You're allowed Cv comes out here for this adiabatic expansion, which is not a constant volume only because this is always true for an ideal gas.

绝热过程写下,这个式子是因为它对理想气体都成立，并没有用到等容过程的条件,只用了理想气体的条件。

麻省理工公开课 - 热力学与动力学课程节选

So that's going to results in an expansion where the new volume new temperature new pressure and an external pressure, p2 which is p2 which is a smaller pressure.

这样经过膨胀,到达了,状态为V2，T2，p2的末态，外界压强也降低到。

麻省理工公开课 - 热力学与动力学课程节选