• id: JTHERM-01-N10
• title: Second Law — Irreversibility and Entropy
• tags: second-law, kelvin-planck, clausius

Two equivalent statements: Kelvin-Planck: No heat engine can convert all absorbed heat into work in a cyclic process (some must be rejected). Clausius: Heat cannot flow from cold to hot without external work. Both statements define the direction of natural processes — the Second Law introduces irreversibility. The concept of entropy ($S$) quantifies this: $dS = dQ_{\text{rev}}/T$. For any natural process: $\Delta S_{\text{total}} \geq 0$ (entropy of the universe never decreases). Reversible processes have $\Delta S_{\text{total}} = 0$.