Deck 20: Electromagnetic Induction

Question 1

A conducting rod slides at 4.0 m/s on metal rails separated by 2.0 m.If a current of 40 mA is generated in the circuit with resistance of 0.36 $\Omega$ ,what is 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

Answer 1

1.8 mT

Question 2

A coil is rotated in a magnetic field about an axis perpendicular to the field.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

Answer 2

2

Question 3

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.

Answer 3

V·s/A.

Question 4

A conducting rod is sliding on metal rails with velocity v.A magnetic field B is into the paper,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?

A) up
B) down
C) up then down
D) down then up
E) Insufficient information given,

Question 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 m T magnetic field is perpendicular to both the rod and its velocity.If the resistance of the rod is 15 m $\Omega$ , 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

Question 6

For the typical "household ac voltage" of 120 V,what is the amplitude of the emf?

A) 120 V
B) 240 V
C) 60.0 V
D) 170 V
E) 330 V

Question 7

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

Question 8

A 20 turn coil of area 10 cm² is placed in a magnetic field so that the normal to its area is in the direction of the field.If the field originally has a value of 0.25 T that increases 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

Question 9

The ac motor and generator were invented by

A) Edison.
B) Tesla.
C) Westinghouse.
D) Maxwell.
E) Hertz.

Question 10

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 m T 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

Question 11

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

Question 12

A conducting rod slides at 3.0 m/s on metal rails separated by 2.0 m.The 0.66 m T magnetic field is into the paper.If the resistance is 220 m $\Omega$ , what is the current in the circuit?

A) 18 mA
B) 9.0 mA
C) 87 $\mu$ A
D) 3.0 $\mu$ A
E) 4.5 mA

Question 13

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) $\pi$ lines/m².

Question 14

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

Question 15

A 20 turn coil of area 10 cm² is placed in a magnetic field so that the normal to its area is in the direction of 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 $\times$ 10^-3 T·m²
C) 25 $\times$ 10^-5 T·m²
D) 50 T · m²
E) 25 T · m²

Question 16

A flat coil has a maximum flux 4.0 $\times$ 10^-3 T·m² through it as its area is perpendicular to a uniform magnetic field.When the coil is rotated 60 $^{\circ}$ (so that its normal makes a 60 $^{\circ}$ angle with the field),what is the flux through it?

A) 4.0 $\times$ 10^-3 T·m²
B) 8.0 $\times$ 10^-3 T·m²
C) 3.5 $\times$ 10^-3 T·m²
D) 2.0 $\times$ 10^-3 T·m²
E) 6.9 $\times$ 10^-3 T·m²

Question 17

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 time variation of the magnetic flux.
C) current divided by the time.
D) the magnetic flux density times the area of the loop.

Question 18

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 T 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

Question 19

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 m T 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

Question 20

The emf produced by an ac generator is given by $\varepsilon$ = $\omega$ NBA sin $\omega$ t.In the SI system,what is the value of $\omega$ for a frequency of 60.0 Hz?

A) 60.0 Hz
B) 120 rad/s
C) 2.00 $\pi$ rad/s
D) 377 rad/s
E) 377 Hz

Question 21

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

Question 22

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.

Question 23

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

Question 24

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 $^{\circ}$ below the horizontal,what is the magnetic flux through the desk surface?

A) 1.1 $\times$ 10^-3 Wb
B) 5.2 $\times$ 10^-4 Wb
C) 3.2 $\times$ 10^-4 Wb
D) 7.3 $\times$ 10^-3 Wb

Question 25

Doubling the diameter of a loop of wire produces what change of induced emf?

A) The induced emf is 4 times as much.
B) The induced emf is twice as much.
C) There is no change in the induced emf.
D) The induced emf is 6.28 times as much.

Question 26

A copper pipe is held vertically and a magnet is dropped down the pipe.The magnet is oriented with the 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 is dropped down the tube will it fall faster or slower?

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

Question 27

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

Question 28

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 $\times$ 10⁴ V
B) 240 V
C) 170 V
D) 120 V
E) 100 V

Question 29

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 2.0 A/s?

A) 3.3 H
B) 33 mH
C) 1.6 H
D) 41 mH
E) 0.41 H

Question 30

Eddy current in conductors

A) step up voltages.
B) dissipate energy.
C) increase currents in circuits.
D) decrease net charges.
E) increase net charges.

