Electromagnetic Induction Exam Prep Giambattista Ch.20

Physics, 9e (Giambattista)

Chapter 20 Electromagnetic Induction

1) A metal rod of length L moves with velocity v, perpendicular to its length, in a magnetic field B, which is perpendicular to both the rod and its velocity. If the length of the rod is doubled, what happens to the electric field in the rod?

A) It stays the same.

B) It doubles.

C) It quadruples.

D) It halves.

E) It quarters.

2) A metal rod of length 2.0 m is moved at 6.0 m/s in a direction perpendicular to its length. A 5.0 mT magnetic field is perpendicular to both the rod and its velocity. What is the magnitude of the electric field in the rod?

A) 12 mV/m

B) 60 mV/m

C) 30 mV/m

D) 15 mV/m

E) 0.0 mV/m

3) A metal rod of length 2.0 m is moved at 6.0 m/s in a direction perpendicular to its length. A 5.0 mT magnetic field is perpendicular to both the rod and its velocity. What is the potential difference between the ends of the rod?

A) 12 mV

B) 60 mV

C) 30 mV

D) 15 mV

E) 0 mV

4) A metal rod of length 2.0 m is moved at 6.0 m/s in a direction perpendicular to its length. A 5.0 mT magnetic field is perpendicular to both the rod and its velocity. What is the magnetic flux swept through per second by the rod?

A) 0.0

B) 15 mT·m2

C) 30 mT·m2

D) 60 mT·m2

E) 12 mT·m2

5) A metal rod of length 2.0 m is moved at 6.0 m/s in a direction perpendicular to its length. A 5.0 mT magnetic field is perpendicular to both the rod and its velocity. If the resistance of the rod is 15 mΩ, what is the current in the rod?

A) 0.0 A

B) 1.0 A

C) 2.0 A

D) 4.0 A

E) 8.0 A

6) A conducting rod is free to slide on horizontal metal rails without friction. At a particular time, it is sliding with velocity v to the right, as shown. A uniform magnetic field B is directed into the page, the separation of the rails is L, and the resistance of the circuit is R. In what direction in the diagram is the current flowing in the resistor at this time?

A) up

B) down

C) up then down

D) down then up

E) Insufficient information given.

7) A conducting rod is free to slide horizontally on frictionless rails separated by 2.0 m. At a particular time, the rod is sliding at 3.0 m/s to the right, as shown. There is a uniform magnetic field of magnitude 0.66 mT directed into the page. If the resistance R is 220 mΩ, what is the current in the circuit at this time?

A) 18 mA

B) 9.0 mA

C) 87 μA

D) 3.0 μA

E) 4.5 mA

8) A conducting rod is free to slide on horizontal, frictionless rails separated by 2.0 m. At a particular time, the rod is sliding at 4.0 m/s to the right, as shown. A uniform magnetic field is directed into the page. If a current of 40 mA is generated in the circuit at this time, and the resistance R is 0.36 Ω, what is the magnitude of the magnetic field between the rails?

A) 0.028 T

B) 0.11 T

C) 14 mT

D) 3.6 mT

E) 1.8 mT

9) The emf produced by an ac generator is given by ε = ωNBA sin ωt. In SI units, what is the value of ω for a frequency of 60.0 Hz?

A) 60.0 Hz

B) 120 rad/s

C) 2.00π rad/s

D) 377 rad/s

E) 377 Hz

10) The SI unit H (the henry) is equal to a

A) V/m.

B) V/A.

C) V·s.

D) V·s/A.

E) V·A/s.

11) For the typical household electricity in the United States and Canada, what is the amplitude of the provided emf?

A) 120 V

B) 240 V

C) 60.0 V

D) 170 V

E) 330 V

12) A generator has 200 turns of wire on a coil of radius 2.00 cm. What rotational speed is necessary to produce an emf of amplitude 4.50 V if the magnetic field in the region of the coil is 0.180 T?

A) 60.0 rad/s

B) 16.0 rpm

C) 100 rpm

D) 500 rpm

E) 950 rpm

13) If the rotation rate of a generator coil is doubled, what happens to the peak emf?

A) It stays the same.

B) It doubles.

C) It quadruples.

D) It halves.

E) It quarters.

14) The ac motor and generator were invented by

A) Thomas Edison.

B) Nikola Tesla.

C) George Westinghouse.

D) James Clerk Maxwell.

