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Thermaldynamics problem, reversible process work done on

  1. Sep 21, 2011 #1
    1. The problem statement, all variables and given/known data

    One mole of a monatomic ideal gas is taken through a reversible process from an initial state "a" to a final state "b" during which the pressure of the gas varies with volume V as shown below. Calculate the work Won[/on](a->b) done on the gas during this process.

    2. Relevant equations

    problem shows a graph which i'll just write out points a and b

    a = (300kPa, 1m^3), b= (500kPa, 2m^3)

    3. The attempt at a solution

    work on = negative integral Pextdv and since this is reversible then P should equal Pext

    I can calculate the temp at a and b, but i don't think that will contribute to anything.

    Sorry this should be easy, but i'm confused
     
  2. jcsd
  3. Sep 21, 2011 #2

    Andrew Mason

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    Is it a straight line from a to b or is it a curve? If it is a curve, we need to know the relationship between P and V at all points from a to b.

    AM
     
  4. Sep 21, 2011 #3
    sorry it's a straight line, and i thought to myself, if there is no heat added or lost, then could i use the fact

    dU = Q + W where Q = 0

    then (3/2)nRdT = dU = W?
     
  5. Sep 21, 2011 #4
    Just to point out, [itex]\Delta[/itex]E = Q - W... not Q + W.
    From introductory general physics, [itex]\Delta[/itex]U = -W.
    Potential energy is equal to negative work. [itex]\Delta[/itex]U = Ua - Ub or [itex]\Delta[/itex]U = -(Ub - Ua)
    W = Ub - Ua

    Thus, [itex]\Delta[/itex]U = -W.

    If I'm not mistaken, correct?
     
  6. Sep 22, 2011 #5

    Andrew Mason

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    How does it increase its volume AND pressure (hence temperature) without positive heatflow into the gas? Is the gas doing positive work on the surroundings or is positive work being done on the gas?

    AM
     
  7. Sep 22, 2011 #6

    Andrew Mason

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    This only applies if the process is adiabatic, which this is not. Whether there is a - sign for W depends on the convention used. There are two different conventions for the first law. You are using the convention I prefer, which is that dW = PdV = work done BY the gas. Some texts use dW = -PdV = work done ON the gas. The OP is using the latter convention. The question actually asks for the work done ON the gas ie [itex]-\int PdV[/itex].

    AM
     
  8. Sep 22, 2011 #7
    Oh, right. Sign conventions always trips me up.
    Thanks for clarifying, and sorry to Liquidxlax for adding confusion :P
     
  9. Sep 22, 2011 #8
    no problem i have no problem with the sign convention, the fact i just made W positive wasn't for a fact i was confused about the convention.

    thanks for the help guys
     
  10. Sep 22, 2011 #9

    Andrew Mason

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    So what was your answer?

    AM
     
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