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Homework Help: Thermal (Entropy / Enthalpy)

  1. May 15, 2009 #1
    1. The problem statement, all variables and given/known data
    Gas has a heat capacity of (Cv = (3/2)nR), intial temp of To, pressure Po and volume Vo.

    Process a to b is a quasistatic isobaric expansion to twice the intial volume

    a) work done,
    b) heat transfer,
    c) change in internal energy,
    d) change in enthalpy
    e) change in entropy
    In terms of n(moles), R, T0

    2. Relevant equations

    W = -P(delta)V
    (delta)U = Q + W = (f/2)nR(delta)T
    PV = nrT
    H = U + PV

    3. The attempt at a solution
    Now this is what ive come up with but im unsure about enthalpy and entropy (this mostly), they seem ok but look wrong. I dont have any answer or similar questions to check it againt first and only isobaric cycle. Looking on the net ive found mostly constant U, while V changes which isnt helpful.

    Attached is my attemp if any1 can shine any light on the last two points or anything else


    Thanks Trent
  2. jcsd
  3. May 16, 2009 #2


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    Homework Helper

    I thought the formula was
    [tex]\Delta S = \int_{T_i}^{T_f} \frac{C_V}{T}\mathrm{d}T[/tex]
    but anyway, you seem to have the right idea with the calculations. :-)
  4. May 16, 2009 #3
    Im basing this [tex]\Delta S = \int_{T_i}^{T_f} \frac{C_P}{T}\mathrm{d}T[/tex], from this

    "When T is changing, it's usually more convenient to write the relation in terms of the heat capacipty at constant volume: dS = (CVdT)/T."

    Now we dont have constant volume so ive discarded that

    Over the page than it says,
    "constant pressure processes in which the temperature changes, we can write Q = CPdT, than integrate to obtain,

    [tex]\Delta S_P= \int_{T_i}^{T_f} \frac{C_P}{T}\mathrm{d}T[/tex]

    This correct or i may have read misunderstood something, i agree it should be CV rather than CP, but from my understanding thats for constant volume??

    If any1 can shine some light thanks heeps

    Cheers Trent
    Last edited: May 16, 2009
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