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Author Topic: What happens to the temperature of an expanding balloon?  (Read 5352 times)

Offline DoctorBeaver

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Discussing the expansion of the universe and its effect on the temperature of the CMBR got me wondering.

If I fill a balloon with cool gas & then leave it to warm up to room temperature, what happens? As the gas warms, it expands causing the balloon to expand. But expansion causes gas to cool, and as gas cools it contracts.

I'm confuddled  ???


 

Offline chris

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What happens to the temperature of an expanding balloon?
« Reply #1 on: 31/05/2008 09:50:35 »
So are you thinking that the universe should mimic the cooling balloon and be shrinking?
 

Offline syhprum

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What happens to the temperature of an expanding balloon?
« Reply #2 on: 31/05/2008 10:24:11 »
The gas in the balloon would only cool on expansion if this was caused by a reduction of the pressure external to the balloon.
 

lyner

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What happens to the temperature of an expanding balloon?
« Reply #3 on: 31/05/2008 12:21:55 »
Chris.
If an expanding gas does work against, say, a piston. It will lose internal energy. That will mean that its temperature will go down because of Energy conservation considerations.
Syphrum
The balloon statement is true but how relevant is it to the Cosmological, situation?
A better analogy than a balloon would be a body of hot air rising in the atmosphere.
 

Offline lightarrow

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What happens to the temperature of an expanding balloon?
« Reply #4 on: 31/05/2008 12:51:31 »
Discussing the expansion of the universe and its effect on the temperature of the CMBR got me wondering.

If I fill a balloon with cool gas & then leave it to warm up to room temperature, what happens? As the gas warms, it expands causing the balloon to expand. But expansion causes gas to cool, and as gas cools it contracts.

I'm confuddled  ???
You simply have to re-read what you have written: if it warms up to room T, it doesn't cool.  :)
 

Offline DoctorBeaver

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What happens to the temperature of an expanding balloon?
« Reply #5 on: 31/05/2008 12:58:25 »
Discussing the expansion of the universe and its effect on the temperature of the CMBR got me wondering.

If I fill a balloon with cool gas & then leave it to warm up to room temperature, what happens? As the gas warms, it expands causing the balloon to expand. But expansion causes gas to cool, and as gas cools it contracts.

I'm confuddled  ???
You simply have to re-read what you have written: if it warms up to room T, it doesn't cool.  :)

That's what I said. I was pointing out an apparent dichotomy.
 

lyner

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What happens to the temperature of an expanding balloon?
« Reply #6 on: 31/05/2008 18:05:40 »
No dichotomy;  add energy and the temperature rises, lose it and the temperature falls.
If the energy is in the form of heat, then the above is obvious but if the energy is in the form of mechanical (i.e. work) then it isn't, perhaps, quite so obvious. A gas expanding, corresponds to losing energy. Being compressed corresponds to gaining energy.

If you heat up your balloon, the temperature will rise AND the balloon will expand; you energy input goes into two forms. The temperature rise would be more if the balloon were not elastic.

As for the Universe 'cooling', it can't cool by losing heat to somewhere else; there is nowhere else to lose it to!
« Last Edit: 31/05/2008 18:08:16 by sophiecentaur »
 

Offline DoctorBeaver

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What happens to the temperature of an expanding balloon?
« Reply #7 on: 31/05/2008 18:21:33 »
A gas expanding, corresponds to losing energy. Being compressed corresponds to gaining energy.

Precisely. As the gas expands it loses energy. That means the elasticity of the balloon would be able to compress it, which will heat it & make it expand, causing it to lose energy...
 

lyner

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What happens to the temperature of an expanding balloon?
« Reply #8 on: 01/06/2008 00:21:01 »
DrB
But the following sentence in the post is the full story. It makes the point that the "elasticity of the balloon" will not compress the air, it will merely absorb some of the energy it was originally supplied with in the form of heat. The balloon will still EXPAND and not be compressed.
Reaction against expansion is not the same thing as compression.
 

Offline DoctorBeaver

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What happens to the temperature of an expanding balloon?
« Reply #9 on: 01/06/2008 10:10:14 »
Don't get all technical & spoil my fun!  [:(!]
 

lyner

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What happens to the temperature of an expanding balloon?
« Reply #10 on: 01/06/2008 15:07:52 »
Sorry Dr B, I can't resist getting technical.
 

Offline DoctorBeaver

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What happens to the temperature of an expanding balloon?
« Reply #11 on: 01/06/2008 20:12:10 »
PAH!
 

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What happens to the temperature of an expanding balloon?
« Reply #11 on: 01/06/2008 20:12:10 »

 

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