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Originally, entropy was named to describe the "waste heat", or more accurately, energy loss, from heat engines and other mechanical devices which could never run with 100% efficiency in converting energy into work. Later, the term came to acquire several additional descriptions, as more was understood about the behavior of molecules on the microscopic level. In the late 19th century, the word "disorder" was used by [[Ludwig Boltzmann]] in developing [[Entropy (statistical views)|statistical views of entropy]] using [[probability theory]] to describe the increased molecular movement on the microscopic level. That was before quantum behavior came to be better understood by [[Werner Heisenberg]] and those who followed. Descriptions of thermodynamic (heat) entropy on the microscopic level are found in statistical thermodynamics and [[statistical mechanics]].
For most of the 20th century, textbooks tended to describe entropy as "disorder", following Boltzmann's early conceptualisation of the [[kinetic energy|"motional" (i.e. kinetic) energy]] of molecules. More recently, there has been a trend in chemistry and physics textbooks to describe [[Entropy (energy dispersal)|entropy as energy dispersal]].<ref name=Lambert>[http://franklambert.net/entropysite.
The mathematics developed in statistical thermodynamics were found to be applicable in other disciplines. In particular, information sciences developed the concept of [[information entropy]], which lacks the Boltzmann constant inherent in thermodynamic entropy.
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*'''An indicator of irreversibility''': fitting closely with the 'unavailability of energy' interpretation is the 'irreversibility' interpretation. Spontaneous thermodynamic processes are irreversible, in the sense that they do not spontaneously undo themselves. Thermodynamic processes artificially imposed by agents in the surroundings of a body also have irreversible effects on the body. For example, when [[James Prescott Joule]] used a device that delivered a measured amount of mechanical work from the surroundings through a paddle that stirred a body of water, the energy transferred was received by the water as heat. There was scarce expansion of the water doing thermodynamic work back on the surroundings. The body of water showed no sign of returning the energy by stirring the paddle in reverse. The work transfer appeared as heat, and was not recoverable without a suitably cold reservoir in the surroundings. Entropy gives a precise account of such irreversibility.
* [[Entropy (energy dispersal)|'''Dispersal''']]: [[Edward A. Guggenheim]] proposed an ordinary language interpretation of entropy that may be rendered as "dispersal of modes of microscopic motion throughout their accessible range".<ref name="Dugdale 101">Dugdale, J.S. (1996). ''Entropy and its Physical Meaning'', Taylor & Francis, London, {{ISBN|0748405682}}, Dugdale cites only Guggenheim, on page 101.</ref><ref name="Guggenheim1949">Guggenheim, E.A. (1949), Statistical basis of thermodynamics, ''Research: A Journal of Science and its Applications'', '''2''', Butterworths, London, pp. 450–454; p. 453, "If instead of entropy one reads number of accessible states, or spread, the physical significance becomes clear."</ref> Later, along with a criticism of the idea of entropy as 'disorder', the dispersal interpretation was advocated by [[Frank L. Lambert]],<ref name=Lambert/><ref name="Lambert2005">{{cite journal |last1=Kozliak |first1=Evguenii I. |last2=Lambert |first2=Frank L.|date=2005 |title="Order-to-Disorder" for Entropy Change? Consider the Numbers!|journal=Chem. Educator |volume=10 |pages= 24–25|url=http://franklambert.net/entropysite.com/order_to_disorder.pdf}}</ref> and is used in some student textbooks.<ref>For example: Atkins, P. W., de Paula J. Atkins' Physical Chemistry, 2006, W.H. Freeman and Company, 8th edition, {{ISBN|9780716787594}}. Brown, T. L., H. E. LeMay, B. E. Bursten, C.J. Murphy, P. Woodward, M.E. Stoltzfus 2017. Chemistry: The Central Science, 10th ed. Prentice Hall, 1248pp, {{ISBN|9780134414232}}. Ebbing, D.D., and S. D. Gammon, 2017. General Chemistry, 11th ed. Centage Learning 1190pp, {{ISBN|9781305580343}}. Petrucci, Herring, Madura, Bissonnette 2011 General Chemistry: Principles and Modern Applications, 10th edition, 1426 pages, Pearson Canada {{ISBN|9780132064521}}.</ref>
:The interpretation properly refers to dispersal in abstract microstate spaces, but it may be loosely visualised in some simple examples of spatial spread of matter or energy. If a partition is removed from between two different gases, the molecules of each gas spontaneously disperse as widely as possible into their respectively newly accessible volumes; this may be thought of as mixing. If a partition, that blocks heat transfer between two bodies of different temperatures, is removed so that heat can pass between the bodies, then energy spontaneously disperses or spreads as heat from the hotter to the colder.
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