So I need, well the pressure is constant, but there's a change in volume.

压强不变，体积变化。

麻省理工公开课 - 热力学与动力学课程节选

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。

麻省理工公开课 - 热力学与动力学课程节选

Then the second derivative gives the change in entropy with respect to the variable that we're differentiating, with respect to which is either pressure or volume.

二阶导数给出熵,随着变量变化的情况，这些变量包括压强或者体积。

麻省理工公开课 - 热力学与动力学课程节选

Now let's change the pressure and temperature and sweep through a whole range of pressures and temperatures and measure the volume in every one of them.

然后改变气压和温度,并且让气压和温度,取便所有可能的数值,测量相应的体积。

麻省理工公开课 - 热力学与动力学课程节选

OK, now we actually would like to simplify this or to write this in terms of not the volume change, v2/v1 but the pressure change. So, we have V2 over V1.

接下来我们将要把问题简化,不用体积变化来描述，而改作用压强变化来描述，现在我们有。

麻省理工公开课 - 热力学与动力学课程节选