Thermal dynamics volume

Second law of thermodynamics 

The second law of thermodynamics sets limits on the direction of processes that can occur in thermodynamic systems, and excludes the possibility of creating a perpetual motion machine of the second kind. In fact, this result was already reached by Sadi Carnot in his work “On the driving force of fire and on machines capable of developing this force” . However, Carnot relied on the ideas of the theory of caloric and did not give a clear formulation of the second law of thermodynamics. This was done in 1850-1851 independently by Clausius and Kelvin . There are several different, but at the same time equivalent formulations of this law.
Kelvin's postulate: "A circular process is impossible, the only result of which would be the production of work by cooling the thermal reservoir" . Such a circular process is called the Thomson-Planck process, and it is postulated that such a process is impossible.
Clausius's postulate: " Heat cannot spontaneously transfer from a less heated body to a hotter body " . The process in which no other change occurs, except for the transfer of heat from a cold body to a hot one, is called the Clausius process. The postulate states that such a process is impossible. Heat can transfer spontaneously in only one direction, from a more heated body to a less heated one, and such a process is irreversible.
Taking as a postulate the impossibility of the Thomson-Planck process, it can be proved that the Clausius process is impossible, and vice versa, from the impossibility of the Clausius process it follows that the Thomson-Planck process is also impossible.
The consequence of the second law of thermodynamics, postulated in these formulations, allows us to introduce for thermodynamic systems one more function of the thermodynamic state �S, called entropy, such that its total differential for quasi-static processes is written as ��=��/� . In combination with temperature and internal energy, introduced in the zero and first principles, entropy constitutes a complete set of quantities necessary for the mathematical description of thermodynamic processes. Only two of the three quantities mentioned, with which thermodynamics adds to the list of variables used in physics, are independent.