Deck 29: Changing Magnetic Fields

Question 1

In the figure,a straight wire carries a current I.The wire passes through the center of a toroidal coil.If the current in the wire is quickly reduced to zero,the induced current through the resistor R is

A) from a to b.
B) from b to a.
C) There is no induced current through the resistor.

Answer 1

from b to a.

Question 2

In the figure,a straight wire carries a steady current I perpendicular to the plane of the page.A bar is in contact with a pair of circular rails,and rotates about the straight wire.The direction of the induced current through the resistor R is

A) from a to b.
B) from b to a.
C) There is no induced current through the resistor.

Answer 2

There is no induced current through the resistor.

Question 3

In the figure,two parallel wires carry currents of magnitude I in opposite directions.A rectangular loop is midway between the wires.The current I is decreasing with time.The induced current through the resistor R is

A) from a to b.
B) from b to a.
C) There is no induced current through the resistor.

Answer 3

from a to b.

Question 4

In the figure,two solenoids are side by side.The switch S,initially open,is closed.The induced current through the resistor R is

A) from a to b.
B) from b to a.
C) There is no induced current through the resistor.

Question 5

In the figure,the inner loop carries a clockwise current I that is increasing.The resistor R is in the outer loop and both loops are in the same plane.The induced current through the resistor R is

A) from a to b.
B) from b to a.
C) There is no induced current through the resistor.

Question 6

In the figure,a bar magnet moves away from the solenoid.The induced current through the resistor R is

A) from a to b.
B) from b to a.
C) There is no induced current through the resistor.

Question 7

In the figure,a C-shaped conductor is in a uniform magnetic field B,which is increasing.The polarity of the induced emf in terminals X and Y is

A) X and Y are at the same potential.
B) X is positive and Y is negative.
C) X is negative and Y is positive.

Question 8

A closed,circular loop has a counter-clockwise current flowing through it as viewed by a person on the right,as shown in the figure.If a second closed circular loop with the same radius approaches this loop with constant velocity along a common axis as shown,in what direction will a current flow in the approaching loop as viewed by the person on the right?

A) clockwise
B) counter-clockwise
C) No current will be induced because the velocity of approach is constant.

Question 9

A large magnetic flux change through a coil must induce a greater emf in the coil than a small flux change.

Question 10

In the figure,two solenoids are approaching each other with speed v as shown.The induced current through the resistor R is

A) from a to b.
B) from b to a.
C) There is no induced current through the resistor.

Question 11

A circular metal ring is situated above a long straight wire,as shown in the figure.The straight wire has a current flowing to the right,and the current is increasing in time at a constant rate.Which statement is true?

A) There is an induced current in the metal ring, flowing in a clockwise direction.
B) There is an induced current in the metal ring, flowing in a counter-clockwise direction.
C) There is no induced current in the metal ring because the current in the wire is changing at a constant rate.

Question 12

A coil lies flat on a tabletop in a region where the magnetic field vector points straight up.The magnetic field vanishes suddenly.When viewed from above,what is the direction of the induced current in this coil as the field fades?

A) counter-clockwise then clockwise
B) clockwise then counter-clockwise
C) clockwise
D) counter-clockwise
E) There is no current induced in the coil.

Question 13

The figure shows three metal coils labeled A,B,and C heading towards a region where a uniform static magnetic field exists.The coils move with the same constant velocity and all have the same resistance.Their relative sizes are indicated by the background grid.As they enter the magnetic field the coils will have an induced electric current in them.For which coil will the current be the greatest?

A) A
B) B
C) C
D) The current is the same in all three cases since all the coils move with the same velocity.
E) There is no induced current in any of the coils since they move at constant velocity.

Question 14

A bar magnet is held vertically with its upper end a little bit below the center of a horizontal metal ring.The upper end of the magnet is its north pole,as shown in the figure.The bar magnet is now dropped.An observer views the ring from above its center.To this observer,how will the induced current in the ring behave as the magnet falls?

A) The current will flow clockwise and be increasing.
B) The current will flow clockwise and be decreasing.
C) The current will flow counter-clockwise and be increasing.
D) The current will flow counter-clockwise and be decreasing.
E) The induced current will be zero.

Question 15

The three loops of wire shown in the figure are all subject to the same uniform magnetic field that does not vary with time.Loop 1 oscillates back and forth as the bob in a pendulum,loop 2 rotates about a vertical axis,and loop 3 oscillates up and down at the end of a spring.Which loop,or loops,will have an emf induced in them?

