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Homework Help: Intro Thermodynamic question

  1. Oct 10, 2005 #1
    Hi all , I have 2 questions, I dunno how to derive the equations, please help.
    Thank you.
    1)A reversible adiabatic expansion of 2.5 moles or He from a volume 9L at 300K to a final volume of 28L.
    Find work(W), heat(Q), internal energy(U) and enthalpy change(H) and final temperature. (He is a monatomic ideal gas)

    2)A reversible isobaric expansion of an ideal gas from P1,V1,T1 to P1,V2,T2, assume Cv = 5/2nR
    Find W, change in internal energy, Heat and enthalpy.

    I know for #1 heat = 0, therefore delta U = W = integral P*dV, but I don't know since we don't know pressure , I don't know how to continue.

    For #2, it is constant pressure, therefore W= -P*delta V
    delta U = Q - (p*delta V)
    Q = delta U + (p*delta V) = enthalpy
    Am I right ? it looks so weird, I need some help thanks !
  2. jcsd
  3. Oct 11, 2005 #2


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    1. Adiabatic work done, [itex]W = \int PdV [/itex]. But [itex]PV^{\gamma} = const, ~C[/itex] gives :

    [tex]W = C~\int \frac{dV}{V^{\gamma}} [/tex]

    Integrate that out, find P using the Ideal Gas Law, use the value of [itex]\gamma[/itex] for an ideal monoatomic gas, and hence find the value of the adiabatic constant C. Plug in values of C, Vf and Vi to find the work done.

    2. For an isobaric process, Q is easily found. Recall what Cp is ?
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