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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

21.3 Transformers and Power Transmission – Quiz

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Transformers and Power Transmission American Physics quiz: free practice for American System Physics. Work through this Transformers and Power Transmission American Physics quiz and get instant feedback on every answer. If you get stuck, review the 21.3 Transformers and Power Transmission — 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.
What is the core of a transformer usually made from?
Laminated soft iron
Solid copper
Steel magnet (permanent)
Plastic insulator
2.
Why does a transformer not work with a steady d.c. input?
d.c. cannot flow through copper coils
A steady current gives a constant flux, so no e.m.f. is induced in the secondary
The core would melt
d.c. makes the secondary current too large
3.
Comparing ideal step-up and step-down transformers, which statement pairs a type with a property correctly?
Step-up - secondary current larger than primary current
Step-down - secondary current larger than primary current
Step-down - Ns greater than Np
Step-up - Vs smaller than Vp
4.
Which pairing of energy loss and the way it is reduced is correct?
Heating of the coils - thin high-resistance wire
Flux leakage - coils placed far apart
Eddy currents in the core - laminated core
Eddy currents - solid iron core
5.
Transformers appear all along the route from a power station to a phone. Which location is matched with the right kind?
Power station output - step-down
Substation near homes - step-up
Doorbell supply - step-up
Phone charger - step-down
6.
A model-railway transformer has 400 turns on its primary and 20 on its secondary and is plugged into 120 V a.c. mains. What p.d. reaches the track?
6.0 V (step-down)
2400 V
12 V
120 V
7.
A student winds 60 turns as the primary and 900 turns as the secondary and connects a 9.0 V a.c. supply. The measured output is 128 V. How does this compare with the ideal prediction?
It equals the ideal value of 128 V
It is above the ideal value of 0.60 V
It is 50% below the ideal 270 V
It is about 5% below the ideal 135 V
8.
An ideal transformer steps 230 V up to 11 500 V. The secondary current is 2.0 A. What is the primary current?
0.040 A
100 A
2.0 A
50 A
9.
An ideal step-down transformer supplies 12 V at 3.0 A to a lamp from 120 V mains. What is the primary current?
30 A
3.0 A
0.30 A
0.030 A
10.
A laptop adapter draws 0.50 A from a 120 V socket and supplies 4.8 A at 12 V to the laptop. What is its efficiency?
96%
104%
10%
9.6%
11.
A 95% efficient transformer delivers 380 W to a load. What power is wasted as heat?
19 W
400 W
20 W
361 W
12.
A wind farm sends 1.0 MW along cables of total resistance 5.0 Ω at 20 kV. How much power is lost as heat in the cables?
12.5 kW
250 W
80 MW
100 W
13.
The transmission current in a cable is halved. What happens to the power lost in the cable?
It halves
It doubles
It falls to one quarter
It is unchanged
14.
The same power is transmitted but the transmission p.d. is raised by a factor of 10. By what factor does the cable loss change?
It is divided by 10
It is multiplied by 10
It is unchanged
It is divided by 100
15.
What is the main reason the grid uses a.c. rather than d.c.?
a.c. has no resistance losses
a.c. travels faster than d.c.
The p.d. of a.c. can easily be changed with transformers
Generators cannot produce d.c.
16.
Which is a disadvantage of transmitting power at hundreds of kilovolts?
Cable losses become very large
Currents become dangerously large
Transformers cannot be used
Tall pylons and very good insulation are needed for safety
17.
What is the output p.d. of the ideal transformer shown?
Dr Dabour Hub Academy - figure 1
120 V
4.8 V
24 V
240 V
18.
The primary of the transformer shown is connected to a 120 V battery (steady d.c.) instead of 120 V a.c. What is the steady output p.d.?
Dr Dabour Hub Academy - figure 2
6.0 V
120 V
0 V
2400 V
19.
A student builds the transformer shown but it becomes hot and has low efficiency. Which is the best improvement?
Dr Dabour Hub Academy - figure 3
Replace the solid core with a laminated soft-iron core
Use a solid copper core
Remove the core completely
Use thinner wire for both coils
20.
In the grid diagram, what type are transformers A and B?
Dr Dabour Hub Academy - figure 4
A is step-down; B is step-up
A is step-up; B is step-down
Both are step-up
Both are step-down
21.
Using the voltages labelled on the grid diagram, calculate Ns/Np for transformer A.
Dr Dabour Hub Academy - figure 5
0.0625
375
16
1/16 000
22.
For the grid shown, transformer B has turns ratio Np/Ns of about
Dr Dabour Hub Academy - figure 6
3333
16
0.0003
120
23.
A student varies the number of secondary turns on a demonstration transformer (400 primary turns, 12 V a.c. primary). The results are shown. Which statement is best supported?
Dr Dabour Hub Academy - figure 7
Vs is exactly equal to the ideal prediction
Vs is proportional to Ns, but slightly below the ideal prediction
Vs is inversely proportional to Ns
Vs does not depend on Ns
24.
In the transformer experiment shown, what is the most likely reason the measured Vs is a little lower than Vp × Ns/Np?
Dr Dabour Hub Academy - figure 8
The frequency is increased in the secondary
The voltmeter reads peak values
Not all the flux from the primary links the secondary
The primary has too few turns
25.
