in low temperature physics,at joule thomson effect hydrogen does not show cooling effect at room temperature why?

Answer 1

Because the inversion temperature is lower than room temperature for hydrogen (and helium) , so that the J-T coefficient is negstive at room temperture. See

http://en.wikipedia.org/wiki/Joule%E2%80%93Thomson_effect#The_Joule.E2.80.93Thomson_.28Kelvin.29_coefficient

It seems to be a characteristic of gases that are very hard to liquify.