The Second Principle of Thermodynamics

Abstract

The impossibility of realizing the perpetual motion is universally accepted as a fundamental principle of Physics. This postulate defines two main categories, where physical processes can be placed: natural processes and unnatural processes. The former includes all the observed processes and the hypothesized processes that do not violate the fundamental principle and are, therefore, possible. The latter includes all the hypothesized processes that violate the principle of the impossibility of perpetual motion, and therefore they cannot occur. This is the starting point for formulating an evolutionary criterion for all-natural processes. To achieve this, a suitable mathematical tool must be developed. The fundamental step is the definition of entropy and of absolute temperature. These are complementary quantities and constitute the basis of the Second Principle, which must be formulated, first, in the frame of closed systems. Before the extension of the fundamental equations of Thermodynamics to continuous systems, the approximation of discontinuous systems and the problem of the conversion of heat into work both for reversible and irreversible engines are discussed. The coefficients of performance of refrigerators and of heat pumps are defined and the problem of the maximum work obtainable from a given configuration immersed in a given environment is briefly treated.