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Author Topic: Why does a gas cool as it expands?  (Read 37751 times)

lyner

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Why does a gas cool as it expands?
« Reply #25 on: 08/09/2009 13:18:13 »
It is a matter of the atomic strikes on atoms, how fast and how frequent.

If a hard ball is moving in a wide space, and "expanse" it seldom strikes things like another one, supposing there are lots there like gas molecules. But bring it closer and there are more hits. If in a metre it moves 5 times per second through it and it is then "confined" between a ten cm space, it strikes each block 50 times per second, I think. In atoms this gives greater heat. By compression.

Like with the Blackbird SR 71 which gets hot and expands traveling through cold air.

And look at spacecraft re-entry. Cold molecules hit hard and fast the surface of the craft, it gets hot.

Gas molecules have no wall sometimes, so they push out and expand and so cool, like breath, blown to cool food.


But temperature is the average kinetic energy. Merely having more collisions doesn't affect the kinetic energy - if they are perfectly elastic. The energy is there when work is done against the gas. This work will normally result in faster molecules but, as was established earlier on in the thread, with non-ideal molecules, some energy will be transferred to electrical potential energy due to distortion of the molecules during the collisions and this may affect how much the average KE (temperature) actually increases.
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Offline Titanscape

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Why does a gas cool as it expands?
« Reply #26 on: 27/09/2009 14:58:16 »
Yes, I find chemistry hard to understand, been a long time, and I did not grasp it well then either. Just my liking for a conversation.
 

Offline CZARCAR

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Why does a gas cool as it expands?
« Reply #27 on: 22/05/2011 21:47:45 »
consider a balloon inside an insulated sphere with a thermometer. The pressure between the baloon & the sphere is lowered so the baloon expands....does the temp change?
 

Offline Phractality

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Why does a gas cool as it expands?
« Reply #28 on: 22/05/2011 23:35:55 »
When you push down on the handle of a bicycle pump, the vertical motion of the piston causes the air molecules to bounce down faster, thus making them hotter. So heat is part of the potential energy of compressed air. After the air cools and you raise the handle, (assuming no gas was released) you don't get back as much energy as you put in because the cooler molecules are not hitting the piston as hard. They bounce off of the rising piston more slowly, so the air then becomes cold.
 

Offline Geezer

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Why does a gas cool as it expands?
« Reply #29 on: 23/05/2011 00:01:20 »
When you push down on the handle of a bicycle pump, the vertical motion of the piston causes the air molecules to bounce down faster, thus making them hotter. So heat is part of the potential energy of compressed air. After the air cools and you raise the handle, (assuming no gas was released) you don't get back as much energy as you put in because the cooler molecules are not hitting the piston as hard. They bounce off of the rising piston more slowly, so the air then becomes cold.

If it was an actual bicycle pump, some of the air would be released into the tire and take heat with it.

If none of the air was released, as long as there in no heat loss from the pump, when the air expands, you will get all the energy back that went into compressing it.
 

Offline CZARCAR

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Why does a gas cool as it expands?
« Reply #30 on: 23/05/2011 16:23:39 »
extramolecular friction?
 

Offline Bored chemist

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Why does a gas cool as it expands?
« Reply #31 on: 23/05/2011 18:56:41 »
the real question is why wouldn't gas cool when it expands? maybe its because of of the recitical matter in the particles naked to the eye but ever so evolving contrary to cinematic elusive propoganda yet profound enough to diliberate a stretch in an inutive colas.  molecules are  niether questioned  nor classified sorry if some of you might not understand yet I have done my best to be understood yet not enough to be reasoned with thank you for reading and hope you find knowledge in  incomp  air   
Did anyone order salad?
 

Offline CZARCAR

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Why does a gas cool as it expands?
« Reply #32 on: 26/05/2011 00:12:28 »
the real question is why wouldn't gas cool when it expands? maybe its because of of the recitical matter in the particles naked to the eye but ever so evolving contrary to cinematic elusive propoganda yet profound enough to diliberate a stretch in an inutive colas.  molecules are  niether questioned  nor classified sorry if some of you might not understand yet I have done my best to be understood yet not enough to be reasoned with thank you for reading and hope you find knowledge in  incomp  air   
Did anyone order salad?
yes, we ordered the brains alad with may or not nays
 

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Why does a gas cool as it expands?
« Reply #32 on: 26/05/2011 00:12:28 »

 

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