An RL circuit in which L = 0.4 H and R = 5 W is connected to a battery with E = 22 V at time t = 0. What energy is stored in the inductor when the current in the circuit is 0.5 A?

1
An RL circuit in which L 0.4 H and R 5 W is
connected to a battery with E 22 V at time t
0. What energy is stored in the inductor when the
current in the circuit is 0.5 A?

1. 500 mJ
2. 1.0 J
3. 2.0 J
4. 5.0 J

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2
You can calculate the induced current in a ring
by setting the emf from a changing flux E
-DØ/Dt equal to the voltage change -LDI/Dt in an
inductor. If the magnetic field through a ring of
area 1 cm2 collapses from 0.02 T to zero and the
inductance of the ring is 10-7 H, what is the
induced current in the ring?

1. 5 A
2. 10 A
3. 15 A
4. 20 A

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3
Superconductors have been discussed as a means
for electrical energy storage. Because they are
resistanceless, a current once started in a loop
would continue without loss. If a current of 104
A were started in a huge toroidal coil of radius
1 km and inductance 50 H, how much electrical
energy (in kW hours) could be stored?

1. 300 kWH
2. 482 kWH
3. 694 kWH
4. 842 kWH

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4
A small airplane with a wing span of 12 m flies
horizontally and due north at a speed of 60 m/s
in a region where the magnetic field of the earth
is 60 µT directed 60 below the horizontal. What
is the magnitude of the induced emf between the
ends of the wing?

1. 50 mV
2. 31 mV
3. 37 mV
4. 44 mV

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5
What is the minimum frequency with which a
200-turn, flat coil of cross-sectional area of
300 cm2 can be rotated in a uniform 30-mT
magnetic field if the maximum value of the
induced emf is to equal 8.0 V?

1. 7.5 Hz
2. 7.1 Hz
3. 8.0 Hz
4. 8.4 Hz

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6
A series circuit contains a 12-V battery, a 2000
W resistor, and a 3 mH inductor. If the switch to
the battery is closed at t 0, find the time
required for the current in the circuit to reach
63 of its final value.

1. 1.5 ms
2. 3.0 ms
3. 4.0 ms
4. 5.0 ms

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7
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 p rads
and is decreasing at the rate of 0.50 rad/s, what
is the magnitude of the emf induced in the loop?

1. zero
2. 13 mV
3. 26 mV
4. 20 mV

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8
A flat coil of wire consisting of 20 turns, each
with an area of 50 cm2, is positioned
perpendicularly to a uniform magnetic field that
increases its magnitude at a constant rate from
2.0 T to 6.0 T in 2.0 s. If the coil has a total
resistance of 0.40 W, what is the magnitude of
the induced current?

1. 70 mA
2. 140 mA
3. 500 mA
4. 800 mA

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9
A planar loop consisting of four turns of wire,
each of which encloses 200 cm2, is oriented
perpendicularly to a magnetic field that
increases uniformly in magnitude from 10 mT to 25
mT in a time of 5.0 ms. What is the resulting
induced current in the coil if the resistance of
the coil is 5.0 W?

1. 60 mA
2. 12 mA
3. 0.24 mA
4. 48 mA

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10
A rectangular loop (area 0.15 m2 ) turns in a
uniform magnetic field with B 0.20 T. At an
instant when the angle between the magnetic field
and the normal to the plane of the loop is p/2
rads and increasing at the rate of 0.60 rad/s,
what is the magnitude of the emf induced in the
loop?

1. 24 mV
2. zero
3. 18 mV
4. 30 mV

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11
The magnet moving past an object will produce
eddy currents in the object if the object is?

1. magnetic material only
2. a conductor
3. an insulator
4. a liquid

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12
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?

1. the magnet enters the loop
2. the magnet is half way through
3. the magnet is leaving the loop
4. never

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13
Electricity may be generated by rotating a loop
of wire between the poles of a magnet. The
induced current is greatest when?

1. the plane of the loop is parallel to the magnetic
   field
2. the plane of the loop is perpendicular to the
   magnetic field
3. the magnetic flux through the loop is a maximum
4. the plane of the loop makes an angle of 45 with
   the magnetic field

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14
The "back emf" of a motor refers to a source of
voltage that?

1. occurs when the motor runs backwards
2. occurs when the motor is used as a generator
3. is biggest when the current through the motor is
   biggest
4. is biggest when the motor turns fastest

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15
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
will be directly proportional to?

1. t
2. r
3. R
4. none of these

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16
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?

1. 3 A
2. 6 A
3. zero
4. there is insufficient information to work this
   problem.

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17
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?

1. the rod is vertical with the positive end higher.
2. the rod is horizontal with the positive end
   toward the north.
3. the rod is horizontal with the positive end
   toward the east.
4. the rod is horizontal with the positive end
   toward the west.

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18
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 amps, the current in the
secondary will be?

1. 3 A
2. 6 A
3. 1.5 A
4. zero

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19
A bar magnet is falling through a loop of wire
with constant velocity with the north pole
entering first. The induced current will be
greatest when the magnet is located so that the
loop is?

