Two kettles can boil the same water. The one that does it faster has the bigger power: it transfers the energy at a greater rate.

power = energy transferred ÷ time
P = E ÷ t
power = work done ÷ time
P = W ÷ t

Power in watts (W), energy or work in joules (J), time in seconds (s). One watt is one joule every second. Recall both: they are not on the sheet for AQA, Edexcel or OCR.

Work done is energy transferred by a force. When a crane lifts a load, the work done on it equals the gravitational potential energy it gains. (Working out work from force × distance, W = F s, is in the forces unit.)

Papers love numbers that are not in J and s.

• 1 kJ = 1000 J, 1 MJ = 1 000 000 J
• 1 kW = 1000 W, 1 MW = 1 000 000 W
• minutes × 60 = seconds, hours × 3600 = seconds

1. A microwave transfers 216 kJ in 4 minutes. What is its power?
2. E = 216 kJ = 216 000 J
3. t = 4 × 60 = 240 s
4. P = 216 000 ÷ 240 = 900 W

1. A 2.5 kW heater runs for 20 minutes. Energy in kJ?
2. E = P t = 2500 W × 1200 s = 3 000 000 J
3. = 3000 kJ

Big energies, like a power station running all day, go in standard form: 600 MW for 5 hours is 6 × 10⁸ × 18 000 = 1.08 × 10¹³ J.