A factory uses a large electric motor to drive a conveyor belt. The motor has a power rating of 500 W. During operation, the motor gets warm and produces a humming sound. Explain how these observations show that the motor is not perfectly efficient, and describe one way the efficiency of the motor could be improved.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Electric motors are used widely in industry. Like all real machines, they transfer some energy in ways that do not directly serve their purpose.
Model answer (4 marks)
The motor’s heating shows that part of the electrical energy is being converted into thermal energy, which does not help drive the belt – a waste energy transfer.
The humming sound indicates that some energy is being released as acoustic energy, also a waste transfer.
Because energy is lost as heat and sound instead of all becoming useful mechanical energy, the useful output is less than the electrical input, so the efficiency is less than 100 %.
One way to improve efficiency is to lubricate the motor’s moving parts, reducing friction and therefore the amount of energy lost as heat.
The humming sound indicates that some energy is being released as acoustic energy, also a waste transfer.
Because energy is lost as heat and sound instead of all becoming useful mechanical energy, the useful output is less than the electrical input, so the efficiency is less than 100 %.
One way to improve efficiency is to lubricate the motor’s moving parts, reducing friction and therefore the amount of energy lost as heat.
Examiner tips
- Show the two waste energy transfers (heat and sound) first, then link to efficiency <100 %.
- Mention a specific improvement such as lubrication to reduce friction.
Mark scheme (4 marks)
- The heat produced by the motor is a waste energy transfer (energy transferred to a form that does not directly serve the purpose of the system).
- The sound produced by the motor is also a waste energy transfer (energy transferred to a form that does not directly serve the purpose of the system).
- Because energy is transferred as heat and sound rather than entirely as useful kinetic energy / mechanical energy, the useful output is less than the total input, so efficiency is less than 1 (or 100%).
- Efficiency could be improved by lubricating the moving parts of the motor to reduce friction, thereby reducing the waste heat energy transfer.
Key terms in this question
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