Irreversible Process
A real process that passes through non-equilibrium states and cannot be reversed without leaving a permanent change in the surroundings.
Formula / equation
ΔS_universe > 0
Unit
J/K
In depth
All spontaneous processes are irreversible, and each one generates entropy. Because the system is never in equilibrium, less work is extracted than the reversible limit allows, with the shortfall dissipated as heat. Friction, free expansion and mixing are all inherently irreversible.
Examples in the real world
A gas rushing into a vacuum; heat flowing across a large temperature difference; a match burning.