reversible and irreversible

[Diana_Diep2I]

What is the clear difference between reversible and irreversible expansion? I am confused about the role external pressure plays on both of these.

[Rohit Ghosh 4F]

A reversible process is one that can be reversed by an infinitely small change in a variable. Irreversible expansion would violate this definition. External pressure is present in both cases.

[Tyler Angtuaco 1G]

It's most helpful to consider what reversible and irreversible expansion is in the context of a system with a piston.

Reversible expansion: If the external pressure and pressure of the system are pretty much equal, a slight increase in external pressure would cause the piston to move slightly in due to it having a slightly greater pressure than the system. Conversely, a slight reduction in external pressure would cause the piston to move out due to it having slightly less pressure than the system. If infinitesimal adjustments like these can be made to bring the pressures back to equilibrium (external pressure=pressure of the system), it is reversible expansion.

Irreversible expansion: The external pressure is already less than the pressure of the system by a non-infinitesimal amount. 5 atm for the piston and 10atm for the system, for example. If the external pressure were to increase by an infinitesimal amount, nothing would really happen. The piston will not move in any further. Thus, this it can be considered irreversible.