The Joule-Thompson coefficent is dT/dp at constant enthalpy. It can be either positive or negative. There is an "inversion temperature" below which the coefficient is positive, so that an expanding gas will cool in an constant-enthapy flow process if it starts out at a temperature below the inversion temperature.
I do not believe you stated the question correctly or clearly.
There is an article explaining this better at
http://en.wikipedia.org/wiki/Joule%E2%80%93Thomson_effect#The_Joule.E2.80.93Thomson_.28Kelvin.29_coefficient
As for why nature is this way, it has to do with molecular attraction forces.
in joule thomson effect,why the temperature of inversion leads to negative sign?
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1 answer