Because it's so important. And I should add and also under reversible work, where the external pressure is equal to the internal pressure.

而且我还要说应该在外部压强,和内部压强相等的,可逆过程中引入它。

麻省理工公开课 - 热力学与动力学课程节选

The circulatory system which we'll talk about in a couple of weeks, the heart, the blood vessels, and the blood work together to form a totally internal function.

例如过几周我们要讲的循环系统,心脏,血管,血液,三者协同工作,形成一个完全在体内发挥作用的体系

耶鲁公开课 - 生物医学工程探索课程节选

And because there is an explicit relationship between u, delta u, q and w, you can always find the easy way to derive the change in internal energy or the heat or the work.

因为Δu，q和w之间,有明确的关系，一般来说,内能或热或功的变化,易于计算。

麻省理工公开课 - 热力学与动力学课程节选