A student uses a joulemeter to investigate the efficiency of an electric motor lifting a load. The joulemeter shows that 600 J of energy is supplied to the motor. The useful energy transferred to the load is 210 J. Explain why the efficiency of this motor cannot equal 1, and describe what happens to the energy that is not usefully transferred.

Eduqas GCSE Physics — 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).

An electric motor is used to lift a heavy load. A joulemeter measures the total energy supplied to the motor.

Model answer (4 marks)

The efficiency cannot be 1 because the law of conservation of energy requires that the total energy supplied must equal the sum of useful energy and all other energy transfers. Some of the 600 J supplied to the motor is always lost as waste energy, so the useful output (210 J) is always less than the input. The energy that is not usefully transferred is dissipated as waste energy, for example as heat or thermal energy in the motor windings and bearings. This waste energy does not perform the intended work of lifting the load and therefore reduces the efficiency below 1.

Examiner tips

  • Use the phrase "law of conservation of energy" to justify why efficiency cannot be 100 %. Mention that some energy is always lost as waste. Identify the waste energy as heat. Show the relationship: efficiency = useful energy / total energy.
  • common_mistakes
  • :
  • Saying the motor is 100 % efficient without recognising energy loss. Confusing efficiency with power. Failing to specify that the lost energy is heat or thermal energy.

Mark scheme (4 marks)

  1. The law of conservation of energy means energy cannot be created or destroyed, only transferred or dissipated
  2. Some energy is always dissipated / transferred as waste energy, so useful output is always less than total input
  3. The energy that is not usefully transferred is a waste energy transfer, for example as heat/thermal energy
  4. This waste energy transfer does not directly serve the purpose of the system / is not a useful energy transfer

Key terms in this question

efficiency · useful energy transfer · joulemeter

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