Deck 29: Electromagnetic Induction and Faradays Law

Question 1

State Lenz's Law.

Answer 1

An induced emf is always in a direction that opposes the original change in flux that caused it.

Question 2

The emf in a conducting rod of length L moving perpendicular to a magnetic field is directly proportional to the speed of the rod.

Answer 2

True

Question 3

An emf is induced in a wire by keeping a stationary magnet near the wire.

Answer 3

False

Question 4

An emf is induced in a wire by changing the current in that wire.

Question 5

For the maximum induced emf in the coil of a generator, the term sin ωt has a value of zero.

Question 6

Explain what happens when the angular speed of the coil in an electric generator is increased.

Question 7

A changing magnetic field can produce an electric current.

Question 8

An emf is induced in a wire by moving the wire near a magnet.

Question 9

A constant magnetic field can be used to produce an electric current.

Question 10

An emf is induced in a wire by changing the current in a nearby wire.

Question 11

The negative sign in the Faraday's equation for electromagnetic induction is related to the direction of the induced emf.

Question 12

If the minus sign were not in Faraday's law it would lead to a violation of the law of conservation of energy.

Question 13

Faraday's law of induction and Lenz's law are actually the same law stated differently.

Question 14

There is no induced emf in a wire that is moving parallel to a magnetic field if the wire is moving in the direction of the magnetic field.

Question 15

There is an induced emf in a wire that is moving parallel to a magnetic field if the wire is moving in the opposite direction to that of the magnetic field.

Question 16

In nuclear power plants, the nuclear energy is used to make steam which is then used to produce electricity.

Question 17

A changing magnetic flux through a closed loop of wire induces an emf in that loop.

Question 18

List the three ways that an emf can be induced in a loop of wire.

Question 19

A constant magnetic flux through a closed loop of wire induces an emf in that loop.

Question 20

According to Lenz's Law, the direction of the induced current in a conducting loop of wire is that which tends to oppose the change that produces it.

Question 21

A 200-loop coil of cross sectional area 8.5 cm² lies in the plane of the paper. Directed out of the plane of the paper is a magnetic field of 0.06 T. The field out of the paper decreases to 0.02 T in 12 milliseconds. What is the direction of the current induced?

A)clockwise to counterclockwise
B)clockwise
C)counterclockwise to clockwise
D)counterclockwise
E)cannot be determined

Question 22

A circular loop of wire is in the plane of the paper. The south pole of a bar magnet is being moved from a position in front of the paper in a direction away from the center of the loop. The direction of the induced current in the loop

A)is clockwise.
B)is north.
C)is counterclockwise.
D)is south.
E)cannot be determined.

Question 23

A circular coil of copper wire is lying flat on a horizontal table. A bar magnet is held with its south pole downward, vertically above the center of the coil. The magnet is released from rest and falls toward the coil. As viewed from above, you can say that, as it falls, the magnet induces

A)counterclockwise current in the loop.
B)clockwise current in the loop.
C)no current in the loop.
D)an emf but no electric current in the loop.
E)Not enough information is provided.

Question 24

A coil lies flat on a horizontal table top in a region where the magnetic field points straight down. The magnetic field disappears suddenly. When viewed from above, what is the direction of the induced current in this coil as the field disappears?

A)counterclockwise
B)clockwise
C)clockwise initially, then counterclockwise before stopping
D)counterclockwise initially, then clockwise before stopping
E)There is no induced current in this coil.

Question 25

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

A)The induced current flows counterclockwise.
B)The induced current flows clockwise.
C)There is no induced current in this coil.
D)The current flows clockwise initially, and then it flows counterclockwise before stopping.
E)The current flows counterclockwise initially, and then it flows clockwise before stopping.

Question 26

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)produce the smallest voltage
E)oppose the effect which produces it

Question 27

FIGURE 29-2
The wire in Fig. 29-2 carries a current I that is decreasing with time at a constant rate. The induced emf in each of the loops is such that

A)no emf is induced in any loop.
B)all loops experience counterclockwise emf.
C)loop A has clockwise emf, loop B has no induced emf, and loop C has counterclockwise emf.
D)loop A has counterclockwise emf, loop B has no induced emf, and loop C has clockwise emf.
E)loop A has counterclockwise emf, loop B clockwise emf, and loop C has clockwise emf.

Question 28

A rectangular coil lies flat on a horizontal surface. A bar magnet is held above the center of the coil with its north pole pointing down. What is the direction of the induced current in the coil?

A)There is no current in the coil.
B)clockwise
C)counterclockwise
D)Not enough information is provided.

