A mobile phone battery transfers energy to the phone's components. Explain why the total energy supplied by the battery is greater than the useful energy transferred to the screen and processor, and describe one way the efficiency of the phone's energy use could be improved.

Edexcel A-Level Physics (9PH0) — 3.1 Charge and current · Explain · 5 marks · View as Markdown

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

A mobile phone uses a rechargeable battery. When the phone is in use, the screen displays images, the processor carries out calculations, and the casing becomes warm to the touch.

Model answer (5 marks)

The battery supplies a total of electrical energy, E_total. Some of this energy is converted into useful work – the light produced by the screen and the calculations performed by the processor – which is the useful energy, E_useful. The remainder is lost as waste energy, mainly as heat that warms the phone’s casing. By the law of conservation of energy, E_total = E_useful + E_waste, so E_total must be greater than E_useful.

Efficiency is defined as η = E_useful / E_total. Because E_waste > 0, η < 1 (or <100 %).

One way to improve efficiency is to reduce the amount of waste heat, for example by adding better thermal insulation or heat‑sinking to minimise heat loss to the surroundings, thereby increasing the proportion of energy that reaches the useful components.

Examiner tips

  • Use the definition of efficiency and show the inequality η<1.
  • Explain that waste heat is the main source of inefficiency.
  • Mention a specific improvement such as better thermal management.
  • Keep the answer concise and use correct terminology.

Mark scheme (5 marks)

  1. The phone produces waste energy transfer (in the form of heat/thermal energy)
  2. By the law of conservation of energy, energy cannot be created or destroyed, so the total input must account for both useful and waste outputs
  3. The useful energy transferred is only the energy that directly serves the purpose of the device (powering the screen and processor)
  4. Efficiency = useful energy transferred by the device ÷ total energy supplied to the device, so because waste energy is produced, efficiency is less than 1 (or 100%)
  5. One valid method to improve efficiency, e.g. thermal insulation to reduce heat lost to surroundings / reducing friction in moving parts via lubrication / recycling waste heat energy as a useful input

Key terms in this question

useful energy transfer · efficiency

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