Power revision notes

Edexcel International GCSE Physics (4PH1), chapter 16: Work and power, specification point 4.16, 4.17. Free lesson.

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Power is the rate of transfer of energy, or the rate of doing work. You calculate it from work done and time taken, and can rearrange the equation to find time.

What power tells you

Power describes how quickly energy is transferred. It is the energy transferred each second, so you need to include the idea of a rate or “per second” in a definition. Two people can do the same work, such as carrying identical boxes up the same stairs, but the one who does it faster has greater power.

The power equation and its units

Use P = W/t to calculate power. The equation is on the equation list, so you need to choose it and use it correctly. Here, W means work done, not the unit watt. P = power, measured in watts (W). W = work done, or energy transferred, measured in joules (J). t = time taken, measured in seconds (s). 1 W = 1 J/s, and 1 kW = 1000 W.

How to calculate power when lifting

When a motor lifts a load, the work done is equal to the load’s gain in gravitational potential energy (GPE). First calculate the work using its mass, the gravitational field strength and the height. Then divide by the time.

Why time affects power

Power depends on both the energy transferred and the time taken. If two motors transfer the same energy, the motor that takes less time has greater power because it transfers energy faster. For the same work, halving the time doubles the power.

How to find time, including with kilowatts

You can rearrange P = W/t to find the time taken. Convert power from kilowatts to watts first, so the units match and the answer comes out in seconds. Convert kilowatts to watts by multiplying by 1000. For lifting, work done is the GPE gained: W = m × g × h.