Another Thermodynamic Function

The interpretation of $S$ suggests that we can construct a new thermodynamic function in the same manner that we considered the physical interpretation enthalpy, $H$, in Eq. .

This new thermodynamic is formed by subtracting off the ``thermal energy'' part from $U$:

$$\input{equations/F-legendre}$$ (12-4)

This should be a function expressing how much compression work can be done by any body at constant $T$. This function is called the Helmholtz free energy.

(But this is getting ahead of ourselves since we have only been discussing ideal gases.)

We have found the entropy is a state function for an ideal gas, a generalization of this entropy for any system makes up the first part of the second law.

The new functions that we have developed must also be state functions--we will find that their interpretation makes them very useful indeed.