I'm a high school student with introductory knowledge about thermodynamics. I'm confused regarding irreversible processes. I was told that in irreversible processes we cannot define intermediate states of the undergoing process. For example if the initial state is $(P_1, V_1, T_1)$ and the final is $(P_2, V_2, T_2)$, I can't say what's the pressure when it's going through that irreversible expansion. Then how can we plot this curve for irreversible process?
Thanks in advance.
EDIT :- The graph represents an isothermal expansion processes the first being irreversible and the second one being reversible. In my opinion I just can't plot the irreversible curve because I don't know the internal pressure of the gas at the instants it's undergoing the process.What I interpret out of the first graph ( if I'm not given the description that it's an irreversible isothermal process) is that the pressure is slowly decreased from P1 to P2 and then the volume is expanded slowly from V1 to V2 (by slow I mean slow enough to reach the reversible limit). What I mean to say is that I think that only equilibrium states can be plotted on the graph. Thanks to all for extending a helping hand.