Introduction to Thermodynamic Property Relations
The purpose of this chapter is to develop the tools necessary to derive the unknown quantities from the measureable ones—i.e. thermodynamic property relations.
• The states of simple compressible substances are normally specified by two independent variables, and other properties are written as a function of those two. In other words, for three properties x, y, and z, we can write z = f (x, y)
• From calculus, we know that we can write a change in the variable z as
where the subscripts denote variables held constant, or dz = M dx N dy
In thermodynamics, the derivatives M and N represent properties of the substance which can be measured or derived.
• If we are evaluating a process where x=constant, then dz = N dy
• For example, if we choose to express pressure as P = P(T,v) then we have
and if we evaluate the change in pressure during a constant temperature process, then we have