1. near either the north or the south pole.
2. near the north pole only
3. near the middle of the magnet.
4. with no acceleration, the induced current is zero.

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20
A bar magnet is falling through a loop of wire
with constant velocity with the north pole
entering first. As the south pole leaves the loop
of wire the induced current will be, as viewed
from above?

1. clockwise
2. counterclockwise
3. zero
4. along the length of the magnet

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21
Two loops of wire are arranged so that a changing
current in one will induce a current in the
other. If the current in the first is increasing
clockwise by one amp every second, the current in
the second loop will?

1. be increasing counterclockwise
2. stay constant
3. increase clockwise also
4. stay zero

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22
A bar magnet is falling through a loop of wire
with constant velocity with the north pole
entering first. As the north pole enters the
wire, the induced current will be, as viewed from
above

1. clockwise
2. counterclockwise
3. zero
4. along the length of the magnet

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23
A 12 V battery is connected in series with a
switch, 6 W resistor and coil. What energy is
stored in the coil when the current is 2.0 A? The
time constant is 4 x 10-4 s.

1. 4.8 x 10-3 J
2. 9.6 x 10-3 J
3. 14.4 x 10-3 J
4. 29 x 10-3 J

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24
An electric motor draws a current of 2.5 A from a
15 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 A. What
is the back emf of the motor when running at
normal speed?

1. 18.8 V
2. 10.0 V
3. 12.5 V
4. 11.3 V

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25
A 12 V battery is connected in series with a
switch, resistor and coil. If the circuit's time
constant is 4 x 10-4 s and the final steady
current after the switch is closed becomes 2.0 A,
what is the value of the inductance?

1. 1.2 mH
2. 2.4 mH
3. 9.6 mH
4. 48 mH

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26
A loop of wire is held in a vertical position at
the equator, with the axis of the loop pointing
in the east-west direction. Which change will
induce the greatest current in the loop?

1. raising the loop to a higher elevation
2. moving the loop north
3. rotating the loop so its axis is vertical
4. rotating the loop so its axis is north-south

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27
By what factor is the self-inductance of an air
solenoid changed if its number of coil turns, N,
is tripled?

1. 1/3
2. 3
3. 6
4. 9

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28
By what factor is the self-inductance of an air
solenoid changed if its cross-sectional area, A,
is tripled?

1. 1/3
2. 3
3. 6
4. 9

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29
What is the self-inductance in a coil which
experiences a 1.5 V induced emf when the current
is changing at a rate of 55 A/s?

1. 83 mH
2. 45 mH
3. 37 mH
4. 27 mH

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30
By what factor is the self-inductance of an air
solenoid changed if its length and number of coil
turns are both tripled?

1. 1/3
2. 3
3. 6
4. 9

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31
An RL series circuit has the following
components 2.5 mH coil, 0.5 ohm resistor, 6.0 V
battery, ammeter and switch. What is the time
constant of this circuit?

1. 12.5 x 10-3 s
2. 5 x 10-3 s
3. 2.5 x 10-2 s
4. 200 s

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32
An RL series circuit has the following
components 2.5 mH coil, 0.5 ohm resistor, 6.0 V
battery, ammeter and switch. After the switch has
been closed for a long time, what is the final
value of the current?

1. 2.5 A
2. 12 A
3. 0.015 A
4. 2400 A

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33
A 0.20 m wire is moved perpendicular to a 0.5
magnetic field at a speed of 1.5 m/s. What emf is
induced across the ends of the wire?

1. 2.25 V
2. 1.0 V
3. 0.60 V
4. 0.15 V

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34
An airplane with a wingspan of 60 m flies
parallel to the earth's surface at a point where
the downward component of the earth's magnetic
field is 0.4 x 10-4 T. If the induced potential
between wingtips is 0.90 V, what is the plane's
speed?

1. 250 m/s
2. 338 m/s
3. 375 m/s
4. 417 m/s

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35
A motor with a coil resistance of 10 ohms is
supplied by a voltage of 90 V. What is the
current in the motor when it is running at its
maximum speed with a back emf of 60 V?

1. zero
2. 3.0 A
3. 6.0 A
4. 15.0 A

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36
A 500 turn circular coil with an area of 0.05 m2
is mounted on a rotating frame which turns at a
rate of 20 rad/s in the presence of a 0.05 T
uniform magnetic field which 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 30 angle to the field?

1. zero
2. 12.5 V
3. 21.6 V
4. 25.0 V

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37
A coil with a self-inductance of 0.75 mH
experiences a constant current build up from zero
to 5.0 A in 0.125 s. What is the induced emf
during this interval?

1. 0.045 V
2. 0.03 V
3. 0.47 V
4. 0.019 V

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38
A 500 turn circular coil with an area of 0.05 m2
is mounted on a rotating frame which turns at a
rate of 20 rad/s in the presence of a 0.05 T
uniform magnetic field which 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 parallel to the field?

