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-   -   Thermo question (http://www.physicsforums.com/showthread.php?t=151017)

DazedNConfused Jan11-07 09:33 PM

thermo question
 
1. The problem statement, all variables and given/known data
Rigid tank volume .5m^3 initially evacuated. Tiny hole develops, air seeps in @ 1 bar and 21 degrees C. Pressure in tank reaches 1 bar, slow leak so temp remains 21 deg C inside tank. What's amount of heat transfer?


2. Relevant equations

Q=T[S(2)-S(1)]
Pv=nRT

3. The attempt at a solution

Ok, I understand this is a reversible isothermal process but it seems that I am not given enough information to solve the problem. Please help to send me in the right direction! Thanks!

FredGarvin Jan12-07 10:47 PM

Have you taken a look at the the first law? You know of the assumptions for the problem, I would also add to treat the air as an ideal gas.

snowJT Jan13-07 12:54 PM

I'm very new to thermo but I haven't seen this formula Q=T[S(2)-S(1)], unless the S's means Volumes, I write it as capital V's

but anyways.. not the point... for rigid tanks, isn't the volume constant so you can solve for Q then use Q to find the missing variables in Pv=nRT

the temperature will give you the pressure, and you would have to treat this as an ideal gas, you're given little v, and I think you can get R from Q, or by the chartes... so then you just have P?

I havn't covered rigid tanks yet, so I'm probably very wrong...

teknodude Jan13-07 06:07 PM

The S probably stands for entropy. He probably got the first equation from the 2nd law of themo, S2 - S1 = Q/Tb + sigma, where sigma will equal zero since it's a reversible process.

PV =nRT works, but i think PV = mRT is a better variation of the ideal gas for this problem.

I'm guessing the others assumption that KE and PE are negligble. Take a control volume and you can see what you need to do from there.

DazedNConfused Jan14-07 12:29 AM

thank you....
 
Thanks for the help - I appreciate the responses.......have returned to school after a LONG break and some of the basics are pretty hazy

I thought that a control volume would only be applied to fluids?

By the S's I was meaning entropy, didn't know how to show using subscripts.

Side question - I know that when you are given two properties of a fluid (such as temperature and pressure) that you should know what state they are in (superheated etc).......but how exactly is that determined?
And, iff you are given both temp and press which table should you be using?

DazedNConfused Jan14-07 12:30 AM

if I do need to do a control volume - how exactly do i do that?


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