Question 29

A coil lies flat on a level table top in a region where the magnetic field vector points straight up. The magnetic field suddenly grows stronger. When viewed from above, what is the direction of the induced current in this coil as the field increases?

A)counterclockwise
B)clockwise
C)clockwise initially, then counterclockwise before stopping
D)counterclockwise initially, then clockwise before stopping
E)There is no induced current in this coil.

Question 30

FIGURE 29-1
The three loops of wire shown in Fig. 29-1 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 induced emf?

A)Loop 1
B)Loop 2
C)Loop 3
D)Loops 1 and 3
E)Loops 2 and 3

Question 31

As a coil is removed from a magnetic field an emf is induced in the coil causing a current to flow within the coil. This current interacts with the magnetic field producing a force which

A)acts at right angles to the coil's motion.
B)acts in the direction of the coil's motion.
C)causes the coil to tend to flip over.
D)acts in the direction opposite to the coil's motion.

Question 32

A circular loop of wire of area 25 cm² lies in the plane of the paper. An increasing magnetic field B is coming out of the paper. What is the direction of the induced current in the loop?

A)There is no current.
B)counterclockwise then clockwise
C)clockwise then counterclockwise
D)clockwise
E)counterclockwise

Question 33

A circular loop of wire is in the plane of the paper. The north pole of a bar magnet is being moved toward the center of the loop from a position in front of the paper. The direction of the induced current in the loop

A)is clockwise.
B)is south.
C)is counterclockwise.
D)is north.
E)cannot be determined.

Question 34

A circular coil of copper wire is lying flat on a horizontal table. A bar magnet is held with its south pole downward, vertically above the center of the coil. The magnet is kept stationary with respect to the coil. As viewed from above, you can say that the magnet induces

A)counterclockwise current in the loop.
B)clockwise current in the loop.
C)no current in the loop.
D)an emf but no electric current in the loop.
E)Not enough information is provided.

Question 35

A circular loop of wire of area 25 cm² lies in the plane of the paper. An decreasing magnetic field B is coming out of the paper. What is the direction of the induced current in the loop?

A)clockwise then counterclockwise
B)counterclockwise then clockwise
C)counterclockwise
D)There is no current.
E)clockwise

Question 36

Magnetic flux depends upon

A)the magnetic field.
B)the orientation of the area with respect to the field.
C)the area involved.
D)all of the above
E)none of the above

Question 37

A long straight wire lies on a horizontal table and carries an ever-increasing current northward. Two coils of wire lie flat on the table, one on either side of the wire. When viewed from above, the induced current circles

A)clockwise in both coils.
B)counterclockwise in both coils.
C)clockwise in the east coil and counterclockwise in the west coil.
D)counterclockwise in the east coil and clockwise in the west coil.
E)no emf is induced in either coil.

Question 38

For the maximum induced emf in the coil of a generator, the term sin ωt has a value of one.

Question 39

A rectangular coil lies flat on a horizontal surface. A bar magnet is held above the center of the coil with its north pole pointing down. If the magnet is dropped from this position what is the direction of the induced current in the coil?

A)There is no current in the coil.
B)counterclockwise
C)clockwise
D)Not enough information is provided.

Question 40

FIGURE 29-2
The wire in Fig. 29-2 carries a current I that is increasing with time at a constant rate. The induced emf in each of the loops is such that

A)no emf is induced in any loop.
B)all loops experience counterclockwise emf.
C)loop A has clockwise emf, loop B has no induced emf, and loop C has counterclockwise emf.
D)loop A has counterclockwise emf, loop B has no induced emf, and loop C has clockwise emf.
E)loop A has counterclockwise emf, loop B clockwise emf, and loop C has clockwise emf.

Question 41

FIGURE 29-11
A cube whose edges are 0.10 m long has one corner at the origin of an xyz-coordinate system as shown in Fig. 29-11. A magnetic field with a strength of 0.40 T is applied along the +x-axis.
(a) What is the magnetic flux through the shaded face of the cube?
(b) What is the magnetic flux through the face of the cube along the yz plane at x = 0?
(c) What is the total magnetic flux through the six faces of the cube?

Question 42

FIGURE 29-5
The two identical bar magnets in Fig. 29-5 are dropped from rest along a vertical line passing through the center of the rings, as shown. The two rings are identical in every respect except that the ring on the right has a small break in it. Calling a_L and a_R the magnitude of the downward accelerations of the magnets on the left and right, respectively, you observe that

A)a_L = a_R.
B)a_L > a_R.
C)a_L < a_R.
D)a_L = a_R = 0.
E)It is not possible to predict the outcome of this experiment with the data given.

