# Explain why the efficiency of a machine that converts electrical energy into mechanical energy must always be less than 100%.

> IB DP Physics Standard Level (2023 syllabus) — A.3 Work, energy and power · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Some input energy is always transferred to the surroundings as thermal energy (heat) due to resistance/friction.
2. 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.
3. Efficiency is defined as the ratio of useful output energy (or power) to total input energy (or power), expressed as a percentage.
4. 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

- [efficiency](https://www.gradenine.co.uk/glossary/efficiency)

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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