Example 1 — Basic efficiency:
A light bulb uses 100 J of electrical energy and produces 8 J of useful light energy. Calculate its efficiency.
- Efficiency = Useful output ÷ Total input = 8 ÷ 100 = 0.08
- As a percentage: 0.08 × 100 = 8%
- Wasted energy: 100 − 8 = 92 J (as heat)
Example 2 — Finding useful output:
A motor is 65% efficient and receives 2,000 J. How much useful energy does it produce?
- Rearrange: Useful output = Efficiency × Total input
- Convert 65% to decimal: 0.65
- Useful output = 0.65 × 2,000 = 1,300 J
Example 3 — Using power:
A machine has input power of 500 W and useful output power of 350 W. Calculate the efficiency.
- Efficiency = Useful output power ÷ Total input power
- Efficiency = 350 ÷ 500 = 0.7 (or 70%)
Quick Check: An engine has an efficiency of 0.30 and a total input of 6,000 J. How much energy is wasted?
Useful output = 0.30 × 6,000 = 1,800 J. Wasted = 6,000 − 1,800 = 4,200 J. You could also calculate wasted as (1 − 0.30) × 6,000 = 0.70 × 6,000 = 4,200 J.
This worked example covers Worked Examples within Efficiency for GCSE Physics. Revise Efficiency in Energy for GCSE Physics with 19 exam-style questions and 4 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 8 of 13 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
Practice questions for Efficiency
Which equation correctly defines efficiency?
Explain one method that can be used to reduce unwanted energy transfers in a machine and state how it reduces waste.