AQA GCSE Physics (8463), Higher tier · Energy › Energy stores and changes
Practise Power. 15 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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Write down the equation that links power (P), time (t) and work done (W).
P = W ÷ t
Write down the equation that links power (P), energy transferred (E) and time (t).
P = E ÷ t
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Power is the rate at which energy is transferred or the rate at which work is done.
(a) Which unit is the same as one watt? Tick (✓) one box.[1]
joule per second
joule second
kilogram per second
newton per metre
(b) An electric motor transfers 3600 J of energy in 45 s. Calculate the power of the motor. Use the equation: power = energy transferred ÷ time[2]
(c) A crane does 72 000 J of work in 60 s. Calculate the power of the crane. Use the equation: power = work done ÷ time[2]
Show the answer and mark scheme
(a)Answer: joule per second
(b)Answer: 80 W
P = 3600 ÷ 45
P = 80 (W)
(c)Answer: 1200 W
P = 72 000 ÷ 60
P = 1200 (W)
Question 2Medium6 marks
Two electric motors, A and B, are used to lift identical loads. Each load has a weight of 400 N and is lifted vertically through 3.0 m. Motor A takes 8.0 s to lift its load. Motor B takes 5.0 s to lift its load.
(a) Calculate the work done on each load.[2]
(b) Calculate the useful power output of motor B.[2]
(c) Explain why motor B has a greater useful power output than motor A.[2]
Show the answer and mark scheme
(a)Answer: 1200 J
W = 400 × 3.0
W = 1200 (J)
(b)Answer: 240 W
P = 1200 ÷ 5.0
P = 240 (W)
(c)Answer: Both motors do the same work, but motor B does it in less time, so it transfers energy at a greater rate.
both motors do the same (amount of) work / transfer the same (amount of) energy
motor B does it in less time / transfers energy at a higher rate
Question 3Hard8 marks
An electric motor is used to raise a lift. The total mass of the lift and its passengers is 1500 kg. The lift rises at a steady speed of 0.60 m/s. The input power to the motor is 12 kW. Gravitational field strength = 9.8 N/kg
(a) Calculate the increase in the gravitational potential energy of the lift and passengers in 1 s.[3]
(b) Calculate the efficiency of the motor. Give your answer as a decimal.[2]
(c) The lift travels a vertical distance of 24 m at the same steady speed. Calculate the energy wasted by the motor during this journey.[3]