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