# Engineering Thermodynamics/Third Law

## Third Law of ThermodynamicsEdit

The Third Law of Thermodynamics extends the definition of Entropy:

**Entropy is zero only in a perfect crystal at absolute zero ( 0 kelvin [- 273.15 degree Celsius ]).**

The Third Law of Thermodynamics can mathematically be expressed as

lim STâ†’0 = 0 (1)

where

S = entropy (J/K)

T = absolute temperature (K)

At a temperature of absolute zero there is no thermal energy or heat. At a temperature of zero Kelvin the atoms in a pure crystalline substance are aligned perfectly and do not move. There is no entropy of mixing since the substance is pure. By -VBM