E) Heinrich Hertz.

15) The unit of magnetic flux is the weber. 1 Wb equals

A) 1 V·m.

B) 1 V·s/A.

C) 1 T·m2.

D) π lines/m2.

16) Faraday's law of induction states that the emf induced in a rectangular loop of wire is proportional to

A) the magnetic flux.

B) the rate of change of the magnetic flux.

C) current divided by the time.

D) the magnetic flux density times the area of the loop.

17) A flat coil has a maximum flux of 4.0 × 10-3 T·m2 through it, with its area perpendicular to a uniform magnetic field. When the coil is rotated 60° (so that its normal makes a 60° angle with the field), what is the flux through it?

A) 4.0 × 10-3 T·m2

B) 8.0 × 10-3 T·m2

C) 3.5 × 10-3 T·m2

D) 2.0 × 10-3 T·m2

E) 6.9 × 10-3 T·m2

18) A coil is rotated in a magnetic field about an axis perpendicular to both the field and the coil axis (normal to the coil area). During each rotation, how many times does the flux through the coil equal zero?

A) 0

B) 1

C) 2

D) 4

E) more than 4

19) A 20 turn coil of area 10 cm2 is placed in a magnetic field so that the normal to the coil's area is in the same direction as the field. If the field has a value of 0.25 T, what is the total flux through the coil?

A) 0.0

B) 5.0 × 10-3 T·m2

C) 25 × 10-5 T·m2

D) 50 T·m2

E) 25 T·m2

20) A 20 turn coil of area 10 cm2 is placed in a magnetic field so that the normal to the coil's area is in the same direction as the field. If the magnitude of the field increases from 0.25 T to 0.35 T in 2.0 s, what is the average emf induced in the coil?

A) 7.0 mV

B) 2.0 mV

C) 1.0 mV

D) 3.5 mV

E) 4.0 mV

21) A horizontal desk surface measures 1.3 m by 1.0 m. If the Earth's magnetic field has a magnitude of 0.44 mT and is directed 65° below the horizontal, what is the magnetic flux through the desk surface?

A) 1.1 × 10-3 Wb

B) 5.2 × 10-4 Wb

C) 3.2 × 10-4 Wb

D) 7.3 × 10-3 Wb

22) Suppose an emf is induced in a loop of wire by rotating the loop in a magnetic field. If the diameter of the loop were twice as large, how would the induced emf compare?

A) The induced emf would be 4 times as much.

B) The induced emf would be twice as much.

C) There would be no change in the induced emf.

D) The induced emf would be 6.28 times as much.

23) The north end of a bar magnet is pushed downward toward a wire loop in the plane of the paper. In which direction is the induced current, and which way is the induced magnetic field?

A) clockwise, into the paper

B) clockwise, out of the paper

C) counter-clockwise, into the paper

D) counter-clockwise, out of the paper

E) There is no induced current.

24) The south end of a bar magnet is pushed downward toward a wire loop in the plane of the paper. In which direction is the induced current, and which way is the induced magnetic field?

A) clockwise, into the paper

B) clockwise, out of the paper

C) counter-clockwise, into the paper

D) counter-clockwise, out of the paper

E) There is no induced current.

25) A circular coil lays flat on a horizontal table in a region where the magnetic field is straight down. The magnetic field is suddenly removed. When viewed from above, what is the direction of the current in the coil as the field is removed?

A) clockwise

B) clockwise initially, then counter-clockwise before going to zero

C) counter-clockwise

D) There is no induced current.

26) The back emf of an electric motor is greatest

A) when the motor is just starting up.

B) when the motor is halfway to operating speed.

C) when the motor is at 71% of the operating speed.

D) when the motor is at full operating speed.

E) at all times since it is a constant when the motor is running at any speed.

27) A dc motor is connected to an emf of 12.0 V. The resistance of its windings is 2.0 Ω. At normal operating speed, the motor delivers 6.0 W of mechanical power. What current does it draw at normal operating speed?

A) 6.0 A

B) 0.50 A

C) 1.0 A

D) 3.0 A

28) If the number of turns on a transformer input is greater than the number on the output, then the transformer

A) outputs greater voltage than at the input.

B) outputs less current than at the input.

C) outputs greater voltage and current than at the input.

D) outputs less voltage than at the input.

E) increases the voltage but does not affect the current.

