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Author Topic: Why does a gas cool as it expands?  (Read 37639 times)

Offline mike2niner4

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Why does a gas cool as it expands?
« on: 29/08/2009 22:01:38 »
I understand that when a gas expands it cools but why does it cool?

Thanks, Mike  :)


 

lyner

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Why does a gas cool as it expands?
« Reply #1 on: 30/08/2009 01:12:18 »
Imagine the gas was in a cylinder and expanded by pushing against a piston. The movement would involve work being done. This work would need energy, which would come from the kinetic energy of the gas molecules hitting the piston- reducing their average speed. Taking KE away corresponds to lowering the temperature of the gas.
 

Offline Geezer

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Why does a gas cool as it expands?
« Reply #2 on: 30/08/2009 04:10:02 »
As Sophiecentaur says, it's because of the kinetic energy of the gas molecules.

It might also help to turn the question on it's head. As in, why does air heat up when it is compressed? - well known in bicycle pumps.

The air contains a certain number of molecules. The molecules have a certain amount of heat energy (in the form of kinetic energy). When the air is compressed the total heat energy now occupies a smaller volume. Unless heat is rapidly removed while the air is being compressed, the air's temperature has to increase because the heat energy per unit volume is increasing.

The expansion process is the exact opposite, so unless heat is added while the air is expanding, the temperature has to decrease.

The compression heating effect can be quite dramatic. Before the development of matches, air compression devices were used for lighting fires.
 

Offline John Chapman

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Why does a gas cool as it expands?
« Reply #3 on: 30/08/2009 10:06:30 »

When the air is compressed the total heat energy now occupies a smaller volume. Unless heat is rapidly removed while the air is being compressed, the air's temperature has to increase because the heat energy per unit volume is increasing.


That's a really good explanation. I've often wondered how that works. What about in diesel engines though? Would the compression ratio be enough to reach the high temperatures needed for combustion using your explanation?

Also, is that effect the same thing that makes an elastic band warm up when you stretch it? It seems sort of the wrong way round. If you cut an elastic band (a chunky one works best) to get a length of elastic then stretch it in front of your face and immediately touch the elastic on your top lip you'll feel that it is noticeably warm. If you let it cool for a few seconds, then relax it and touch your top lip again it is now noticeably cold. Doesn't the elastic band fills the same volume whether it is stretched or relaxed? I'm not sure. But how does that fit into your explanation above?
 
« Last Edit: 30/08/2009 10:09:38 by John Chapman »
 

Offline chris

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Why does a gas cool as it expands?
« Reply #4 on: 30/08/2009 11:15:13 »
Hi John

we did this experiment as a Kitchen Science.

http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/make-a-rubber-fridge/

The bottom line is that when you stretch the rubber band to begin with you are doing work to straighten out the randomly-coiled polymer chains in the rubber. You are therefore adding energy to the rubber so it becomes hotter.

When you let you the process is reversed. Now the rubber does work pulling the straightened chains back  to their initial compact configuration. Since the elastic has done work, using energy, the rubber must lose energy in the process and therefore it becomes colder.

In the link above Dave's put a nice interactive animation that enables you to demonstrate this for yourself.

Chris
 

Offline John Chapman

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Why does a gas cool as it expands?
« Reply #5 on: 30/08/2009 13:03:39 »
That's a fun computer model. Clever old Dave.

I'm still not clear about this, though? Are you saying that something will necessarily get warmer if you put energy into it? Will a wrist watch get warmer if you wind it up or a jar of pickles if you lift it on to a higher shelf?

Sorry for highjacking your thread, mike2niner4. But this is a vaguely similar question to the original one, which I think has now been answered by Geezer's very plausible sounding explanation.
 
 

lyner

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Why does a gas cool as it expands?
« Reply #6 on: 30/08/2009 16:21:13 »
geezer
Quote
the air's temperature has to increase because the heat energy per unit volume is increasing.
But the mass of gas is the same. Unless you say that the specific heat capacity changes, the temperature shouldn't change by that argument.
The argument must hang on work done on or by the gas and the resulting change in internal energy.
This is very standard 'book work' concerning adiabatic changes.
 

