# Why do reversible processes have to be quasi-static?

I have already read two posts on this topic (here and here) and was still confused on what it is about a quasi-static process that makes it reversible. The answer that has gotten me closest to understanding this was the following:

A reversible process is the one that can be made flow backwards. It is intuitive to think that it can be made flow backwards at any time we wish. But if the system were in a non-equilibrium state, one would need to wait a bit until it goes to equilibrium before trying to drive it back. So, it does not satisfy our desire to have the system under the control at any time.

But I still wasn't clear on how to interpret parts of this answer. For example, is the reason why a process involving non-equilibrium states is irreversible because going from non-equilibrium to equilibroium always involves an increase in entropy and therefore reversing it is prohibited by the second law? I'd appreciate it if someone could help me understand the contents of the answer above and this topic in general.

It needs to be made clear what "reversible" means in this context. A process is thermodynamically reversible if it can be perfectly reversed (so it retraces all past macroscopic states in opposite order) by an arbitrarily small change in system conditions, such as temperature or pressure somewhere.

For example, air inside a cylinder with a movable piston pushed back by Earth's atmosphere and pushed/pulled by some other external force (a hand, a motor) can undergo a compression process or expansion process depending on which direction the piston is moving. When this process can be perfectly reversed by a very small change in the external force acting on the piston, the process is thermodynamically reversible.

This can only happen when the piston is moving very slowly, otherwise gas inside would produce eddy currents, pressure and heat waves, so-called non-equilibrium states that can't be made to be exactly retraced by some macroscopic action. So thermodynamic reversibility requires that processes are very slow.

Process being very slow is not always enough to be thermodynamically reversible, mostly because of friction. Consider what happens when the piston experiences large static friction on its outer edge. No matter how slow the piston moves, the friction will still be there, dissipating work into heat, thus heating up the cylinder and piston and the air layers close to them. Then it would be impossible to reverse the compression/expansion process exactly just by small change of force, because in both directions of motion, work is lost to heating up the cylinder. So to approach thermodynamically reversible motion, roughly speaking it has to be both slow enough to prevent non-equilibrium states, and friction has to be minimized.

In an irreversible process, there are transport and/or chemical reaction processes occurring within the system that proceed at finite rates, all of which generate entropy. These include

1. Heat conduction
2. Viscous friction/dissipation
3. Molecular diffusion
4. Chemical reaction at finite rate

These phenomena all occur microscopically (and, in most cases, are distributed non-uniformly spatially), and are thus not reversible on the macro scale.