A student uses an electric kettle to heat water for a hot drink. The kettle is designed to transfer electrical energy to the water. Explain what happens to the energy in this system, including reference to useful energy transfer, waste energy transfer, and the conservation of energy.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric kettle is plugged into the mains and switched on. The kettle heats the water inside it. The outside of the kettle also becomes warm to the touch, and a humming sound can be heard whilst it operates.
Model answer (4 marks)
The kettle converts electrical energy into thermal energy in the water. The electrical energy supplied by the mains is transferred to the water as useful heat, raising its temperature.
Some of the input energy is lost as waste. Heat is conducted through the kettle’s outer casing to the surrounding air, and a small amount is emitted as sound (the humming).
The total energy output – useful heat in the water plus waste heat and sound – equals the electrical energy input. Thus the energy is conserved.
Energy cannot be created or destroyed; it is only transferred or transformed, so the total energy of the system remains constant.
Some of the input energy is lost as waste. Heat is conducted through the kettle’s outer casing to the surrounding air, and a small amount is emitted as sound (the humming).
The total energy output – useful heat in the water plus waste heat and sound – equals the electrical energy input. Thus the energy is conserved.
Energy cannot be created or destroyed; it is only transferred or transformed, so the total energy of the system remains constant.
Examiner tips
- Use the word "converted" or "transferred" to show the change of form.
- Mention both useful (heat to water) and waste (heat to air, sound).
- State that the sum of useful and waste equals the input to demonstrate conservation.
Common mistakes
- Saying the kettle *creates* energy instead of converting it.
- Forgetting to mention the sound as a waste form of energy.
- Claiming that energy is lost from the system rather than transferred to the surroundings.
Mark scheme (4 marks)
- The useful energy transfer is electrical energy being transferred to thermal energy (heat) in the water.
- Some energy is wasted — transferred as heat to the surroundings (e.g. through the warm outer casing) and/or as sound.
- The total energy output (useful + waste) equals the total electrical energy input — energy is conserved.
- Energy cannot be created or destroyed, only transferred, stored or dissipated — the total energy of the closed system remains constant.
Key terms in this question
useful energy transfer · waste energy transfer · conservation of energy
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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