Third Law of Thermodynamics
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[edit] Definition
The Third Law of Thermodynamics states that, by definition,
- The entropy of a pure substance in equilibrium approaches zero as the absolute temperature approaches zero.
This is also known as Nernst's Theorem after Walther Nernst, who first stated an alternate form of this law. According to the Second Law of Thermodynamics, it should be impossible in practice to cool a substance to zero entropy. A flawless diamond cooled in liquid helium probably comes as close as is reasonably possible.
[edit] Discussion
While it is possible to use entropy as a comparative concept, the Third Law provides a convenient zero point that simplifies calculations. It is calculated with reference to absolute zero, at -273.15 degrees Celsius or -459 degrees Fahrenheit. At absolute zero, the molecules in a substance have the smallest possible thermal motion and the smallest possible kinetic energy.
The Third Law is thus not so much a law of nature as a convention fixed upon by scientists: but it is not an entirely arbitrary convention, because it assigns zero entropy to those things with the least conceivable amount of entropy.
[edit] Misconceptions
Since some things, such as the core of a sun or a thermonuclear explosion, can have extremely high temperatures, many people assume that it should be possible to have equally low temperatures. However, absolute zero provides an absolute limit on how cold it can get. Misunderstandings of this idea have showed up in "The Golden Apples of the Sun" by Ray Bradbury and in the Sylvester Stallone/Wesley Snipes film Demolition Man.
