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 2 3 4 25% 25% 25% 25% 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 1. 500 mJ 2. 1.0 J 3. 2.0 J 4. 5.0 J
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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?
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1. 500 mJ
2. 1.0 J
3. 2.0 J
4. 5.0 J
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?
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1. 5 A
2. 10 A
3. 15 A
4. 20 A
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?
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1. 300 kWH
2. 482 kWH
3. 694 kWH
4. 842 kWH
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?
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1. 50 mV
2. 31 mV
3. 37 mV
4. 44 mV
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?
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1. 7.5 Hz
2. 7.1 Hz
3. 8.0 Hz
4. 8.4 Hz
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.
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1. 1.5 ms
2. 3.0 ms
3. 4.0 ms
4. 5.0 ms
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?
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1. zero
2. 13 mV
3. 26 mV
4. 20 mV
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?
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1. 70 mA
2. 140 mA
3. 500 mA
4. 800 mA
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?
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1. 60 mA
2. 12 mA
3. 0.24 mA
4. 48 mA
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 π/2 rads and increasing at the rate of 0.60 rad/s, what is the magnitude of the emf induced in the loop?
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1. 24 mV
2. zero
3. 18 mV
4. 30 mV
The magnet moving past an object will produce eddy currents in the object if the object is?
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1. magnetic material only
2. a conductor
3. an insulator
4. a liquid
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?
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1. the magnet enters the loop
2. the magnet is half way through
3. the magnet is leaving the loop
4. never
Electricity may be generated by rotating a loop of wire between the poles of a magnet. The induced current is greatest when?
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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
The "back emf" of a motor refers to a source of voltage that?
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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
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?
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1. t
2. r
3. R
4. none of these
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?
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1. 3 A
2. 6 A
3. zero
4. there is insufficient information to work this problem.
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?
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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.
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?
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1. 3 A
2. 6 A
3. 1.5 A
4. zero
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?
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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.
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?
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1. clockwise
2. counterclockwise
3. zero
4. along the length of the magnet
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?
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1. be increasing counterclockwise
2. stay constant
3. increase clockwise also
4. stay zero
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
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1. clockwise
2. counterclockwise
3. zero
4. along the length of the magnet
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.
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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
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?
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1. 18.8 V
2. 10.0 V
3. 12.5 V
4. 11.3 V
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?
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1. 1.2 mH
2. 2.4 mH
3. 9.6 mH
4. 48 mH
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?
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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
By what factor is the self-inductance of an air solenoid changed if its number of coil turns, N, is tripled?
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1. 1/3
2. 3
3. 6
4. 9
By what factor is the self-inductance of an air solenoid changed if its cross-sectional area, A, is tripled?
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1. 1/3
2. 3
3. 6
4. 9
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?
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1. 83 mH
2. 45 mH
3. 37 mH
4. 27 mH
By what factor is the self-inductance of an air solenoid changed if its length and number of coil turns are both tripled?
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1. 1/3
2. 3
3. 6
4. 9
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?
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1. 12.5 x 10-3 s
2. 5 x 10-3 s
3. 2.5 x 10-2 s
4. 200 s
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?
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1. 2.5 A
2. 12 A
3. 0.015 A
4. 2400 A
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?
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1. 2.25 V
2. 1.0 V
3. 0.60 V
4. 0.15 V
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?
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1. 250 m/s
2. 338 m/s
3. 375 m/s
4. 417 m/s
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?
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1. zero
2. 3.0 A
3. 6.0 A
4. 15.0 A
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?
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1. zero
2. 12.5 V
3. 21.6 V
4. 25.0 V
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?
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1. 0.045 V
2. 0.03 V
3. 0.47 V
4. 0.019 V
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?
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1. zero
2. 125 V
3. 216 V
4. 250 V
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?
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1. zero
2. 12.5 V
3. 21.6 V
4. 25.0 V
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?
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1. 12.5 V
2. 125 V
3. 188 V
4. 250 V
How is the energy stored in a current carrying inductor related to its self-inductance, L?
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1. directly proportional to L2
2. directly proportional to L1/2
3. directly proportional to L
4. inversely proportional to L
How is the energy stored in a current carrying inductor related to the current value, I?
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1. directly proportional to I2
2. directly proportional to I1/2
3. directly proportional to I
4. inversely proportional to I
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?
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1. 5 Wb
2. 0.45 Wb
3. 0.25 Wb
4. 0.135 Wb
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?
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1. 5 Wb
2. 0.52 Wb
3. 0.39 Wb
4. 0.225 Wb
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?
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1. zero
2. one time constant
3. reciprocal of one time constant
4. ten time constants
The basic function of the electric generator is which one of the following conversion processes?
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1. mechanical energy to electrical
2. electrical energy to mechanical
3. low voltage to high or vice versa
4. alternating current to direct
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?
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1. Faraday
2. Lenz
3. Ampere
4. Volta
The function of the electric motor is which one of the following conversion processes?
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1. mechanical energy to electrical
2. electrical energy to mechanical
3. low voltage to high or vice versa
4. alternating current to direct
The back emf in an electric motor is its maximum value under which of the following conditions?
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1. motor speed is zero
2. current is a maximum
3. voltage is a maximum
4. motor speed is maximum
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?
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1. zero
2. one time constant
3. at a time t = L/R
4. ten time constants
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?
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1. 3.9 A
2. 2.4 A
3. 1.5 A
4. 0.9 A
The self-inductance of a solenoid increases under which of the following conditions?
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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
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?
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1. zero
2. battery voltage divided by inductance
3. battery voltage times inductance
4. battery voltage divided by resistance
What is the stored energy in a 0.50 mH coil carrying a current of 4.0 A?
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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
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?
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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
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?
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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
The operation of a tape player to play music depends on which of the following?
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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
The operation of an electric motor depends on which of the following effects?
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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
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?
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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
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?
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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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