Power is the amount of energy transferred per second and is measured in watts (W).
Power can be calculated using the equation: power = current × potential difference (P = I V).
At a constant p.d., a larger electric current transfers energy more quickly than a smaller current.
Domestic appliances plugged into the mains supply in the UK have a p.d. of 230 V across them.
The fuse with the smallest rating for the required current (calculated using the equation: P = I² R) should be used.

Common misconception

Pupils often use the terms power and energy interchangeably.

Use concrete examples to show pupils that a greater power transfers energy more quickly.

Keywords

Power - the amount of energy transferred each second
Watts (W) - the unit of power; equal to the number of joules of energy transferred each second
Kilowatts (kW) - a unit of power; 1 kilowatt is equal to 1000 watts
Fuse - a thin wire that melts when a dangerously large current flows through it

Comparing the heating effect of different power settings on a microwave oven is a good way to illustrate that power is proportional to the amount of energy transferred per second.

Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.

Which of the following are ways of writing the definition of power?

Correct answer: rate of energy transfer

amount of energy used up per second

Correct answer: amount of energy transferred per second

time taken to use up one joule of energy

Q2.

A mains circuit includes a fuse. Which of the following makes a fuse ‘blow’?

Correct answer: a circuit current that is too high

a circuit resistance that is too high

a room temperature that is too high

Q3.

What is the potential difference across the resistor in the circuit shown?

0.75 V

1.0 V

Correct answer: 1.5 V

2.5 V

3.0 V

Q4.

What is the missing current in the circuit shown?

Correct answer: 1.0 A

1.5 A

3.5 A

5.0 A

Q5.

A cell is connected to a lamp and a current flows. What happens to the current through the cell if another lamp is added in parallel with the first lamp?

It stays the same.

Correct answer: It increases.

It decreases.

Q6.

An ammeter shows that the current through a resistor is 0.40 A and a voltmeter shows that the p.d. across it is 1.5 V. A pupil calculates the resistance. Which of these answers should they give?

3 Ω

4 Ω

3.7 Ω

Correct answer: 3.8 Ω

3.75 Ω

Exit quiz

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6 Questions

Q1.

A lamp has power of 7 W. Which of the following explains what this means?

The lamp transfers 1 J in 7 s.

Correct answer: The lamp transfers 7 J in 1 s.

The lamp transfers 1 W in 7 s.

The lamp transfers 7 W in 1 s.

Q2.

When an iron is working correctly, the current through it varies between 1.6 A and 3.3 A. Which of the following fuse sizes should be used in the plug of this iron?

3 A

Correct answer: 5 A

13 A

Q3.

Which of the following could cause a circuit breaker to switch off a circuit?

no appliances being used

too many appliances being connected in series

Correct answer: too many appliances being connected in parallel

Q4.

The current through a lamp is 1.5 A and the p.d. across it is 12 V. The power of the lamp is W.

Correct Answer: 18, 18 W, 18W, 18 watts, eighteen

Q5.

What is the current through a 600 W lawnmower that is plugged into the mains?

0.38 A

Correct answer: 2.6 A

4.7 A

6.0 A

Q6.

A 1300 W iron and a 1000 W hairdryer are plugged into a mains circuit, in parallel. What is the current through a circuit breaker in the consumer unit?

0.10 A

0.77 A

1.3 A

5.3 A

Correct answer: 10 A