A Levels Physics (9702)•9702/12/M/J/20
Explanation
Efficiency Formula for Energy Conversion
Steps:
- Identify total input energy from fuel: X.
- Determine useful output: kinetic energy = X - Y (since Y is thermal waste).
- Recall efficiency definition: useful output divided by total input.
- Calculate: (X - Y) / X.
Why A is correct:
- Efficiency is the ratio of useful kinetic energy to total fuel energy input, per the standard physics formula η = (output / input) × 100%.
Why the others are wrong:
- B inverts the ratio, yielding a value greater than 1, which exceeds 100% efficiency.
- C simplifies to 1 (or 100%), ignoring thermal losses Y.
- D compares input to waste, unrelated to efficiency.
Final answer: A
Topic: Energy conservation
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