This topic summary covers Knowledge Organiser: Efficiency 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 14 of 14 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 14 of 14
Practice
19 questions
Recall
4 flashcards
Knowledge Organiser: Efficiency
Key Equations
- Efficiency = Useful output ÷ Total input
- Efficiency (%) = (Useful ÷ Total) × 100
- Wasted = Total input − Useful output
- Same formula works with power (W)
Key Terms
- Efficiency — ratio of useful output to total input (no unit)
- Useful output — energy in intended form
- Wasted energy — unintended outputs (mainly heat)
- Sankey diagram — width = energy amount
- Dissipated — spread into surroundings as heat
Key Rules
- Efficiency is always less than 100%
- Energy is conserved (never lost)
- Wasted energy goes to thermal store
- Efficiency has NO unit
Improving Efficiency
- Lubrication — reduces friction
- Insulation — reduces heat loss
- Streamlining — reduces air resistance
- Superconductors — reduce electrical resistance
Common Mistakes
- Efficiency greater than 1 (or 100%): Efficiency can never exceed 1.0 (100%) — if your answer is above this, check you divided useful by total, not the other way round
- Using the wrong form of the equation: Both forms are correct — useful energy ÷ total energy, OR useful power ÷ total power — choose based on what data is given
- Saying wasted energy disappears: Wasted energy is transferred to the thermal store of the surroundings — it is not destroyed
- Forgetting to multiply by 100 for percentage: The decimal form (e.g. 0.75) and percentage form (75%) are both valid — make sure you give the correct form
- Assuming more efficient = more powerful: Efficiency is a ratio — a low-power device can be 100% efficient while a high-power device is only 50% efficient
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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.
Quick Recall Flashcards
19 questions on Efficiency — practise free
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