This is a point that is often confusing, because you can think, well maybe I could calculate what the internal pressure is even for this very rapid process.

这一点可能让你们很困惑，因为你们可以想象这个过程，我也可以计算在这快速的变化中,内部压强是多大。

麻省理工公开课 - 热力学与动力学课程节选

Remember it's the external pressure that's important. In this case, because it's a reversible process, the external pressure turns out to be always the same as the internal pressure.

是过程非常缓慢,比如我把体积，从V1压缩到V2时，我做得十分缓慢,在过程中的任何一点。

麻省理工公开课 - 热力学与动力学课程节选