Entropy and Maxwell's Demon
Main Concept
Entropy is a measure of the number of microscopic configurations corresponding to a given macroscopic state. For example, the entropy of a gas (microscopically, a simple system containing molecules moving around in a container) is at a maximum when the gas is at thermal equilibrium (microscopically, each molecule has the same energy). The Second Law of Thermodynamics is a postulate derived from statistical mechanics and verified by experiment which states that the entropy of an isolated system can never decrease.
Maxwell's demon is a physical thought experiment by the renowned physicist James Clerk Maxwell which purports to demonstrate a method of decreasing entropy, which would therefore violate the Second Law of Thermodynamics. If a hypothetical demon were to direct all the fast particles to one side and all the slow particles to the other using a door (but not expending work), then the entropy of the system would decrease as the temperature difference between the sides increases. Now you can test your skills as the demon and attempt to decrease the entropy by opening and closing the door at the appropriate moment.
Click "Play" and adjust the speed to your liking. Use the "Open Door" check box to change the door status.
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