Lessons of thermodynamics typically focus on only four classes of ideal gas processes: isothermal, isobaric, isochoric, and adiabatic. These are all specific cases of polytropic processes, thermodynamic processes with a constant specific heat, which are better known to most engineers than physics teachers because of their engineering applications.

Randall Knight prepared a resource for physics teachers that summarizes the properties of polytropic processes. The author hopes highlighting this class for physics teachers will expand the range of explorable examples they can provide students during lessons of thermodynamics.

“I felt it was important to increase the awareness of polytropic processes so that they can be used as examples in the teaching of thermodynamics,” Knight said.

One example that can be taught at the introductory physics level is the compression and expansion of gases in industrial applications. While most physics textbooks describe these processes as adiabatic, Knight explained they are better described as polytropic processes. They almost always have a negative specific heat, a property of polytropic processes in which the work done by the gas exceeds the heat input.

“This will be interesting to anyone who teaches thermodynamics at either the introductory level or a more advanced class,” Knight said. “It will provide them with a new set of examples of thermodynamics processes and, in the situations of negative specific heat, the opportunity to make a clear distinction about the different roles that work and heat play in ideal gas processes.”

Source: “All about polytropic processes,” by Randall Knight, The Physics Teacher (2022). The article can be accessed at https://doi.org/10.1119/5.0077026.