# The Effect Of The Joule Thomson Effect

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2.1 Joule Thomson effect: 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.
The experimental set up is shown in figure 2(a). it consists of a thermally insulated tube fitted with a po-rous plug and two weightless and frictionless pistons X and Y. two sensitive thermometers are fitted on both the sides of the porous plug to record temperature.
Consider that a certain amount of the gas is passed through the porous plug by slow movement of piston X, i.e. a volume V1 of the gas at pressure P1 be forced through the plug on the left side of the plug. The gas in the right hand chamber is allowed to expand to volume V2 and pressure P2 by moving piston Y. The change in the temperature is recorded from the thermometers.
Most of the gases are found to undergo cooling on expansion through the porous plug. However, helium and hydrogen are exceptions as these get warmed.
The gas is compressed on the left hand side. Thus, work is done on the gas. It is equal to P1V1. The work done on the right hand side by the gas is equal to P2V2. Total work done by the gas = P1V1 – P2V2
As q = 0, the work done by the gas lowers its internal energy and constantly temperature falls.
ΔE = P1V1-P2V2
E2- E1 = P1V1-P2V2 ( E2 + P2V2 ) – ( E1 + P1V1 ) = 0 or H2 – H1