Third law of thermodynamics
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The third law of thermodynamics was developed by Walther Nernst and is thus sometimes referred to as Nernst's theorem.
This states that the entropy of a system at zero absolute temperature is a well-defined constant. This is because a system at zero temperature exists in its ground state, so that its entropy is determined only by the degeneracy of the ground state. OR It states that 'it is impossible by any procedure, no matter how idealised, to reduce any system to the absolute zero of temperature in a finite number of operations'.
A special case of this is systems with a unique ground state, such as crystal lattices. The entropy of a perfect crystal lattice as defined by Nernst's theorem is zero (since ln(1) = 0). However this disregards the fact that real crystals must be grown at finite temperature and possess an equilibrium defect concentration. When cooled down they are generally unable to achieve complete perfection.
See also
- Thermodynamics: Thermodynamic entropy, Timeline of thermodynamics, statistical mechanics, and random processes
- Other: Adiabatic process, Ground state
cs:3. termodynamický zákon da:Termodynamikkens 3. lov et:Termodünaamika kolmas seadus nl:Derde wet van de thermodynamica de:Nernst-Theorem