Clausius , Rudolf Julius Emmanuel

(1822–1888) German physicist

Clausius, who was born in Köslin (now Koszalin in Poland), studied at the University of Berlin and obtained his doctorate from Halle in 1848. He was professor of physics at the Royal Artillery and Engineering School, Berlin (1850–55) and professor of mathematical physics at Zurich (1855–67). He then transferred to the University of Würzburg (1867) and, from there, moved to Bonn (1869).

He is noted for his formulation of what is now known as the second law of thermodynamics. Clausius arrived at this by considering the theorem of Sadi Carnot on heat engines and attempting to reconcile this with the mechanical theory of heat, which was developing at the time. In 1850 he published a famous paper Uber die bewegende Kraft der Wärme (On the Motive Force of Heat), in which he first introduced the principle that “it is impossible by a cyclic process to transfer heat from a colder to a warmer reservoir without net changes in other bodies.” An alternative statement of this, the second law, is “heat does not flow spontaneously from a colder to a hotter body.” The second law of thermodynamics was independently recognized by Lord Kelvin.

Clausius gave the law a mathematical statement in 1854 and published a number of papers on the topic over the next few years. In 1865 he introduced the term entropy as a measure of the availability of heat. The change in entropy of a system is the heat absorbed or lost at a given temperature divided by the temperature. An increase in entropy corresponds to a lower availability of heat for performing work.

The second law of thermodynamics is one of the fundamental principles of physics. It describes the fact that, although the total energy in a system is conserved, the availability of energy for performing work is lost. Clausius showed that in any nonideal (irreversible) process the entropy increased. The first and second laws of thermodynamics are encapsulated in his famous statement, “The energy of the universe is a constant; the entropy of the universe always tends toward a maximum.”

Clausius also followed the work of James Joule on the kinetic theory of gases, introducing the ideas of effective diameter and mean free path (the average distance between collisions). A contribution to electrochemistry was his idea that substances dissociated into ions on solution. In the field of electrodynamics he produced a theoretical expression for the force between two moving electrons – a formula later used by Hendrik Lorentz.