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Homework Help: Thermodynamic Signs

  1. Apr 19, 2008 #1
    1. The problem statement, all variables and given/known data
    A thermodynamic system changes isochorically from state A to state B where the temperature is greater than state A. The system then expands isobarically to state C where the temperature is greater than state B. From state C the system returns to its original state A linearly. Select Zero, Positive, or Negative for all selections.
    W C → A
    Q A → B
    W B → C
    ∆U C → A
    Q B → C
    ∆U A → B


    2. Relevant equations
    ∆U = Q + W
    W = WORK ON GAS
    Q = HEAT ADDED TO GAS
    W = -P∆V

    3. The attempt at a solution
    W C → A +
    Q A → B +
    W B → C -
    ∆U C → A 0
    Q B → C 0
    ∆U A → B +

    WHAT AM I DOING WRONG?

    (GRAPH ATTATCHED)
     

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    Last edited: Apr 19, 2008
  2. jcsd
  3. Apr 19, 2008 #2

    Mapes

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    Determine ∆U for B → C, then find which one of your results is inconsistent regarding the B → C process.
     
  4. Apr 19, 2008 #3
    It's W = -P∆V. The volume is increasing because the temperature is increasing. So the work is negative. So I can't find anything inconsistent regarding the B → C process...
     
  5. Apr 19, 2008 #4
    anyone??
     
  6. Apr 19, 2008 #5
    really, i need tp know the answer
     
  7. Apr 19, 2008 #6

    Andrew Mason

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    Make sure that you are using the right convention for W. The convention is to use W done by the gas as positive and W done on the gas as negative. The first law is:

    [tex]\Delta Q = \Delta U + W[/tex] or

    [tex]\Delta U = \Delta Q - W[/tex]

    Also, we can't tell what you are doing wrong without knowing your reasoning.

    ie. What is your reason for saying that:

    1. the work done by the gas from C to A is positive?

    2. the heat flow into the gas from A to B is positive?

    3. the work done by the gas from B to C is negative?

    4. the change in internal energy of the gas from C to A is 0?

    5. the heat flow into the gas from B to C is 0?

    6. the change in internal energy of the gas from A to B is positive?

    AM
     
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