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American System Physics

A Physics Toolkit

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Representing Motion

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Accelerated Motion

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Forces in One Dimension

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Displacement and Force in Two Dimensions

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Motion in Two Dimensions

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Gravitation

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Rotational Motion

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Momentum and Its Conservation

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Energy and Its Conservation

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Thermal Energy

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States of Matter

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Vibrations and Waves

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Sound

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Fundamentals of Light

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Reflection and Refraction

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Interference and Diffraction

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Electrostatics

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Electric Current and Circuits

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Magnetic Fields

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Electromagnetic Induction and Electromagnetism

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Quantum Theory and the Atom

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Solid-State Electronics

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Nuclear and Particle Physics

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Quiz

21.2 Generators and Alternating Current – Quiz

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Generators and Alternating Current American Physics quiz: free practice for American System Physics. Work through this Generators and Alternating Current American Physics quiz and get instant feedback on every answer. If you get stuck, review the 21.2 Generators and Alternating Current — Notes lesson. More practice: American System Physics — all units. Syllabus: OpenStax free science textbooks.

  • Questions count: 50
  • Passing grade: A-
  • Time limit:
Start Quiz
1.
At the instant pictured, sides A and B of the generator coil sweep straight across the horizontal field lines. What is the size of the induced e.m.f. now?
Dr Dabour Hub Academy - figure 1
Zero, because sides A and B are in the field
Zero, because the coil is parallel to the field
Maximum (peak), because sides A and B move at right angles to the field lines
Half the peak value
2.
The diagram shows the coil with its plane perpendicular to the field. Why is the induced e.m.f. zero at this instant?
Dr Dabour Hub Academy - figure 2
The flux through the coil is zero
The coil has stopped rotating
The brushes have lost contact
Sides A and B are moving parallel to the field lines, so they cut none
3.
The coil in the diagram has turned so its plane is at 30° to the field lines (measured from the parallel position). How does the e.m.f. compare with the peak value E0?
Dr Dabour Hub Academy - figure 3
It is zero
It is E0 cos 30° ≈ 0.87 E0
It equals E0
It is E0 sin 30° = 0.50 E0
4.
The oscilloscope trace shows the output of an a.c. generator. What is the frequency of the output?
Dr Dabour Hub Academy - figure 4
20 Hz
200 Hz
50 Hz
12.5 Hz
5.
Using the same oscilloscope trace, what is the rms output p.d.?
Dr Dabour Hub Academy - figure 5
6.0 V
12 V
8.49 V
4.24 V
6.
For the trace shown, what is the period of the signal?
Dr Dabour Hub Academy - figure 6
5.0 ms
10 ms
2.0 ms
20 ms
7.
The trace shows the output of a generator. What kind of connector does this generator have, and how many rotations per second does its coil make?
Dr Dabour Hub Academy - figure 7
Slip rings; 25 rotations per second
A split-ring commutator; 50 rotations per second
A split-ring commutator; 25 rotations per second
Slip rings; 50 rotations per second
8.
Trace X was recorded from a generator. The coil speed is then doubled. Compared with trace X, the new trace has
