Paddle-wheels, friction, and moving-boundary work

Abstract Moving-boundary work, also called pressure-volume work or expansion work, is perhaps the main form of work discussed in introductory courses about thermodynamics. Here, we take a particular definition of this kind of thermodynamic work. On this basis, we show with simple reasoning that significant phenomena involving dissipation of mechanical energy, often mentioned as well in those courses but with a vague formalization, can be traced back right to moving-boundary processes. We refer in particular to the conversion of external work into internal energy in paddle-wheel experiments such as the one conducted by J. P. Joule in 1850, and to other processes involving friction.