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In thermodynamics (a branch of physics), entropy, symbolized by S,[3] is a measure of the unavailability of a system’s energy to do work.[4][5] It is a measure of the randomness of molecules in a system and is central to the second law of thermodynamics and the fundamental thermodynamic relation, 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. The word "entropy" is derived from the Greek e?t??p?a "a turning toward" (e?- "in" + t??p? "a turning").[6] When a system's energy is defined as the sum of its "useful" energy, (e.g., that is used to push a piston), and its "useless energy", i.e., that energy which cannot be used for external work, then entropy may be visualized as the "scrap" or "useless" energy whose energetic prevalence over the total energy of a system is directly proportional to the absolute temperature of the considered system. (Note the product "TS" in the Gibbs free energy or Helmholtz free energy relations).
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