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1 | |
Lenz's law is a consequence of the fact that energy must be conserved. |
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| | A) | True |
| | B) | False |
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2 | |
The magnitude of an induced emf is directly proportional to the rate at which magnetic flux lines are cut by a conductor. |
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| | A) | True |
| | B) | False |
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3 | |
A generator is a device that converts magnetic energy to electric energy. |
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| | A) | True |
| | B) | False |
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4 | |
A transformer can be used for alternating current only. |
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| | A) | True |
| | B) | False |
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5 | |
The back emf in the armature of an electric motor is zero when the motor is started and rises as the motor’s speed increases. |
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| | A) | True |
| | B) | False |
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6 | |
If the armature of a generator is rotating with a constant angular velocity in a constant magnetic field, the induced emf will be constant. |
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| | A) | True |
| | B) | False |
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7 | |
For an ac generator, the maximum current occurs when the induced emf is a minimum. |
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| | A) | True |
| | B) | False |
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8 | |
Every motor is also a generator. |
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| | A) | True |
| | B) | False |
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9 | |
In a shunt-wound motor, the output speed can be controlled by varying the input current to either the field windings or the armature windings. |
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| | A) | True |
| | B) | False |
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10 | |
A series-wound motor is preferable if a large starting torque is desirable. |
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| | A) | True |
| | B) | False |
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11 | |
When a wire moves perpendicular to a magnetic field, the induced emf does not depend on the |
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| | A) | velocity of the wire |
| | B) | resistance of the wire |
| | C) | flux density of the magnetic field |
| | D) | orientation of the wire |
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12 | |
A wire parallel to the paper moves downward into a flux density directed into the paper. The induced current will be |
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| | A) | directed to the left |
| | B) | zero because the motion is perpendicular to the field |
| | C) | directed to the right |
| | D) | opposite to the induced emf, according to Lenz's law |
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13 | |
Which of the following is not a component of a simple ac generator? |
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| | A) | Field magnet |
| | B) | Split-ring commutator |
| | C) | Armature |
| | D) | Brushes |
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14 | |
The instantaneous emf produced by a simple ac generator is a maximum when the angular displacement of the loop is an |
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| | A) | integral multiple of π/2 |
| | B) | odd multiple of π/2 |
| | C) | even multiple of π |
| | D) | odd multiple of π |
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15 | |
A coil of wire of area 0.3 m2 has 90 turns of wire and is suspended with its plane perpendicular to a uniform magnetic field. An emf of -2 V is induced
when the coil is flipped parallel to the field in 0.4 s. The flux density of the field
is approximately |
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| | A) | 0.01 T |
| | B) | 0.02 T |
| | C) | 0.03 T |
| | D) | 0.04 T |
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16 | |
An armature in an ac generator consists of 600 turns, each having an area of 0.4 m2. The coil rotates in a field of flux density 2 x 10-3 T. What must the
rotational frequency of the armature be to induce a maximum emf of 12 V? |
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| | A) | 200 rpm |
| | B) | 226 rpm |
| | C) | 239 rpm |
| | D) | 478 rpm |
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17 | |
A wire 0.2 m long moves at a constant speed of 5 m/s in a direction that is 37° with respect to a magnetic field of 0.2 T directed perpendicular to the wire. The induced emf is approximately |
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| | A) | 0.12 V |
| | B) | 0.16 V |
| | C) | 0.20 V |
| | D) | 0.24 V |
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18 | |
A 120-V shunt motor has a field resistance of 200 Ω
and an armature resistance of 12 Ω. At full speed, the back emf of the motor is 90 V. The operating current
is approximately |
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| | A) | 3 A |
| | B) | 6 A |
| | C) | 12 A |
| | D) | 16 A |
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19 | |
A transformer has 400 secondary turns and only 50 primary turns. If an ac voltage of 120 V is connected to the primary coil, the output voltage is |
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| | A) | 15 V |
| | B) | 240 V |
| | C) | 430 V |
| | D) | 960 V |
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20 | |
A magnetic flux changing at the rate of one will induce an emf of one for each of the conductor. |
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21 | |
For a dc motor in operation, the net voltage delivered to the armature coils is equal to the less the . |
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22 | |
The major components of a simple ac generator are the , , and with . |
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23 | |
Three types of motors discussed in this text are the , , and . |
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24 | |
The current drawn by an electric motor is a maximum when the motor is . |
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25 | |
If an ideal transformer, the power output is equal to the . |
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26 | |
In a step-down transformer, the number of turns in the secondary coil must be the number of turns in the primary coil. |
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27 | |
A simple transformer consists of three essential parts: , , and . |
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28 | |
In a(n) , the field windings and the armature windings are connected in parallel. |
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