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Homework Help: A thermodynamics problem

  1. Jan 5, 2009 #1
    1. The problem statement, all variables and given/known data

    Two moles of an monoatomic ideal gas are experiencing an adiabatic transformation; during this time the temperature modifies from [tex]T1=400K[/tex] to [tex]t2=277^o C[/tex].
    What's the value of the mechanical work exchanged by the gas with the exterior?

    2. Relevant equations

    The mechanical work's formula, Poisson's equations

    3. The attempt at a solution

    First i wrote this
    Then wrote Poisson's equations, replaced the temperature with the given values, then find out V1 in function of V2 and p1 in function of p2.
    Replacing them in the first equation would get me a wrong answer (2.6 = 1)

    I haven't done any thermodynamics exercise for 3 years and i quite forgot the stuff.
    Sorry for the weird terms i may have used, don't know them exactly in english.
  2. jcsd
  3. Jan 5, 2009 #2

    The section Titled "Derivation of Discrete Formula" has all of the equations you should need to derive a law.

    Remember that the ratio of specific heat capacities of a mono-atomic ideal gas is related to the number of degrees of freedom. http://en.wikipedia.org/wiki/Monatomic_gas

    You should end up with:

    dW = alpha *nR(T2 - T1)

    ... where gamma = (alpha + 1) / alpha (gamma is the ratio http://en.wikipedia.org/wiki/Heat_capacity_ratio)

    This gives the magnitude of dw as: 3068 J to 4 significant figures.

    Let me know if this helps.
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