A electric kettle has a power-rating of 3 kW. When it is used, some energy is usefully transferred to heat the water, but some energy is wasted. Explain what is meant by useful energy transfer and waste energy transfer in this context, and describe one way in which the efficiency of the kettle could be improved.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Useful energy transfer is the part of the electrical energy that is converted into heat in the water, so that the water is boiled. Waste energy transfer is the part of the electrical energy that is converted into other forms that do not heat the water, such as heat lost to the air around the kettle or sound.
One way to improve the efficiency is to add thermal insulation to the kettle body or lid. This reduces the amount of heat that escapes to the surroundings, so a larger proportion of the input energy is used to heat the water.
One way to improve the efficiency is to add thermal insulation to the kettle body or lid. This reduces the amount of heat that escapes to the surroundings, so a larger proportion of the input energy is used to heat the water.
Examiner tips
- Use the exact terms ‘useful energy transfer’ and ‘waste energy transfer’.
- Explain that insulation reduces heat loss to the surroundings.
- Show the link between reduced waste and higher efficiency.
Mark scheme (4 marks)
- Useful energy transfer is the transfer of energy that directly serves the purpose of the kettle, i.e. heating the water
- Waste energy transfer is the transfer of energy to a form that does not directly serve the purpose of the kettle, e.g. heating the surroundings / sound
- Correct method to improve efficiency stated, e.g. adding thermal insulation to the kettle body / lid
- Correct explanation of why the method improves efficiency, i.e. it reduces waste energy transfer so a greater proportion of the input energy is usefully transferred
Key terms in this question
useful energy transfer · waste energy transfer · efficiency · power-rating
Related
- All WJEC GCSE Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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