These are going to be fairly common-sense definitions, but they're important, and when you get to a problem set, really nailing down what the system is, not more, nor less, in terms of the amount of stuff, that's part of the system, it's going to be often very crucial.

这些是,很常识性的定义，但是很重要，当你要解决问题时，务必要搞清楚系统是什么，它包含多少物质，不能多也不能少，这往往,是非常重要的。

麻省理工公开课 - 热力学与动力学课程节选