Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Recall and use the equation: power (watt, W) = energy transferred (joule,…
Full specification wording
Topic 10 – Electricity and circuits: 10.29 Recall and use the equation: power (watt, W) = energy transferred (joule, J) ÷ time taken (second, s)
The equation links three quantities: power in watts (W), energy transferred in joules (J) and time taken in seconds (s).
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 27 of 39
Study notes
From the checked teaching notes for this specification point.
The idea
The equation links three quantities: power in watts (W), energy transferred in joules (J) and time taken in seconds (s). Write it as P = E ÷ t, where P is power, E is energy transferred and t is time taken. To find power, divide the energy by the time: a motor transferring 3600 J in 12 s has P = 3600 J ÷ 12 s = 300 W. To find energy, multiply power by time: a 500 W heater running for 20 s transfers E = 500 W × 20 s = 10 000 J. To find time, divide energy by power: t = E ÷ P. Always convert time to seconds first, so 2 minutes becomes 120 s, and keep units consistent before substituting.
What earns the marks
- Recalls the equation power = energy transferred ÷ time taken.
- Uses the correct units of watt, joule and second in the equation.
- Rearranges the equation to calculate energy transferred from power and time.
- Rearranges the equation to calculate time taken from energy and power.
- Substitutes values with time converted to seconds and states the answer with the correct unit.
Common mistakes and how to put them right
Multiplies energy by time instead of dividing.
Put it right: Divide energy by time to obtain power.
Leaves time in minutes before substituting.
Put it right: Convert to seconds so the answer is in watts.
Omits the unit on the final answer.
Put it right: State the result in W, J or s to match the quantity calculated.
Exam tips
- Write the equation, then rearrange it before substituting numbers.
- Convert all times to seconds at the start to avoid unit errors.
- Check the unit of your answer against the quantity you were asked to find.
Try the lesson
The start of the Topic 10 – Electricity and circuits: 10.29 Recall and use the equation: power (watt, W) = energy transferred (joule, J) ÷ time taken (second, s) lesson, made from checked teaching notes for this specification point.
Have a go first
A lamp transfers 600 J of energy in 20 s. What is the power of the lamp?
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