Well, we start out with our usual system, and we have a bunch of little pebbles on it.

还是那个系统，活塞上面放了很多小石子。

麻省理工公开课 - 热力学与动力学课程节选

The piston has a cross-sectional area, a, and the force pressure is force per volume area.

我们以活塞系统为例，来说明膨胀功。

麻省理工公开课 - 热力学与动力学课程节选

Here there's an expansion going on, so the system is doing work to the external world.

这里发生了膨胀,所以系统对外界做功，活塞被推出来。

麻省理工公开课 - 热力学与动力学课程节选