## Thermodynamics help needed!

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NDR008
NDR008
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Joined: Thu May 20, 2004 11:04 am
Location: Bristol-Europe

### Thermodynamics help needed!

I got stuck during a question, and I am starting to panic coz i feel left for too late to get cracking on the books - worked most question, but this has me puzzled.

C = celcius
m3 = metre cubed

A certain gas for which (gamma) = 1.26 and the molar mass is 26kg/kmol, expands reversibly from 727C, 0.003m3 to 2C, 0.6m3, according to a linear law on the T-s diagram. Calculate the work done per kg of gas and sketch the process on a T-s diagram.

Now.. W = integration (p) dV
now I guess the equations for p must derived from "a linear law on the T-s diagram"

So Ts = c?
I don't know I get stuck, somehow I am guessing pV = mRT comes into play.
R = Universal R / Molar mass
gamma = Cp/Cv....
I donno, I really can't see it yet... Please someone help me

West
West
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Joined: Tue Jan 06, 2004 11:42 pm
Location: San Diego, CA
I assume that this is a constant pressure, isentropic process. You might want to assume the gas is ideal, also.

If calorically perfect, you might be able to use w = -Cp(T2 - T1), which is also equal to (u2 - u1); T in Kelvin

PV = nRT might work if the pressure is constant at both 727C and at 3C, then u can use w = P(v2 - v1)
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NDR008
NDR008
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Joined: Thu May 20, 2004 11:04 am
Location: Bristol-Europe
Yeah... i think you're on to something thanks.
Now all I got to do is use:
Cp - Cv = R
R = universal R / molar mass,
Cp/Cv - Gamma
and then i can use the rest.

I'll try and get back at you.

NDR008
NDR008
0
Joined: Thu May 20, 2004 11:04 am
Location: Bristol-Europe
Damn, I don't think the constant pressure can be assumed.

If you know the temperature and volume occupied of a substance, how can you find its pressure?