I am studying Thermodynamics in my first year and I don't quite understand the process of isothermal reversible/irreversible expansion. Below is a P-V graph I found on Google, just to help me illustrate my question here.
My question is: why does the irreversible process take the path under the curve (the red route)? Why can't it take the path above the curve (the blue route)? I tried to figure out why this is not happening. For a process to be spontaneous, the overall entropy (system and surrounding) should be positive. But I couldn't see how entropy comes anywhere in this graph. Please shed some light on this.