Example 1 — Basic power calculation:
A person does 2,400 J of work climbing stairs in 40 seconds. Calculate their power output.
- Write the formula: P = W/t
- Substitute values: P = 2,400 / 40
- Calculate: P = 60 W
Example 2 — Rearranging to find time:
A 1,500 W hairdryer transfers 90,000 J. How long was it switched on?
- Rearrange: t = E/P
- Substitute: t = 90,000 / 1,500
- Calculate: t = 60 s
Example 3 — Electrical power:
A lamp has a current of 0.5 A and operates at 12 V. Calculate its power.
- Formula: P = IV
- Substitute: P = 0.5 × 12
- Calculate: P = 6 W
Example 4 — Energy cost (kWh):
A 2 kW electric heater is used for 3 hours. Electricity costs 30p per kWh. Find the cost.
- Energy = P × t = 2 × 3 = 6 kWh
- Cost = 6 × 30p = 180p = £1.80
Quick Check: A 50 W electric motor runs for 2 minutes. How much energy does it transfer?
E = P × t = 50 × 120 = 6,000 J. Remember to convert minutes to seconds: 2 min = 120 s.
This worked example covers Worked Examples within Power for GCSE Physics. Revise Power in Energy for GCSE Physics with 14 exam-style questions and 25 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 7 of 12 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
Practice questions for Power
Which of the following is the correct definition of power?
A microwave oven transfers 54,000 J of energy. It has a power rating of 900 W. Calculate how long the microwave was switched on. Use the equation: power = energy transferred / time Give your answer in seconds (s).