physical property of the state of a system, measure of disorder
In thermodynamics, entropy is a measure of a thermodynamic system's disorder. The entropy of the system varies directly with any reversible change in heat and inversely with the net temperature of the system. (The concept of entropy has somewhat different meanings in information theory, economics, and other disciplines.)
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- If I took a heavy weight on the floor here and pushed it, it would slide and stop. ... So, a frictional effect seems to be irreversible. ... a frictional effect ... is the result of enormous complexity of the interaction of the block with the wood ... the jiggling of the atoms inside the wood of the block is changed into disorganized irregular wiggle-waggles of the atoms in the wood.
- Richard Feynman, (July 11, 2018)"Richard Feynman's Lecture Entropy (Part 01)". EduBloq, YouTube. (quote at 6:47 of 21:31 in video)
- Because entropy is not really a classical quantity, we must build quantum mechanics into the definition. ...
It suffices to define entropy as the logarithm of the number of quantum states accessible to a system.
- Charles Kittel, (1989). "How to define entropy". Nature 339 (6221). DOI:10.1038/339170a0.
- Investigations of the entropy of substances at low temperatures have produced very important information regarding the structure of crystals, the work of Giauque and his collaborators being particularly noteworthy. For example, the observed entropy of crystalline hydrogen shows that even at very low temperatures the molecules of orthohydrogen in the crystal are rotating about as freely as in the gas; ... subsequent to this discovery the phenomenon of rotation of molecules in crystals was found to be not uncommon.
- Linus Pauling, (1935). "The structure and entropy of ice and of other crystals with some randomness of atomic arrangement". Journal of the American Chemical Society 57 (12): 2680-2684.