Reversible vs. Irreversible Processes?

[Belicia Tang 1B]

Hi,

I am having trouble understanding what makes a process reversible vs irreversible. According to the textbook, a reversible process is one that can take place in EITHER direction. That is, a process that can be reversed by an infinitely small change in external pressure. An irreversible process is one in which an infinitely small change in the external pressure does not reverse the direction of travel of the piston. So would a piston with external pressure = internal pressure be considered a reversible process, and a piston with external pressure not equal to internal pressure be an irreversible process?



Thanks!

[Jamie_Lin_3I]

Hello!
I had trouble with this topic as well, so it took me a while to grasp the idea.
An irreversible process, like you said, is a case when the pressure of the system is equal to the external pressure or when the difference between the pressure of the system and the external pressure is so insignificant that a small increase of pressure (or as the textbook said, infinitesimal change) in either the external pressure or the pressure of the system will cause a piston to move in the appropriate direction.

-> So let's say the pressure of the system is equal to the external pressure, then if the pressure of the system increase by a very small increment/infinitesimal change then the piston would move out. Similarly, if I increase the external pressure by a small increment, the piston would move in. As the name says, the system is reversible, meaning it could move in either direction because even small increment of change could reverse the direction the piston is moving.

For an irreversible process, like the name says, an infinitesimal change can not reverse the direction of the piston in a cylinder. In the book, it talks about a cylinder where the pressure of the system is 2 atm and the external pressure is 1 atm. Here, the difference between the external pressure and the pressure of the system is so great that an infinitesimal change could not possibly reverse the process.

Hopefully that helps! Not quite sure if it made it clearer or just more confusing.