A factory uses an electric motor to lift heavy crates from the ground floor to a storage shelf. The motor is supplied with 6000 J of energy. Only 4200 J of this is usefully transferred to gravitational potential energy in the crates. Explain why the motor cannot transfer all of the supplied energy usefully, and describe TWO ways in which the efficiency of the motor could be improved.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric motor in a factory lifts heavy crates onto storage shelves. The motor receives 6000 J of electrical energy from the mains supply. Of this, 4200 J is transferred usefully as gravitational potential energy.
Model answer (5 marks)
The motor cannot transfer all of the supplied energy usefully because energy is conserved – the missing energy must appear as other forms such as heat or sound. In the motor, friction in the moving parts and electrical resistance cause energy to be lost as heat, and some is also radiated as sound. Thus the efficiency is less than 1.
Two ways to improve the efficiency are:
1. Lubricate the moving parts to reduce friction, which lowers the amount of energy lost as heat.
2. Provide thermal insulation around the motor so that less heat is transferred to the surroundings, keeping more energy within the motor system.
Two ways to improve the efficiency are:
1. Lubricate the moving parts to reduce friction, which lowers the amount of energy lost as heat.
2. Provide thermal insulation around the motor so that less heat is transferred to the surroundings, keeping more energy within the motor system.
Examiner tips
- Use the term "conservation of energy" and explain that the missing energy appears as heat or sound. Show the two improvement methods with clear reasoning. Keep the answer concise and use the exact terminology from the mark scheme.
Common mistakes
- Claiming the energy is "lost" instead of "transferred to waste forms". Forgetting to mention friction or electrical resistance as the cause of heat. Using vague phrases like "improve efficiency" without specifying how.
Mark scheme (5 marks)
- Energy cannot be created or destroyed (conservation of energy), so the missing energy must be transferred to waste forms rather than lost.
- Some energy is transferred as heat (and/or sound) due to friction in the motor's moving parts, making it a waste energy transfer.
- The efficiency of the motor is less than 1 (or 100%) because some of the input energy is wasted.
- Lubrication of the motor's moving parts reduces friction, so less energy is wasted as heat, increasing efficiency.
- Thermal insulation around the motor reduces heat transfer to the surroundings, meaning less energy is dissipated as waste, increasing efficiency.
Key terms in this question
efficiency · gravitational potential energy
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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