Thermodynamics help needed!

Post anything that doesn't belong in any other forum, including gaming and topics unrelated to motorsport. Site specific discussions should go in the site feedback forum.
NDR008
NDR008
0
Joined: Thu May 20, 2004 11:04 am
Location: Bristol-Europe

Thermodynamics help needed!

Post

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 :P

West
West
0
Joined: Tue Jan 06, 2004 11:42 pm
Location: San Diego, CA

Post

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)
Bring back wider rear wings, V10s, and tobacco advertisements

NDR008
NDR008
0
Joined: Thu May 20, 2004 11:04 am
Location: Bristol-Europe

Post

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

Post

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?