Dr Dabour Hub Academy - figure 8
twice the peak height and the same number of cycles
the same peak height and twice as many cycles
half the peak height and twice as many cycles
twice the peak height and twice as many cycles on screen
9.
The graph shows a generator's output. At which time is the coil plane perpendicular to the magnetic field?
Dr Dabour Hub Academy - figure 9
10 ms
5 ms
15 ms
25 ms
10.
Using the graph of the generator output, what are the rms e.m.f. and frequency?
Dr Dabour Hub Academy - figure 10
10.0 V and 50 Hz
7.07 V and 50 Hz
7.07 V and 20 Hz
14.1 V and 25 Hz
11.
Which set of entries completes the table?
P: peak; Q: zero; R: peak with opposite sign
P: zero; Q: peak; R: zero
P: peak; Q: zero; R: peak, same sign
P: half peak; Q: zero; R: half peak
12.
Which conclusion is best supported by the table of generator results?
Peak e.m.f. is independent of speed
Frequency is inversely proportional to rotation rate
Peak e.m.f. is directly proportional to rotation rate
Peak e.m.f. is proportional to the square of the speed
13.
What entry replaces the question mark in the table?
Straight horizontal line
Only negative halves
Square wave
Both halves above the axis
14.
The screen is 8 divisions high (4 above centre) and 10 divisions wide. Which student chose the best settings to measure the peak and period?
B
A
C
D
15.
Using the repeat readings in the table, what is the mean frequency?
20.0 Hz
50.0 Hz
0.050 Hz
100 Hz
16.
Which reading in the table is most likely anomalous?
The reading at 8 turns per second
The reading at 2 turns per second
The reading at 6 turns per second
The reading at 10 turns per second
17.
What energy transfer takes place in an a.c. generator?
Kinetic energy to electrical energy
Electrical energy to kinetic energy
Thermal energy to chemical energy
Electrical energy to magnetic energy only
18.
What is the purpose of the slip rings in an a.c. generator?
To reverse the connections every half-turn
To keep each end of the coil connected to the same external terminal
To increase the magnetic field
To convert a.c. into d.c.
19.
Each labelled part of a hand-cranked alternator has one job. Which part is matched with its job correctly?
Split-ring commutator - gives an alternating output
Slip rings - reverse the connections every half-turn
Brushes - carbon contacts linking the rotating coil to the external circuit
Brushes - produce the magnetic field
20.
Two cycle lamps are lit by different machines. Which machine is correctly paired with the output it gives?
a.c. generator - constant steady d.c.
d.c. dynamo - sinusoidal with negative halves
a.c. generator - one direction only
d.c. dynamo - one direction, varying from zero to maximum
21.
A European wall socket has a peak p.d. of 325 V. What rms value would be printed on appliances there?
460 V
230 V
163 V
325 V
22.
US mains is quoted as 120 V. What is the peak p.d.?
85 V
120 V
240 V
170 V
23.
A heater takes an rms current of 5.0 A from a 120 V rms supply. What is the average power?
300 W
600 W
1200 W
850 W
24.
A 60 Ω resistor is connected to a supply of peak p.d. 170 V. What is the average power dissipated?
482 W
2.83 W
241 W
341 W
25.
On an oscilloscope, one cycle of a signal from a hydroelectric generator takes 16.7 ms. At what frequency is it running?
16.7 Hz
60 Hz
0.060 Hz
120 Hz
26.
A demonstration generator has a 100-turn coil of area 0.010 m² turning 50 times per second in a 0.20 T field. Using E0 = NBAω, find its peak e.m.f.
62.8 V
10.0 V
44.4 V
0.628 V
27.
A generator produces a peak e.m.f. of 12 V at 20 Hz. The speed is increased so the frequency becomes 50 Hz. What is the new peak e.m.f.?
12 V
4.8 V
30 V
75 V
28.
Why is it harder to turn a generator when it is supplying a large current?
The magnets become weaker
Friction in the brushes increases with current
The coil becomes heavier
The induced current produces a force opposing the rotation (Lenz's law)
29.
Large power-station generators rotate an electromagnet inside fixed coils. What is the main advantage?
The large output current does not pass through sliding contacts
It produces d.c. directly
It needs no energy input
It removes the need for a magnetic field
30.
A generator's output sine wave has peak 8.0 V and frequency 25 Hz. A stronger magnet is fitted and the speed is unchanged. Which output is possible?
Peak 8.0 V, frequency 40 Hz
Peak 12 V, frequency 40 Hz
Peak 5.0 V, frequency 25 Hz
