Maxwell's demon

Maxwell's demon is a character in an 1867 thought experiment by the Scottish physicist James Clerk Maxwell, meant to raise questions about the second law of thermodynamics. This law forbids (among other things) two bodies of equal temperature, brought in contact with each other and isolated from the rest of the Universe, from evolving to a state in which one of the two has a significantly higher temperature than the other. The second law is also expressed as the assertion that entropy never decreases.

Related Topics:
1867 - Thought experiment - Scottish - Physicist - James Clerk Maxwell - Second law - Thermodynamics - Law - Temperature - Entropy

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Maxwell described his thought experiment in this way:

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:"... 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. 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. Now let us suppose that such a vessel is divided into two portions, A and B, by a division in which there is a small hole, and that a being, who can see the individual molecules, opens and closes this hole, so as to allow only the swifter molecules to pass from A to B, and only the slower molecules to pass from B to A. He will thus, without expenditure of work, raise the temperature of B and lower that of A, in contradiction to the second law of thermodynamics." http://users.ntsource.com/~neilsen/papers/demon/node3.html

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In other words, Maxwell imagines two containers, A and B, filled with the same gas at equal temperatures, placed next to each other. A little 'demon' guards a trapdoor between the two containers, observing the molecules on both sides. When a faster-than-average molecule from A flies towards the trapdoor, the demon opens it, and the molecule will fly from A to B. Thus, the average speed of the molecules in B will have increased, while the molecules in A will have slowed down on average. However, since average molecular speed corresponds to temperature, the temperature in A will have decreased and in B will have increased; this is contrary to the second law of thermodynamics.

Related Topics:
Gas - Molecules - Speed

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Is Maxwell correct? Could such a demon, as he describes it, actually violate the second law? One of the most famous responses to this question was suggested in 1929 by Leó Szilárd. Szilárd pointed out that a real-life Maxwell's Demon would need to have some means of measuring molecular speed, and that the act of acquiring information would require an expenditure of energy. Szilárd's insight was expanded upon in 1982 by Charles H. Bennett, who argued that to determine what side of the gate a molecule must be on, the demon must store information about the state of the molecule. Eventually, the demon will run out of information storage space and must begin to erase the information that has been previously gathered. Erasing information is a thermodynamically irreversible process that increases the entropy of a system. Therefore, according to Bennett, Maxwell's demon reveals a deep connection between thermodynamics and information theory http://www.ulearntoday.com/magazine/physics_article1.jsp?FILE=maxwelldemon.

Related Topics:
1929 - Leó Szilárd - 1982 - Charles H. Bennett - Information theory

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Real-life versions of Maxwellian demons (with their entropy-lowering effects, of course duly balanced by increase of entropy elsewhere) occur in living systems, such as the ion channels and pumps that make our nervous systems work, including the human brain. Single atom traps allow an experimenter to control the state of individual quanta in the same way as Maxwell's demon. Molecular-sized mechanisms are no longer found only in biology; they are also the subject of the emerging field of nanotechnology.

Related Topics:
Ion channel - Pumps - Nervous system - Brain - Nanotechnology

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