A) loop 1 only
B) loop 2 only
C) loop 3 only
D) loops 1 and 2
E) loops 2 and 3

Question 16

A capacitor is charging in a simple RC circuit with a dc battery.Which one of the following statements about this capacitor is accurate?

A) There is a magnetic field between the capacitor plates because charge travels between the plates by jumping from one plate to the other.
B) There is no magnetic field between the capacitor plates because no charge travels between the plates.
C) There is a magnetic field between the capacitor plates, even though no charge travels between them, because the magnetic flux between the plates is changing.
D) There is a magnetic field between the capacitor plates, even though no charge travels between them, because the electric flux between the plates is changing.
E) The magnetic field between the capacitor plates is increasing with time because the charge on the plates is increasing.

Question 17

A circular loop of wire lies in the plane of the paper.An increasing magnetic field points out of the paper.What is the direction of the induced current in the loop?

A) counter-clockwise then clockwise
B) clockwise then counter-clockwise
C) clockwise
D) counter-clockwise
E) There is no current induced in the loop.

Question 18

The long straight wire in the figure carries a current I that is decreasing with time at a constant rate.The circular loops A,B,and C all lie in a plane containing the wire.The induced emf in each of the loops A,B,and C is such that

A) no emf is induced in any of the loops.
B) a counterclockwise emf is induced in all the loops.
C) loop A has a clockwise emf, loop B has no induced emf, and loop C has a counterclockwise emf.
D) loop A has a counter-clockwise emf, loop B has no induced emf, and loop C has a clockwise emf.
E) loop A has a counter-clockwise emf, loops B and C have clockwise emfs.

Question 19

In the figure,a copper bar is in contact with a pair of parallel metal rails and is in motion with
Velocity ν.A uniform magnetic field is present pointing downward,as shown.The bar,the rails,
And the resistor R are all in the same plane.The induced current through the resistor R is

A) from a to b.
B) from b to a.
C) There is no induced current through the resistor.

Question 20

The figure shows a bar magnet moving vertically upward toward a horizontal coil.The poles of the bar magnets are labeled X and Y.As the bar magnet approaches the coil it induces an electric current in the direction indicated on the figure (counter-clockwise as viewed from above).What are the correct polarities of the magnet?

A) X is a south pole, Y is a north pole.
B) X is a north pole, Y is a south pole.
C) Both X and Y are north poles.
D) Both X and Y are south poles.
E) The polarities of the magnet cannot be determined from the information given.

Question 21

What is the energy density in the magnetic field 25 cm from a long straight wire carrying a current of 12 A? (μ₀ = 4π × 10^-7 T • m/A)

A) 7.3 × 10^-5 J/m³
B) 3.7 × 10^-5 J/m³
C) 3.6 × 10^-4 J/m³
D) 1.2 × 10^-4 J/m³
E) The density cannot be determined without knowing the volume.

Question 22

A closed loop conductor that forms a circle with a radius of 2.0 m is located in a uniform but changing magnetic field.If the maximum emf induced in the loop is 5.0 V,what is the maximum rate at which the magnetic field strength is changing if the magnetic field is oriented perpendicular to the plane in which the loop lies?

A) 0.40 T/s
B) 2.5 T/s
C) 0.080 T/s
D) 5.0 T/s

Question 23

Suppose that you wish to construct a simple ac generator having an output of 12 V maximum when rotated at 60 Hz. A uniform magnetic field of 0.050 T is available. If the area of the rotating coil is
100 cm²,how many turns do you need?

A) 8
B) 16
C) 32
D) 64
E) 128

Question 24

A 2.0-m long conducting wire is formed into a square and placed in the horizontal xy-plane.A uniform magnetic field is oriented 30.0° above the horizontal with a strength of 9.0 T.What is the magnetic flux through the square?

A) 1.1 T • m²
B) 1.9 T • m²
C) 2.3 T • m²
D) 18 T • m²

Question 25

The coil in a 60-Hz ac generator has 125 turns,each having an area of 3.0 × 10^-2 m² and is rotated in a uniform 0.12-T magnetic field.What is the peak output voltage of this generator?

A) 170 V
B) 120 V
C) 200 V
D) 110 V
E) 220 V

Question 26

A rectangular coil having N turns and measuring 15 cm by 25 cm is rotating in a uniform 1.6-T magnetic field with a frequency of 75 Hz.The rotation axis is perpendicular to the direction of the field.If the coil develops a sinusoidal emf of maximum value 56.9 V,what is the value of N?

