That's the work that the system is doing to the environment. I'm doing work to the environment. The work is negative, which means that work is being done to the environment.

功的大小,就是矩形的面积,这样我们就实现了一台热机，冷却，加热，加热,冷却，系统回到了初态。

麻省理工公开课 - 热力学与动力学课程节选

So what I'm doing here is I'm cooling, and then when I'm sitting at a fixed volume and I'm increasing the pressure, what am I doing? I'm heating, right?

对了，因此在这里，需要冷却,而在这里，当体积不变增加压强时？

麻省理工公开课 - 热力学与动力学课程节选