1. zero
2. 125 V
3. 216 V
4. 250 V

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39
A 500 turn circular coil with an area of 0.05 m2
is mounted on a rotating frame which turns at a
rate of 20 rad/s in the presence of a 0.05 T
uniform magnetic field which 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 90 angle to the field?

1. zero
2. 12.5 V
3. 21.6 V
4. 25.0 V

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40
A circular coil with an area of 0.05 m2 is
wrapped with 500 turns of wire and is placed in a
uniform magnetic field perpendicular to its
plane. If the field changes in value from -0.10 T
to 0.15 T in an interval of 0.5 s, what average
voltage is induced in the coil?

1. 12.5 V
2. 125 V
3. 188 V
4. 250 V

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41
How is the energy stored in a current carrying
inductor related to its self-inductance, L?

1. directly proportional to L2
2. directly proportional to L1/2
3. directly proportional to L
4. inversely proportional to L

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42
How is the energy stored in a current carrying
inductor related to the current value, I?

1. directly proportional to I2
2. directly proportional to I1/2
3. directly proportional to I
4. inversely proportional to I

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43
A uniform 1.5 T magnetic field passes
perpendicular through the plane of a wire loop
0.3 m2 in area. What flux passes through the loop?

1. 5 Wb
2. 0.45 Wb
3. 0.25 Wb
4. 0.135 Wb

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44
A uniform 1.5 T magnetic field passes through the
plane of a wire loop 0.3 m2 in area. What flux
passes through the loop when the direction of the
1.5 T field is at a 30 angle to the normal of
the loop plane?

1. 5 Wb
2. 0.52 Wb
3. 0.39 Wb
4. 0.225 Wb

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45
In a circuit made up of inductor, resistance,
ammeter, battery and switch in series, at which
one of the following time intervals after the
switch is closed will the rate of current
increase be greatest?

1. zero
2. one time constant
3. reciprocal of one time constant
4. ten time constants

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46
The basic function of the electric generator is
which one of the following conversion processes?

1. mechanical energy to electrical
2. electrical energy to mechanical
3. low voltage to high or vice versa
4. alternating current to direct

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47
The principle or law which says that an induced
emf in a circuit loop produces a current whose
magnetic field opposes further change of magnetic
flux, is credited to which one of the following
scientists?

1. Faraday
2. Lenz
3. Ampere
4. Volta

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48
The function of the electric motor is which one
of the following conversion processes?

1. mechanical energy to electrical
2. electrical energy to mechanical
3. low voltage to high or vice versa
4. alternating current to direct

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49
The back emf in an electric motor is its maximum
value under which of the following conditions?

1. motor speed is zero
2. current is a maximum
3. voltage is a maximum
4. motor speed is maximum

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50
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
time intervals, as measured after the switch is
closed?

1. zero
2. one time constant
3. at a time t L/R
4. ten time constants

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51
A motor with an internal resistance of 10 ohm,
connected to a 24 V battery of negligible
resistance, produces a back emf of 15 V and hence
will carry what current?

1. 3.9 A
2. 2.4 A
3. 1.5 A
4. 0.9 A

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52
The self-inductance of a solenoid increases under
which of the following conditions?

1. solenoid length is increased
2. cross sectional area is decreased
3. number of coils per unit length is decreased
4. number of coils is increased

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53
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?

1. zero
2. battery voltage divided by inductance
3. battery voltage times inductance
4. battery voltage divided by resistance

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54
What is the stored energy in a 0.50 mH coil
carrying a current of 4.0 A?

1. 2.0 x 10-3 J
2. 4.0 x 10-3 J
3. 8.0 x 10-3 J
4. 12.0 x 10-3 J

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55
In a primary/secondary coil combination, the
current in the secondary is a maximum at the
moment that which one of the following conditions
is met in the primary?

1. current is maximum in positive direction
2. current is maximum in negative direction
3. rate of current change is maximum
4. voltage is maximum in positive direction

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56
A sensitive galvanometer is connected to a wire
loop which is placed within the magnetic field of
a strong horseshoe magnet. The galvanometer shows
a deflection under what conditions?

1. when the wire is moved parallel to the field
2. when the wire is moved perpendicularly to the
   field
3. when neither wire nor magnet are moving
4. when the wire's axis is parallel to the field

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57
The operation of a tape player to play music
depends on which of the following?

1. the Doppler effect
2. the photoelectric effect
3. the force acting on a current-carrying wire in a
   magnetic field
4. induced current from the motion of a magnet past
   a wire

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58
The operation of an electric motor depends on
which of the following effects?

1. the Doppler effect
2. the photoelectric effect
3. the force acting on a current-carrying wire in a
   magnetic field
4. induced current from the motion of a wire in a
   magnetic field

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59
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?

1. enhance the effect which produces it
2. produce a greater heating effect
3. produce the greatest voltage
4. oppose the effect which produces it

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60
A coil with a self-inductance of 1.5 mH will show
a rate of current change of what value when
inducing an emf of 0.30 V?

1. 200 A/s
2. 0.45 x 10-4 A/s
3. 0.05 A/s
4. 0.30 A/s

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