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[Physltest] [Phys-L] Re: expansion of gases



----- Original Message -----
From: "Anthony Lapinski" <anthony_lapinski@PDS.ORG>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Monday, January 03, 2005 4:30 PM
Subject: expansion of gases


| My physics colleague brought a puzzling idea to me today regarding
| cosmology. We all know that when a gas expands, it cools. By conservation
| of energy, the expanding gas cools and its surroundings warm up.

Whether the surroundings warm up is accidental to the expansion of the gas.
There will be conductive heat transfer only if there is imperfect insulation
and a temperature difference.

| But what happens when a gas expands in a vacuum? If it cools, is the total
| energy still conserved?

An ideal gas would not cool. Only if there are interparticle forces among
the gas particles will a temperature change happen. In that case total
energy - potential plus kinetic - is conserved. Think, for example, of
two like electric particles moving apart .

| More specifically, what about the Big Bang? If the
| Universe was very hot in the beginning and expanded (into a vacuum), then
| it should cool. And it has -- to 2.7 K. But does this violate conservation
| of energy? We were both baffled. Can anyone help us with this apparent
| dilemma?

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
http://www.winbeam.com/~trebor/
trebor@winbeam.com
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