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

But my only criterion is if you talk in your sleep, now that's not allowed because talking is not allowed.

我唯一的要求就是别说梦话,因为我的课上不准说话的

耶鲁公开课 - 基础物理课程节选