Physical significance of entropy or of the second law

by Joseph Frederic Klein

(12) Another condition for the process considered is furnished by the First Law. For each element of time

, where

is the inner energy of the body and

the work expended upon it in an element of time by external means.

Now let us consider a more special case in which the external pressure at each instant is equal to the pressure p of the supposedly stationary body. Then the external work will be represented by

and then it follows that