A electric motor is used to lift a heavy crate. During this process, 2400 J of electrical energy is supplied to the motor. The motor transfers 1800 J of energy as useful gravitational potential energy to the crate, and the remainder is wasted. Explain what is meant by a useful energy transfer and a waste energy transfer in this context, and state the law that governs the total energy involved in this process.

Pearson Edexcel International GCSE Physics (4PH1) — 4.1 Energy transfers · Explain · 4 marks · View as Markdown

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

An electric motor receives 2400 J of electrical energy. It transfers 1800 J of this energy as gravitational potential energy to a crate being lifted. The remaining energy is transferred as thermal energy to the surroundings.

Model answer (4 marks)

A useful energy transfer is the part of the energy that performs the intended work – here the 1800 J that is converted into gravitational potential energy of the crate. A waste energy transfer is the part that does not contribute to the intended work – here the 600 J that is lost as thermal energy to the surroundings. The total energy is governed by the law of conservation of energy, which states that energy cannot be created or destroyed, only transferred or transformed, so the input 2400 J equals the sum of useful 1800 J and waste 600 J.

Examiner tips

  • Use the exact terms ‘useful energy transfer’ and ‘waste energy transfer’.
  • Show the calculation 2400 J – 1800 J = 600 J.
  • State the conservation of energy law in your own words.

Mark scheme (4 marks)

  1. A useful energy transfer is the transfer of energy that directly serves the purpose of the system — here, the 1800 J transferred to gravitational potential energy in the crate.
  2. A waste energy transfer is the transfer of energy to a form that does not directly serve the purpose of the system — here, the 600 J transferred as thermal energy to the surroundings.
  3. The waste energy transferred equals 2400 − 1800 = 600 J (thermal energy to surroundings).
  4. The law of conservation of energy states that energy cannot be created or destroyed, only transferred, stored or dissipated — so the total energy output (1800 J + 600 J) equals the total energy input (2400 J).

Key terms in this question

useful energy transfer · waste energy transfer · gravitational potential energy

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