# Engineering Thermodynamics/Third Law

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## Third Law of Thermodynamics[edit]

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 ST→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