# 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

.
Roll your mouse over this box to close.
• Homework problem hints and answers
• Get Help from Dr. B in the LT Blog
• 120 day membership

Get it ALL for \$5 US

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

• 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.