entropy

n. measure of the level of disorder in a system; amount of unavailable energy in a system (Thermodynamics)

For other uses, see: information entropy (in information theory) and entropy (disambiguation). In physics, entropy, symbolized by S,( from the Greek μετατροπή (metatropi) meaning "transformation"), is a measure of the unavailability of a  system’s energy to do work. Entropy is central to the second law of thermodynamics and the combined law of thermodynamics, which deal with physical processes and whether they occur spontaneously. Spontaneous changes, in isolated systems, occur with an increase in entropy. Spontaneous changes tend to smooth out differences in temperature, pressure, density, and chemical potential that may exist in a system, and entropy is thus a measure of how far this smoothing-out process has progressed. In short Entropy is a function of a quantity of heat which shows the possibility of conversion of that heat into work. The increase in entropy is small when heat is added at high temperature and is greater when heat is added at lower temperature. Thus for maximum entropy there is minium availability for conversion into work and for minimum entropy there is maximum avilability for conversin into work.
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(n.)
A certain property of a body, expressed as a measurable quantity, such that when there is no communication of heat the quantity remains constant, but when heat enters or leaves the body the quantity increases or diminishes. If a small amount, h, of heat enters the body when its temperature is t in the thermodynamic scale the entropy of the body is increased by h / t. The entropy is regarded as measured from some standard temperature and pressure. Sometimes called the thermodynamic function.
  

term employed in theories about disordered artworks - the more you know about, the less entropy it seems to have

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Entropy [from Greek entropia turned in] The second law of thermodynamics, enunciated by mathematical physicist Clausius (1822-1888), which states that heat cannot of itself pass from a colder to a warmer body; also that it is impossible, by means of inanimate material agencies, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest surrounding objects. The process of cooling is considered irreversible, and the energy is said to have passed into an unavailable form. The entropy of such a cooling system is said to have attained a maximum -- all energy is "turned in" or run-down. The final result of such a process on a universe supposed to be dissipating its energy in the form of heat, would be to reduce all bodies to the same temperature; hence there could be no further transference of heat or energy among them and a state of quiescence or deadness would ensue. Such has been the scientific view, which assumes that the sun is a hot body cooling -- a view not held by theosophy.
Further, it is a mere assumption that a process is irreversible, made on no better ground than that we have not yet been able to reverse it. For example, if all the waters of the earth ran down into the sea, and no other agency intervened, we should soon have a universal dead-level and no more running water. But this result is prevented by the effects of evaporation and other causes collaborating with it. It is easy to fall into errors by considering only a portion of the facts; but we live in a living universe and not in a soulless heat-engine.