Hi people, I have a question. Where can we apply the first law of thermodynamics? I mean is it applicable between 2 equilibrium points? Or between a reversible process? When we consider free expansion of a gas into vacuum, we apply the first law. But since the process isn't reversible (or even defined), we apply it between the final and initial equilibrium points and hence conclude ΔU=0. So my question is is it applicable between 2 equilibrium points, for a reversible process, or for a general irreversible/reversible process?