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Homework Help: 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.

    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
  2. jcsd
  3. Feb 19, 2007 #2
    The biggest trick to solving thermodynamics problems is to always include units in your equations so that you can see if the end result makes sense. You are throwing around so many numbers without units that you don't realize you have an answer that doesn't make sense. The pressure is given in pounds per square inch and the volume is given in feet cubed. Your method for the first one is okay, but it looks like you have some converting to do before you plug numbers in.

    For the second one you have similar problems mixing units. Also always make sure never to use celcius/farenheit, you want to convert them to kelvin/rankin. After you get passed units, you have 2 states described in the problem each which you can describe with an equation. After you describe each state with an equation I'm sure you'll be able to see where to go from there.
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