/secondary coil combination, the current in the secondary is a maximum at the moment that which one of the following conditions is met in the primary?
a. current is maximum in positive direction b. current is maximum in negative direction c. rate of current change is maximum d. voltage is maximum in positive direction
11. The principle or law that says “an induced emf in a circuit loop produces a current whose magnetic field opposes further change of magnetic flux” is credited to:
a. Faraday b. Lenz c. Ampere d. Volta
12. The basic function of the electric generator is which of the following processes?
a. mechanical energy to electrical b. electrical energy to mechanical c. low voltage to high or vice versa d. alternating current to direct
13. The function of the electric motor is which one of the following conversion processes?
a. mechanical energy to electrical b. electrical energy to mechanical c. low voltage to high or vice versa d. alternating current to direct 14. The back emf in an electric motor is its maximum value under which condition?
a. motor speed is zero b. current is a maximum c. voltage is a maximum d. motor speed is maximum
15. In a circuit made up of inductor L, resistance R, ammeter, battery and switch in series, the current is greatest at which time interval, as measured after the switch is closed?
a. zero b. one time constant c. at a time t = L/R d. ten time constants
16. In a circuit made up of inductor, resistance, ammeter, battery and switch in series, at which time interval after the switch is closed is the rate of current increase greatest?
a. zero b. one time constant c. reciprocal of one time constant d. ten time constants
17. How is the energy stored in a current carrying inductor related to its self‑inductance, L?
a. directly proportional to L2 b. directly proportional to L1/2 c. directly proportional to L d. inversely proportional to L
18. How is the energy stored in a current carrying inductor related to the current value, I?
a. directly proportional to I2 b. directly proportional to I1/2 c. directly proportional to I d. inversely proportional to I
19. A uniform 1.5 T magnetic field passes perpendicular through the plane of a wire loop 0.3 m in area. What flux passes through the loop?
a. 5 Wb b. 0.45 Wb c. 0.25 Wb d. 0.135 Wb
20. A uniform 1.5 T magnetic field passes through the plane of a wire loop 0.3 m2 in area. What flux passes through the loop when the direction of the 1.5 T field is at a 30˚ angle to the normal of the loop plane?
a. 5 Wb b. 0.52 Wb c. 0.39 Wb d. 0.225 Wb 21. A square coil with sides 2 cm long is wrapped with 2500 turns of wire. A uniform magnetic field perpendicular to its plane is turn 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 b. .125 V c. 2.0 V d. 2.5 V
22. A 500 turn circular coil with an area of 0.05 m2 is mounted on a rotating frame which turns at a rate of 20 rad/s in the presence of a 0.05 T uniform magnetic field which is perpendicular to the axis of rotation. What is the instantaneous emf in the coil at the moment that the normal to its plane is parallel to the field?
a. zero b. 125 V c. 216 V d. 250 V
23. A 500 turn circular coil with an area of 0.05 m2 is mounted on a rotating frame which turns at a rate of 20 rad/s in the presence of a 0.05 T uniform magnetic field which is perpendicular to the axis of rotation. What is the instantaneous emf in the coil at the moment that the normal to its plane is at a 90˚ angle to the field?
a. zero b. 12.5 V c. 21.6 V d. 25.0 V
24. A 500 turn circular coil with an area of 0.05 m2 is mounted on a rotating frame which turns at a rate of 20 rad/s in the presence of a 0.05 T uniform magnetic field which is perpendicular to the axis of rotation. What is the instantaneous emf in the coil at the moment that the normal to its plane is at a 30˚ angle to the field?
a. zero b. 12.5 V c. 21.6 V d. 25.0 V
25. A 0.20 m wire is moved parallel to a 0.5 magnetic field at a speed of 1.5 m/s. What emf is induced across the ends of the wire?
a. 2.25 V b. 1.0 V c. 0.60 V d. zero 26. An airplane with a wingspan of 60 m flies parallel to the earth’s surface at a point where the downward component of the earth's magnetic field is 0.4 ´ l0-4 T. If the induced potential between wingtips is 0.90 V, what is the plane’s speed?
a. 250 m/s b. 338 m/s c. 375 m/s d. 417 m/s
27. A motor with a coil resistance of 10 ohms is supplied by a voltage 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 b. 3.0 A c. 6.0 A d. 15.0 A
28. A coil with a self‑inductance of 0.75 mH experiences a constant current build up from zero to 5.0 A in 0.125 s. What is the induced emf during this interval?
