Deck 20: Induced Voltages and Inductance

Question 1

A uniform 8.00-T magnetic field passes perpendicularly through the plane of a wire loop 0.320 m² in area.What flux passes through the loop?

A)25.6 T⋅m²
B)2.56 T⋅m²
C)25.0 T⋅m²
D)0.819 T⋅m²
E)7.68 T·m²

Answer 1

2.56 T⋅m²

Question 2

A double loop of wire (making 2 turns)is in the x-y plane centered at the origin.A uniform magnetic field is increasing at a constant rate in the positive z-direction.Viewed from the positive z-axis,in which direction is the induced magnetic field in the loop?

A)in the positive z-direction
B)in the negative z-direction
C)There is no induced field because of the double loop.
D)There is no induced field because the rate of change of the magnetic field is constant.

Answer 2

in the negative z-direction

Question 3

A 460-turn solenoid of length 30.0 cm and radius 4.00 cm carries a current of 3.50 A. What is the magnetic field strength inside the coil at its midpoint?
What is the magnetic flux through a circular cross-sectional area of the solenoid at its midpoint?

A)​1.02E-5 T/m²
B)​3.05E-6 T/m²
C)​2.49E-7 T/m²
D)​3.39E-5 T/m²

Answer 3

​3.39E-5 T/m²

Question 4

A circular loop of wire has its radius reduced in half in time Δt.A uniform magnetic field is at an angle of 60° to the plane of the coil,and the magnetic field doubles its intensity in the same time interval Δt.During this interval,what happens to the flux through the coil?

A)It increases.
B)It decreases.
C)It remains the same.
D)More information is needed to make this conclusion.

Question 5

A loop of area 0.384 m² is in a uniform 0.0565-T magnetic field.If the flux through the loop is 6.10 × 10-³ T·m²,what angle does the normal to the plane of the loop make with the direction of the magnetic field?

A)73.7°
B)89.3°
C)16.3°
D)76.0º
E)This is not possible.

Question 6

A coil is placed in a changing magnetic field and an emf is induced.What happens to the induced emf if the rate of change of magnetic field quadruples?

A)There is no change.
B)The emf doubles.
C)The emf quadruples.
D)The emf increases by a factor of 16.

Question 7

A coil is placed in a magnetic field and has a flux Φ_B through it.The coil is stressed so that its area reduces to 75% of its original value.If the plane of the coil stays the same and the flux through it remains the same,how must the magnetic field change?

A)It must increase by 25%.
B)It must increase by 33%.
C)It must increase by 125%.
D)It must decrease by 25%.

Question 8

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

Question 9

If the induced current in a wire loop were such that the flux it produces were in the same direction as the change in external flux causing the current,which of the following conservation laws would end up being violated?

A)momentum
B)charge
C)energy
D)angular momentum

Question 10

Three loops of wire,one circular,one rectangular,and one square,are made from identical lengths of wire.If the loops are in the same increasing magnetic field perpendicular to the plane of the coils,which loop has the greatest induced emf?

A)the circular one
B)the rectangular one
C)the square one
D)All three would have the same emf induced.

Question 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.

Question 12

A uniform 1.72-T magnetic field passes through the plane of a wire loop 0.46 m² in area.What flux passes through the loop when the direction of the 1.72-T field is at a 38.4° angle to the normal of the loop plane?

A)2.9 T⋅m²
B)0.49 T⋅m²
C)0.62 T⋅m²
D)0.63 T⋅m²
E)0.79 T⋅m²

Question 13

"GFI" stands for:

A)grand flux indicator.
B)ground forcing indicator.
C)ground fault interrupter.
D)gauss-free invention.

Question 14

The magnetic field inside a circular loop of wire of radius 30 cm increases at a constant rate from 2.0 × 10^-3 T to 3.5 × 10^-3 T in 0.47 s.What is the induced emf in the wire?

