A electric motor is used to lift boxes onto a warehouse shelf. Explain why the energy transferred to the motor from the electrical supply is always greater than the energy gained by the boxes as gravitational potential energy.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric motor in a warehouse is connected to the mains supply. It lifts heavy boxes from the floor onto a shelf above. Engineers notice that the motor becomes warm during use.
Model answer (4 marks)
The electric motor receives a certain amount of electrical energy from the mains. Only part of this energy is converted into useful work – the increase in gravitational potential energy of the boxes. The remainder is lost as waste energy. In the motor this waste energy is mainly dissipated as heat in the windings and the surrounding air, which is why the motor becomes warm. Thus the energy supplied by the mains is always greater than the gravitational potential energy gained by the boxes because the motor’s efficiency is less than 100 % and the unused energy is transferred to the thermal store of the motor and its surroundings.
Examiner tips
- Use the word ‘efficiency’ to link input, useful output and waste energy.
- Mention that the motor’s heat is a waste energy transfer.
- Show that total energy is conserved – input = useful + waste.
- Keep the answer concise – 4 marks only.
Common mistakes
- Saying the motor is 100 % efficient; ignoring heat loss.
- Confusing the electrical supply energy with the motor’s internal energy.
- Not recognising that the waste energy is heat, not just ‘unused’ energy.
Mark scheme (4 marks)
- Some energy is transferred as a waste energy transfer / not all input energy becomes useful energy
- Energy is dissipated / transferred to the thermal store of the motor and surroundings (as heat)
- The useful energy transfer is only the gravitational potential energy gained by the boxes
- Energy is conserved overall, so total input equals useful output plus waste — efficiency cannot reach 100%
Key terms in this question
gravitational potential energy
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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