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

Pressure, the piston goes down by some volume l.

我们只讨论膨胀功。

麻省理工公开课 - 热力学与动力学课程节选

So expansion work, for instance, or compression work, is if you have a piston with a gas in it. All right, you put a pressure on this piston here, and you compress the gas down. This is compression work.

力乘以距离，就是功，功有许多种：,电可以做功,驱动电机与电风扇,磁力也可以做功。

麻省理工公开课 - 热力学与动力学课程节选

This is the pressure that you're applying against the piston, not the pressure of the gas.

最后等于Pext乘以体积,面积乘以距离得到体积。

麻省理工公开课 - 热力学与动力学课程节选