Deck 31: Electromagnetic Induction

Question 1

Electric fields can be generated by

A) charges.
B) changing magnetic fields.
C) both of the above.
D) none of the above; electric fields can be neither created nor destroyed.

Answer 1

both of the above.

Question 2

A statement equivalent to Lenz's law is the following:

A) Induced currents produce magnetic fields that reinforce the magnetic field that induced them in the first place.
B) Induced currents produce magnetic fields that reinforce the change in the magnetic field that induced them in the first place.
C) Induced currents produce magnetic fields that oppose the magnetic field that induced them in the first place.
D) Induced currents produce magnetic fields that oppose the change in the magnetic field that induced them in the first place.

Answer 2

Induced currents produce magnetic fields that oppose the change in the magnetic field that induced them in the first place.

Question 3

A circular coil lies in the horizontal plane, having a bar magnet suspended above the coil's center with its south pole pointing down. When the magnet is released and approaches the coil, it induces

A) no current in the coil.
B) a current in the clockwise direction (when viewed from above).
C) a current in the counterclockwise direction (when viewed from above).
D) a large current in the coil.
E) a small current in the coil.

Answer 3

a current in the clockwise direction (when viewed from above).

Question 4

An adjustable single loop positioned in the plane of the page is placed in a magnetic field B = 1.5 T pointing out of page. When the cross-sectional area of the loop is reduced from 0.50 m² to 0.35 m² in 0.30 s, the average emf induced in the coil is

A) 75 V.
B) 30 V.
C) 23 V.
D) 0.75 V
E) 0.30 V.

Question 5

An adjustable single loop positioned in the plane of the page is placed in a magnetic field B = 1.5 T, making an angle of 30^o with the page. When the cross-sectional area of the loop is reduced from 0.50 m² to 0.35 m² in 0.30 s, the average emf induced in the coil is

A) 87 V.
B) 65 V.
C) 0.87 V.
D) 0.65 V.
E) 0.38 V.

Question 6

A circular loop is placed in a magnetic field, with the plane of the loop perpendicular to the field lines. Which of the following will not cause a current to be induced in the loop?

A) Crushing the loop.
B) Rotating the loop about an axis perpendicular to the field lines.
C) Pulling the loop out of the field.
D) Keeping the orientation of the loop fixed and moving it along the field lines.

Question 7

A rod lies in the plane of the page and moves at constant velocity, perpendicular to the rod, toward the top of the page, through a magnetic field that points out of the page. The correct statement concerning the potential induced across the rod is

A) no potential is induced across the rod.
B) the top part of the rod is at lower potential than the lower part.
C) the top part of the rod is at higher potential than the lower part.
D) the right end of the rod is at higher potential than the left end.

Question 8

A conducting square loop is placed around a bar magnet. As the loop moves away from the magnet,

A) the magnet and the loop attract one another.
B) the magnet and the loop repel one another.
C) the magnet repels the loop, but the loop attracts the magnet.
D) the magnet attracts the loop, but the loop repels the magnet.
E) there is neither attraction nor repulsion between the loop and the magnet.

Question 9

A circular loop of wire is rotated at constant angular speed about its symmetry axis, whose direction can be varied, in a uniform magnetic field pointing vertically upward. To get a maximum induced emf, the loop's axis of rotation should

A) make an angle of 30^o with the vertical.
B) make an angle of 45^o with the vertical.
C) make an angle of 60^o with the vertical.
D) be vertical.
E) be horizontal.

Question 10

A circular loop of wire is rotated at constant angular speed about its symmetry axis, whose direction can be varied, in a uniform magnetic field pointing vertically upward. To get no induced emf, the loop's axis of rotation should

A) make an angle of 30^o with the vertical.
B) make an angle of 45^o with the vertical.
C) make an angle of 60^o with the vertical.
D) be vertical.
E) be horizontal.

Question 11

A conducting coil is rotated at constant speed in a uniform magnetic field as shown in the figure here. The time required by the loop to make a full rotation is T. The graph that most likely represents the current generated in the coil as a function of time is

A) A
B) B
C) C
D) D
E) E

Question 12

A conducting coil is rotated at constant speed in a uniform magnetic field as shown in the figure here. The time required by the loop to make a full rotation is T. The graph that most likely represents the force exerted on an electron within the coil as a function of the angle $\theta$ made by the coil's plane with the magnetic field is

A) A
B) B
C) C
D) D
E) E

Question 13

A conducting coil is rotated at constant speed in a uniform magnetic field as shown in the figure here. The time required by the loop to make a full rotation is T. The current generated in the coil would increase

A) if the angular speed of the coil increases.
B) if the area of the coil increases.
C) if the strength of the magnetic field increases.
D) All of the above are correct.
E) None of the above id correct.

