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Homework Help: Determine the amount of energy transfer by work in btu

  1. Jan 28, 2013 #1
    1. The problem statement, all variables and given/known data
    A closed system consisting of 14.5 lb of air undergoes a polytropic process from p1 = 80 lbf/in2, v1 = 4 ft3/lb to a final state where p2= 20 lbf/in2, v2 = 11 ft3/lb. Determine the amount of energy transfer by work, in Btu, for the process

    2. Relevant equations
    W = ∫v1v2p dv = ∫v1v2 c/vn dv

    3. The attempt at a solution

    Using the relationship p1v1n = p2vn2

    I solved for n with:

    n = ln(p2/p1)/ln(v2/v1) which gives me

    n = -1.3704 but other people I've spoken to have gotten +1.3704 which, when used in:

    W = p2v2 - p1v1/(1-n)

    gives me 563714.9 lbf-ft, which then gives me 725 Btu when converted to Btu.

    Why do I get -1.3704 instead of a positive?
  2. jcsd
  3. Jan 28, 2013 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    You did not solve for n correctly. It's
    n = ln(p1/p2)/ln(V1/V2) > 0.
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