29) A transformer has 200 turns on its primary and 12 turns on its secondary. If the input voltage is 2000 V, what is the output voltage?

A) 3.3 × 104 V

B) 240 V

C) 170 V

D) 120 V

E) 100 V

30) An ideal transformer has 200 turns on its primary and 12 turns on its secondary. If the power input to the transformer is 120 kW, what is the power output?

A) 0.600 kW

B) 7.20 kW

C) 10.0 kW

D) 120 kW

E) 240 kW

31) An ideal transformer has an input voltage of 20,000 V and an output voltage of 260 V. If the input current is 26.0 A, what is the output current?

A) 1.70 A

B) 10.0 A

C) 100 A

D) 200 A

E) 2000 A

32) An ideal transformer supplies 100 kW of power to a 120 V circuit. If the input voltage is 10,000 V, what is the input current?

A) 1.2 mA

B) 10 A

C) 120 A

D) 240 A

E) 830 A

33) What number of turns on the secondary is needed to reduce the voltage from 120 V to 9.0 V if there are 40 turns on the primary of a step-down transformer?

A) 3

B) 9

C) 18

D) 120

E) 530

34) A transformer with a turns ratio of 20/1 is used to step down the standard 170 V amplitude ac line voltage. What is the peak voltage in the secondary?

A) 6.0 V

B) 7.3 V

C) 170 V

D) 8.5 V

35) Eddy currents in conductors

A) step up voltages.

B) dissipate energy.

C) increase currents in circuits.

D) decrease net charges.

E) increase net charges.

36) A hollow copper pipe is held vertically, and a magnet is dropped down the pipe. The magnet is oriented with its north pole down. What direction do the eddy currents flow as viewed from above the pipe? If copper bar of approximately the same size and weight as the magnet is dropped down the tube instead, will it fall faster or slower than the magnet?

A) CCW above the magnet and CW below the magnet; copper bar will fall slower

B) CCW above the magnet and CW below the magnet; copper bar will fall faster

C) CW above the magnet and CCW below the magnet; copper bar will fall slower

D) CW above the magnet and CCW below the magnet; copper bar will fall faster

37) If the number of turns is tripled for an inductor of fixed length and fixed cross-sectional area, what happens to its inductance?

A) It stays the same.

B) It triples.

C) It becomes 9 times the original value.

D) It decreases by a factor of 3.

E) It decreases by a factor of 9.

38) A solenoid of length 4.00 cm and cross-sectional area 2.00 cm2 has 80 turns. What is its self-inductance?

A) 160 μH

B) 120 μH

C) 80.0 μH

D) 40.2 μH

E) 0.500 μH

39) A 33 mH inductor has a current going through it that is increasing at a rate of 2.3 A/s. What is the magnitude of the induced emf in this coil?

A) 33 mV

B) 76 mV

C) 13 mV

D) 95 mV

E) 2.5 V

40) What is the inductance of a coil that has an emf of 0.82 V induced in it while the current through it is changing at a rate of 2.0 A/s?

A) 3.3 H

B) 33 mH

C) 1.6 H

D) 41 mH

E) 0.41 H

41) An inductor has 200 turns/cm and a cross-sectional area of 1.8 cm2. If its inductance is 32 mH, what is its length?

A) 21 cm

B) 15 cm

C) 35 cm

D) 64 cm

E) 72 cm

42) If the magnetic energy stored by a 0.50 H inductor is 3.6 J, what is the current through it?

A) 0.13 A

B) 0.90 A

C) 1.8 A

D) 3.8 A

E) 5.4 A

43) What is the magnetic energy stored by a 300 mH inductor with a 2.0 A current going through it?

A) 0.60 J

B) 1.2 J

C) 30 mJ

D) 0.15 J

E) 0.075 J

44) Reducing the magnetic field to 71% of its previous value changes the magnetic energy density in the field to ________% of its original value.

A) 39

B) 50

C) 71

D) 85

E) 15

45) The current in a 0.40 H inductor increases form 2.0 A to 3.0 A in 1.5 s. The energy stored by the inductor during this process increases by

A) 3.6 J.

B) 1.8 J.

C) 1.2 J.

D) 1.0 J.

E) 0.6 J.

46) What is the magnetic energy density in a volume where the magnetic field is 4.0 mT?

A) 3.2 × 10-3 J/m3

B) 3.2 J/m3

C) 6.4 J/m3

D) 13 J/m3

E) 13 × 10-3 J/m3

47) A 0.40 H inductor has a current that rises from zero to 3.0 A in 6.0 s. What is the average power required to accomplish this?