a. 0.045 V b. 0.03 V c. 0.47 V d. 0.019 V
29. What is the self‑inductance in a coil which experiences a 1.5 V induced emf when the current is changing at a rate of 55 A/s?
a. 83 mH b. 45 mH c. 37 mH d. 27 mH
30. By what factor is the self‑inductance of an air solenoid changed if its number of coil turns, N. is tripled?
a. 1/3 b. 3 c. 6 d. 9
31. By what factor is the self‑inductance of an air solenoid changed if its cross‑sectional area, A, is tripled?
a. 1/3 b. 3 c. 6 d. 9 32. 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 b. 3 c. 6 d. 9
33. An RL series circuit has the following components: 2.5 mH coil, 0.5 W resistor, 6.0 V battery, ammeter and switch. What is the time constant of this circuit?
a. 12.5 ´ 10‑3s b. 5 ´ 10‑3s c. 2.5 ´ 10‑2s d. 200s
34. An RL series circuit has: 2.5 mH coil, 0.5 W resistor, 6.0 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 b. 12 A c. 0.015 A d. 2400 A
35. A 12 V battery is connected in series with a switch, resistor and coil. If the circuit’s time constant is 4 ´ 10‑4s and the final steady current after the switch is closed becomes 2.0 A, what is the value of the inductance?
a. 1.2 mH b. 2.4 mH c. 9.6 mH d. 48 mH
36. A 12 V battery is connected in series with a switch, 6 W resistor and coil. What energy is stored in the coil when the current is 2.0 A? The time constant is 4 ´ 10-4s.
a. 4.8 ´ 10‑3 J b. 9.6 ´ 10‑3 J c. 14.4 ´ 10‑3 J d. 29 ´ 10‑3 J 37. An electric motor draws a current of 2.5 A from a 15 V battery when it runs at normal speed. If the motor’s armature is “jammed” so that it cannot rotate, the current suddenly rises to 10 A. What is the back emf of the motor when running at normal speed?
a. 18.8 V b. 10.0 V c. 12.5 V d. 11.3 V
38. A loop of wire is held in a vertical position at the equator. The axis of the loop points in the east‑west direction. Which change will induce the greatest current in the loop?
a. raising the loop to a higher elevation b. moving the loop north c. rotating the loop so its axis is vertical d. rotating the loop so its axis is north‑south
39. A bar magnet is falling through a loop of wire with constant velocity. The south pole enters first. As the magnet leaves the wire, the induced current (as viewed from above):
a. is clockwise b. is counterclockwise c. is zero d. is along the length of the magnet
40. A bar magnet is falling through a loop of wire with constant velocity. The north pole enters first. The induced current will be greatest when the magnet is located so that:
a. the loop is near either the north or the south pole. b. the loop is near the north pole only c. the loop is near the middle of the magnet. d. with no acceleration, the induced current is zero.
41. 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 b. counterclockwise c. zero d. along the length of the magnet
42. Two loops of wire are arranged so that a changing current in one will induce a current in the other. If the current in the first is increasing clockwise by one amp every second, the current in the second loop will:
a. be increasing counterclockwise b. stay constant c. increase clockwise also d. stay zero
43. 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 amps, the current in the secondary will be:
a. 3 A b. 6 A c. 1.5 A d. zero
44. 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 b. 6 A c. zero d. there is insufficient information to work this problem.
