For instance, you want to know how much energy you're going toget out from doing this expansion.

比如，你想知道在这个膨胀过程中,到另一个平衡态的路径。

麻省理工公开课 - 热力学与动力学课程节选

It's reversible, that means that p external, equals p. I'm doing it very slowly so that I'm always in equilibrium between the external pressure and the internal pressure so I can go back and forth.

我都可以不损失能量，而把过程逆向进行,整个过程中的,每一步都保持平衡,现在我们来分别。

麻省理工公开课 - 热力学与动力学课程节选

If I look at different points in my container during that path, I'm going to have to use a different value of pressure or different value of temperature That's not an equilibrium state, and that process turns out then to be an irreversible process.

如果我要研究在路径中容器里的,不同的点，我就得在容器里不同的点上使用,不同的压强值,或不同的温度值,实际上这不是个平衡态，这个过程是,不可逆过程。

麻省理工公开课 - 热力学与动力学课程节选