A) 15 W

B) 9.0 W

C) 1.5 W

D) 0.60 W

E) 0.30 W

48) A 1.7 H inductor has a current of 6.6 A flowing through it. The current reduces to 3.3 A over a period of 1.6 s. What is the average power the inductor supplies to the circuit as the current reduces?

A) 17 W

B) 37 W

C) 9.3 W

D) 28 W

E) 46 W

49) What electric field has the same energy density as a 1.0 T magnetic field?

A) 1.0 V/m

B) 3.0 × 108 V/m

C) 9.0 × 1016 V/m

D) 1.1 × 10-17 V/m

E) 3.3 × 10-9 V/m

50) A solenoid of length 2.8 cm and diameter 0.75 cm is wound with 160 turns per cm. When the current through the solenoid is 0.20 A, what is the magnetic flux through one of the windings of the solenoid?

A) 2.5 × 10-7 Wb

B) 1.8 × 10-7 Wb

C) 0.88 × 10-7 Wb

D) 1.2 × 10-7 Wb

51) An ideal solenoid has a length of 25 mm. If the windings are compressed so that the length is reduced to ½ of its original length how does the inductance change?

A) ¼ of original

B) to ½ of original

C) twice the original

D) quadruple the original

52) A series LR circuit consists of a battery, resistor, inductor and a switch. When the switch is closed the circuit has a time constant of 8.0 s. If the resistance and inductance of the circuit are both doubled, what is the resulting time constant?

A) 8.0 s

B) 16 s

C) 32 s

D) 4.0 s

E) 2.0 s

53) A series LR circuit includes a 9.00 V battery, a 30.0 mH inductor, and a resistance of 5.00 Ω. What is the time constant of this circuit?

A) 115 s

B) 170 s

C) 4.20 ms

D) 6.00 ms

E) 8.30 ms

54) A series LR circuit includes a 9.0 V battery, a resistance of 0.20 Ω, and an inductor. When the switch is closed, the circuit exhibits a time constant of 200 ms. What is the inductance of the circuit?

A) 42 mH

B) 15 mH

C) 32 mH

D) 40 mH

E) 22 mH

55) A series LR circuit includes a 9.0 V battery, a resistance of 0.50 Ω, and an inductance of 0.80 H. When the switch is closed, how long does it take for the current to reach one half of its maximum value?

A) 0.80 s

B) 1.6 s

C) 2.3 s

D) 1.1 s

E) 0.55 s

56) A series LR circuit includes a 9.0 V battery, a resistance of 0.50 Ω, and an inductance of 0.80 H. What is the current 2.0 s after the switch is closed?

A) 18 A

B) 13 A

C) 6.4 A

D) 5.2 A

E) 4.3 A

57) A series LR circuit includes a 9.0 V battery, a resistance of 0.50 Ω, and an inductance of 0.80 H. What is the induced emf 2.0 s after the switch has been closed?

A) 0.0

B) 9.0 V

C) 5.2 V

D) 2.6 V

E) 19 mV

58) A series LR circuit includes a 9.0 V battery, a resistance of 0.50 Ω, and an inductance of 0.80 H. What is the energy stored by the inductor 2.0 s after the switch is closed?

A) 33 mJ

B) 30 mJ

C) 13 J

D) 66 J

E) 33 J

59) A series LR circuit includes a 9.00 V battery, a resistance of 0.500 Ω, and an inductance of 0.800 H. What is the greatest energy stored by the inductor after the switch is closed?

A) 33.0 mJ

B) 13.0 mJ

C) 130 J

D) 65.0 J

E) 33.0 J

60) After the switch is closed in a series LR circuit, what is the current when a time equal to four time constants has elapsed?

A) (1/5) Imax

B) (15/16) Imax

C) (1/16) Imax

D) 0.018 Imax

E) 0.98 Imax

61) The maximum induced emf in a generator whose coils are rotating at 240 rpm is 35 V. If, instead, it is desired to generate a maximum emf of 45 V but with half the magnetic field, what rotation rate must be used?

A) 380 rpm

B) 190 rpm

C) 310 rpm

D) 165 rpm

E) 620 rpm

62) A generator is designed to produce a maximum emf of 170 V while rotating at 3200 rpm. The coils of the generator are circular with diameter 7.50 cm. Assuming the magnetic field in the generator is 0.0750 T, what is the number of loops needed in the generator coil?

