Power: how fast energy is transferred

By UK GCSE Physics · independent study · myAudience

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Power: how fast energy is transferred

Power is energy transferred per unit time, P=E/t. It can also be expressed as work done per time, P=W/t. One watt means one joule per second. The unit W for watt and the symbol W sometimes used for work mean different things; context resolves the notation.

Two motors can perform the same lifting work but have different powers. The faster motor transfers the required energy in less time. A high power does not mean an appliance necessarily uses more energy over every period: both power and running time determine total transfer.

Rearrange the relationship when needed: E=Pt and t=E/P. Seconds are needed with watts and joules. A minute is 60 seconds, so a calculation that inserts minutes without conversion produces a numerical error. For heating or lifting, also consider how much of the input becomes the intended transfer.

Worked example: A motor transfers 600 J in 3 s. Find its power and compare a motor doing the same work in 6 s.

First motor: P=600/3=200 W.

Second motor: P=600/6=100 W.

Both transfer 600 J, but the first transfers it twice as fast.

Answer: 200 W versus 100 W

Check your understanding: Does a 200 W device running for 10 s transfer the same energy as 100 W running for 20 s?

Answer: Yes: both transfer 2000 J.

Why: Energy depends on power multiplied by time, not on power alone.