A student uses an electric kettle to heat water. Explain why the efficiency of the kettle can never reach 100%, and describe ONE way in which the design of the kettle could be improved to increase its efficiency.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric kettle is designed to transfer electrical energy to heat the water inside it. In practice, some energy is also transferred to the surrounding air and to the kettle's outer casing.
Model answer (5 marks)
The kettle cannot reach 100 % efficiency because, by the conservation of energy, the electrical energy supplied must equal the sum of all energy transfers. Some of this energy is inevitably lost as heat to the surrounding air and to the kettle’s outer casing, so the useful energy that actually heats the water is always less than the total input.
One way to improve efficiency is to add thermal insulation to the outer casing. Insulation reduces heat loss to the surroundings, so a larger proportion of the input electrical energy is transferred to the water, increasing the overall efficiency.
One way to improve efficiency is to add thermal insulation to the outer casing. Insulation reduces heat loss to the surroundings, so a larger proportion of the input electrical energy is transferred to the water, increasing the overall efficiency.
Examiner tips
- Explain the conservation of energy and identify waste heat as the reason for <100 % efficiency; give a specific design improvement and explain how it reduces waste heat; use correct terminology such as ‘thermal insulation’ and ‘heat loss’
Common mistakes
- Claiming the kettle can reach 100 % efficiency; ignoring the conservation of energy; not specifying how the improvement reduces heat loss
Mark scheme (5 marks)
- Energy cannot be created or destroyed (conservation of energy), so the total energy input equals the total energy output
- Some energy is transferred to the surroundings / outer casing as waste energy transfer (e.g. heat to the air)
- Because some energy is wasted, the useful output energy transfer is always less than the total input energy
- Named design improvement — e.g. adding thermal insulation to the outer casing / using insulating materials
- Explanation of how the improvement reduces waste energy transfer / increases the proportion of energy used to heat the water
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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