Using the table, what happens to the efficiency as the load current increases?
It falls from 96% to 88%
It rises from 88% to 96%
It stays at 96%
It falls from 96% to 20%
26.
For test 2 in the table, how much power is wasted in the transformer?
120 W
9.6 W
110 W
4.6 W
27.
Which set of entries completes the table?
P: thin wire; Q: solid core; R: coils far apart
P: laminated core; Q: thick wire; R: steel core
P: thick low-resistance copper wire; Q: laminated core; R: coils wound close together on one core
P: higher frequency; Q: d.c. supply; R: air core
28.
Which transformer in the table gives the largest output p.d.?
B (24 V)
A (60 V)
C (50 V)
D (12 V)
29.
Comparing input and output powers, which of the four listed transformers wastes the smallest fraction of its input?
W (95%)
X (96%)
Z (94%)
Y (97.5%)
30.
Which sequence correctly describes how a transformer works?
a.c. in primary → changing field → core links flux to secondary → changing flux in secondary → e.m.f. induced
d.c. in primary → steady field → e.m.f. induced in secondary
Current flows through core → secondary heats → e.m.f.
Secondary induces primary → field reverses → d.c. output
31.
A transformer with more turns on its secondary than its primary is called a _____ transformer.
step-up
step-down
laminated
rectifying
32.
A transformer changes 120 V to 6.0 V. The primary has 1000 turns. The secondary must have _____ turns.
50
20
20 000
500
33.
An ideal transformer has turns ratio Ns/Np = 1/20. If the primary current is 0.50 A, the secondary current is _____.
0.025 A
0.50 A
20 A
10 A
34.
A village receives 200 kW sent at 10 kV through 8.0 Ω lines. The fraction of the power wasted in the lines is _____.
0.08%
1.6%
16%
40%
35.
The transformer shown has a turns ratio Ns/Np of _____ and is a step-down transformer.
Dr Dabour Hub Academy - figure 9
20
0.5
760
0.05 (1:20)
36.
Using the table, X and Y are _____.
X = 10 A, Y = 2.0 kW
X = 250 A, Y = 250 kW
X = 50 A, Y = 10 kW
X = 10 A, Y = 0.40 kW
37.
In a transformer the primary and secondary coils are electrically connected to each other by wires. True or false?
True
True only in step-up transformers
False, energy is transferred by the changing magnetic field in the core
False, energy is transferred by sound waves
38.
The alternating e.m.f. induced in the secondary coil has the same frequency as the primary supply. True or false?
False, a step-up transformer increases the frequency
False, the frequency is divided by the turns ratio
True only for step-down transformers
True
39.
A neon-sign transformer with 150 primary turns and 1200 secondary turns, fed from a 12 V a.c. source, produces 96 V. True or false?
False, it produces 1.5 V
True
False, it produces 12 V
False, it produces 192 V
40.
A doorbell transformer (2000 turns primary, 50 turns secondary) on a 230 V supply gives an output of 5.75 V. True or false?
False, it gives 9200 V
False, it gives 46 V
True
False, it gives 11.5 V
41.
A substation transformer takes 2.0 A at 230 V and gives 36 A at 11.5 V, so it is 90% efficient. True or false?
False, it is 111% efficient
True
False, it is 5% efficient
False, it is 18% efficient
42.
A student measures an output power of 52 W from a transformer whose input power is 50 W. This is a possible real result. True or false?
True
True only for step-up transformers
False, because transformers are always exactly 50% efficient
False, because output power cannot exceed input power (energy is conserved)
43.
Transmitting at a high p.d. reduces cable losses because the high p.d. makes the cable resistance smaller. True or false?
False, because it reduces the current, and the loss depends on I²R
True
True only for a.c.
False, because high p.d. increases losses
44.
High-voltage transmission lets companies use thinner, cheaper cables. True or false?
False, it needs thicker cables
True
False, cable thickness never matters
True only for d.c.
45.
A 0.20 A load is connected to the secondary of the transformer shown; assuming no losses, 1.0 A flows in its primary. True or false?
Dr Dabour Hub Academy - figure 10
False, it is 0.040 A
True
False, it is 0.20 A
False, it is 5.0 A
46.
If transformer A in the grid diagram is ideal and the station supplies 2000 A, the cable current is 125 A. True or false?
Dr Dabour Hub Academy - figure 11
False, it is 32 000 A
False, it is 2000 A
False, it is 8.0 A
True
47.
From the graph, the ideal prediction for Ns = 100 is 3.0 V, and the measured value is about 3% lower. True or false?
Dr Dabour Hub Academy - figure 12
False, the ideal value is 2.9 V
False, it is 30% lower
False, the ideal value is 0.30 V
True
48.
The table shows that increasing the transmission p.d. by a factor of 5 reduces the cable loss by a factor of 25. True or false?
True
False, by a factor of 5
False, the loss rises
False, by a factor of 10
49.
Transformer C in the table is an isolating transformer: output p.d. equals input p.d. True or false?
False, it is step-up
False, it is step-down
True
False, it gives zero output
50.
The soft-iron core concentrates the magnetic field and links it to the secondary coil. True or false?
False, the core blocks the field
True
False, the core carries current between coils
True only for d.c.
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