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From: rwt@dcs.ed.ac.uk (Rainer Thonnes)
Subject: Re: Too much Pressure!
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Date: Thu, 8 Aug 1996 12:22:51 GMT
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In article <4ub39h$3ah@scoop.eco.twg.com>,
mike@vishnu.eco.twg.com (This space intentionally left blank) writes:
> 
> The formula for gas is: "P*V=N*R*T" (Presure times Volume is equal to the
> Number of moles of gas times the gas constant (R) times the Temperature. 
> From this it should be obvious that an increase in presure must cause an
> increase in the number of moles of gas (clearly impossible) or a rise in
> temperature and/or a decrease in volume to maintain the equality.

The thing is, though, that when we compress a fixed quantity (in terms of
mass or moles, not volume) of gas, we *expect* the volume to decrease.
Your formula has too many degrees of freedom, what we need is something
else (experts please provide) to tell us just what is going to happen
(and why) when we, say, double the pressure in a container full of air
(by pushing a piston rather forcing in more air).  What happens is that
we don't quite manage to halve the volume, but instead it gets hot.  If
we then keep the pressure constant and let the apparatus cool down to its
original temperature, then the volume will shrink to half its original.

Similarly, in a petrol engine, say, if we force the volume down to a
tenth of its original, because it heats up, the pressure required will be
rather more than 10 atmospheres.

My question is:  What determines the ratio of achieved volume to expected
original-temperature volume when applying a given pressure, or what
determines the ratio of required pressure to expected original-temperature
pressure when compressing a gas to a given volume?

[I've been good and trimmed the newsgroups a fair bit]
