Power and Efficiency — AQA GCSE Physics
Power is the rate at which energy is transferred or work is done. It links to efficiency.
Power
Power is the rate of energy transfer (or the rate of doing work), measured in watts (W). One watt is one joule per second (1 W = 1 J/s).
$$P = \frac{E}{t} \qquad P = \frac{W}{t}$$
- P = power (W), E = energy transferred (J), W = work done (J), t = time (s).
A more powerful device transfers the same amount of energy in less time.
Worked example
A motor transfers 600 J of energy in 5 s.
- Power = 600 ÷ 5 = 120 W.
Power and efficiency
Efficiency can be calculated using power: $$\text{efficiency} = \frac{\text{useful power output}}{\text{total power input}}$$
For example, if a 100 W lamp transfers 20 W as useful light, its efficiency = 20 ÷ 100 = 0.2 = 20%; the rest is wasted as heat.
Comparing devices
Two devices can do the same job but with different power ratings. The one with the higher power does the work faster, but if it wastes more energy it may be less efficient.
Exam tips
- Power = energy transferred ÷ time (or work done ÷ time), in watts.
- 1 watt = 1 joule per second.
- Learn the power version of the efficiency equation.
- Higher power = faster transfer, but check efficiency separately.