In a thermodynamic process, entropy—which is a measure of the system’s disorder or the number of microscopic configurations consistent with its macroscopic state—behaves differently depending on the type of process:
- For an isolated system:
Entropy never decreases; it either increases or remains constant. This is a statement of the Second Law of Thermodynamics. In natural (spontaneous) processes, the total entropy of an isolated system always increases until it reaches equilibrium, where entropy is maximized. - For a reversible process:
Entropy of the system remains constant. Reversible processes are idealized processes that happen infinitely slowly, with the system always close to equilibrium. They do not generate entropy. - For an irreversible process:
Entropy increases. Real processes are irreversible (due to friction, unrestrained expansion, heat flow through finite temperature differences, etc.), causing entropy generation inside the system or its surroundings. - In an open or closed system exchanging heat with surroundings:
The system’s entropy can increase or decrease depending on heat flow, but the total entropy change of system plus surroundings is never negative.