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Need help with thermodynamics

  1. Feb 17, 2007 #1
    Need help with thermodynamics!!

    im studying building systems engineering technology. yeh i know it's only a technology program at the cegep level but when i'm done with it, i'm gonna do software engineering in university. anyways i'm having trouble to understand two problems in the book. if anyone can help me with this, it would be greatly appreciated.

    1) Determine the gage pressure if the atmospheric pressure is 14.7 psia, the gas constant is 96 ft-lb/lb R, and the specific volume is 10 ft^3/lbm.
    The answer is 31.9 psig

    2) 70 lbs mass of gas are contained in a rigid container at 200 psia and 80 F. The gas is then expanded to fill a 2000 ft^3 volume at a pressure pf 20 psia and a temperature of 70 F. Determine the volume of the rigid container.
    The answer is 203.7 ft^3.

    Can someone show me the steps on how they got the answer cuz i already know the answer. i just need help on how to solve the problem.

    for the first question, the formula is pv = mrt. we'll i know right away to find P = mRT. i would multiply 96ft-lb*700 * R (I dunno how to get that)/ Volume (I also dont know how to get that but i know it's something to do with the specific volume of 10 ft^3)

    for the second question, i multiplied (70)(80)/200. and from there i'm lost and dunno what to do.
    Last edited: Feb 17, 2007
  2. jcsd
  3. Feb 17, 2007 #2


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    Staff: Mentor

    For #1, what's the definition of specific volume...?
    For #2, you were taught another version of the ideal gas equation for dealing with that situation - find it in your notes/book. Hint: pv/mrt=constant.

    Btw, we have a homework section...
  4. Feb 17, 2007 #3
    specific volume is the volume per unit mass. (ft^3/slug)
    for number 1ok i think when it says the specific volume is 10 ft^3/lbm, i did 1/(10ft^3/lbm) to get the density.
    then i mutiplied (0.1)(96)(700) = 6720. but the answer is 31.9 psig so i know i'm really off even though i didn't even finished my answer.
    Last edited: Feb 18, 2007
  5. Feb 19, 2007 #4
    As russ_watters pointed out, there is another variation of the ideal gas law. The one you posted PV = mRT is one variation, and you can rewrite the equation to solve this problem. It should also be in your textbook.

    Look at the units that you have. You are given ft and the answer is in psig which is pounds per square inch gage. Also they are asking for the gage pressure, not absolute and you don't need to find density.

    Also yes specific volume is the volume per unit mass. (ft^3/slug) in BG units. But the question is in EE units.
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