A factory uses an electric motor to lift heavy crates. The motor has a power rating of 500 W but only 350 W is used to raise the crates. Explain why the efficiency of the motor cannot equal 1 (100%), and describe two ways the manufacturer could improve the efficiency of the motor.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric motor in a factory is used to lift crates. Not all of the electrical energy supplied to the motor is transferred usefully — some is wasted as heat and sound during operation.
Model answer (4 marks)
The efficiency cannot be 1 (100 %) because, according to the first law of thermodynamics, energy supplied to the motor is either converted into useful mechanical work or lost as waste energy such as heat and sound; it cannot all become useful work.
To improve the efficiency the manufacturer could:
1. Reduce friction by better lubrication of the moving parts, which lowers the amount of energy lost as heat.
2. Lower the current through the motor (or use conductors with lower resistance) to reduce I²R heating of the windings, thereby decreasing waste heat.
Alternatively, the motor could recover some of the waste energy (e.g. by using a heat exchanger to capture waste heat) and convert it into useful work, increasing the useful output relative to the total input.
To improve the efficiency the manufacturer could:
1. Reduce friction by better lubrication of the moving parts, which lowers the amount of energy lost as heat.
2. Lower the current through the motor (or use conductors with lower resistance) to reduce I²R heating of the windings, thereby decreasing waste heat.
Alternatively, the motor could recover some of the waste energy (e.g. by using a heat exchanger to capture waste heat) and convert it into useful work, increasing the useful output relative to the total input.
Examiner tips
- Use the term ‘efficiency cannot be 1 because energy is lost as heat/sound – reference the first law of thermodynamics’ to gain full marks for the first part.
- Show two specific improvement methods: lubrication to reduce friction and reducing I²R losses; mention energy recovery if you have space.
- Use correct units (W) and keep the answer concise and focused on the question.
Mark scheme (4 marks)
- The efficiency cannot equal 1 (100%) because some energy is transferred as waste (e.g. heat/sound) rather than as useful energy transfer, and energy cannot be created or destroyed.
- Lubrication of the motor's moving parts reduces friction, which reduces waste heat energy transfer, improving efficiency.
- Reducing the current through the motor decreases power lost to heating of wires (since power loss = I²R), reducing waste energy transfer.
- Recycling or recovering waste energy (e.g. capturing waste heat) means energy that would be dissipated is reused, increasing the useful output relative to total input.
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
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- Decode the mark scheme abbreviations →
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