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'[OT:] Thermodynamics question'
2005\10\30@164252 by  A recent discussion with a colleague left me looking for a formula to
calculate the temperature of a given gas as a function of volume.

I understand PV=nRT, but what if I only change the volume and want to
calculate both the new temp and pressure? I expect there's an equation that
relates specific heat, temperature and volume, but I haven't been able to
find it with google (probably a bad choice of search terms), a little help?

-Denny  Denny Esterline wrote:
> A recent discussion with a colleague left me looking for a formula to
> calculate the temperature of a given gas as a function of volume.
>
> I understand PV=nRT, but what if I only change the volume and want to
> calculate both the new temp and pressure? I expect there's an equation that
> relates specific heat, temperature and volume, but I haven't been able to
> find it with google (probably a bad choice of search terms), a little help?
>
> -Denny
>
This may give you a starting point

http://www.grc.nasa.gov/WWW/K-12/airplane/eqstat.html

I think this is why so many of my weather texts start with
something along the lines "Assuming a unit volume......"

Jim  At 04:42 PM 10/30/2005 -0500, you wrote:
>A recent discussion with a colleague left me looking for a formula to
>calculate the temperature of a given gas as a function of volume.
>
>I understand PV=nRT, but what if I only change the volume and want to
>calculate both the new temp and pressure? I expect there's an equation that
>relates specific heat, temperature and volume, but I haven't been able to
>find it with google (probably a bad choice of search terms), a little help?

Try adding "adiabatic" to your search terms.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff  ----- Original Message -----
From: "Denny Esterline" <firmware tds.net>
To: <piclist mit.edu>
Sent: Sunday, October 30, 2005 3:42 PM
Subject: [OT:] Thermodynamics question

> A recent discussion with a colleague left me looking for a formula to
> calculate the temperature of a given gas as a function of volume.
>
> I understand PV=nRT, but what if I only change the volume and want to
> calculate both the new temp and pressure? I expect there's an equation
that
> relates specific heat, temperature and volume, but I haven't been able
to
> find it with google (probably a bad choice of search terms), a little
help?

Presenting the one and only combined gas law:
(P1 x V1) / (P2 x V2) = T1/T2  > > A recent discussion with a colleague left me looking for a formula to
> > calculate the temperature of a given gas as a function of volume.
> >
> > I understand PV=nRT, but what if I only change the volume and want to
> > calculate both the new temp and pressure? I expect there's an equation
> that
> > relates specific heat, temperature and volume, but I haven't been able
> to
> > find it with google (probably a bad choice of search terms), a little
> help?
>
> Presenting the one and only combined gas law:
> (P1 x V1) / (P2 x V2) = T1/T2
>

But that's just a simplified case of PV=nRT. PV=nRT can be rewitten to
P1 V1         P2 V2
----------  = ----------
n1 R T1       n2 R T2

When n (number of moles of gas) is constant, n1=n2 and that term cancels.
And since R is a constant, it cancels- leaving your equation.

What I'm looking for is an expression that relates temperature to quantity
of gas, volume, specific heat (a property of a specific gas) and some
quantity of heat (calories, joules, BTUs, whatever)

I need to be able to answer questions like: what is the temperature and
presure of a mole of gas X in a container of size Y if it contains Z
calories of heat. And: If a sample of gas X at temperature and pressure Y
and Z is compressed to volume W, what is the final temperature and presure.
PV/T is not enough to solve these.

-Denny

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