Thermodynamic Entropy

Per the second law of thermodynamics, real processes contain irreversibilities like heat transfer and friction. Entropy is a thermodynamic property of a system used to quantify the irreversibility of a process or cycle.

Entropy is given as:

• Intrinsic property: $\text{s }[\text{kJ}/\text{kgK}]$
• Extrinsic property: $\text{S [kJ/K]}$

Specifically, entropy is a measure of the number of possible ways energy can be distributed in a system of molecules.

For a perfect crystal of a pure substance, microscopic state is certain (“third law”), such that ${\lim }_{T\to 0}S=0$.

Increase of Entropy Principle

The total entropy (system + surroundings) never decreases. This can be expressed as:

$$S_{\text{gen}}=\Delta S_{\text{tot}}=\Delta S_{\text{sys}}+\Delta S_{\text{surr}}\ge 0$$

Some cases for $S_{\text{gen}}$:

$$S_{gen}=\{\begin{array}{ll}>0& \text{Irreversible process}\\ =0& \text{Reversible process}\\ <0& \text{Impossible process}\end{array}$$