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Clausius, Rudolf

law heat entropy universe

[klow zeeus] (1822–88) German theoretical physicist: a founder of thermodynamics, especially linked with its Second Law.

Clausius’s father was a Prussian pastor and proprietor of a small school which the boy attended. Later he went to the University of Berlin to study history, but changed to science; his teachers included and . He was short of money, which delayed his graduation, but his ambition was to teach university physics and he did so at Zürich, Würzburg and Bonn. In the Franco-Prussian War of 1870 he and his students set up an ambulance service and he was badly wounded.

By the 1850s a major problem had arisen in heat theory: results were accepted, but while he believed correctly that, when a heat engine produces work, a quantity of heat ‘descends’ from a higher to a lower temperature, he also believed that it passed through the engine intact. The First Law of Thermodynamics, largely due to , visualizes some heat as being lost in a heat engine and converted into work. This apparent conflict was solved by Clausius, who showed in 1850 that these results could both be understood if it is also assumed that ‘heat does not spontaneously pass from a colder to a hotter body’ (the Second Law of Thermodynamics). The next year arrived at the same law, differently expressed, and there are now several other equivalent formulations of the same principle. Clausius developed this concept, of the tendency of energy to dissipate, and in 1865 used the term entropy ( S ) for a measure of the amount of heat lost or gained by a body, divided by its absolute temperature. One statement of the Second Law is that ‘the entropy of any isolated system can only increase or remain constant’. Entropy was later seen as a measure of a system’s disorder. The Second Law generated much controversy, but Clausius, and Thomson led a vigorous and successful defence, although we would not now fully accept Clausius’s crisp summaries of ‘the energy of the universe is constant’ (First Law) and ‘the entropy of the universe tends to a maximum’ (Second Law), thereby predicting a ‘heat-death’ for the universe.

Clausius also did valuable work on the kinetic theory of gases, where he first used the ideas of ‘mean free path’ and ‘effective molecular radius’ which later proved so useful. In the field of electrolysis, Clausius was the first to suggest (in 1851) that a salt exists as ions in solution before a current is applied. In each area he attacked, he showed outstanding intuition, and his work led to major developments by others; but Clausius was strangely little interested in these developments.

Claxton, Rozelle [next]

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