Explain why a kettle transfers electrical energy to thermal energy more usefully than a filament lamp transfers electrical energy to light energy.

AQA GCSE Physics (8463) — 4.2.4 Energy transfers in everyday appliances · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A kettle and a filament lamp are both everyday electrical appliances. The kettle is designed to heat water, while the filament lamp is designed to produce light.

Model answer (4 marks)

A kettle is designed to heat water, so the electrical energy is converted mainly into useful thermal energy. In a filament lamp most of the electrical energy is lost as heat rather than light; the filament’s resistance rises with temperature, increasing the heat produced. Consequently a kettle transfers a larger proportion of its electrical energy to the intended useful form (thermal energy) than a filament lamp does to light.

Examiner tips

Use the word ‘useful’ to link the appliance’s purpose to the energy transfer.
Show that the lamp loses energy to heat by mentioning the resistance rise with temperature.
Keep the answer concise – 4 marks can be earned with two short sentences.
Use UK spelling (e.g. ‘useful’).

Common mistakes

Confusing the kettle’s output as light instead of heat.
Failing to mention the resistance increase in the filament.
Using vague terms like ‘more energy’ without explaining why it is less useful.

Mark scheme (4 marks)

The kettle's useful energy output is thermal energy, which is the intended/desired energy transfer
The filament lamp transfers most of its electrical energy to thermal (heat) energy rather than light, so most energy is wasted
The filament's resistance increases as the filament's temperature increases, causing more energy to be dissipated as thermal energy
Therefore the kettle transfers a greater proportion of electrical energy to its intended useful form (thermal energy) compared to the filament lamp transferring energy to light

Key terms in this question

electrical energy · thermal energy · filament lamp