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Homework Help: Find the Work Done and the Heat Transfer

  1. Feb 6, 2013 #1
    \1. The problem statement, all variables and given/known data
    Air is compressed in a piston-cylinder assembly from p1 = 10 lbf/in2, T1 = 500°R to a final volume of V2 = 1ft3 in a process described by pv1.25 = constant. The mass of air is 0.5 lb. Assuming ideal gas behavior and neglecting kinetic and potential energy effects, determine the work and heat transfer, each in Btu, using a) constant specific heats evaluated at 500°R and b) data from the table. Compare the results

    The table is titled 'Ideal Gas properties of air

    2. Relevant equations

    Cv = constant

    pv = nRT

    Q = ΔE + W

    E = U + KE + PE

    W = ∫P dv

    3. The attempt at a solution

    I'm not sure where to begin with this question. I think I need to find the initial volume first but I'm not sure what the best way to do this is. I had thought to use:

    pv = nRT

    But then I'd have to convert my 0.5 lb of air into moles which would require me to convert everything else into English units. Is there a simpler way?
  2. jcsd
  3. Feb 6, 2013 #2

    Simon Bridge

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    Science Advisor
    Homework Helper

    Start with what you know about the kind of process that can be described as ##pv^{1.25}=\text{const}##.

    The only way to avoid converting things is to find an expression of the equations that uses the things you want.
  4. Feb 7, 2013 #3
    You need to find the value of the gas constant expressed in units of (psi)(ft3)/((lb-mole)(degree R)). Look it up with Google. Converting 0.5 lb to lb-moles is easy, since you just divide by 29. Then you are ready to apply the ideal gas law to calculate the initial volume in ft3.
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