Question 43

FIGURE 29-10
A rectangular coil with N turns, length L, and width w, as shown in Fig. 29-10, is rotating in a magnetic field B with an angular frequency ω. If the area of the coil is A, what is the induced emf in the coil?

A)ε = NB (Lw) ω sin ωt
B)ε = NBA ω sin ωt
C)ε = NBA (2πf) sin 2πft
D)All of these answers are correct.
E)None of these answers is correct.

Question 44

A transformer is a device used to

A)transform an alternating current into a direct current.
B)transform a direct current into an alternating current.
C)increase or decrease an ac voltage.
D)increase or decrease a dc voltage.

Question 45

FIGURE 29-3
Fig. 29-3 above shows a bar magnet moving upward along the vertical axis 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. What are the correct polarities of the magnet?

A)X = south , Y = north
B)X = negative, Y = positive
C)X = positive, Y = negative
D)X = north, Y = south
E)The polarities of the magnet cannot be determined from the information given.

Question 46

FIGURE 29-6
A metallic rod moves at constant speed in the positive x direction inside a uniform magnetic field as shown in Fig. 29-6. Positive and negative charges build up on the rod as indicated. What is the direction of the magnetic field?

A)negative x
B)negative y
C)negative z
D)positive y
E)positive z

Question 47

FIGURE 29-7
Fig. 29-7 shows 3 metallic frames labeled A, B, and C heading towards a region where a uniform magnetic field exists. The frames move with the same constant velocity. Their relative sizes are indicated by the background grid. As they enter the magnetic field the frames will have an induced electric current along their perimeter. For which frame will the current be the greatest?

A)A
B)B
C)C
D)The current is the same in all 3 cases.
E)There is no induced current when the frames move at constant speed.

Question 48

FIGURE 29-10
A circular loop of wire is rotated at constant angular speed about an axis whose direction can be varied. In a region where a uniform magnetic field points straight down, what must be the orientation of the loop's axis of rotation if the induced emf is to be a maximum?

A)Any horizontal orientation will do.
B)It must make an angle of 45° to the vertical.
C)It must make an angle of 60° to the vertical.
D)It must be vertical.
E)none of the given answers

Question 49

Generators that were used in the past to supply electricity in automobiles have been replaced by

A)batteries.
B)armatures.
C)dynamos.
D)alternators.
E)commutators.

Question 50

Electric power is transmitted over long distances at high voltage because

A)electricity moves faster at higher voltage.
B)the lines can be on taller poles.
C)lightning does more damage.
D)there is less power lost.
E)electricity moves slower at higher voltage.

Question 51

A horizontal rod (oriented in the east-west direction) is moved northward at constant velocity through a magnetic field that points straight down. Make a statement concerning the potential induced across the rod.

A)The west end of the rod is at higher potential than the east end.
B)The east end of the rod is at higher potential than the west end.
C)The top surface of the rod is at higher potential than the bottom surface.
D)The bottom surface of the rod is at higher potential than the top surface.
E)The potential is uniform across the rod.

Question 52

FIGURE 29-4
Fig. 29-4 shows the time evolution of a uniform magnetic field. Four particular instants labeled t_A to t_D are also identified on the graph. The field passes through a circular coil whose normal is parallel to the direction of the field. At what time does the current induced in the coil have the largest value?

A)t_A
B)t_B
C)t_C
D)t_D
E)The current is the same at all these times.

Question 53

FIGURE 29-8
A metallic frame moving along the positive direction enters a region of space with a uniform magnetic field pointing in the positive z direction as shown in Fig. 29-8. In what direction should a force be applied to the frame to keep it moving at a constant speed while it is entering the field?

A)negative x
B)positive x
C)positive y
D)negative y
E)positive z

Question 54

A coil is rotating at constant speed within a uniform magnetic field. What can be said about the magnitude of the emf induced in the coil?

A)The magnitude of the induced emf reaches its maximum value whenever the plane of the coil is perpendicular to the direction of the field.
B)The induced emf is zero whenever the plane of the coil makes a zero angle with respect to the field.
C)The induced emf has a constant value.
D)The induced emf is zero since the speed of the coil is constant.
E)The magnitude of the induced emf reaches its maximum value whenever the plane of the coil is parallel to the direction of the field.

Question 55

FIGURE 29-10
A circular loop of wire is rotated at constant angular speed about an axis whose direction can be varied. In a region where a uniform magnetic field points straight down, what must be the orientation of the loop's axis of rotation if the induced emf is to be zero?

