A filament light bulb transfers electrical energy into light. Explain why the filament light bulb is not 100% efficient and describe what happens to the energy that is not usefully transferred.

WJEC GCSE Physics (Wales) — 1.6 Distance, speed and acceleration · Explain · 4 marks · View as Markdown

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

A filament light bulb is connected to the mains electricity supply. Its purpose is to provide light for a room.

Model answer (4 marks)

The filament bulb converts electrical energy into light energy.
1. The conversion is not 100 % efficient because a large part of the electrical energy is lost as heat.
2. The heat is produced by the resistance of the filament.
3. Energy is conserved, so the total output (light + heat) equals the electrical input.
4. The waste heat is transferred to the surrounding air, warming the room.

Examiner tips

  • Use the word ‘efficiency’ and mention heat as the waste product.
  • Show the energy flow: electrical → light + heat.
  • Explain conservation of energy briefly.
  • Mention the heat warming the surroundings to show understanding of energy transfer.

Common mistakes

  • Saying the bulb is 100 % efficient; ignoring the heat loss.
  • Not linking the heat to the filament’s resistance.
  • Failing to mention that the heat is transferred to the room, not just ‘lost’.

Mark scheme (4 marks)

  1. The useful energy transfer is electrical energy to light (energy)
  2. Some energy is transferred as heat/thermal energy, which is a waste energy transfer
  3. Because of conservation of energy, energy cannot be created or destroyed, so the total output energy equals the input energy but not all of it is useful
  4. The waste thermal energy is dissipated/transferred to the surroundings, warming the air around the bulb

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