gives expressions for the work done on various simple systems in a reversible change. We have already discussed a simple compressible fluid. An elastic rod is described to good approximation by its length $L$ and the tension $f$. To stretch the rod by $\mathrm{d} L$ against this force, the work done is clearly $f \mathrm{~d} L$. The surface tension in a liquid surface is, by definition, the parameter $\sigma$ such that the work required to increase the area of the surface by $\mathrm{d} A$ is $\sigma \mathrm{d} A$. "These cases are all straightforward. The expressions for electric and magnetic systems are discussed at the end of the chapter (Section 14.5).