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Of course in the reversible case, you're always pushing against an external pressure, which is essentially equal to the internal pressure.
当然在可逆的情况下，外压力，总是，等于内部压力。
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In this case here, our property is the value of the pressure times the volume, times the molar volume. That's the property.
或者电阻,对气体来说，它的特性是气体压强。
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And we combine this with first law, which for the case of pressure volume changes we write as this.
结合第一和第二定律,对于压强体积功我们可以这样写。
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Wrev w reversible. in this case here let's see this is it's a pressure is going down.
曲线下的面积，可逆过程的功,这个过程中。
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Well, I can, for any given case, measure the pressure, determine the entropy and I'll know what the slope of change will be.
好的，对特定的情况，我可以测量压强，确定熵,并且知道这就是变化的速率。
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How many moles of gas are there in each case, in reactants and products? If that changes, of course you know that the pressure in there is going to change at constant volume if the amount of gas in there is changing.
在反应物和生成物中，各有多少摩尔的气体？,如果它发生了变化,当然在等体条件下，如果气体的总量,发生了变化，压强也会发生变化。
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In this case it relates the volume to the pressure and the temperature.
现在假想我们需要用,理想气体定律来设计一台机器。
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So if delta v is negative, in this case delta v is negative, OK, delta v is negative, pressure is a positive number, negative times negative is positive, work is greater than zero.
也就是当活塞,压缩气体的时候，做的功是正的，按照这一规定,我们需要在这里，加上一个负号，这是习惯规定。
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Remember it's the external pressure that's important. In this case, because it's a reversible process, the external pressure turns out to be always the same as the internal pressure.
是过程非常缓慢,比如我把体积，从V1压缩到V2时，我做得十分缓慢,在过程中的任何一点。
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