Question 31

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

A) 160 $\mu$ H
B) 120 $\mu$ H
C) 80.0 $\mu$ H
D) 40.0 $\mu$ H
E) 0.500 $\mu$ H

Question 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) 1.20 mA
B) 10.0 A
C) 120 A
D) 240 A
E) 830 A

Question 33

If the number of turns on the 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.

Question 34

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

Question 35

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.

Question 36

A dc motor is connected to a emf of 12.0 V.The resistance of its windings is 2.0 $\Omega$ .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

Question 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

Question 38

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.

Question 39

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.

Question 40

A transformer with a turns ratio of 20:1 is used to step down 120 vac,what is the peak voltage in the secondary?

A) 6.00 V
B) 7.33 V
C) 170 V
D) 8.50 V

Question 41

A series LR circuit includes a 9.0 V battery,a resistance of 0.20 $\Omega$ ,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

Question 42

A series LR circuit includes a 9.0 V battery,a resistance of 0.50 $\Omega$ ,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

Question 43

A series LR circuit includes a 9.0 V battery,a resistance of 0.50 $\Omega$ ,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

Question 44

A series LR circuit includes a 9.0 V battery,a resistance of 0.50 $\Omega$ ,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

Question 45

A series LR circuit includes a 9.0.V battery,a resistance of 0.500 $\Omega$ ,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

Question 46

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 $\times$ 10^-7 Wb
B) 1.8 $\times$ 10^-7 Wb
C) 0.88 $\times$ 10^-7 Wb
D) 1.2 $\times$ 10^-7 Wb

Question 47

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)I_max
B) (15/16)I_max
C) (1/16)I_max
D) 0.018 I_max
E) 0.98 I_max

Question 48

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.

Question 49

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

Question 50

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

Question 51

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

Question 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

Question 53

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

Question 54

A series LR circuit includes a 9.00 V battery,a 30.0 mH inductor,and a resistance of 5.00 $\Omega$ .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

Question 55

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

A) 1.0 V/m
B) 3.0 $\times$ 10⁸ V/m
C) 9.0 $\times$ 10¹⁶ V/m
D) 1.1 $\times$ 10^-17 V/m
E) 3.3 $\times$ 10^-9 V/m

Question 56

An ideal solenoid has a length of 25 mm.If the windings are compressed so that the length is reduced to 1/2 of its original length how does the inductance change?

A) 1/4 of original
B) to 1/2 of original
C) twice the original
D) quadruple the original

Question 57

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

Question 58

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

A) 3.2 $\times$ 10^-3 J/m³
B) 3.2 J/m³
C) 6.4 J/m³
D) 13 J/m³
E) 13 $\times$ 10^-3 J/m³

Question 59

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

Question 60

A series LR circuit includes a 9.0 V battery,a resistance of 0.50 $\Omega$ ,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

Question 61

A coiled loop of wire is in the field region between the pole tips of an electromagnet.Which of the following things would NOT result in an induced EMF in the coil?

A) Rotation of the coil
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.

Question 62

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

Question 63

A conducting ring is placed such that it is lying flat in the x-y 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 x-y 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 x-y plane.

Question 64

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 what percentage of its final value

A) 100%
B) 90%
C) 37%
D) 63%
E) 50%

Question 65

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 what percentage of its final value

A) 63%
B) 14%
C) 37%
D) 40%
E) 100%

Question 66

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.

Question 67

Induced emf in a straight segment of wire that passes through a magnetic field depends on all of the following,except:

A) the orientation of the wire with respect to the magnetic field direction
B) its length,its resistance
C) its speed
D) the magnetic field strength

Question 68

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 7.5 cm.Assuming the magnetic field in the generator is 0.075 T,what is the number of loops needed in the generator coil?

A) 160
B) 1530
C) 6120
D) 4810
E) 25

Question 69

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) 610 rpm

Question 70

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

Question 71

A square conducting loop is situated such that it is vertical,in the x-z plane.The earth's gravitational field points downward (in the -z direction).The loop is dropped,and maintains its orientation in the x-z plane.A magnetic field pointing in the +y direction exists and is uniform with respect to time,but varies as a function of z: B(z) = B_o(10 m - z).The loop is dropped and allowed to fall under the influence of gravitation.You observe the loop from 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.

Question 72

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 at this time 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

Question 73

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

A) the number of turns the primary and secondary windings have
B) the resistance of the windings
C) the voltage in the primary winding
D) the properties of the core material

Question 74

A generator is designed to produce a maximum emf of 170 V while rotating at 3200 rpm.The 1525 coils of the generator are circular with diameter 7.5 cm.Assuming the magnetic field in the generator is 0.075 T,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