![]() For we have seen that molecules in a vessel full of air at uniform temperature are moving with velocities by no means uniform, though the mean velocity of any great number of them, arbitrarily selected, is almost exactly uniform. if we conceive of a being whose faculties are so sharpened that he can follow every molecule in its course, such a being, whose attributes are as essentially finite as our own, would be able to do what is impossible to us. His description of the experiment is as follows: Maxwell conceived a thought experiment as a way of furthering the understanding of the second law. The second law is also expressed as the assertion that in an isolated system, entropy never decreases. The second law of thermodynamics ensures (through statistical probability) that two bodies of different temperature, when brought into contact with each other and isolated from the rest of the Universe, will evolve to a thermodynamic equilibrium in which both bodies have approximately the same temperature. William Thomson (Lord Kelvin) was the first to use the word "demon" for Maxwell's concept, in the journal Nature in 1874, and implied that he intended the Greek mythology interpretation of a daemon, a supernatural being working in the background, rather than a malevolent being. In his letters and books, Maxwell described the agent opening the door between the chambers as a "finite being". It appeared again in a letter to John William Strutt in 1871, before it was presented to the public in Maxwell's 1872 book on thermodynamics titled Theory of Heat. The thought experiment first appeared in a letter Maxwell wrote to Peter Guthrie Tait on 11 December 1867. Other researchers have implemented forms of Maxwell's demon in experiments, though they all differ from the thought experiment to some extent and none have been shown to violate the second law. ![]() Most scientists argue, on theoretical grounds, that no practical device can violate the second law in this way. It stimulated work on the relationship between thermodynamics and information theory. The concept of Maxwell's demon has provoked substantial debate in the philosophy of science and theoretical physics, which continues to the present day. This would decrease the total entropy of the two gases, without applying any work, thereby violating the second law of thermodynamics. Because the kinetic temperature of a gas depends on the velocities of its constituent molecules, the demon's actions cause one chamber to warm up and the other to cool down. As individual gas molecules (or atoms) approach the door, the demon quickly opens and closes the door to allow only fast-moving molecules to pass through in one direction, and only slow-moving molecules to pass through in the other. In the thought experiment, a demon controls a small massless door between two chambers of gas. In his first letter Maxwell called the demon a "finite being", while the Daemon name was first used by Lord Kelvin. It was proposed by the physicist James Clerk Maxwell in 1867. Maxwell's demon is a thought experiment that would hypothetically violate the second law of thermodynamics. Schematic figure of Maxwell's demon thought experiment
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