The Second Principle

For producing motive power in a cyclic operation, a heat engine is required to operate between two thermal reservoirs at different temperatures. What is nowadays known as Planck’s statement of the second principle of thermodynamics was first perceived by Carnot and was the main difficulty for the acceptance of the equivalence between heat and work and of the principle of energy conservation. In the reverse sense, as stated by Clausius, heat cannot be transferred from a cold to a hot body by itself. Although with different words and worries, the underlying meaning behind both Plank and Clausius’s statements is the same: nature has an arrow. We start this chapter by presenting the role of dissipation when reversible and irreversible processes are compared. This subject is followed by the presentation of the statements of the second principle and the demonstration of their equivalence and by an analysis of the works of Carnot and Clapeyron and of reversible cycles other than Carnot’s cycles. Real processes are irreversible and irreversible processes are always dissipative and sources of entropy, as presented in Sect. 2.5. The chapter ends with a section on the meaning of entropy following the outstanding findings of Clausius, Maxwell, and Boltzmann.