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Maxwell's demon, artwork. This thought experiment was devised by the Scottish physicist James Clerk Maxwell (1831-1879) to question the Second Law of Thermodynamics. The law states that the entropy of an isolated system will always increase, which means that a hot region cannot draw heat from a colder region. Maxwell's demon (not seen) operates a door between a hot and cold gas, opening it (centre left) to allow high-energy, hot molecules (red) through. This would violate the second law. However, as the demon requires energy to distinguish between high and low energy molecules, and to operate the door, the entropy of the system will still increase.
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