Offline John Chapman

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Why does a gas cool as it expands?
« Reply #7 on: 30/08/2009 16:35:25 »
Ahhh, I've heard about adiabatic cooling in respect of meteorology. Moving air hits a mountain or hill and is forced upwards. With higher altitude comes lower pressure and the air expands and cools. Cold air holds less water vapour than warm air and at some point up the mountain the air will reach it's dew point (the water saturation point of the air) and the water vapour will condense out as cloud. This is why cartoon mountains are often drawn with a cloud circling it's peak.
 
 

Offline Turveyd

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Why does a gas cool as it expands?
« Reply #8 on: 31/08/2009 00:15:04 »
Nice,  when you compress the gas,  the heat energy compresses which creates heat,  which is then lost as this will resume to room temperature,  so when you release the gas your releasing gas with less heat nice.

Never considered that before.

I like science.

 

Offline Geezer

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Why does a gas cool as it expands?
« Reply #9 on: 31/08/2009 06:41:26 »
But the mass of gas is the same. Unless you say that the specific heat capacity changes, the temperature shouldn't change by that argument.
The argument must hang on work done on or by the gas and the resulting change in internal energy.
This is very standard 'book work' concerning adiabatic changes.
Sophiecentaur

The question was "why does the gas cool", not "how much does the gas cool". Certainly the work done on or by the gas can lead us to the temperature of the gas at a certain pressure and volume, but it hardly explains why the temperature of the gas changes.

We could also say, well, P times V is proportional to T, because it's a well known law, but that does not provide too much insight either.

The temperature of a gas is proportional to the kinetic energy in the molecules of the gas. If we enclose a certain number of molecules of the gas and reduce the volume of the enclosure (which does require work) there are still the same number of molecules in the enclosure, but they now occupy a smaller space. This increases the kinetic energy of the enclosed gas, and hence the temperature. The energy per unit volume has also increased.
 

lyner

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Why does a gas cool as it expands?
« Reply #10 on: 31/08/2009 16:13:28 »
The reason "why" it cools is that the molecules have lost average KE. This is 'because' they have lost it in collisions with the retreating walls of the container* - their recoil speed is less than their approach speed, having transferred some energy to the container by doing work. There may be a deeper 'why' to answer but, in terms of kinetic theory, I can't think of one.

*an actual container is not necessary for this argument to apply - you can replace the walls of the container by molecules of a surrounding gas.
 

Offline Turveyd

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Why does a gas cool as it expands?
« Reply #11 on: 31/08/2009 20:29:54 »
It cools when you compress it,  as there is the same heat in a smaller area if you felt a can / oxygen cylinder when it's being compressed it's warm / hot,  so when it leaves it's lost the temperature already.

If you put your can in the oven and heat it to 100c most likely all through then the gas should come out near room temperatue.


Don't try this the can will explode :)

 

Offline Geezer

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Why does a gas cool as it expands?
« Reply #12 on: 01/09/2009 03:37:35 »
It cools when you compress it,  as there is the same heat in a smaller area if you felt a can / oxygen cylinder when it's being compressed it's warm / hot,  so when it leaves it's lost the temperature already.

If you put your can in the oven and heat it to 100c most likely all through then the gas should come out near room temperatue.


Don't try this the can will explode :)

Turveyd:
Ah - well yes. When you compress gas it heats, and the heat can escape, if you let it. The situation we were debating does not allow the heat to escape. If you look back in the thread you'll notice that Sophiecentaur mentioned adiabatic which is the term used to describe a process where there is no heat transfer.

If there is heat transfer, the point I made about compression and expansion being reversible would not be valid. As you point out, if energy is allowed to escape from the can in the form of heat, you would have to add back the energy that escaped if you wanted the air to return to the original temperature when it expanded again.

Sophiecentaur can provide a more rigorous explanation if required.
« Last Edit: 01/09/2009 06:05:09 by Geezer »
 

Offline Bored chemist

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Why does a gas cool as it expands?
« Reply #13 on: 01/09/2009 20:20:06 »
Imagine I have some gas in a container. At the moment all the gas is in one side of the container and it's held there by a thin wall. On the other side of the wall (i.e. in the other half of the container) there's a vacuum.
Now imagine I take the thin wall away. The molecules of gas that would have hit it carry on to the other half of the container at exactly the same speed. They hit it and bounce off- just as they would have done from the wall I took away.
After a very short time the molecules are randomly distributed. They occupy twice the volume they did so the pressure is halved.
But no work has been done on or by the gas. It hasn't gained or lost any energy. It is, therefore at exactly the same temperature as it was before.

