If we want to know the total heat added to the system, we can measure it, which is the straightforward thing, but sometimes you want to calculate in advance, or sometimes you want to calculate it on an exam.

如果想知道给系统的总热量，我们可以测量，这是很直接的事情，但有时候我们想事先计算它，或者需要在考试中计算。

麻省理工公开课 - 热力学与动力学课程节选