Where can we apply the first law of thermodynamics?

[kulkajinkya]

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?

 

[DrClaude]

The first law of thermodynamics is just a particular formulation of the conservation of energy. Therefore, it applies at all times. It's just that separating energy into work and heat, which you do in thermodynamics, can be difficult for transient processes. When studying equilibrium thermodynamics, it is more common to apply it at the different points where the system is in equilibrium.