Question 14

A conducting bar is slid at constant velocity toward the positive x direction, along two conducting rods connected across a resistor R, as shown in the figure here. The graph that most likely shows the current generated by the apparatus is

A) A
B) B
C) C
D) D
E) E

Question 15

A conducting bar is slid at constant velocity toward the positive x direction, along two conducting rods connected across a resistor R, as shown in the figure here. The current generated by the apparatus increases if the following quantities increase except for

A) the speed of the bar.
B) the strength of the magnetic field.
C) the distance between the two conducting rods.
D) the resistance of the resistor.
E) All of the above increase the current generated by the apparatus; there are no exceptions.

Question 16

A conducting bar is slid at constant velocity toward the positive x direction, along two conducting rods connected across a resistor R, as shown in the figure here. At moment t₁, the conducting bar is slid at constant velocity toward the negative x direction. The graph that most likely shows the current generated by the apparatus is

A) A
B) B
C) C
D) D
E) E

Question 17

When the number of loops of wire in a coil is doubled, assuming all the other factors stay constant, the induced emf

A) becomes four times larger.
B) becomes two times larger.
C) stays the same.
D) becomes two times smaller.
E) becomes four times smaller.

Question 18

A long solenoid is surrounded by a circular coil. If the current changes in the solenoid, the emf induced in the coil as measured between the terminals of the coil is directly proportional to all of the following except

A) the number of turns in the solenoid.
B) the number of turns in the coil.
C) the radius of the coil.
D) All of the above are true.

Question 19

A long solenoid is surrounded by a circular coil. The solenoid is replaced with an ideal toroid that produces the same average magnetic field in its interior as was present in the solenoid (and no magnetic field outside its enclosed region). The emf induced in the coil if the current in the toroid changes at the same rate is

A) considerably greater than for the case of the solenoid.
B) approximately equal to the case of the solenoid.
C) considerably less than for the case of the solenoid.
D) zero because there is no magnetic field at the location of the coil.

Question 20

A solid conducting cube is placed on your page near the left side and oriented square with the page. When the cube is moved toward the right side of the page, in a uniform magnetic field pointing out of the page, an emf develops between the two surfaces

A) nearest the top and the bottom of the page.
B) nearest the left and right sides of the page.
C) parallel to the face and the backside of the page.
D) None of the above is correct.

Question 21

A solid conducting cube is placed on your page near the left side and oriented square with the page. When the cube is moved toward the right side of the page, in a uniform magnetic field pointing out of the page, an emf develops between the two surfaces. Charges

A) continue to flow for as long as the cube moves.
B) flow until the charge separation creates an opposite force on the charges, after which the flow ceases.
C) oscillate back and forth between the relevant surfaces.
D) are not flowing in this case.

Question 22

The "henry" has dimensions of

A) volt·second/ampere.
B) ampere volt/second.
C) ampere second/volt.
D) amperes/volt·second.

Question 23

Inductance refers to the tendency for an electrical element to resist

A) current flow.
B) change in current flow.
C) charge buildup.
D) all of the above.
E) none of the above.

Question 24

A simple RL circuit contains a 4.0- $\Omega$ resistor and an 32-H inductor. The circuit's time constant is

A) 130 s.
B) 8.0 s.
C) 4.0 s.
D) 2.0 s.
E) 0.13 s.

Question 25

A series RL circuit with inductance L and resistance R is connected to an emf V. After a time the current reaches a final value of 2.0 A. A second series circuit is identical except that the inductance is 2L. When the second circuit is connected to the same emf V, the final value of the current will be

A) 1.0 A
B) 2.0 A.
C) 4.0 A.
D) 8.0 A.

Question 26

A series RL circuit with inductance L, resistance R, and a switch is connected to an emf V. The current in the circuit is zero when the switch is closed at $t = 0$ s. The current increases to 70% of its final value in 4.0 s. The time constant of the circuit is

A) 2.8 s.
B) 5.7 s.
C) 8.5s.
D) 11 s.
E) 18 s.

Question 27

A series RL circuit with inductance 6.0 mH and resistance 20 $\Omega$ is connected to a 12.0-V battery of negligible internal resistance. The current in the circuit 4.0 $\mu$ s after the switch is closed is

A) 1.0 mA.
B) 4.0 mA.
C) 7.9 mA.
D) 0.40 A.
E) 0.59 A.

Question 28

A series RL circuit with inductance 6.0 mH and resistance 20 $\Omega$ is connected to a 12.0-V battery of negligible internal resistance. The rate of change of current in the circuit 4.0 $\mu$ s after the switch is closed is

A) 2.0 * 10³ A/s.
B) 1.0 * 10³ A/s.
C) 7.2 A/s.
D) 3.0 A/s.
E) 0.60 A/s.

Question 29

A series RL circuit with inductance L and resistance R is connected to an emf V. After a time the current reaches a final value of 5.0 A. A second series circuit connected to the same emf V is identical except that the inductance is 4L. The current in the second circuit is

A) 0.80 A.
B) 1.3 A.
C) 1.8 A.
D) 5.0 A.
E) 20 A.

Question 30

A series RL circuit with inductance L and resistance R is connected to an emf V. After a time the current reaches a final value of 5.0 A. A second series circuit connected to the same emf V is identical except that the resistor is 4R. The current in the second circuit is

A) 0.80 A.
B) 1.3 A.
C) 1.8 A.
D) 5.0 A.
E) 20 A.

