when you compress a gas it gets hot, when you expand a gas it gets cold, I know why, but after searching the net, I have yet to read a simple explanation.
google refrigeration....wiki
Simple but correct answer: When a gas is compressed, the molecules are much closer together, increasing the friction between them as they move. That friction creates heat.
The same principle in reverse is used in cooling systems: a compressed gas is allowed to expand, cooling as it does so and thus pulling heat from the air or water as they pass through the coil.
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I would have to say that compressing molecules will create more friction thus increasing energy=heat. Molecules that have more space between them will be cooler. That's as simple as I can get it. Anything more starts into all the laws of gaseous states and critical temperatures and liquification.
no its not friction, any volume of gas contains a volume of heat, you must think of heat as quantity x temperature, if you compress it ten times then its temperature must increase ten times, then because heat flows from high to low temperature nine units of heat will be lost to outside ambient conditions, that one remaining unit of heat if allowed to decompress back again, will be ten times cooler.
All: Please see any physics book. There is, for example, a more complete if necessarily roundabout explanation at http://books.google.com/books?id=MJWsO2rMpWkC&pg=PA167&lpg=PA167&dq=why+does+a+gas+heat+up+under+compression%3F&source=bl&ots=Un-N_9MP0V&sig=OTnbcDiLaPyzMvwLCI4XSvvDjzY&hl=en&ei=V3g7SunJA9SytweFotj9Dw&sa=X&oi=book_result&ct=result&resnum=3
The books that have this information tend to state either friction among the molecules themselves or with the surrounding walls of the cylinder. The heat comes from more molecules hitting the walls at greater speed...
neesee's response of the general formula holds in ideal condition, but does not get into where the heat comes from, which was the point of the initial question, yes? Or maybe some of us read it another way.
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