A)9.0 ×10^-4 V
B)2.3 ×10^-4 V
C)1.8 ×10^-3 V
D)1.8 ×10^-4 V

Question 15

A 460-turn solenoid of length 30.0 cm and radius 4.00 cm carries a current of 3.50 A. What is the magnetic field strength inside the coil at its midpoint?
​What is the magnetic field strength inside the coil at its midpoint?

A)​2.02E-3 T
B)​6.74E-3 T
C)​6.07E-4 T
D)​4.95E-5 T

Question 16

A 460-turn solenoid of length 30.0 cm and radius 4.00 cm carries a current of 3.50 A. What is the magnetic field strength inside the coil at its midpoint?
The units T·m²/s are equivalent to:

A)W.
B)V.
C)N/m.
D)webers.

Question 17

A coil in a magnetic field encloses a flux of 0.196 T·m² when the angle between the normal to the coil and the direction of the magnetic field is 62.0°.What flux would go through the coil if the angle were changed to 38.9°?

A)0.118 T·m²
B)0.123 T·m²
C)0.539 T·m²
D)0.325 T·m²
E)0.139 T·m²

Question 18

A changing current in one loop of wire can induce a current in another loop of wire even though the wires do not touch.The induced current appears almost instantaneously for loops close together.How can this energy exchange happen?

A)This is one of those things for which science has no adequate explanation yet.
B)The second loop is affected by the thermal exchange from the heating of the first loop.
C)It is a gravitational effect on the electrons.
D)It is done via an electromagnetic wave.

Question 19

A sensitive ammeter is connected to a wire loop and placed within the magnetic field of a strong horseshoe magnet.The ammeter 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 is moving.
D)the wire's axis is parallel to the field.

Question 20

The magnetic field inside a circular loop of wire of radius 30 cm increases at a constant rate from 2.0 × 10^-3 T to 3.5 × 10^-3 T in 0.47 s.If the resistance of the loop is 0.31 Ω,what is the current in the loop?

A)7.4 × 10^-4 A
B)5.8 × 10^-4 A
C)2.9 × 10^-3 A
D)5.8 × 10^-3 A

Question 21

A straight wire lies along the y-axis initially carrying a current of 10 A in the positive y-direction.The current decreases and reverses to 10 A in the negative y-direction,the change in current happening at a uniform rate.In the 1st quadrant a square conducting coil has 2 sides parallel to the y-axis and the other 2 sides parallel to the x-axis.The side of the coil nearest and parallel to the straight wire is at a distance equal to the length of one of the sides of the square.As the current is going from 10 A in one direction to 10 A in the other,in which direction is the induced current in this side of the square coil nearest to the straight wire?

A)It is in the positive y-direction.
B)It is in the negative y-direction.
C)At first it is in the positive y-direction,but after the current passes through zero,it is in the negative y-direction.
D)At first it is in the negative y-direction but after the current passes through zero,it is in the positive y-direction.

Question 22

A 41-turn square coil of area 0.074 m² and a 123-turn circular coil are both placed perpendicular to the same changing magnetic field.The voltage induced in each of the coils is the same.What is the area of the circular coil?

A)0.22 m²
B)0.82 m²
C)0.025 m²
D)0.074 m²
E)0.67 m²

Question 23

A wire loop of radius 0.250 m is placed in a uniform magnetic field of magnitude 0.740 T. The field is directed perpendicular to the plane of the loop.

-If the area of the loop increases at a rate of area 1.50 * 10^-3 m²/s,what is the magnitude of the emf induced in the coil?

A)1.11 mV
B)3.00 mV
C)0.218 mV
D)0.589 mV

Question 24

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.

Question 25

A plane with a wingspan of 35.0 m is flying horizontally at a speed of 280 m/s over a location where the Earth's magnetic field is 45.0 μT at 55° below the horizontal.What voltage is generated between the wingtips?

