Entropy

 

A thermodynamic state or property that measures the degree of disorder or randomness of a system.

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A measure of uncertainty regarding the state of a system: for example, a gas molecule at an unknown location in a large volume has a higher entropy than one known to be confined to a smaller volume. Free energy can be extracted in converting a low-entropy state to a high-entropy state: the (time-average) pressure exerted by a gas molecule can do useful work as a small volume is expanded to a larger volume. In the classical configuration space picture, any molecular system can be viewed as a single-particle gas in a high-dimensional space. In the quantum mechanical picture, entropy is described as a function of the probabilities of occupancy of different members of a set of alternative quantum states. Increased information regarding the state of a system reduces its entropy and thereby increases its free energy, as shown by the resulting ability to extract more work from it. An illustrative contradiction in the simple textbook view of entropy as a local property of a material (defining an entropy per mole, and so forth) can be shown as follows: The third law of thermodynamics states that a perfect crystal at absolute zero has zero entropy*; this is true regardless of its size. A piece of disordered material, such as a glass, has some finite entropy G0 > 0 at absolute zero. In the local-property view, N pieces of glass, even (or especially) if all are atomically identical, must have an entropy of NG0. If these N pieces of glass are arranged in a regular three-dimensional lattice, however, the resulting structure constitutes a perfect crystal (with a large unit cell); at absolute zero, the third law states that this crystal has zero entropy, not NG0. To understand the informational perspective on entropy, it is a useful exercise to consider (1) what the actual entropy of such crystal is as a function of N, with and without information describing the structure of the unit cell, (2) how the third law can be phrased more precisely, and (3) what this more precise statement implies for the entropy of well-defined aperiodic structures. Note that any one unit cell in the crystal can be regarded as a description of all the rest.

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A measure of the disorder of a physical system.

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Denotes the degree of disorder in a closed system and the probability if a reaction will take place or not. In a closed system entropy will increase until the equilibrium is reached. The thermodynamic definition of entropy is related to the fact, that the transformation of heat into work is limited: "There is no device that can transform heat withdrawn from a reservoir completely into work with no other effect." (Lord Kelvin)

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A measure of the disorder of a closed system. The second law of thermodynamics states that the entropy (and disorder) increases as time moves forward. [Encyclopedia Nanotech]

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A measure of the disorder of a closed system. The second law of thermodynamics states that the entropy (and disorder) increases as time moves forward

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A measure of the disorder of a closed system. The second law of thermodynamics states that the entropy (and disorder) increases as time moves forward. [Encyclopedia Nanotech]

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A measure of the disorder of a closed system. The second law of thermodynamics states that the entropy (and disorder) increases as time moves forward.

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