That's different when you have continuous values-- you don't have P because it's always zero.

和离散型随机变量的分布不同的是,连续型随机变量的分布中,某一点的概率值始终是零

耶鲁公开课 - 金融市场课程节选

And so they defined them, p after many experiments, the limit of this 0 delta T delta p and the limit of delta p goes to zero as the Joule-Thomson coefficient.

他们定义了这些量，以及它们的范围，ΔT比Δ,Δp的极限趋近于,叫做焦耳-汤姆逊系数。

麻省理工公开课 - 热力学与动力学课程节选