# Explain why no real heat engine operating between two fixed temperature reservoirs can exceed the efficiency of a Carnot engine operating between the same two reservoirs.

> IB DP Physics Higher Level (2023 syllabus) — B.4 Thermodynamics (HL only) · Explain · 4 marks

## Mark scheme (4 marks)

1. A Carnot engine is fully reversible, meaning all processes are quasi-static and no entropy is generated within the engine itself.
2. Any real engine contains irreversible processes (such as friction, heat losses across finite temperature differences, or turbulence) which generate entropy.
3. By the second law of thermodynamics, entropy of an isolated system (or the universe) cannot decrease, so irreversible processes increase the total entropy.
4. Because more heat must be rejected to the cold reservoir (to account for extra entropy produced), less work is extracted from the same heat input, so efficiency is lower than the Carnot efficiency.

## Key terms

- [Carnot engine](https://www.gradenine.co.uk/glossary/carnot-engine)

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-no-real-heat-engine-3c03f230) · Published by Druglandscape Ltd.