It's constant pressure. OK, so now, last time you looked at the Joule expansion to teach you how to relate derivatives like du/dV.

这是恒压的，好，上节课你们,学习了焦耳定律,以及怎样进行导数间的变换。

麻省理工公开课 - 热力学与动力学课程节选

OK, we're going to do this reversibly, which means we're going to slowly change the external pressure very, very slightly at a time, so that at every point we're basically in equilibrium, p2/ until the pressure reaches a new smaller pressure p2.

整个过程保持可逆，外界压强,变化得很慢，每一个瞬间,都保持平衡，直到压强减小到末态值。

麻省理工公开课 - 热力学与动力学课程节选

This is going to end up at a different temperature by the way. You saw this last time in a slightly different way. Last time what you saw is we compared isothermal and adiabatic paths that ended up at the same final pressure, and what you saw is that therefore they ended up in different final volumes.

末态温度是不一样的,上次你们看到的,和这个有一点不一样,上次我们比较的是末态压强,相等的等温过程和绝热过程,因此它们的末态,体积是不一样的。

麻省理工公开课 - 热力学与动力学课程节选