Question: What are the units of ? What are the units of ?
Imagine that there is a completely homogeneous system that has been arbitrarilry divided into small volumes of size :
Consider what is left of the internal energy after we subtract off the ``mechanical'' or ``compression'' energy:
So that we can define a new state function (that is also extensive),
It is sensible to interpret as the ``thermal energy'' at constant pressure. In other words, we divide the internal energy for a simple fluid into two parts--one part corresponding to that stored as compressive energy ( ) and another part the thermal energy . Another way to see this is:
In other words
for an ideal gas: