Second Law of Thermodynamics

In a thermodynamically isolated system, the total thermodynamic entropy does not decrease over time; spontaneous processes either increase total entropy or leave it unchanged when they are reversible, establishing a preferred direction for macroscopic energy dispersal and limiting complete conversion of heat into work.

In a macroscopic thermodynamic context, the Second Law of Thermodynamics asserts that for an isolated system (no exchange of matter or energy) the thermodynamic entropy does not decrease over time; typical spontaneous processes increase total entropy, which determines the directionality of heat flow and the irreversibility of macroscopic transformations.