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Homework Help: Entropy, thermaldynamics

  1. Oct 5, 2011 #1
    1. The problem statement, all variables and given/known data

    A cylinder contains 1000mol o He gas at an initial temp o 2000k and initial pressure of 1MPa. The He gas is now cooled to a final temp o 500k in a reversible process in which the volume and pressure are constrained to vary as PV3 = constant. Assume that the He is a monatomic ideal gas. Denote the initial and final states of the gas by A and B, respectively.

    a) Find the initial volume VA of the gas

    b) use the 1st law o thermodynamics to show that

    dQin = (3/2)nRdT + (PAVA3dV)/V3

    c) eleminate P from the two process equations PV = nRT and PV3 = PAVA3

    and hence that

    blah blah blah

    d) Use dS/dQinrev/T to find the entropy change ΔS = SB - SA

    2. Relevant equations

    all given in the question

    3. The attempt at a solution

    I solved a) b) c) with no trouble, but i'm just uncertain about d)

    doing the integral i found that the change in entropy ΔS = -1.152x104 and i was wondering i this is reasonable, because i thought entropy is supposed to be ≥0.

    I thought to myself that the ΔSuniverse = 0 such that ΔS + ΔSsurrounding = 0

    would this be a good assumption?
  2. jcsd
  3. Oct 5, 2011 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Yes; that's what "reversible" implies.
  4. Oct 5, 2011 #3
    derp :facepalm: thanks for the help
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