45. A metal rod is falling toward the surface of the earth near the equator. As it falls, one end of the rod becomes positively charged due to the motional emf of the rod through the earth's magnetic field. The rod is oriented so that:
a. the rod is vertical with the positive end higher. b. the rod is horizontal with the positive end toward the north. c. the rod is horizontal with the positive end toward the east. d. the rod is horizontal with the positive end toward the west
46. The magnetic field going through a stationary circular loop increases from zero to 5 T in a certain time t. The induced current in the loop will depend on the radius of the loop, r, the resistance of the loop, R, and the time, t. If two of these stay constant, the induced current will be directly proportional to:
a. t b. r c. R d. none of these
47. If a bar magnet is falling through a loop of wire, the induced current in the loop of wire sets up a field which exerts a force on the magnet. This force between the magnet and the loop will be attractive when:
a. the magnet enters the loop b. the magnet is half way through c. the magnet is leaving the loop d. never 48. Electricity may be generated by rotating a loop of wire between the poles of a magnet. The induced current is greatest when:
a. the plane of the loop is parallel to the magnetic field b. the plane of the loop is perpendicular to the magnetic field c. the magnetic flux through the loop is a maximum d. the plane of the loop makes an angle of 45˚ with the magnetic field
49. The “back emf” of a motor refers to a source of voltage that:
a. occurs when the motor runs backwards b. occurs when the motor is used as a generator c. is biggest when the current through the motor is biggest d. is biggest when the motor turns fastest
50. The magnet moving past an object will produce eddy currents in the object if the object:
a. is magnetic material only b. is a conductor c. is an insulator d. is a liquid
51. A flat coil of wire consisting of 20 turns, each with an area of 50 cm2, 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 W, what is the magnitude of the induced current?
a. 70 mA b. 140 mA c. 500 mA d. 800 mA
52. A planar loop consisting of four turns of wire, each of which encloses 200 cm2, 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 W?
a. 60 mA b. 12 mA c. 0.24 mA d. 48 mA
53. A rectangular loop (area = 0.15 m2) turns in a uniform magnetic field with B = 0.20 T. At an instant when the angle between the magnetic field and the normal to the plane of the loop is π/2 rads and increasing at the rate of 0.60 rad/s, what is the magnitude of the emf induced in the loop?
a. 24 mV b. zero c. 18 mV d. 30 mV
54. A circular loop (area = 0.20 m2) turns in a uniform magnetic field with B = 0.13 T. At an instant when the angle between the magnetic field and the normal to the plane of the loop is π rads and is decreasing at the rate of 0.50 rad/s, what is the magnitude of the emf induced in the loop?
a. zero b. 13 mV c. 26 mV d. 20 mV
55. A large jetliner with a wing span of 40 m flies horizontally and due north at a speed of 300 m/s in a region where the magnetic field of the earth is 60 µT directed 50˚ below the horizontal. What is the magnitude of the induced emf between the ends of the wing?
a. 250 mV b. 350 mV c. 550 mV d. 750 mV
56. What is the minimum frequency with which a 200‑turn, flat coil of cross sectional area of 300 cm2 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. .03 Hz b. 2.9 Hz c. 8.0 Hz d. 8.4 Hz
57. A series circuit contains a 12‑V battery, a 2000 W resistor, and a 3 µH 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 b. 3.0 ms c. 4.0 ms d. 5.0 ms
58. 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 104 A were started in a huge toroidal coil of radius 1 km and inductance 50 H. how much electrical energy (in kW hours) could be stored?
a. 300 kWh b. 482 kWh c. 694 kWh d. 842 kWh
59. An RL circuit has L = 0.4 H and R = 5 W. It is connected to a battery with E = 22 V at time t = 0. Find the energy stored in the inductor when the current in the circuit =0.5 A.
a. 50 mJ b. 1.0 J c. 2.0 J d. 5.0 J
60. You can calculate the induced current in a ring by setting the emf from a changing flux E = –Dj/Dt equal to the voltage change –LDI/Dt in an inductor. If the magnetic field through a ring of area 1 cm2 collapses from 0.02 T to zero and the inductance of the ring is 10‑7H. what is the induced current in the ring?
a. 5 A b. 10 A c. 15 A d. 20 A
Notes:
# Ans Difficulty # Ans Difficulty
1. B 1 31. B 1 2. D 1 32. B 2 3. C 1 33. B 2 4. D 1 34. B 2 5. A 2 35. B 2 6. D 1 36. A 2 7. D 2 37. D 3 8. D 1 38. C 2 9. B 2 39. B 2 10. C 1 40. B 2 11. B 1 41. A 2 12. A 1 42. A 3 13. B 1 43. D 2 14. D 1 44. D 2 15. D 2 45. C 2 16. A 1 46. D 2 17. C 1 47. C 2 18. A 1 48. A 1 19. B 2 49. D 1 20. C 2 50. B 1 21. A 2 51. C 2 22. A 3 52. D 2 23. D 3 53. C 3 24. B 3 54. A 3 25. D 2 55. C 3 26. C 2 56. B 2 27. B 2 57. A 2 28. B 2 58. C 2 29. D 2 59. A 2 30. D 1 60. D 3
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