Entropy

Definition

Entropy ($S$) is a measure of the molecular disorder or randomness of a system. It is a state function that quantifies the unavailability of a system’s energy to do work.

Formula

For a reversible process: $dS = \frac{\delta Q_{rev}}{T}$

Relationships

• Related: Second Law of Thermodynamics, Internal Energy, Enthalpy
• Topic: Thermodynamics

Physical Interpretation

• Macro: Degree of irreversibility in a process.
• Micro: $S = k_B \ln \Omega$ (Boltzmann’s entropy formula), where $\Omega$ is the number of microstates.