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Summary Article: Clausius, Rudolf Julius Emanuel (1822-1888)
from The Hutchinson Dictionary of Scientific Biography

Place: Germany

Subject: biography, physics

German theoretical physicist who is credited with being one of the founders of thermodynamics, and with originating its second law. His great skill lay not in experimental technique but in the interpretation and mathematical analysis of other scientists' results.

Clausius was born in Köslin in Pomerania (now Koszalin in Poland) on 2 January 1822. He obtained his schooling first at a small local school run by his father, and then at the Gymnasium in Stettin. He entered the University of Berlin in 1840, and obtained his PhD from the University of Halle in 1848. Clausius then taught at the Royal Artillery and Engineering School in Berlin, and in 1855 became professor of physics at the Zürich Polytechnic. He returned to Germany in 1867 to become professor of physics at the University of Würzburg, and then moved to Bonn in 1869 where he held the chair of physics until his death in Bonn on 24 August 1888.

In 1870 the Franco-Prussian war stimulated Clausius to organize a volunteer ambulance service run by his students. He was wounded during the course of these activities, and the injury caused him perpetual pain. This, combined with the death of his wife in 1875, probably served to reduce his productivity during his later years. However, his scientific achievements were rewarded with many honours, including the award of the Royal Society's Copley Medal in 1879. Clausius died in Bonn on 24 August 1888.

Sadi Carnot, Benoit Clapeyron (1799-1864), and Lord Kelvin had made contributions to the theory of heat and to changes of state. Clausius examined the caloric theory, eventually rejecting it in favour of the equivalence of heat and work. Drawing particularly on Carnot's and Kelvin's concept of the continuous degradation or dissipation of energy, Clausius formulated (in a paper published in 1850) the second law of thermodynamics and introduced the concept of entropy.

The word entropy derives from the Greek word for transformation, of which there are two types according to Clausius. These are the conversion of heat into work, and the transfer of heat from high to low temperature. Flow of heat from low to high temperature produces a negative transformation value, and is contrary to the normal behaviour of heat. Clausius deduced that transformation values can only be zero, which occurs only in a reversible process, or positive, which occurs in an irreversible process. Clausius therefore concluded that entropy must inevitably increase in the universe, a formulation of the second law of thermodynamics. This law can also be expressed by the statement that heat can never pass of its own accord from a colder to a hotter body.

Entropy is considered to be a measure of disorder, and of the extent to which energy can be converted into work. The greater the entropy, the less energy is available for work. Clausius was opposed in his views by a number of scientists, but James Clerk Maxwell gave Clausius considerable support in scientific argument on the subject.

Clausius did other work on thermodynamics, for example by improving the mathematical treatment of Hermann Helmholtz's law on the conservation of energy, which is the first law of thermodynamics; and by contributing to the formulation of the Clausius-Clapeyron equations that describe the relationship between pressure and temperature in working changes of state.

The second area to which Clausius made important contributions was the development of the kinetic theory of gases, which Maxwell and Ludwig Boltzmann had done so much to establish. From 1857 onwards, Clausius examined the inner energy of a gas, determined the formula for the mean velocity and mean path-length of a gas molecule, provided support for Avogadro's work on the number of molecules in a particular volume of gas, demonstrated the diatomic nature of oxygen, and ascribed rotational and vibrational motion (in addition to translational motion) to gas molecules. Clausius also studied the relationship between thermodynamics and kinetic theory.

Clausius' third major research topic was the theory of electrolysis. In 1857, he became the first to propose that an electric current could induce the dissociation of materials, a concept which was eventually established by Svante Arrhenius. Clausius also proposed that Ohm's law applies to electrolytes, describing the relationship between current density and the electric field.

Clausius was clearly a brilliant theoretician, but he showed a curious lack of interest in the developments that arose from his work. The results of Boltzmann, Maxwell, Willard Gibbs (1839-1903), and other scientists and the advancements in the field of thermodynamics, statistical mechanics, and kinetic theory seem to have gone completely unnoticed by Clausius in his later years.

One interesting consequence of the second law of thermodynamics is that as entropy increases in the universe, less and less energy will be available to do work. Eventually a state of maximum entropy will prevail and no more work can be done: the universe will be in a static state of constant temperature. This idea is called ‘the heat death of the universe’ and although it seems to follow logically from the second law of thermodynamics, this view of the future is by no means accepted by cosmologists.

© RM, 2018. All rights reserved.

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