A)Any horizontal orientation will do.
B)It must make an angle of 45° to the vertical.
C)It must make an angle of 60° to the vertical.
D)It must be vertical.
E)none of the given answers

Question 56

An electric generator transforms

A)chemical energy into electrical energy.
B)electrical energy into mechanical energy.
C)mechanical energy into electrical energy.
D)direct current into alternating current.
E)alternating current into direct current.

Question 57

FIGURE 29-11
A 200-loop coil of cross sectional area 8.5 cm² lies in the plane of the paper. Directed out of the plane of the paper is a magnetic field of 0.06 T. The field out of the paper decreases to 0.02 T in 12 milliseconds.
(a) What is the magnitude of the change in magnetic flux enclosed by the coil?
(b) What is the average voltage induced in the coil as the flux is changing?
(c) If the coil has a resistance of 4.0 ohms, what is the magnitude of the current in the coil?

Question 58

If the number of turns on the secondary coil of a transformer are less than those on the primary, the result is a

A)step-down transformer.
B)step-up transformer.
C)120-V transformer.
D)220-V transformer.
E)a dc transformer.

Question 59

In a transformer, the power input

A)is larger than the power output.
B)is equal to the power output.
C)is smaller than the power output.
D)can be either larger or smaller than the power output.

Question 60

Electric fields produced by changing magnetic fields

A)start on positive charges and end on negative charges.
B)start on negative charges and end on positive charges.
C)are conservative.
D)are nonconservative.
E)do not exist.

Question 61

FIGURE 29-13
The windings of a motor have a resistance of 4.0 Ω. The counter emf of this motor when connected to a 120-V line and running at full speed is 104 V.
(a) What is the start up current of this motor?
(b) What is the current in the motor when running at full speed?

Question 62

A circular loop of radius 0.10 m is rotating in a uniform magnetic field of 0.20 T. Find the magnetic flux through the loop when the plane of the loop and the magnetic field vector are parallel.

A)zero
B)3.1 × 10^-3 T∙m²
C)5.5 × 10^-3 T∙m²
D)6.3 × 10^-3 T∙m²
E)9.2 × 10^-3 T∙m²

Question 63

FIGURE 29-12
A conducting rod whose length is 25 cm is placed on a U-shaped metal wire that has a resistance R of 8 Ω as shown in Fig. 29-12. The wire and the rod are in the plane of the paper. A constant magnetic field of strength 0.4 T is applied perpendicular and into the paper. An applied force moves the rod to the right with a constant speed of 6 m/s.
(a) What is the magnitude of the induced emf in the wire?
(b) What is the magnitude and direction of the induced current in the wire?

Question 64

A conducting loop in the form of a circle is placed perpendicular to a magnetic field of 0.50 T. If the area of the loop decreases at a rate of 3.0 × 10^-3 m² /s, what is the induced emf in the loop?

A)1.7 mV
B)4.3 mV
C)5.5 mV
D)0 mV
E)1.5 mV

Question 65

A square coil of wire with 15 turns and an area of 0.40 m² is placed parallel to a magnetic field of 0.75 T. The coil is flipped so its plane is perpendicular to the magnetic field in 0.050 s. What is the magnitude of the average induced emf?

A)6.0 V
B)12 V
C)36 V
D)45 V
E)90 V

Question 66

FIGURE 29-13
A DC motor of internal resistance 6.0 Ω is connected to a 24-V power supply. The operating current is 1.0 A.
(a) What is the start-up current?
(b) What is the back emf when the motor is running at full speed?
(c) What is the back emf when the motor is running at half speed?

Question 67

A loop of radius r = 3 cm is placed parallel to the xy-plane in a uniform magnetic field = ( 0.75 T) . The resistance of the loop is R = 18 Ω. Starting at t = 0, the magnitude of the field decreases uniformly to B = 0 in 0.15 seconds. What is the magnitude of the electric current produced in the loop during that time?

A)0.8 mA
B)3.9 mA
C)1.7 mA
D)2.1 mA
E)0.2 mA

Question 68

A 20-turn coil of area 0.32 m² is placed in a uniform magnetic field of 0.055 T so that the perpendicular to the plane of the coil makes an angle of 30° with respect to the magnetic field. The flux through the coil is

A)1.8 × 10^-2 Tm².
B)1.8 × 10^-1 Tm².
C)8.8 × 10^-2 Tm².
D)8.8 × 10^-3 Tm².
E)1.5 × 10^-2 Tm².

