EME 504
Foundations in Sustainability Systems

3.3 Second Law of Thermodynamics: Entropy of an Isolated System Never Decreases

“You can't shovel manure into the rear end of a horse and expect to get hay out of its mouth”

W. White. "Thermodynamics." In Geochemist.

There are many ways to explain the second law of thermodynamics. Some are more colorful than others! For more formal statements:

  • It is impossible to construct a machine that is able to convey heat by a cyclical process from one reservoir at a lower temperature to another at a higher temperature unless work is done by some outside agency. In other words, air conditioning is never free.
  • Heat cannot be entirely extracted from a body and turned into work. That is why computers will always need cooling systems.
  • Every system left to itself will, on the average, change toward a condition of maximum probability. Another classical example of this statement starts with imagining a system made up of two closed chambers separated by a removable partition. Assume that each chamber contains a different gas. If the partition that physically separates these chambers is removed, the gases will eventually mix with each other. Re-inserting the partition will not result in unmixing of the gases. Returning to the initial conditions may or may not be possible. But, even if it is possible, energy will be needed for this process to occur.

This last statement brings us to the idea of entropy, or the measurement of disorder in a system. The second law of thermodynamics states that entropy always grows in isolated systems. Notice that entropy does not necessarily increase in an open system.