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Reversible Expansion and Compression

Slow Compression of a Gas

If a fluid expands infinitely slowly, in this case by removing tiny weights from the piston one at a time, the piston never has any significant velocity.

The process path consists of a series of equilibrium states.

In chapter 4, we called this type of process a
quasi-equilibrium process.
We can now see that this is also a

reversible process

Lesson
6E Blog

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Lesson 6E: page 2 of 25

In order to compress or expand a fluid infinitely slowly will take forever. So, no real compression or expansion can be reversible.
But if the compression or expansion occurs very slowly, it can be a reasonable approximation of a reversible process.
The neat thing about a reversible process is that the system is always at equilibrium. That means there are no unbalanced driving forces.
It also means that we can discuss the state of the system at all times during the process because it is always at equilibrium.
So, the process path on a phase diagram, such as a PV Diagram, is just a series of equilibrium states.
This makes the process easy to analyze. Most of the equations we have developed that relate the properties of a system to one another are only strictly applicable when the system is at equilibrium.
This is good news. It means that we have all the tools we need to begin our analysis of reversible thermodynamic cycles.
Let’s begin with the Carnot Cycle.

Reversible Expansion and Compression

reversible process

Ch 6, Lesson E, Page 2 - Reversible Expansion and Compression