Second Law of Thermodynamics
The total entropy of the universe increases in every spontaneous process.
Formula / equation
ΔS_universe > 0
Unit
J/K
In depth
The second law establishes the direction of time in physical processes and explains why energy spontaneously disperses. A system's own entropy may fall, but only if the surroundings gain more, which is how living organisms and refrigerators maintain internal order at the cost of greater disorder outside.
Examples in the real world
Heat never flows spontaneously from a cold body to a hot one; a shattered glass never reassembles.