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Somebody say it real loud. Yes, the system is working on the surroundings, right? It's pushing out against them.
有人很大声的回答了，对，系统在对外做功，他把外界向外推开。
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Thermodynamics is asking you, what work does this thing do on the surroundings or the surroundings do on the system?
热力学在问你们，到底是系统对环境做功,还是环境对系统做功？
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They're both expansions. They'll both have net work done on the surroundings, but not the same amount, right.
是不同的，他们都是膨胀,都对外做功，但数量不同，对吧？
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No, we call that work positive work. All right, so that means we need to put a negative sign right here, by convention.
历史上有过不同的习惯，这里我们规定,如果环境对系统做功。
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OK, so w, first of all, work is being done to the system on the left hand side here.
好的，首先,我们通过左边对系统做功。
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W1 So I have my initial state, my final state, W1 my initial state, my final state here.
再加热，做功，对系统做的总功是。
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Delta v is positive in this direction here, the work is negative. So work on the system is positive. Work done by the system is negative.
功是负的，总而言之,外界对系统做功的符号为正。
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Here there's an expansion going on, so the system is doing work to the external world.
这里发生了膨胀,所以系统对外界做功，活塞被推出来。
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It's an expansion, so we're doing work against the environment, or to the environment.
压强降低，系统对，外界做功。
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So, the system does work on the surroundings, hence the minus sign.
系统对环境做功，因此符号是负的。
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You know the heat isn't going to flow from a cold body to a hot body without putting some work in to make that happen.
大家知道热量不会自发地,从冷的物理流动到热的物理,除非对它做功。
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And there is no work done on the surroundings because pressure is zero.
这里系统没有对环境做功,因为压强是零。
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Well the system got its work from the surroundings.
环境对系统做功。
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There might be a mass here. This could be a car.
系统对外界做功。
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w we have the environment doing work on the system OK, now we've assumed that epsilon prime is greater than epsilon.
大于零，that，is，in，this，one,在这里环境对系统做功,好，现在我们假设,ε’比ε大。
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The total work is less than zero.
说明系统对外界做功。
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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。
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