Convention, OK, this negative sign is just a pure convention. You just got to use it all the time. If you use an old textbook, written when I was taking thermodynamics, they have the opposite convention, and it's very confusing. But now we've all agreed on this convention, and work is going to be with the negative sign here.

系统对外界做功的符号为负,这就是我们,在本课中的规定，纯粹出于统一符号的需要，有些教科书中，可能会使用相反的符号规定,注意不要混淆了，这门课上,我们将一直使用。

麻省理工公开课 - 热力学与动力学课程节选

Common objects can be pure or impure; it just depends whether or not they've been in contact with a source of impurity or not. Alright?

普通的事物可以是纯洁的或者不纯洁的;,原因只取决于它们是否,和一些不纯洁的来源有接触，对吧？

耶鲁公开课 - 旧约导论课程节选

Just to give you a sense of what that means, if the 264 figure is pure covalency and the 344 is partial ionic character, what fraction of the 608 is 344? 344, which is partial ionic character, 57% divided by 608 times 100 is 57%.

它只是给你一个这样的概念，264千焦每摩是完全共价成分的键能,而344是离子成分的，344除以608是多少，344，是不完全的离子特性，除以608乘以100得到。

麻省理工公开课 - 固态化学导论课程节选