B.4.7 (HL)—Irreversibility

Irreversibility

A real process is irreversible when it cannot be exactly undone so that both system and surroundings return to their original states. Real isolated processes are almost always irreversible.

Entropy signature

For a real isolated process, total entropy increases: $$\Delta S_{\mathrm{total}}>0$$. The process has a preferred direction even though energy is conserved.

Physical examples

Friction, free expansion, mixing and heat flow through a finite temperature difference are irreversible because they increase the number of accessible microstates or disperse energy.

Common trap

Irreversible does not mean energy disappears. The first law still holds; irreversibility describes entropy and the impossibility of complete reversal.