A machine takes in 500 J and delivers 400 J of useful output. Its efficiency is
Correct answer: A. 80 per cent
- A. 80 per cent
- B. 125 per cent
- C. 100 per cent
- D. 20 per cent
Explanation
Dividing 400 by 500 and multiplying by 100 gives 80 per cent, with the remaining 100 J lost mainly as heat. Dividing the wrong way round gives 125 per cent, which should be recognised at once as impossible. The 20 per cent figure is the fraction wasted rather than the efficiency.
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About Energy Losses and Efficiency
Efficiency compares useful output energy or power with the total input, usually expressed as a percentage. The topic covers energy losses through friction, heating, resistance and sound, along with power ratings and Sankey diagrams. Efficiency is not the same as conservation of energy, because lost energy is transferred rather than destroyed.
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