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Definition: Entropy is the quantitative measure of disorder in a system. The concept comes out of thermodynamics, which deals with the transfer of heat energy within a system. Instead of talking about some form of "absolute entropy," physicists generally talk about the change in entropy that takes place in a specific thermodynamic process.

Calculating Entropy

In an isothermal process, the change in entropy (delta-S) is the change in heat (Q) divided by the absolute temperature (T):

delta-S = Q/T
In any reversible thermodynamic process, it can be represented in calculus as the integral from a processes initial state to final state of dQ/T.

The SI units of entropy are J/K (joules/degrees Kelvin).

Entropy & The Second Law of Thermodynamics

One way of stating the second law of thermodynamics is:
In any closed system, the entropy of the system will either remain constant or increase.
One way to view this is that adding heat to a system causes the molecules and atoms to speed up. It may be possible (though tricky) to reverse the process in a closed system (i.e. without drawing any energy from or releasing energy somewhere else) to reach the initial state, but you can never get the entire system "less energetic" than it started ... the energy just doesn't have anyplace to go.

Misconceptions about Entropy

This view of the second law of thermodynamics is very popular, and it has been misused. Some argue that the second law of thermodynamics means that a system can never become more orderly. Not true. It just means that in order to become more orderly (for entropy to decrease), you must transfer energy from somewhere outside the system, such as when a pregnant woman draws energy from food to cause the fertilized egg to become a complete baby, completely in line with the second line's provisions.
Also Known As: Disorder, Chaos, Randomness (all three imprecise synonyms)
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