An electric motor is used to lift boxes in a warehouse. Explain why the electric motor is not 100% efficient, and describe one way in which the efficiency of the motor could be improved.

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).

Electric motors are used in warehouses to lift heavy boxes from floor level to storage shelves. During operation, the motor becomes warm to the touch.

Model answer (4 marks)

The motor converts electrical energy into mechanical work, but not all of the input energy becomes useful work. Some of the energy is lost as waste heat (and a small amount as sound) from resistive heating in the windings and friction in the moving parts. The only useful energy is the gravitational potential energy gained by the boxes. The efficiency can be increased by lubricating the moving parts, which reduces friction and therefore reduces the amount of energy lost as heat.

Examiner tips

  • Use the word "waste" to describe the lost energy.
  • Mention both heat and sound as forms of waste energy.
  • State that the useful output is the potential energy of the lifted boxes.
  • Show that lubrication reduces friction and improves efficiency.

Common mistakes

  • Confusing electrical input with mechanical output; forgetting to mention heat as the main waste.
  • Failing to specify that the useful energy is the potential energy of the boxes.
  • Not linking lubrication directly to reduced friction and higher efficiency.

Mark scheme (4 marks)

  1. The motor produces waste energy transfer (to the surroundings)
  2. The waste energy transfer is in the form of heat/thermal energy (and/or sound)
  3. The useful energy transfer is only the gravitational potential energy gained by the boxes
  4. Efficiency can be improved by lubrication (of moving parts) to reduce friction

Key terms in this question

efficiency

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