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Year 11
OCR
Foundation

Power of an appliance or electric circuit

I can describe how the power of an appliance or electric circuit depends on current and potential difference.

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New
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Year 11
OCR
Foundation

Power of an appliance or electric circuit

I can describe how the power of an appliance or electric circuit depends on current and potential difference.

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Lesson details

Key learning points

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

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

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.

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).

Lesson video

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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?
An image in a quiz
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?
An image in a quiz
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