A solar-powered water pump is used to raise water from a well. The pump has a useful energy output of 1 200 J and a total energy input of 5 000 J. Describe what is meant by useful energy transfer and waste energy transfer in this context, and explain why the total energy in the system is still conserved despite waste energy being produced.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A solar-powered water pump converts energy from sunlight to raise water from a well. For every 5 000 J of energy supplied to the pump, 1 200 J is transferred usefully.
Model answer (4 marks)
Useful energy transfer is the part of the energy that is used to do the intended work – here the 1 200 J that raises the water, increasing its gravitational potential energy.
Waste energy transfer is the part of the energy that is not used for the intended work – for example the heat lost to the surroundings by friction in the pump.
Energy is conserved because it cannot be created or destroyed, only transferred between stores.
The waste energy (heat) plus the useful energy output together equal the total input of 5 000 J, so the overall energy balance is maintained.
Waste energy transfer is the part of the energy that is not used for the intended work – for example the heat lost to the surroundings by friction in the pump.
Energy is conserved because it cannot be created or destroyed, only transferred between stores.
The waste energy (heat) plus the useful energy output together equal the total input of 5 000 J, so the overall energy balance is maintained.
Examiner tips
- Use the exact terms ‘useful energy transfer’ and ‘waste energy transfer’. Explain the waste as heat or friction. Show the energy balance 1 200 J + waste = 5 000 J. Mention conservation of energy explicitly.
Common mistakes
- Confusing useful energy with total energy. Forgetting to identify the waste form (heat) or to state that energy is conserved.
Mark scheme (4 marks)
- Useful energy transfer is the transfer of energy that directly serves the purpose of the system — here, the energy transferred to gravitational potential energy of the raised water.
- Waste energy transfer is the transfer of energy to a form that does not directly serve the purpose of the system — for example, thermal energy lost to the surroundings due to friction in the pump.
- Energy is conserved because it is never created or destroyed; it is only transferred from one store to another.
- The waste energy (thermal/heat energy dissipated to the surroundings) plus the useful energy output together equal the total energy input of 5 000 J, so no energy is lost overall.
Key terms in this question
useful energy transfer · waste energy transfer
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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