Question 69

A ten loop coil of area 0.23 m² is in a 0.047 T uniform magnetic field oriented so that the maximum flux goes through the coil. The average emf induced in the coil is

A)1.1 × 10^-3 V.
B)0.
C)1.1 × 10^-2 V.
D)1.1 × 10^-4 V.
E)1.1 × 10^-1 V.

Question 70

FIGURE 29-13
A power transmission line 50 km long has a total resistance of 0.60 Ω. A generator produces 100 V at 70 A. In order to reduce energy loss due to heating of the transmission line, the voltage is stepped up with a transformer with a turns ratio of 100:1.
(a) What percentage of the original energy would be lost if the transformer were not used?
(b) What percentage of the original energy is lost when the transformer is used?

Question 71

A ten loop coil of area 0.23 m² 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. The average emf induced in the coil during the 0.34 s is

A)3.2 × 10^-3 V.
B)0.
C)3.2 × 10^-2 V.
D)3.2 × 10^-1 V.
E)1.0 V.

Question 72

A constant magnetic field of 0.50 T is applied to a rectangular loop of area 3.0 × 10^-3 m². If the area of this loop changes from its original value to a new value of 1.6 × 10^-3 m² in 1.6 s, what is the emf induced in the loop?

A)1.6 × 10^-2 V
B)0 V
C)7.5 × 10^-2 V
D)4.4 × 10^-4 V
E)9.0 × 10^-2 V

Question 73

A ten loop coil of area 0.23 m² is in a 0.047 T uniform magnetic field oriented so that the maximum flux goes through the coil. This flux is

A)3.4 × 10^-2 Tm².
B)1.1 × 10^-3 Tm².
C)3.4 × 10^-1 Tm².
D)1.1 × 10^-2 Tm².
E)1.1 × 10^-1 Tm².

Question 74

A circular loop of radius 0.10 m is rotating in a uniform magnetic field of 0.20 T. Find the magnetic flux through the loop when the plane of the loop and the magnetic field vector are perpendicular.

A)zero
B)3.1 × 10^-3 T∙m²
C)5.5 × 10^-3 T∙m²
D)6.3 × 10^-3 T∙m²
E)9.2 × 10^-3 T∙m²

Question 75

A circular loop of radius 0.10 m is rotating in a uniform magnetic field of 0.20 T. Find the magnetic flux through the loop when the plane of the loop and the magnetic field vector are at an angle of 30°.

A)zero
B)3.1 × 10^-3 T∙m²
C)5.5 × 10^-3 T∙m²
D)6.3 × 10^-3 T∙m²
E)9.2 × 10^-3 T∙m²

Question 76

FIGURE 29-13
A conducting bar moves along frictionless conducting rails connected to a 4.00-Ω resistor as shown in Fig. 29-13. The length of the bar is 1.60 m and a uniform magnetic field of 2.20 T is applied perpendicular to the paper.
(a) What is the applied force required to move the bar to the right with a constant speed of 6.00 m/s?
(b) At what rate is energy dissipated in the 4.00 Ω resistor?

Question 77

A uniform magnetic field is applied perpendicular to the plane of a 60-turn circular coil with a radius of 6.0 cm and a resistance of 0.60 Ω. If the magnetic field increases from 0.20 T to 1.8 T in 0.20 s, what is the emf induced in that coil?

A)7.2 V
B)5.4 V
C)9.2 V
D)12 V
E)0 V

Question 78

A conducting loop in the form of a circle is placed perpendicular to a magnetic field of 0.50 T. If the area of the loop increases at a rate of 3.0 × 10^-3 m² /s, what is the induced emf in the loop?

A)4.3 mV
B)0 mV
C)1.5 mV
D)1.7 mV
E)5.5 mV

Question 79

The flux through a coil changes from 4.0 × 10^-5 Wb to 5.0 × 10^-5 Wb in 0.10 s. What emf is induced in this coil?

A)5.0 × 10^-4 V
B)4.0 × 10^-4 V
C)1.0 × 10^-4 V
D)0 V
E)none of the given answers

Question 80

The area of a rectangular loop of wire is 3.6 × 10^-3 m². The loop is placed in a magnetic field that changes from 0.20 T to 1.4 T in 1.6 s. The plane of the loop is perpendicular to the direction of the magnetic field. What is the magnitude of the induced emf in that loop?

A)2.8 × 10^-3 V
B)2.7 × 10^-3 V
C)0 V
D)1.8 × 10^-3 V
E)3.0 × 10^-3 V