A)3.6E-1 V
B)2.5E-1 V
C)6.3E-1 V
D)4.4E-1 V

Question 26

An airplane with a wingspan of 79.0 m flies parallel to the Earth's surface at a point where the downward component of the Earth's magnetic field is 0.490 × l0−⁴ T.If the induced potential between wingtips is 0.875 V,what is the plane's speed?

A)54.3 m/s
B)33.9 m/s
C)226 m/s
D)17 900 m/s
E)111 m/s

Question 27

A planar loop consisting of seven turns of wire,each of which encloses 200 cm²,is oriented perpendicularly to a magnetic field that increases uniformly in magnitude from 14 × 10^-3 T to 38 × 10^-3 T in a time of 8.0 × 10^-3 s.What is the resulting induced current in the coil if the resistance of the coil is 5.0 Ω?

A)1.2E-2 A
B)1.2E+0 A
C)1.3E-1 A
D)8.4E-2 A
E)7.7E-1 A

Question 28

A bar magnet is falling through a loop of wire with constant velocity.The north pole enters first.The induced current will be greatest in magnitude 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.

Question 29

A wire loop of radius 0.250 m is placed in a uniform magnetic field of magnitude 0.740 T. The field is directed perpendicular to the plane of the loop.
If the field reverses its direction,and its magnitude changes to 0.100 T in time 1.06 s,what is the magnitude of the average induced emf in the loop during this time?

A)156 mV
B)119 mV
C)185 mV
D)741 mV

Question 30

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.

Question 31

A circular loop is suspended in a uniform magnetic field as shown in the figure.
​
If the loop area increases,what affect does this have on the induced current?

A)The induced current is clockwise.
B)The induced current is counterclockwise.
C)There is no induced current.
D)We would need to know the magnitude of the magnetic field.

Question 32

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.

Question 33

A flat coil of wire consisting of 15 turns,each with an area of 40 cm²,is positioned perpendicularly to a uniform magnetic field that increases its magnitude at a constant rate from 1.5 T to 5.1 T in 2.0 s.If the coil has a total resistance of 0.20 Ω,what is the magnitude of the induced current?

A)7.65E-1 A
B)5.40E+1 A
C)5.40E-1 A
D)8.64E-2 A
E)2.16E-1 A

Question 34

A bar magnet is held stationary while a circular loop of wire is moved toward the magnet at constant velocity at position A as in the figure.
The loop passes over the magnet's center at position B and moves away from the magnet at position C.Viewing the loop from the left as indicated in the figure,for which position is the induced current zero?

A)Position A
B)Position B
C)Position C
D)The induced current is never zero.

Question 35

A square coil,enclosing an area with sides 2.75 cm long,is wrapped with 1 875 turns of wire.A uniform magnetic field perpendicular to its plane is turned on and increases to 0.390 T during an interval of 1.70 s.What average voltage is induced in the coil?

A)0.325 V
B)0.553 V
C)1.73 V
D)0.118 V
E)1.82 V

Question 36

A straight wire of length
Is oriented east-west and is in a magnetic field B pointing north.The wire is moving downward at a constant speed v.Which end of the rod is positively charged?

A)neither
B)the east end
C)the west end
D)both ends

Question 37

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 halfway through.
C)the magnet is leaving the loop.
D)never.

Question 38

A straight wire of length l is oriented east-west and is in a magnetic field B pointing north.The wire is moving downward at a constant speed v.If the resistance of the rod is R,what is the current through the rod?

A)R/Blv
B)Blv/R
C)B²l²v²/R²
D)not given

Question 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.

Question 40

A bar magnet is held above the center of a conducting ring in the horizontal plane.The magnet is dropped so it falls lengthwise toward the center of the ring.Will the falling magnet be attracted toward the ring or be repelled by the ring due to the magnetic interaction of the magnet and the ring?

A)It will be attracted.
B)It will be repelled.
C)It will be attracted only if the north end of the magnet is the leading end as it falls toward the ring.
D)It will be attracted only if the south end of the magnet is the leading end as it falls toward the ring.

