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The second law of thermodynamics states that entropy will always increase over time within a closed system, defining a closed system as one in which neither matter nor energy may enter or leave.
The second law of thermodynamics states that entropy will always increase over time within a closed system, defining a closed system as one in which neither matter nor energy may enter or leave.
==Definitions==
===Thermodynamic definition===
In classical [[thermodynamics]], if a small amount of energy dQ is supplied to a system from a reservoir held at temperature T, the change in entropy is given by
<math>
dS=\frac{dQ}{T}
</math>
===Statistical mechanics definition 1===
If a system can be arranged in W different ways, the entropy is
<math> S= k_B \log W </math>
where <math>k_B</math> is Boltzmann's constant.
===Statistical mechanics definition 2===
Label the different states a thermodynamic system can be in by <math>i=1,2,3\ldots N</math>. If the probability of finding the system in state i is <math>p_i</math>, the entropy is
<math>
S=-k_B \sum_i^N p_i \log p_i
</math>
This definition is closely related to ideas in [[information theory]], where the definition of information content is very similar to the definition of entropy.
==See also==
==See also==