We can use what we know about the statistical side of thermodynamics to give a simple interpretation to a change dU:
we see that because δwrev = -PdV
See also section 17-5 :In this section it is shown that we can manipulate the partition function to find the pressure of a system by calculating the above moment of the distribution. Again we take the derivative of the logarithm of the partition function Q, this time versus V and show that the result resembles the last equation pretty closely (apart from a factor ). Thus we get:
Once again we can find an important quantity of our system by manipulating the partition function Q.