Thus far we have discussed a number of "process variables" (variables that may be measured and used to characterize the operation of a process). It is helpful, prior to discussing state variables to group these variables into two categories:
An extensive variable is one which depends on system size (like mass or volume).
While extensive variables are useful for characterizing the specific system being analyzed, they are not general and (as we will see later in the course) do not help us to determine the "state" of the system.
An intensive variable is one which does not depend on system size (like temperature, pressure, or density).
While it may not be immediately obvious, intensive variables tell us much more about the system than extensive variables. In particular, the temperature and pressure are two of the most critical intensive variables.
Explain the difference between intensive and extensive variables.
A ratio of extensive variables will yield an intensive variable! (For example, mass/volume -- two extensive variables -- gives density, an intensive variable) This is one way to understand why intensive variables "tell you more".