Question 41

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 a maximum

Question 42

A large jetliner with a wingspan of 51.0 m flies horizontally and due north at a speed of 440 m/s in a region where the magnetic field of the earth is 48.4 µT directed 30.0° below the horizontal.What is the magnitude of the induced emf between the ends of the wing?

A)9.41E-1 mV
B)1.09E+0 mV
C)5.43E-1 mV
D)3.26E+1 mV
E)1.06E-2 mV

Question 43

A rectangular loop (area = 0.250 m²)turns in a uniform magnetic field with B = 0.210 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 0.530 rad/s,what is the magnitude of the emf induced in the loop?

A)5.25E-2 V
B)zero
C)2.78E-2 V
D)9.91E-2 V
E)1.47E-2 V

Question 44

A motor has an internal resistance of 12.0 Ω.When running,the motor has a back emf of 29 V and draws a current of 3.7 A.What is the supply voltage in this case?

A)29 V
B)44 V
C)73 V
D)15 V
E)32 V

Question 45

The wiring in a motor has resistance of 1.2 Ω and produces a back emf of 0.62 V when connected to a 10.3-V battery of negligible resistance.Find the current flow through the motor.

A)9.1 A
B)8.6 A
C)0.52 A
D)8.1 A
E)8.0 A

Question 46

An electric motor draws a current of 2.60 A from a 18.0-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 10.0 A.What is the back emf of the motor when running at normal speed?

A)4.68 V
B)10.0 V
C)22.7 V
D)13.3 V
E)133 V

Question 47

A 420-turn circular coil with an area of 0.0650 m² is mounted on a rotating frame,which turns at a rate of 22.3 rad/s in the presence of a 0.0550-T uniform magnetic field that 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 perpendicular to the field?

A)33.5 V
B)zero
C)0.0673 V
D)747 V
E)1.50 V

Question 48

A 580-turn circular coil with an area of 0.0550 m² is mounted on a rotating frame,which turns at a rate of 17.0 rad/s in the presence of a 0.0400-T uniform magnetic field that 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 50.0° angle to the field?

A)zero
B)21.7 V
C)18.9 V
D)16.6 V
E)0.0751 V

Question 49

A motor with a coil resistance of 12.4 Ω is attached to a voltage supply of 66.4 V.What is the current in the motor when it is running at its maximum speed with a back emf of 22.6 V?

A)zero
B)3.53 A
C)7.18 A
D)5.35 A
E)543 A

Question 50

A 0.200-m wire is moved parallel to a 0.500-T magnetic field at a speed of 1.50 m/s.What emf is induced across the ends of the wire?

A)2.25 V
B)1.00 V
C)0.600 V
D)zero

Question 51

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

Question 52

What is the minimum frequency with which a 300-turn,flat coil of cross sectional area 370 cm² can be rotated in a uniform 25-mT magnetic field if the maximum value of the induced emf is to equal 8.6 V?

A)0.20 Hz
B)2.2 Hz
C)4.9 Hz
D)31 Hz
E)0.031 Hz

Question 53

The basic function of the electric generator is which 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

Question 54

When a voltage is generated by rotating a coil in a magnetic field at a constant rate,the period of the voltage equals the time that it takes for the coil to rotate through ________ radians.

A)1
B)π/2
C)π
D)2π

Question 55

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

Question 56

In the United States,the value of ω for commercially generated power is ________ in SI units.

A)50
B)60
C)120
D)377

Question 57

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.

Question 58

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.

Question 59

If the rotating coil used in a generator has its radius doubled,what change results in the amplitude of the voltage produced?

A)It doubles.
B)It quadruples.
C)It is increased by a factor 2^3/2.
D)It stays the same.

Question 60

A coil is rotated in a magnetic field generating an emf given by emf = NBAω sin θ,where θ = ωt.At which angle θ is the emf one-third of its maximum value?

