Explain why the efficiency of a machine that converts electrical energy into mechanical energy must always be less than 100%.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A simple electric motor is used to lift a load vertically at constant speed. The motor is connected to a power supply and draws a constant current.
Model answer (4 marks)
Efficiency is the ratio of useful output energy to total input energy. In an electric motor, electrical energy is supplied, but some of it is always lost as heat through resistance and friction. Because these losses mean the useful mechanical energy is always less than the electrical energy supplied, the denominator (input) is larger than the numerator (output). Therefore the ratio is always less than 1, i.e. less than 100%.
Examiner tips
- Define efficiency as output ÷ input ×100%; mention losses; show why ratio <1.
- Use terms: electrical energy, mechanical energy, heat, resistance, friction.
- Keep answer concise and to the point.
Common mistakes
- Confusing input and output energies; writing efficiency >100%.
- Forgetting to mention resistance or friction as sources of loss.
Mark scheme (4 marks)
- Some input energy is always transferred to the surroundings as thermal energy (heat) due to resistance/friction.
- Energy is conserved overall, but some energy is transferred in a non-useful form (to the surroundings), so not all input energy becomes useful output energy.
- Efficiency is defined as the ratio of useful output energy (or power) to total input energy (or power), expressed as a percentage.
- Because the denominator (input energy) always exceeds the numerator (useful output energy) due to unavoidable dissipation, the ratio is always less than 1 (i.e. less than 100%).
Key terms in this question
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →