The thermodynamic systems considered here are those that can be described by using only two macroscopic variables. For this purpose the specific volume v and the specific internal energy u are used while the pressure function p(u,v) is supposed to be known. It is shown that by a suitable mathematical transformation of variables, the pair (u,v) can be replaced by the pair of functions temperature T(u,v) and entropy s(u,v) that are defined simultaneously by a proposed generic system of differential equations without using the concept of heat. To show the authenticity of these definitons some solutions of this generic system of differential equations are given and in addition to, the special case of the perfect gas is obtained as one of the results. It is proposed to add to the conventional concepts of work and heat a third constitutive component, the throttling. With it the local efficiency η of a differential themodynamic change can be defined. In addition this allows for the deduction of the relation (pdv + du) ≥ 0 as a consequence. The heat-work energy interactions with a thermodynamic system are taking place mostly through an enclosing boundary wall. As these two different forms of energy flow cross the boundary wall, they undergoe some transformation. As a result, the heat and work flow through the wall from/to the outer space to/from the system considered may change, although its algebraic sum, the total energy transfered, retains its initial value. Vector notation is used to facilitate the description of this transformation of energy crossing a boundary wall.

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