ABSTRACT

In ther mo dy na mic investigations it is worthwhile to distinguish between the extensive variables (volume, energy) and intensive variables (e.g., temperature, pressure). An extensive variable must satisfy two conditions. First, its time dependence is described by the generic balance equation:

where J is the fl ux, and G is a generalized source term. Second, it must be additive, in the sense that if Xa and Xb are values of the variables for two systems ‘a’ and ‘b’, the variable has the value ba YX + for the combined system, consisting of the union of the two. A general rule of ther mo dy na mics states that all the interactions of the ther mo dy namic system with its environment take place through the fl ows of extensive variables. Mass, energy, the number of molecules and total volume are examples of extensive properties. That property is summarized in the First Law.