Because it's so important. And I should add and also under reversible work, where the external pressure is equal to the internal pressure.

而且我还要说应该在外部压强,和内部压强相等的,可逆过程中引入它。

麻省理工公开课 - 热力学与动力学课程节选

Here there's an expansion going on, so the system is doing work to the external world.

这里发生了膨胀,所以系统对外界做功，活塞被推出来。

麻省理工公开课 - 热力学与动力学课程节选

So there's p v work and it's given by the integral minus p external dv or just the integral from one to two of dw.

我们知道对于pv系统来说,功w可以写成黑板上的两种不同形式。

麻省理工公开课 - 热力学与动力学课程节选

And it's p external and p external is zero, so there's no work.

外部的p是零，所以结果是零，没有做功。

麻省理工公开课 - 热力学与动力学课程节选