The power of an appliance is the energy it transfers each second. One watt (W) is one joule per second. For anything in a circuit:

power = potential difference × current
P = V × I

Power in watts (W), p.d. in volts (V), current in amps (A). Recall this one (AQA, Edexcel and OCR).

1. A kettle on the 230 V mains takes a current of 10 A
2. P = 230 × 10
3. P = 2300 W = 2.3 kW

A power rating in kW has to become watts before you use it: 2.3 kW = 2300 W.

Put V = I R into P = V I and you get a second power equation. It is useful when you know the resistance instead of the p.d.:

power = (current)² × resistance
P = I² × R

Recall this one too. Only the current is squared.

1. A car rear-window heater has a resistance of 1.5 Ω and a current of 8 A
2. P = 8² × 1.5 = 64 × 1.5
3. P = 96 W

Rearranging: R = P ÷ I², and (Higher) I = √(P ÷ R).

Currents in mA must be changed to amps before squaring: 50 mA = 0.050 A, so I² = 0.0025, not 2500.

A plug fuse melts if the current gets too big. It must be rated just above the normal current: too low and it melts every time you switch on, too high and it may not melt in a fault. UK plug fuses come in 3 A, 5 A and 13 A.

1. A vacuum cleaner is rated 920 W on the 230 V mains
2. I = P ÷ V = 920 ÷ 230 = 4 A
3. 3 A is too small, so fit the 5 A fuse