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And so what happens when we remove these stops is that the piston slams up against the next pair of stops.
把挡板移开后活塞会,向上移动直道第二个挡板处。
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The piston has a cross-sectional area, a, and the force pressure is force per volume area.
我们以活塞系统为例，来说明膨胀功。
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So initially, we're going to have our piston, there's the plug sitting here.
所以一开始，这是个活塞，这里是个节流阀。
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So I've got, this piston here is compressed, and I slowly, slowly increase the volume, drop the temperature.
这样，这个活塞就被压缩了，然后我再很慢很慢地增加它的体积，降低温度。
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So we remove the stops, and the piston moves up, and now we remove one of these weights.
我们移走挡板，活塞向上移动，移走一个重物。
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We have a weight on the piston, and that weight is chosen so that p2 it acts as though p external is equal to p2.
让活塞上放一重物，让它的效果刚好,等同于外压强。
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And so we know the force for a particular mass, and we know the area of the piston.
我们知道每个质量对应的力，也知道活塞的面积。
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V2 Now we could put stops here at V2, but this piston would just kiss the stops.
现在我们可以把挡板放在这里，这时体积是，活塞会刚好碰到挡板。
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This piston is being brought out, so we expect 0 the work to be negative, negative. And we start o V2 ut with zero volume. We end up with a volume p2 of V2, and the external pressure is constant to p2.
所以我们可以想象功是负的,开始的时候体积是,最终的容积是,外界的压力恒为。
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So, the piston moves up again, one weight.
活塞再次上升。
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We're going to take a piston here which is going to be insulated.
容器与气缸,都是绝热的。
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We have a piston. We have a pair of stops.
这有一个活塞，两个挡板。
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Pressure, the piston goes down by some volume l.
我们只讨论膨胀功。
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Pistons in the car, right, so the piston goes up.
比如说这是汽车发动机。
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Then I'm going to release these little pegs here and my piston is going to shoot up now p1 because p2 is less than p1.
现在移开插销,活塞就会往上冲,因为p2小于。
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So expansion work, for instance, or compression work, is if you have a piston with a gas in it. All right, you put a pressure on this piston here, and you compress the gas down. This is compression work.
力乘以距离，就是功，功有许多种：,电可以做功,驱动电机与电风扇,磁力也可以做功。
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This is the pressure that you're applying against the piston, not the pressure of the gas.
最后等于Pext乘以体积,面积乘以距离得到体积。
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Our piston on the right side here fully out, and the piston on my right side, your left side, fully inside. There's no gas p1 on that side here. So there's p2 sitting here.
左边的这个活塞完全在外面，而右边的左边的这个活塞完全在里面，这边没有一点空气,这边是P2，这边是。
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