Physics. [f. Gr. τροπή transformation (lit. ‘turning’), after the analogy of ENERGY. First proposed by Clausius (1865), in Ger. form entropie.
Clausius (Pogg. Ann. CXXV. 390), assuming (unhistorically) the etymological sense of energy to be ‘work-contents’ (werk-inhalt), devised the term entropy as a corresponding designation for the ‘transformation-contents’ (verwandlungsinhalt) of a system.]
The name given to one of the quantitative elements that determine the thermodynamic condition of a portion of matter.
In Clausius’ sense, the entropy of a system is the measure of the unavailability of its thermal energy for conversion into mechanical work. A portion of matter at uniform temperature retains its entropy unchanged so long as no heat passes to or from it, but if it receives a quantity of heat without change of temperature, the entropy is increased by an amount equal to the ratio of the mechanical equivalent of the quantity of heat to the absolute measure of the temperature on the thermodynamic scale. The entropy of a system = the sum of the entropies of its parts, and is always increased by any transport of heat within the system: hence ‘the entropy of the universe tends to a maximum’ (Clausius). The term was first used in Eng. by Prof. Tait (see quot. 1868), who however proposed to use it in a sense exactly opposite to that of Clausius. In this he was followed (with an additional misunderstanding: see quot. 1875) by Maxwell and others; but subsequently Tait and Maxwell reverted to the original definition, which is now generally accepted.
1866. Phil. Mag., Ser. IV. XXXII. July, 1. On the Determination of the Energy and Entropy of a Body. By Professor R. Clausius.
1868. Tait, Sketch Thermodynamics, 29. We shall … use the excellent term Entropy in the opposite sense to that in which Clausius has employed it,—viz., so that the Entropy of the Universe tends to zero.
1875. Maxwell, Th. Heat (ed. 4), 189, note. In former editions of this book the meaning of the term Entropy, as introduced by Clausius, was erroneously stated to be that part of the energy which cannot be converted into work. The book then proceeded to use the term as equivalent to the available energy…. In this edition I have endeavoured to use Entropy according to its original definition by Clausius.
1885. Watson & Burbury, Math. Th. Electr. & Magn., I. 245. As in the working of a heat engine, the entropy of the system must be diminished by the process, that is, there must be equalisation of temperature.