Peak 12 V, frequency 25 Hz
31.
For the trace shown, the peak p.d. is _____.
Dr Dabour Hub Academy - figure 11
10 V
5.0 V
20 V
2.5 V
32.
Using the table of results, X and Y are _____.
X = 3.0 V, Y = 15 Hz
X = 4.5 V, Y = 15 Hz
X = 4.5 V, Y = 10 Hz
X = 6.0 V, Y = 30 Hz
33.
The rms value of an alternating p.d. equals its peak value divided by _____.
2
√2
π
√3
34.
An a.c. current has a peak value of 5.0 A. Its rms value is _____.
3.54 A
2.50 A
7.07 A
5.00 A
35.
A simple two-pole generator coil turns 3600 times per minute. The output frequency is _____.
3600 Hz
120 Hz
30 Hz
60 Hz
36.
A bicycle-wheel alternator: 250 turns, area 4.0 × 10^-3 m², field 0.050 T, 30 rotations per second. The rms output is about _____.
9.42 V
6.66 V
1.50 V
13.3 V
37.
In the diagram the coil plane is perpendicular to the field, so the flux through the coil is at its maximum and the induced e.m.f. is also at its maximum. True or false?
Dr Dabour Hub Academy - figure 12
False, because the flux is maximum but its rate of change is zero, so the e.m.f. is zero
True
False, because the flux is zero and the e.m.f. is maximum
True only for a coil with many turns
38.
A student wants to measure the peak of trace X more precisely. Changing the Y-gain to 1.0 V/div so the trace fills more of the screen would help. True or false?
Dr Dabour Hub Academy - figure 13
True
False, the Y-gain changes the actual p.d.
False, it would only change the period
True only if the time-base is also changed
39.
The results in the table would be more reliable if a motor with a speed control turned the generator instead of a hand crank. True or false?
False, hand turning is more precise
True
False, speed does not affect the output
True only for d.c. dynamos
40.
Using E0 = NBAω, all four generators in the table have the same peak e.m.f. True or false?
False, W is the largest
False, Z is the largest
True
False, Y is the smallest
41.
Of the generators in the table, Z produces the output with the shortest period. True or false?
False, Y has the shortest period
False, all periods are equal
False, W and X have the shortest period
True
42.
The frequency of an alternating current is the number of complete cycles per second. True or false?
True
False, it is the time for one cycle
False, it is the peak current
True only for mains electricity
43.
A bench power pack gives an alternating output whose peak is 24 V; a d.c. supply of about 17 V would heat a resistor at the same average rate. True or false?
False, it would need 24 V d.c.
False, it would need 12 V d.c.
True
False, it would need 34 V d.c.
44.
A 120 V rms a.c. supply delivers the same average power to a lamp as a steady 120 V d.c. supply. True or false?
False, the a.c. supply delivers more because its peak is 170 V
False, a.c. delivers half the power
True only at 60 Hz
True
45.
The average value of a sinusoidal alternating current over one complete cycle is zero, yet it still heats a resistor. True or false?
True
False, the average current is the peak value
False, a.c. cannot heat resistors
True only for square waves
46.
A small wind-turbine alternator produces a signal repeating every 2.5 ms, which corresponds to 400 Hz. True or false?
False, it corresponds to 2.5 Hz
False, it corresponds to 40 Hz
True
False, it corresponds to 0.4 Hz
47.
Doubling the number of turns on a generator coil (same speed) doubles both the peak e.m.f. and the frequency. True or false?
True
False, both stay the same
False, the frequency doubles but the peak is unchanged
False, because the peak doubles but the frequency is unchanged
48.
To reduce uncertainty in the period, it is better to measure the time for several cycles on the screen and divide by the number of cycles. True or false?
False, one cycle is always more accurate
False, the period cannot be found from a trace
True
True only with a d.c. signal
49.
In a simple a.c. generator the two long sides of the coil move in opposite directions, so their induced e.m.f.s add round the coil. True or false?
False, the e.m.f.s cancel
True
True only when the coil is perpendicular to the field
False, only one side cuts field lines
50.
For a two-pole generator, one complete rotation of the coil produces one complete cycle of a.c. True or false?
False, it produces two cycles
False, it produces half a cycle
True
True only for dynamos
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