in low temperature physics,at joule thomson effect hydrogen does not show cooling effect at room temperature why?
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.
In low temperature physics, the Joule-Thomson effect refers to the change in temperature of a gas when it undergoes expansion or compression without exchanging heat with its surroundings. In the case of hydrogen gas, it typically does not exhibit a cooling effect at room temperature when subjected to the Joule-Thomson effect.
To understand why, we need to consider the behavior of hydrogen molecules at different temperatures. At low temperatures, hydrogen gas behaves differently compared to other gases such as nitrogen or helium. This is because hydrogen possesses a unique combination of properties, including its low molecular mass and the quantum nature of its particles.
Normally, when a gas expands, it cools down due to the reduction in the average kinetic energy of its molecules. However, in the case of hydrogen at room temperature, its intermolecular forces, known as van der Waals forces, dominate over the kinetic energy. These forces cause the molecules to stick together and interact strongly, reducing the effect of molecular motion and preventing a significant decrease in temperature during expansion.
Additionally, hydrogen molecules exhibit quantum behavior due to their low mass. This quantum behavior affects the energy levels and the available states for the molecules, leading to unique phenomena. In the case of the Joule-Thomson effect, the quantum effects in hydrogen become important at low temperatures, making it more resistant to cooling compared to other gases under similar conditions.
To experimentally investigate the cooling effect of hydrogen at room temperature, one could set up a Joule-Thomson apparatus. This typically involves passing high-pressure hydrogen gas through a small orifice or throttling valve, which causes the expansion of the gas. By monitoring the temperature on both sides of the valve, one can observe any cooling or heating effects.
In conclusion, hydrogen does not exhibit a significant cooling effect at room temperature in the Joule-Thomson apparatus due to the dominance of intermolecular forces and the quantum properties of its molecules.