Week 12: The Third Law of Thermodynamics and its Uses

The third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches absolute zero. The entropy of a system at absolute zero is typically zero, and in all cases is determined only by the number of different ground states it has. Specifically, the entropy of a pure crystalline substance (perfect order) at absolute zero temperature is zero. This statement holds true if the perfect crystal has only one state with minimum energy.Basically, the third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches absolute zero and the key points are:

• At zero temperature, the system must be in a state with the minimum thermal energy.

• Mathematically, the absolute entropy of any system at zero temperature is the natural log of the number of ground states times Boltzmann's constant (kB).

• For the entropy at absolute zero to be zero, the magnetic moments of a perfectly ordered crystal must themselves be perfectly ordered.

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