Throttling Ideal Gas

starryskiesx

A quick question about throttling:

I've read that for an ideal gas, if you force it through a capillary into a vacuum and allow it to equilibrate, it will experience no temperature change. I was wondering if this is correct, and if anything changes when you're forcing it through with a piston for example.

andrien

http://en.wikipedia.org/wiki/Joule%E...Thomson_effect

Enthalpy

Even with an ideal gas, this sort of thought experiment is very misleading and about impossible to make in practice...

First, the gas would take the temperature of the capillary.

Then, the gas that at some time remains in the bottle pushes the gas that goes out, gives a work, and loses internal heat. It cools down, while the flowing gas receives some work that it may transform into internal heat, depending on the experiment, or into kinetic energy, or into work against the external pressure.

It's only after a very long time that the gas may regain some form of equilibrium - between the part flown first out and the part flown last - that the temperature is uniform, and then, with a ideal gas, you may observe the same temperature as before the expansion.

So it's much more a question of setup and time than of the gas being ideal. In two words: be careful.