Joule-Thomson showed that when a compressed gas is forced through a porous plug into a region of low pressure, there is appreciable cooling.
The phenomenon of producing lowering of temperature when a gas is made to expand adiabatically from a region of high pressure into a region of low pressure, is known as Joule - Thomson effect.
When the gas is allowed to escape into a region of low pressure the molecules move apart rapidly against the intermolecular attractive forces. In this case work is done by the gas molecules at the expense of internal energy of the gas. Therefore cooling occurs as the gas expands. This reduction in the temperature is generally referred as Joule-Thomson effect and is used in the liquefaction of gases.
Inversion temperature (Ti)
The Joule-Thomson effect is obeyed by a gaseous system only when its temperature is below a characteristic value. The characteristic temperature below which a gas expands adiabatically into a region of low pressure through a porous plug with a fall in temperature is called as inversion temperature (Ti).
Ti is characteristic of a gas and it is related to the Vanderwaal's constant `a' and `b',
2a Ti = 2a / Rb
At the inversion temperature there is no Joule Thomson effect (ie) there is neither fall nor rise in temperature. Only when the temperature of the gas is below the inversion temperature there is a fall in temperature during adiabatic expansion. If the temperature of the gas is above Ti there is a small rise in temperature. For gases like H2 and He whose Ti values are very low -80 o C and -240 o C respectively, these gases get warmed up instead of getting cooled during the Joule-Thomson experiment. These gases will obey Joule-Thomson effect only when they are cooled to a temperature below these Ti values.
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