The temperature of an ideal gas expanding into a vacuum does not change.
You need to be clear about answering questions like "why does the gas cool?"

Sometimes it doesn't.
 

lyner

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Why does a gas cool as it expands?
« Reply #14 on: 02/09/2009 00:18:14 »
Correct. But this is a very 'ideal' situation, not often encountered - certainly not naturally - in the weather, for instance.
Most scenarios involve some work being done, though. A piston or a nozzle involve energy loss. A gas which expands by exerting a force times a distance will get  cool.  Then there are Van de Wall's forces which mean that a non ideal gas will still cool down - even when exploding into a vacuum.
 

Offline Bored chemist

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Why does a gas cool as it expands?
« Reply #15 on: 02/09/2009 19:40:10 »
If you let high pressure hydrogen or helium out of a cylinder they warm up.
 

lyner

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Why does a gas cool as it expands?
« Reply #16 on: 02/09/2009 22:44:33 »
I never knew that. Go on then. Give us an explanation.
Are some special conditions necessary?
The energy must come from somewhere. He is monatomic so how....?
« Last Edit: 02/09/2009 22:46:50 by sophiecentaur »
 

Offline Nizzle

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Why does a gas cool as it expands?
« Reply #17 on: 03/09/2009 06:01:32 »
I think it warms up because other molecules from the environment bump into the high pressure Hydrogen or Helium (which has very little movement/Ek) and transfer some kinetic energy that way.

Or are we still talking about a vacuum environment here?
 

Offline Bored chemist

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Why does a gas cool as it expands?
« Reply #18 on: 03/09/2009 06:55:48 »
 

lyner

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Why does a gas cool as it expands?
« Reply #19 on: 03/09/2009 09:34:42 »
How dumb of me. I remember being here before.  It's that inversion temperature thing in the Joule Kelvin effect. A very nice argument to explain it.
 

Offline cyberphlak

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Why does a gas cool as it expands?
« Reply #20 on: 03/09/2009 23:58:36 »
I think you would both benefit from and enjoy learning about enthalpy and entropy. In all cases, energy is either 1. remaining at rest, 2. being transferred into or away from something. No work can be done without energy. Since energy is never created or destroyed, it is only transferred. This simple explanation is a good place to start when considering the activity of energy force - to include heat.
 

Offline Geezer

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Why does a gas cool as it expands?
« Reply #21 on: 04/09/2009 01:17:33 »
I think you would both benefit from and enjoy learning about enthalpy and entropy. In all cases, energy is either 1. remaining at rest, 2. being transferred into or away from something. No work can be done without energy. Since energy is never created or destroyed, it is only transferred. This simple explanation is a good place to start when considering the activity of energy force - to include heat.

Rats! I knew it. Sooner or later somebody was bound to bring up the "E" words (enthalpy and entropy). Apparently, due to some cruel genetic trick, or more likely sheer laziness, I have never been able to get my head around either term. Entropy, kind of, but Enthalpy?? I always assumed they were entirely artificial constructs designed to trap poor undergraduate students.

Is there an "Idiots Guide to Entropy and Enthalpy" somewhere? If there isn't, somebody could make a fortune selling them.
 

Offline cyberphlak

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Why does a gas cool as it expands?
« Reply #22 on: 04/09/2009 01:40:38 »
It appears daunting at first glance. If you want to email me, I will send you some simple ways of looking at it. In short, lets use heat, a cold body will take heat from the environment around it just as a warm body will "give" heat to its environment. Nature seeks balance in all things and energy, in all its forms, is no exception.

Another example - electrolysis of water. Faraday correctly states that it takes a minimum of 1.23 volts to split the water molecule. This doesn't happen in your living room because there are no perfect electrolytes. On the other hand, if you put the solution in a hot environment, some of the energy for dissociation comes from the heat energy and now you see current flow at a lower voltages. How this occurs is explained by the "E" words.
« Last Edit: 04/09/2009 01:43:26 by cyberphlak »
 

Offline Titanscape

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Why does a gas cool as it expands?
« Reply #23 on: 04/09/2009 14:12:05 »
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.
« Last Edit: 05/09/2009 06:58:02 by Titanscape »
 

Offline Elton Hesbon

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Why does a gas cool as it expands?
« Reply #24 on: 08/09/2009 09:45:13 »
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   
 

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Why does a gas cool as it expands?
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