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Process function

A process function, process quantity, or a path function is a physical quantity that describes the transition of a system from an equilibrium state to another equilibrium state. As an example, mechanical work and heat are process quantities because they describe quantitatively the transition between equilibrium states of thermodynamic systems. It depends on the work done and how the state is reached.

Path functions depend on the path taken to reach one state from another. Different routes give different quantities. Examples of path functions include work, heat and length. In contrast to path functions, state functions are independent of the path taken.

Let us take an example: there is three ways for the work to be done from point 1 (p1,v1) to point 2 (p2,v2).It can either be done by following the path A ,B or C.

Actually, the work done is the area under the graph. If we take the path A, the work done will be different from the path B and path C. In decreasing order, the area will be path C > path B > path A. Here we see that the work done is a path function; meaning work done depends on the path taken.