The second law of thermodynamics

The second law is the law that gives time a direction: in an isolated system total entropy can never decrease. This section states its entropic formulation ($\Delta S_\text{tot} \geq 0$), gives an operational definition of what reversible and irreversible mean, and draws out the practical consequences — why heat flows from hot to cold, why a gas never spontaneously returns to half a container, why two bodies move towards equilibrium and never separate on their own.

• Entropic statement
• Reversible and irreversible
• The consequences of the second law