Question 31

A series RL circuit with inductance L and resistance R is connected to an emf V. After a time the current reaches a final value of 6.0 A. A second series circuit connected to the same emf V is identical except that its inductance is 4L. The ratio of the rate of change of current in the first circuit to that in the second circuit is

A) 4
B) 2
C) 1
D) 0.5
E) 0.25

Question 32

A series RL circuit with inductance L and resistance R is connected to an emf V. After a time the current reaches a final value of 6.0 A. A second series circuit connected to the same emf V is identical except that its resistance is 4R. The ratio of the rate of change of current in the first circuit to that in the second circuit is

A) 4
B) 2
C) 1
D) 0.5
E) 0.25

Question 33

A series RL circuit with inductance L and resistance R is connected to an emf V. A second series circuit connected to the same emf V is identical except that its resistance is 4R. The ratio of the time constant of the first circuit to the time constant of the second circuit is

A) 4
B) 2
C) 1
D) 0.5
E) 0.25

Question 34

A series RL circuit with inductance L, resistance R, and a switch is connected to an emf V. The switch is closed at $t = 0$ s. The graph that most likely shows the current in the circuit as a function of time is

A) A
B) B
C) C
D) D
E) E

Question 35

A series RL circuit with inductance L, resistance R, and a switch is connected to an emf V. The switch had been connected to the closed position long enough for the current in the circuit to reach its maximum value. The switch is opened at $t = 0$ s. The graph that most likely shows the current in the circuit as a function of time is

A) A
B) B
C) C
D) D
E) E

Question 36

A series RL circuit with inductance L, resistance R, and a switch is connected to an emf V. The switch is closed at $t = 0$ s. The graph that most likely shows the change in current in the circuit as a function of time is

A) A
B) B
C) C
D) D
E) E

Question 37

Five series RL circuits are identical except for the values of their inductance. They are all connected to the same emf V. After the switch was open in all circuits for a long time, the switch is closed at $t = 0$ s. The graph that most likely shows the current in the circuit with the larger inductance value is

A) A
B) B
C) C
D) D
E) E

Question 38

Five series RL circuits are identical except for the values of their resistance. They are all connected to the same emf V. After the switch was open in all circuits for a long time, the switch is closed at $t = 0$ s. The graph that most likely shows the current in the circuit with the larger resistance value is

A) A
B) B
C) C
D) D
E) E

Question 39

Five series RL circuits are identical except for the values of their resistance. They are all connected to the same emf V. The switch had been connected to the closed position long enough for the current in the circuits to reach its maximum value. The switch is opened at $t = 0$ s. The graph that most likely shows the current in the circuit with the larger resistance value is

A) A
B) B
C) C
D) D
E) E

Question 40

Five series RL circuits are identical except for the values of their inductance. They are all connected to the same emf V. The switch had been connected to the closed position long enough for the currents in the circuit to reach its maximum value. The switch is opened at $t = 0$ s. The graph that most likely shows the current in the circuit with the larger inductance value is

A) A
B) B
C) C
D) D
E) E

Question 41

The self-induction of a coil is a proportionality constant that relates

A) electric field to current.
B) electric flux to current.
C) magnetic flux to current.
D) magnetic field to current.

Question 42

Energy in stored in a electric field by

A) a resistor.
B) an inductor.
C) a capacitor.
D) a motor.

Question 43

Energy in stored in a magnetic field by

A) a resistor.
B) an inductor.
C) a capacitor.
D) a motor.

Question 44

A 6.0-mH coil carries a current of 3.0 A. The energy stored in the coil's magnetic field is

A) 54 mJ.
B) 27 mJ.
C) 54 $\mu$ J.
D) 27 $\mu$ J.
E) 9.0 mJ.

Question 45

A region in space is subject to a 100-G magnetic field and a 2.00 *10⁶ N/C electric field. The magnetic energy density in this region is

A) 11.0 J/m³.
B) 18.0 J/m³.
C) 21.8 J/m³.
D) 39.8 J/m³.
E) 57.8 J/m³.

Question 46

A region in space is subject to a 100-G magnetic field and a 2.00*10⁶ N/C electric field. The electric energy density in this region is

A) 11.0 J/m³.
B) 18.0 J/m³.
C) 21.8 J/m³.
D) 39.8 J/m³.
E) 57.8 J/m³.

Question 47

A region in space is subject to a 100-G magnetic field and a 2.00 *10⁶ N/C electric field. The total energy density in this region is

A) 11.0 J/m³.
B) 18.0 J/m³.
C) 21.8 J/m³.
D) 39.8 J/m³.
E) 57.8 J/m³.

Question 48

A series RL circuit with inductance 6.0 mH and resistance 20 $\Omega$ is connected to a 12.0-V battery of negligible internal resistance. The energy stored in the magnetic field of the inductor 8.0 s after the switch is closed is

A) 4.0 mJ.
B) 8.0 mJ.
C) 9.0 mJ.
D) 11 mJ.
E) 22 mJ.