A) 160

B) 1530

C) 6120

D) 4810

E) 25

63) A generator is designed to produce a maximum emf of 170 V while rotating at 3200 rpm.  The 1531 coils of the generator are circular with diameter 7.5 cm. What is the necessary magnetic field for the generator?

A) 0.0079 T

B) 0.30 T

C) 0.075 T

D) 0.24 T

64) A coiled loop of wire is in the field region between the poles of an electromagnet, with the axis of the coil aligned with the magnetic field. Which of the following things would NOT result in an induced EMF in the coil?

A) Rotation of the coil about an axis perpendicular to the coil axis

B) Pulling the coil partly out of the field region

C) Increasing or decreasing the current in the electromagnet

D) Flexing the coil and changing its shape, keeping the perimeter the same

E) None of these choices will work.

65) A conducting ring is placed such that it is lying flat in the xy-plane. A magnetic field exists in the +z direction, and its strength is decreasing with time. Which of the following occurs? 

A) No current is induced in the ring.

B) A current is induced, in the counter-clockwise direction, as viewed from above the xy-plane.

C) The situation is unclear because we do not know the initial conditions.

D) A current is induced, in the clockwise direction as viewed from above the xy-plane.

66) A square conducting loop is situated such that it is vertical, in the xz-plane. The Earth's gravitational field points downward (in the –z direction). The loop is dropped and maintains its orientation in the xz-plane. A magnetic field pointing in the +y direction exists and is uniform with respect to time, but it varies as a function of z: By(z) = B0(10 m − z). The loop is dropped and allowed to fall under the influence of gravitation. You observe the loop from the +y direction (i.e., looking toward the −y direction). Which of the following occurs?

A) The situation is unclear because we do not know the initial conditions.

B) There is a clockwise induced current.

C) There is a counter-clockwise induced current.

D) There is no induced current.

67) Lenz's law arises because of the following basic principle

A) Gauss's law.

B) the conservation of momentum.

C) Coulomb's law.

D) the conservation of energy.

E) the conservation of charge.

68) Electric fields that arise from changing magnetic fields have all the properties below except

A) they terminate on electric charges.

B) they cannot be described by an electric potential.

C) their field lines take the form of closed loops.

D) work done by the field along a closed path may be non-zero. 

69) The induced emf in a straight segment of wire that moves through a region with a magnetic field depends on all of the following, except

A) the orientation of the wire with respect to the magnetic field direction.

B) the length of the wire.

C) the resistance of the wire.

D) speed of the wire.

E) the magnetic field strength.

70) A loop of wire is moved through a region of uniform magnetic field. As it is moved, its orientation with respect to the magnetic field direction does not change. The induced current in the loop

A) depends on the shape of the loop.

B) depends on the magnitude of the field.

C) depends on the speed with which it is moved.

D) is zero.

E) depends on the acceleration of the loop.

71) A horse and rider travel in a region in which the vertical component of the Earth's magnetic field is 0.035 mT, while the horizontal component is 0.022 mT. Attached to the horse's bridle is a metal bit, a horizontal cylinder approximately 15 cm long, in the horse's mouth. If the horse is galloping at 15 m/s, what is the induced potential difference between the ends of the bit?

A) 79 mV

B) 93 mV

C) 50 µV

D) 93 µV

E) 50 mV

F) 79 µV

72) In a transformer, the relationship between the primary and secondary currents depends on all of the following quantities except

A) the number of turns in the primary and secondary windings.

B) the resistance of the windings.

C) the voltage in the primary winding.

D) the properties of the core material.

73) The principle of operation that transformers depend upon is the induction of current in the secondary windings due to

A) motion of the primary winding relative to the secondary.

B) a varying magnetic field.

C) interaction of the core with the windings.

D) a varying electric field.

74) An LR circuit is prepared for an experiment, initially with its switch open. The time constant is that time after switch closure at which the current reaches approximately what percentage of its final value?

A) 100%

B) 90%

C) 37%

D) 63%

E) 50%

75) An LR circuit is prepared for an experiment, initially with its switch open. The time constant is that time after switch closure at which the energy stored in the inductor reaches approximately what percentage of its final value?

A) 63%

B) 14%

C) 37%

D) 40%

E) 100%