Types of Work

Thermodynamics is almost always taught for fluids only--i.e. systems that do only $PdV$ work. I think this is not terribly useful because it gives the impression that thermo can only be applied to fluids and this is a mistake! Even more, because, as students, professional scientists were taught thermodynamics as fluids, they tend to reduce every system they study to a fluid--and this can be a mistake as well!

However, there is a simple reason for this historical method of teaching thermodynamics: the language and symbols become very complicated if we develop thermodynamics in the most general way. And this makes it difficult to learn anything at all--so, my object is to teach you how to apply thermodynamics and extend the lessons to more general cases. To extend it to more general cases, we need to consider other ways that work can be done on a body, then you can work out useful relations in new cases by following the methods that we will develop in this course.

The rate at which work is done is always of the form $F dx$ where $dx$ represents the change in the extent of some quantity and $F$ is a force that resists the change. This should be familiar for the case of a springs and mechanical objects. Perhaps, it is not so familar to think of $dx$ as a generalization. For instance, we might want to think of $dx$ as the `change in number of water molecules in a polymer' or we might want to think of $dx$ as the `change in the the charge held in a capacitor', or the `change in the number of Lithium ions in an anode.' Each one of these examples represents a different way for a material to store internal energy--and each one has an intensive variable (a generalized) force associated with it.

What follows is a discussion of some other types of work that can be done on a system. The details about polarization, magnetization, and elasticity are abstracted from the very useful book by Nye.