A solar-powered water pump is used to move water from a river to a storage tank. The pump has a power rating of 500 W but only transfers 300 W as useful energy to the water. Explain what is meant by 'useful energy transfer' and 'waste energy transfer' in this context, and describe how the efficiency of the pump could be improved.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A solar-powered water pump transfers energy from sunlight to raise water from a river to a storage tank. Not all of the energy input is transferred usefully.
Model answer (4 marks)
Useful energy transfer is the part of the solar energy that is converted into the energy required to lift the water – i.e. the increase in gravitational potential energy of the water in the storage tank.
Waste energy transfer is the part of the solar energy that is lost to other forms – for example heat and sound produced by friction in the pump motor and other components.
The efficiency of the pump could be improved by reducing friction – for example by lubricating the moving parts – which reduces the amount of energy lost as heat and therefore increases the proportion of useful energy.
Efficiency can never be greater than 1 (or 100 %) because the total output energy can never exceed the input energy; it can only be transferred or dissipated, not created.
Waste energy transfer is the part of the solar energy that is lost to other forms – for example heat and sound produced by friction in the pump motor and other components.
The efficiency of the pump could be improved by reducing friction – for example by lubricating the moving parts – which reduces the amount of energy lost as heat and therefore increases the proportion of useful energy.
Efficiency can never be greater than 1 (or 100 %) because the total output energy can never exceed the input energy; it can only be transferred or dissipated, not created.
Examiner tips
- Use the exact terms ‘useful energy transfer’ and ‘waste energy transfer’ as defined in the mark scheme.
- Explain the physical meaning of each – energy that raises the water vs. energy lost as heat/sound.
- Show a clear link between reducing friction and increasing efficiency.
- Mention the 100 % limit on efficiency to demonstrate understanding of energy conservation.
Common mistakes
- Confusing useful energy with total input energy; forgetting to specify that useful energy is the potential energy of the water.
- Failing to mention that waste energy is lost as heat/sound due to friction.
- Claiming efficiency can exceed 100 % or ignoring the conservation of energy principle.
Mark scheme (4 marks)
- Useful energy transfer is the transfer of energy that directly serves the purpose of the system — here, moving/raising the water (gravitational potential energy).
- Waste energy transfer is the transfer of energy to a form that does not directly serve the purpose of the system — here, energy lost as heat/sound due to friction in the pump motor.
- Lubrication of moving parts in the pump reduces friction, reducing waste (thermal) energy transfer and improving efficiency.
- Efficiency can never exceed 1 (or 100%) because energy is conserved — it can only be transferred or dissipated, not created; so the useful output can never exceed the total input.
Key terms in this question
useful energy transfer · waste energy transfer · efficiency
Related
- All WJEC GCSE Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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