Rudolph Julius Emmanuel Clausius

Born Jan. 2, 1822, in Köslin, Pomerania; died Aug. 24, 1888, in Bonn. German physicist. One of the founders of thermodynamics and the molecular kinetic theory of heat.

Clausius studied at the University of Berlin (from 1840). From 1850 to 1857 he taught in Berlin and Zürich. He was a professor at the universities of Zürich (from 1857), Würzburg (from 1867), and Bonn (from 1869). Clausius was the first to understand and analyze the profound ideas of S. Carnot and to assess their importance in the theory of heat and heat engines. Developing Carnot’s ideas, he (simultaneously with W. Thomson [Lord Kelvin]) gave in 1850 the first formulation of the second law of thermodynamics: “Heat cannot of itself pass from a colder to a hotter body.” Clausius proved that there is no method for transferring heat from a colder body to a hotter body without the occurrence of some changes in nature that would compensate for such a transfer. Clausius introduced the concept of entropy in 1865. Having mistakenly extended the principle of the increase of entropy of a closed system to the entire universe, he formulated the hypothesis of the thermal death of the universe.

Clausius conducted basic studies in the molecular kinetic theory of heat. His studies promoted the introduction of statistical methods into physics. He succeeded in explaining from a unified point of view such superficially different phenomena in gases as internal friction, heat conductivity, and diffusion. He introduced the concept of an ideal gas and the concept of the mean free path of a molecule, first calculating the length of this path in 1860. He proved the virial theorem (1870), which relates the mean kinetic energy of a system of particles to the forces acting within such a system. He devised the kinetic theory of the transition of a substance from one state of aggregation to another and in 1850 substantiated the equation connecting the change in melting point with the change in pressure (Clausius-Clapeyron equation).

Clausius made an important contribution to the theory of electrolysis (1857). He theoretically substantiated the Joule-Lenz law and developed the thermodynamic theory of thermoelectricity (1853). Developing the ideas of the Italian scientist O. F. Mossotti (1847), Clausius worked out a theory of polarization of dielectrics on the basis of which he established the relationship between dielectric constant and polarizability (1879; Clausius-Mossotti equation).

Clausius was a foreign member of the London Royal Society (1868) and a corresponding member of the Paris Academy of Sciences (1865).