Free expansion

[manasa933]

If q=0, and work is different in two conditions (one of which is free expansion and the other one isn't), then U will be different in both cases. Isn't this against the statement that U is a state function?

[Jenny Cheng 2K]

No work is done in free expansion because there is no external pressure; for example, free expansion occurs when a gas pushes against a piston and expands into a vacuum (in which no work is done). For a gas that is expanding by pushing against a piston and an external pressure (no free expansion), there is work done. For internal energy, you are still looking at the final and initial conditions of the system. I think it's important to consider the conditions of the system.

[Magdalena Palavecino 1A]

So no work is done but there still is a change in internal energy due to the change in volume?