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

We've changed the external pressure slowly, and again this is isothermal.

我们慢慢改变外部压强，这个过程也是绝热的。

麻省理工公开课 - 热力学与动力学课程节选

p We want p equal to p external for the entire expansion, p2 and p external is decreased steadily from p1 to p2.

在膨胀过程中保持p等于外部，外部p很快从p1衰减到。

麻省理工公开课 - 热力学与动力学课程节选

So, the work for this process is the integral, pdv or minus the integral, V1, V2, p dV.

此过程中的做功等于负的积分，从v1到v2，外部。

麻省理工公开课 - 热力学与动力学课程节选