A student is investigating ways to improve the efficiency of a model electric motor used to lift a small mass. The motor wastes energy as heat due to friction in its moving parts, and also loses energy as sound. Explain how the efficiency of the motor can be increased, and describe what effect this would have on the useful energy transferred by the motor.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A model electric motor is supplied with 200 J of electrical energy. It transfers 80 J of energy as useful work lifting a mass. The remaining energy is wasted as heat and sound.
Model answer (5 marks)
Efficiency is the ratio of useful output energy to total input energy.
Lubrication of the moving parts reduces friction.
Reducing friction means less energy is wasted as heat.
Reducing waste energy increases the efficiency of the motor.
More of the total input energy is transferred as useful energy (lifting the mass), so the useful energy transferred increases.
Lubrication of the moving parts reduces friction.
Reducing friction means less energy is wasted as heat.
Reducing waste energy increases the efficiency of the motor.
More of the total input energy is transferred as useful energy (lifting the mass), so the useful energy transferred increases.
Examiner tips
- Use the definition of efficiency first, then explain the cause of loss and how to reduce it, finally state the effect on useful energy.
- Include the key terms: efficiency, friction, lubrication, heat, useful energy.
- Keep each point concise and directly linked to the mark scheme points.
Common mistakes
- Confusing efficiency with power or speed.
- Omitting the definition of efficiency or the role of friction.
- Not linking the increase in efficiency to an increase in useful energy transferred.
Mark scheme (5 marks)
- Efficiency is the ratio of useful output energy to total input energy
- Lubrication of the moving parts reduces friction
- Reducing friction means less energy is wasted as heat
- Reducing waste energy increases the efficiency of the motor
- More of the total input energy is transferred as useful energy (lifting the mass), so the useful energy transferred increases
Key terms in this question
efficiency · useful energy transfer · friction
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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