We want to do that because we have too many variables here. We've already got dV p we'll get rid of p as an additional variable and replace it with V which is already in here.

我们之所以要那么做,是因为这儿有太多变量了,我们已经有了dV，我们要把,作为额外的变量消去，用已存在的V代换它。

麻省理工公开课 - 热力学与动力学课程节选