A student uses an electric hair dryer to dry their hair. The hair dryer produces warm moving air, but the outer casing also becomes hot during use. Explain why the hair dryer cannot have an efficiency of 1 (100%), and describe one way in which the efficiency of electrical appliances like the hair dryer could be increased.

OCR GCSE Physics A: Gateway Science (J249) — P7 Energy · Explain · 4 marks · View as Markdown

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

An electric hair dryer transfers electrical energy to produce a warm, fast-moving airstream. Its purpose is to dry hair by directing warm air onto it. After several minutes of use, the plastic casing of the hair dryer feels noticeably warm to the touch.

Model answer (4 marks)

The hair dryer converts electrical energy into useful energy – the warm, fast‑moving air that dries the hair. Some of the electrical energy is also lost as heat in the plastic casing, which is not useful for drying. Because the total energy supplied must equal the useful energy plus the wasted energy, the efficiency (useful output ÷ total input) can never reach 1 (100%).

Efficiency can be increased by reducing energy lost as heat, for example by using wires with lower resistance (or better insulation) so that less power is dissipated as heat in the wires, or by lubricating moving parts to reduce friction and the associated heat loss.

Examiner tips

  • Use the term "useful energy" for the warm air and "waste energy" for the casing heat.
  • Explain that conservation of energy means total input = useful + waste, so efficiency < 1.
  • Mention a specific improvement such as lower‑resistance wires or lubrication of moving parts.

Common mistakes

  • Saying the efficiency can be 1 because the device works.
  • Confusing the warm air with the casing heat as both useful.
  • Not linking the waste heat to the conservation of energy principle.

Mark scheme (4 marks)

  1. The useful energy transfer is the warm moving air (kinetic and thermal energy of the air) directed at the hair
  2. The heating of the casing is a waste energy transfer — energy is dissipated to the surroundings as heat and does not serve the purpose of the appliance
  3. Because of the conservation of energy, total energy input always equals useful output plus waste energy transfer, so efficiency (useful output ÷ total input) must always be less than 1
  4. Efficiency can be increased by reducing the current in the wires (which reduces heating of wires / reduces power loss due to resistance), OR by using lubrication on moving parts to reduce friction and heat loss

Key terms in this question

efficiency

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