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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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All right, so I'm burning some energy, I'm burning some fuel to do this somehow, to get that work to happen.
而这个路径需要先加热再冷却，这样，我需要能量来源，比如燃料，来完成做功。
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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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And what we saw last time is that this was necessary to make things work, this part of it in particular.
上节课我们讲,这是对外做功的必要条件，特别是这个部分。
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One is that he observed when people were machining cannon barrels, a lot of heat was generated, and there was a lot of work done.
第一种说法说焦耳发现,制作加农炮筒时，会产生大量的热，同时这中间有大量的做功。
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Because we did work at constant pressure, and so it's just volume difference times pressure.
因为是在恒压下做功，所以功就等于体积变化乘以压力。
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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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So is this system doing work on the surroundings or are the surroundings doing work on the system?
那么是这个系统向外界做功呢,还是外界向系统做功呢？
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And so, again, we see a temperature increase, and we know the work, and the temperature increase, it's a constant pressure thing.
好，我们看到温度升高了，然后我们有做功量和温度的升高量，这是一个恒定压力下的值。
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And what we'll discover is that when we allow the gas to expand reversibly, we get the most work. This is neat.
最后将发现,当气体可逆膨胀的时候，对外做功最大。
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And what you'll find is that the maximum work out is obtain for a reversible path.
对于各种可能的中间路径，我们将发现,做功做多的是可逆路径。
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It cools down because I'm doing work but the energy has to go somewhere, right?
因为膨胀过程中,对外做功？
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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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The force, as this weight drops is constant, mgh and so the work is just going to be m g h, h where this is h.
当重物掉落时候的力是恒定的，因此做功就是，这段长度是。
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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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We could, write the work that we generate -w that comes out as negative w.
的热机相连,热机运行做功。
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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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OK, so that means heat is being imparted to the system, right, from the surroundings in a manner that compensates exactly the amount of work done.
好，这表示系统,从外界吸热,吸收的热量与系统,对外做功的值相等。
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well since the gas is expanding into vacuum no work is done.
因为气体是向真空膨胀,因此这个过程不做功。
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And so we have the work coming from two steps.
因此我们把做功过程分成了两步。
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You couldn't just run something successfully in a cycle and get work out of it, using the heat from the hot reservoir, without also converting some of the heat that came in to heat that would flow into a cold reservoir.
如果一样东西在循环工作过程中,只有热从高温热源中流出，而没有热流入低温热源，那么此过程,不可能对外做功,不可能把所有热。
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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's an interplay between the energy inside the gas which is the heat energy which is allowing me to do all that work to be outside, and so I'm using up some of the energy that's inside the gas to do the work on the outside.
即使没有热传递,能量也能以,做功的形式传递出去，气体的一部分内能,转化成了功。
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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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