German physicist who reconciled the results of Joule with the theories of Sadi Carnot by abandoning the idea that heat was conserved. He stated formally the equivalence of heat and work (First Law of Thermodynamics ) and developed the concept of entropy (which he named in 1865) to explain the directionality of physical processes. He discovered the fact that entropy can never decrease in a physical process and can only remain constant in a reversible process, a result which became known as the Second Law of Thermodynamics .

With Maxwell, he developed the kinetic theory of gases. In "Über die Art der Bewegung welche wir Wärme nennen" ("On the Kind of Motion which we Call Heat" (1857), he provided a full account of the kinetic theory of molecular motions. This was the first systematic treatment of the kinetic theory. It used probabilistic arguments, introduced the concept of mean free path, and correlated temperature and velocity. Clausius resolved this paradox of Buys-Ballot by explaining the motion of particles in terms of a random walk resulting from many collisions. Clausius also coined the word "virial" used in the virial equations.

Clausius also extended Clapeyron's equation and in 1870, derived the virial theorem, which states that for a bound gravitational system,

where K is the kinetic energy and U is the gravitational potential energy.

Carnot (Sadi), Clapeyron, Joule, Maxwell

Additional biographies: MacTutor (St. Andrews)