A)0°
B)19.5°
C)6.4°
D)70.5°
E)60.0º

Question 61

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 3.0 A,the current in the secondary will be

A)3.0 A.
B)6.0 A.
C)1.5 A.
D)zero.

Question 62

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

Question 63

The current in a coil with a self-inductance of 2.4 mH increases from 0 to 2.66 A in a tenth of a second.What is the induced emf in the coil?

A)64 mV
B)6.4 mV
C)1 700 mV
D)5.1 V
E)130 mV

Question 64

By what factor is the self-inductance of an air solenoid changed if only its number of coil turns,N,is multiplied by five?

A)1/5
B)5
C)15
D)25
E)1/25

Question 65

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 can be directly proportional to

A)t.
B)r.
C)R.
D)none of these

Question 66

In a circuit made up of inductor L,resistance R,ammeter,battery and switch in series,the current is greatest at which of the following times,as measured after the switch is closed?

A)zero
B)one time constant
C)at a time t = L/R
D)ten time constants

Question 67

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

Question 68

A coil with a self-inductance of 0.90 mH experiences a constant current buildup from zero to 15 A in 0.65 s.What is the induced emf during this interval?

A)8.78E-3 V
B)2.08E-2 V
C)8.78E+2 V
D)2.56E-2 V
E)2.31E-2 V

Question 69

In a circuit made up of inductor,resistance,ammeter,battery and switch in series,at which of the following times 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

Question 70

What is the self-inductance in a coil that experiences a 2.30-V induced emf when the current is changing at a rate of 160 A/s?

A)6.96E-1 H
B)3.31E-2 H
C)160 H
D)1.44E-2 H
E)3.68E+1 H

Question 71

An RL series circuit has the following components: 9.2-mH coil,1.2-Ω resistor,16-V battery,ammeter and switch.What is the time constant of this circuit?

A)1.3E+2 s
B)7.7E-3 s
C)1.4E-3 s
D)6.9E-4 s
E)​1.2E-1 s

Question 72

By what factor is the self-inductance of an air solenoid changed if its length and number of coil turns are both multiplied by seven?

A)1/7
B)7
C)1
D)49
E)1/49

Question 73

The self-inductance of a solenoid increases under which of the following conditions?

A)Only the solenoid length is increased.
B)Only the cross sectional area is decreased.
C)Only the number of coils per unit length is decreased.
D)Only the number of coils is increased.

Question 74

One time constant after an RL circuit has its switch closed,how does the current I in it compare to the maximum current I_max that occurs for this circuit?

A)I > I_max/2
B)I = I_max/2
C)I < I_max/2
D)Without knowing R and L,this cannot be determined.

Question 75

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.

Question 76

An air-core inductor has 920 turns/m and an internal volume of 3.0 cm³.What is its inductance?

A)3.2 μH
B)2.8 mH
C)0.35 H
D)32 μH
E)Insufficient information is given.

Question 77

A 13-V battery is connected in series with a switch,resistor and coil.If the circuit's time constant is 1.2 × 10−⁴ s and the final steady current after the switch is closed becomes 2.5 A,what is the value of the inductance?

A)3.9E-3 H
B)6.2E-4 H
C)2.3E-1 H
D)2.7E-3 H
E)2.3E-5 H

Question 78

The unit of inductance,the henry,is equivalent to

A)V·s/A.
B)V/m.
C)J/C.
D)none of the units given.

Question 79

An RL series circuit has: 4.8-mH coil,2.8-Ω resistor,1.0-V battery,ammeter and switch.After the switch is closed for a long time,find the final value of the current.

A)7.4E+1 A
B)3.6E-1 A
C)1.3E-2 A.
D)2.8E+2 A
E)1.3E-2 A

Question 80

By what factor is the self-inductance of an air solenoid changed if only its cross-sectional area,A,multiplied by eight?

A)1/8
B)8
C)24
D)64
E)1/64