Thermodynamics from Carnot to Clausius and Kelvin

Abstract

In 1824 Sadi Carnot (1824) published a little read volume, entitledRéflexions sur la Puissance Motrice du Feu, in which he formulated an ideal cycle that would describe the operation of a steam engine. Carnot’s first claim is that the operation of the engine does not depend upon the nature of the working substance. To him it was evident that

the reasoning would have been the same for all other gaseous substances, and even for all other bodies susceptible of changes in temperature through successive contractions and dilatations, which comprehends all natural substances, or at least all those which are adapted to realize the motive power of heat.

The gas is enclosed in a cylinder with a movable piston, and there are two heat reservoirs, A, and B, at temperatures t _A > t _B. The original plate appearing in Réflexions is shown in Fig. 2.1.

In reference to the figure, Carnot enumerates the four steps in his cycle. Body A is the hot reservoir, and the initial volume of the body of air is abcd. When the system comes in contact with A the piston gradually rises to ef . The contact is such that the temperature remains constant throughout. A is then removed, and the piston continues to rise from ef to gh. The expansion produces a fall in temperature until it becomes equal to the body B. Compression then occurs until the piston arrives at cd. But, because the system is in contact with B, the temperature remains constant. B is then removed and compression brings the piston to ik with a rise in temperature until it is equal to body A. The pressure–volume graphical representation, which is still used today, can be found in Clapeyron’s paper that appeared in 1834, 2 years after Carnot’s death.