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Practise 23 exam-style questions on Power for AQA GCSE Physics. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
Give the unit of power.
The watt (W).
One watt is one joule of energy transferred every second. Larger powers are often given in kilowatts (1 kW = 1000 W).
watt
allow W; allow joules per second / J/s
The table shows the power of some appliances that are connected to the 230 V mains supply, and the current in each one.
Calculate the value of X.
P = V I, so I = P ÷ V
X = 920 ÷ 230 = 4.0 A
Every appliance has the same 230 V across it, so the current is proportional to the power. The kettle, with the largest power, has by far the largest current.
X = 920 ÷ 230
allow 920 = 230 × I; an answer of 4.0 scores 2 marks
4.0 (A)
allow 4 (A)
An electric kettle has a power of 2.5 kW. It is switched on for 3.0 minutes.
Calculate the energy transferred by the kettle.
P = 2.5 kW = 2500 W
t = 3.0 × 60 = 180 s
E = P t = 2500 × 180
E = 450 000 J
Power is the energy transferred per second, so energy = power × time. Use watts and seconds to get the energy in joules.
P = 2500 W and t = 180 s
both needed; an answer of 450 000 (J) scores 3 marks
E = 2500 × 180
allow ecf from incorrect conversions
450 000 (J)
allow 4.5 × 10⁵ (J); allow 450 kJ
A 12 V car headlamp has a power of 55 W when it is lit.
Calculate the current in the headlamp and its resistance when lit.
Give both answers to 2 significant figures.
P = V I, so I = P ÷ V = 55 ÷ 12 = 4.583... A = 4.6 A (2 s.f.)
V = I R, so R = V ÷ I = 12 ÷ 4.583... = 2.618... Ω = 2.6 Ω (2 s.f.)
Find the current from P = V I first, then the resistance from V = I R. Keep the unrounded current in your calculator for the second step and round both answers only at the end.
55 = 12 × I
an answer of 4.6 A and 2.6 Ω scores 5 marks
I = 4.58 (A)
allow 4.6 (A)
R = 12 ÷ 4.58
allow 55 = 4.58² × R; allow ecf from their current
R = 2.62 (Ω)
allow 2.6 (Ω); allow 2.61 (Ω) from using 4.6 A; allow ecf
both answers given to 2 significant figures: 4.6 A and 2.6 Ω
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