# Explain why the Carnot cycle is described as a reversible cycle and discuss what this implies about the total entropy change of the universe during one complete Carnot cycle.

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

> The Carnot cycle consists of two isothermal processes and two adiabatic processes, all carried out quasi-statically.

## Mark scheme (4 marks)

1. A reversible process is one carried out quasi-statically (infinitely slowly) so the system is always in thermodynamic equilibrium, with no dissipative effects such as friction or turbulence.
2. During each isothermal process, heat is exchanged reversibly with a reservoir at the same temperature as the working substance, so there is no temperature difference driving heat transfer irreversibly.
3. The entropy change of the universe (system plus surroundings) during each step is zero because every process is reversible, so the total entropy change of the universe over one complete Carnot cycle is zero.
4. This zero total entropy change means the Carnot cycle achieves the maximum possible efficiency for a heat engine operating between the same two temperatures; any real cycle produces entropy in the universe and therefore must have a lower efficiency.

## Key terms

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

## 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-the-carnot-cycle-is-5ac92159) · Published by Druglandscape Ltd.