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Third law
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Entropy approaches a constant as T approaches zero.
As temperature approaches absolute zero, the entropy of a perfect crystal approaches zero, and absolute zero cannot be reached in a finite number of steps.
Experiments have reached nanokelvin.
Heat capacities vanish at T → 0.
In more detail
The third law of thermodynamics states that the entropy of a perfect crystal approaches zero as the temperature approaches absolute zero. It sets a reference point for entropy.
Real-world uses
- Low-temperature physics uses the third law to interpret measurements.
- It explains why reaching absolute zero is impossible in a finite number of steps.
- Low-temperature physicists use the third law to explain why absolute zero cannot be reached in a finite number of steps.
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Frequently asked questions
Can we reach absolute zero?
No. Cooling becomes increasingly difficult, and the third law prevents reaching exactly zero.
Is entropy zero at absolute zero?
For a perfect crystal, yes, since there is only one microstate.
Can I reach absolute zero?
No. The third law means you can approach it as closely as you like, but not reach it exactly.
Related topics in Heat & Thermodynamics
Entropy change · Statistical entropy · Free energy · Maxwell relations · Clausius–Clapeyron equation · Joule–Thomson effect