ABSTRACT
This chapter discusses the significance of the entropy. It focuses on its “physical” meaning, that is to say on its practical meaning instead of considering entropy under some mathematical angles as it has been done so far. The chapter examines the link between entropy and disorder. In a cycle carried out by the device, the fluid constituting the system is constituted by 1 mole of an ideal gas. It is contained in a cylinder fitted with a weightless and frictionless piston, thus permitting reversible processes to be performed. The fluid exchanges heat with the two exterior sources at different temperatures. These sources are large reservoirs which remain at the same temperature despite the exchanges of heat. It is interesting to notice that when heat is given to the system at high temperature, its entropy increase is lesser than when it is given at colder temperature. In the first case, its energy is of lesser entropy than in the second one.
