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Homework Help: Adiabatic expansion problem

  1. Nov 4, 2012 #1
    1. The problem statement, all variables and given/known data
    A 4.00-L sample of a diatomic ideal gas with specific heat
    ratio 1.40, confined to a cylinder, is carried through a
    closed cycle. The gas is initially at 1.00 atm and at 300 K.
    First, its pressure is tripled under constant volume.
    Then, it expands adiabatically to its original pressure.
    Finally, the gas is compressed isobarically to its original
    volume. (a) Draw a PV diagram of this cycle. (b) Determine
    the volume of the gas at the end of the adiabatic
    expansion. (c) Find the temperature of the gas at the
    start of the adiabatic expansion. (d) Find the temperature
    at the end of the cycle. (e) What was the net work done on the gas for this cycle?

    2. Relevant equations

    3. The attempt at a solution
    Part A is a graph of a typical adiabatic expansion. Part B I used PV^γ is constant and found V=8.77 L. Part C I used PV = nRT and got 900K.

    I am stuck on part D, I know it should be 300 K, but I want to know why the equation TV^(γ-1) = constant isnt working. Lastly for part E, I don't know how to find the area under the curve without P as a function of V.
  2. jcsd
  3. Nov 4, 2012 #2


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    that equation gives the temperature at the end of the adiabatic expansion. It is not the end of the cycle: You have one isobaric compression left.

    No need to integrate. It is a cycle, the gas returns back to its original state, so the change of internal energy is zero. According to the First Law, the heat Q and W, the work done on the gas, add up to zero. You can calculate the heat exchange for each process: It is a diatomic gas, what are Cv and Cp? You also can determine the amount of gas.

  4. Nov 4, 2012 #3

    Andrew Mason

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    Is the adiabatic expansion quasi-static?

  5. Nov 4, 2012 #4
    Thanks guys!

    I completely overlooked that it was asking for the temperature at the original spot...I was thinking at the end of the adiabatic expansion.

    I'll give part E a try.

    Yes it is quasi-static.
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