A) 2
B) 4
C) 6
D) 8
E) 10

Question 27

A 50-cm wire placed in an east-west direction is moved horizontally to the north with a speed of
2)0 m/s. The horizontal component of the earth's magnetic field at that location is 25 μT toward the north and the vertical component is 50μT downward.What is the emf induced between the ends of the wire?

A) 10 µV
B) 20 µV
C) 30 µV
D) 40 µV
E) 50 µV

Question 28

A ten-loop coil having an area of 0.23 m² and a very large resistance is in a 0.047-T uniform magnetic field oriented so that the maximum flux goes through the coil.The coil is then rotated so that the flux through it goes to zero in 0.34 s.What is the magnitude of the average emf induced in the coil during the 0.34 s?

A) 0.0032 V
B) 0.00 V
C) 0.032 V
D) 0.32 V
E) 1.0 V

Question 29

You are designing a generator to have a maximum emf of 8.0 V. If the generator coil has 200 turns and a cross-sectional area of 0.030 m²,what should be the frequency of the generator in a uniform magnetic field of 0.030 T?

A) 7.1 Hz
B) 7.5 Hz
C) 8.0 Hz
D) 22 Hz
E) 44 Hz

Question 30

At a certain instant the current flowing through a 5.0-H inductor is 3.0 A.If the energy in the inductor at this instant is increasing at a rate of 3.0 J/s,how fast is the current changing?

A) 0.20 A/s
B) 0.40 A/s
C) 0.10 A/s
D) 0.80 A/s

Question 31

Wire is wound on a square frame,30 cm by 30 cm,to form a coil of 7 turns.The frame is mounted on a horizontal shaft through its center (perpendicular to the plane of the diagram),as shown in the figure.The coil is in clockwise rotation,with a period of 0.060 s.A uniform,horizontal,magnetic field of magnitude 0.40 T is present.At a given instant,the plane of the coil forms a 60° angle with the horizontal,as shown.At that instant,what is the magnitude of the emf induced in the coil?

A) 13 V
B) 23 V
C) 2.1 V
D) 3.6 V
E) 26 V

Question 32

An insulated wire of diameter 1.0 mm and negligible resistance is wrapped tightly around a cylindrical core of radius 5.0 cm and length 30 cm to build a solenoid. What is the energy stored in this solenoid when a current I = 0.20 A flows through it? (μ₀ = 4π × 10^-7 T • m/A)

A) 1.2 × 10^-4 J
B) 9.6 × 10^-4 J
C) 4.8 × 10^-4 J
D) 2.4 × 10^-4 J
E) 5.9 × 10^-5 J

Question 33

How much energy is stored in a room 3.0 m by 4.0 m by 2.4 m due to the earth's magnetic field with a strength of 5.0 × 10^-5 T? (μ₀ = 4π × 10^-7 T • m/A)

A) 570 mJ.
B) 29 mJ.
C) 10 mJ.
D) 100 mJ.

Question 34

A solenoid of length 0.700 m having a circular cross-section of radius 5.00 cm stores 6.00 µJ of energy when a 0.400-A current runs through it.What is the winding density of the solenoid?
(μ₀ = 4π × 10^-7 T • m/A)

A) 865 turns/m
B) 472 turns/m
C) 1080 turns/m
D) 104 turns/m
E) 327 turns/m

Question 35

What is the self-inductance of a solenoid 30.0 cm long having 100 turns of wire and a cross-sectional area of 1.00 × 10^-4 m²? (μ₀ = 4π × 10^-7 T • m/A)

A) 4.19 nH
B) 4.19 pH
C) 4.19 µH
D) 4.19 mH
E) 4.19 H

Question 36

At what rate would the current in a 100-mH inductor have to change to induce an emf of 1000 V in the inductor?

A) 100 A/s
B) 1 A/s
C) 1000 A/s
D) 10,000 A/s
E) 10 A/s

Question 37

For a long ideal solenoid having a circular cross-section,the magnetic field strength within the solenoid is given by the equation B(t)= 5.0t T,where t is time in seconds.If the induced electric field outside the solenoid is 1.1 V/m at a distance of 2.0 m from the axis of the solenoid,find the radius of the solenoid.

A) 0.30 m
B) 77 m
C) 0.94 m
D) 9.0 m

Question 38

A circular loop of radius 0.10 m is rotating in a uniform external magnetic field of 0.20 T. Find the magnetic flux through the loop due to the external field when the plane of the loop and the magnetic field vector are
(a)parallel.
(b)perpendicular.
(c)at an angle of 30° with each other.