Lecture_19

# Lecture_19 - Lecture 19 Isenthalpic expansion Joule-Thomson...

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Lecture 19 — Isenthalpic expansion Joule-Thomson process: Model for expansion of a gas through a small hole. Consider a insulated cylinder, with two opposing pistons, held with pressures P 1 and P 2 < P 1 , and with a third, porous piston between the other two. The cylinder is initially prepared with the porous piston sealed o ff , pushed up next to the second piston, and the gas on the high-pressure side (between the porous piston and the P 1 piston). (sketch) Now we allow the gas to trickle through the porous piston onto the low-pressure side, in an irreversible process, with no heat flow in or out of the system. The final state is with the porous piston pushed up against the P 1 piston, and all the gas between the porous piston and the P 2 piston. (sketch) Why is this a good model for expansion of a gas through a small hole? Because: 1) If it happens quickly enough, the gas does not have time to pick up heat from surroundings — Q = 0. 2) If

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