Second Law of Thermodynamics
The law asserting that spontaneous physical processes increase total entropy in isolated systems, placing fundamental limits on heat engine efficiency.
Governing formula
ΔS_total ≥ 0
SI unit
J/K
In depth
Stated by Clausius (heat cannot spontaneously flow from cold to hot without work) and Kelvin-Planck (no heat engine can convert heat entirely into work without rejecting waste heat).
Examples in the real world
A drop of food coloring spreading irreversibly throughout a glass of water, never spontaneously recollecting into a single drop.