Other Free Encyclopedias » Online Encyclopedia » Encyclopedia - Featured Articles » Contributed Topics from A-E

Carnot, (Nicolas Léonard) Sadi

heat paper carnot’s thermodynamics

[kah®noh] (1796–1832) French theoretical physicist: a founder of thermodynamics through his theoretical study of an idealized heat engine.

Carnot’s family was unusual. His father, Lasare Carnot (1753–1823), was the ‘Organizer of Victory’ for the Revolutionary Army in 1794 and became Napoleon’s minister of war; unusually, he left politics for science in 1807 and did good work in pure and applied mathematics and in engineering. Sadi had one brother, Hippolyte, also a politician, whose son became president of France. Sadi was educated by his father and at the École Polytechnique, and served in the army as an engineer, leaving it as a captain in 1828. He was a cholera victim in the Paris epidemic of 1832.

His scientific work was highly original, and the single paper he published before his early death did much to create the new science of thermodynamics. His paper was Reflections on the Motive Power of Fire (1824) and it originated in Carnot’s interest in steam engines, which had been developed by British engineers and, as the nationalistic Carnot realized, were generating an industrial revolution in the UK. However, their theory was non-existent and their efficiency very low. Carnot set out to deduce if the efficiency could be improved and whether steam was the best ‘working substance’. His paper is a brilliant success, despite the fact that he used the caloric theory of heat, which presumed it to be a ‘subtle fluid’. (This did not affect the main answers and, incidentally, Carnot’s notes show that long before his death he was converted to modern heat theory.) He also used the correct idea that perpetual motion is impossible, a fact of experience.

In his paper, Carnot considers an idealized steam engine, frictionless, with its working substance passing from heat source to heat sink through a series of equilibrium states, so that it is truly reversible. The pressure–volume changes in it constitute a Carnot cycle. He was able to show that the efficiency of such an engine depends only on the temperature (T1 ) of the heat source and the temperature (T2 ) of the heat sink; that the maximum fraction of the heat energy convertible into work is (T1 – T2 ) /T2 ; and that it does not depend at all on the working substance (Carnot’s theorem). These ideas, which were eventually to mean so much for both engineers and theoreticians, were too abstract to attract much interest in 1824. In 1849 when saw the paper he realized its importance, and he and made it widely known. The paper contains ideas linked with the laws of conservation of energy and the First Law of Thermodynamics, and led Thomson and Clausius towards the Second Law. Later still, and others were to use thermodynamic ideas to forecast whether chemical reactions will occur.

Carolee [next] [back] Carney, Harry (Howell)

User Comments

Your email address will be altered so spam harvesting bots can't read it easily.
Hide my email completely instead?

Cancel or