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Q & A: why expanding air cools

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Most recent answer: 02/09/2018
Q:
why does air cool as it expands
- farah barakat (age 16)
saudi arabia
A:

If the air is pushing on something (a piston, other air,...) as it expands, it's doing work on that other thing. That means some eenrgy is going from the air to whatever it's pushing on. The energy comes out of the little thermal jiggles of the air molecules, leaving them cooler.

If the air expands into a vacuum, however, there's nothing to push on and it doesn't cool down, at least at ordinary pressures. For highly compressed air another effect occurs. The molecules attract each other enough so that it takes significant amounts of energy for them to pull apart. That causes some cooling even when the air is expanding into a vacuum. For extremely condensed liquid air, the molecules are pressed together so much that their interaction energy goes up, not down. Then expnasion can leave the air hotter.

Mike W.


(published on 02/09/2018)

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