QuizPower and energy · 7 min
Quiz: power and energy
8 questions. Answer each one, then open Show the answer to check yourself. In Litmux Academy the quiz is marked and counts towards your certificate.
Question 1What is the power of a device taking 2 A from 12 V?
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Answer: 24 W
P = V × I = 24 W.
Question 2Which formula gives the power in a resistor when you know the current through it?
- P = V / I
- P = I² × R
- P = R / I
- P = I / R
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Answer: B. P = I² × R
P = V × I with V = I × R gives P = I² R.
Question 3What power does a 470 Ω resistor take with 10 V across it?
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Answer: 0.213 W
P = V² / R = 100 / 470 = 0.213 W.
Question 4A resistor calculation gives 0.3 W. Which rating should you choose?
- 1/4 W
- 1/2 W
- 1 W
- 0.3 W exactly
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Answer: C. 1 W
Twice 0.3 W is 0.6 W, so the next standard rating up: 1 W.
Question 5A 12 V 50 Ah battery stores how much energy?
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Answer: 600 Wh
Wh = V × Ah = 12 × 50 = 600 Wh.
Question 6What does a prepaid electricity meter count?
- Watts
- Volts
- Kilowatt-hours (energy)
- Amperes
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Answer: C. Kilowatt-hours (energy)
You pay for energy: kWh.
Question 7A 1500 W heater runs for 2 hours. How many kWh?
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Answer: 3 kWh
1.5 kW × 2 h = 3 kWh.
Question 8Why does the grid send power at very high voltages?
- High voltage travels faster
- For the same power, the current is smaller, so the I² R loss in the lines is much smaller
- Transformers only work at high voltage
- It is safer
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Answer: B. For the same power, the current is smaller, so the I² R loss in the lines is much smaller
P = V × I: higher V, lower I; the loss I² R falls with the square of the current.