Maxwell's demon

Max·well's demon

M0163150 (măks′wĕlz′, -wəlz)n. An imaginary creature who is able to sort hot molecules from cold molecules without expending energy, thus bringing about a general decrease in entropy and violating the second law of thermodynamics.

[After James Clerk Maxwell.]

Thesaurus

Noun

Maxwell's demon - an imaginary creature that controls a small hole in a partition that divides a chamber filled with gas into two parts and allows fast molecules to move in one direction and slow molecules to move in the other direction through the hole; this would result in one part of the container becoming warmer and the other cooler, thus decreasing entropy and violating the second law of thermodynamics

Maxwell's demon

An imaginary being whose action appears to contradict the second law of thermodynamics, which identifies the natural direction of change with the direction of increasing entropy. There has always been a certain degree of discomfort associated with the acceptance of the law, particularly in relation to the time reversibility of physical laws and the role of molecular fluctuations. In 1867, J. C. Maxwell considered, in this connection, the action of “a finite being who knows the paths and velocities of all the molecules by inspection.” This being was later referred to as a demon by Lord Kelvin, and the usage has been generally adopted. See Entropy, Thermodynamic principles, Time, arrow of

The activity of Maxwell's demon can be modeled by a trapdoor in a partition between two regions full of gas at the same pressure and temperature. The trapdoor needs to be restrained by a light spring to ensure that it is closed unless it is struck by molecules traveling from the left (see illustration). Its hinging is such that molecules traveling from the right cannot open it. The essential point of Maxwell's vision was that molecules striking the trapdoor from the left would be able to penetrate into the right-hand region but those present on the right would not be able to escape back into the left-hand region. Therefore, the initial equilibrium state of the two regions, that of equal pressures, would be slowly replaced by a state in which the two regions acquired different pressures as molecules accumulated in the right-hand region at the expense of the left-hand region. Only a slightly more elaborate mechanical arrangement is needed to change the apparatus to one in which the temperatures of the two regions move apart. In each case, the demonic trapdoor appears to be contriving a change that is contrary to the second law, for an implication of that law is that systems in either mechanical equilibrium (at the same pressure) or thermal equilibrium (at the same temperature) cannot spontaneously diverge from equilibrium.

Type of device that emulates mechanically the actions of Maxwell's demon

As frequently occurs in science, the resolution of a paradox or the elimination of an apparent conflict with a firmly based law depends on a detailed analysis of the proposed arrangement. Numerous analyses of this kind have shown that the activities of Maxwell's demon do not in fact result in the overthrow of the second law.

Maxwell's demon

[′mak‚swelz ′dē·mən] (thermodynamics) demon of Maxwell