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Homework Help: External Work done by a tank containing air

  1. Aug 12, 2014 #1
    1. The problem statement, all variables and given/known data

    A tank containing air is stirred by a paddle wheel. The work input to the paddle wheel is 9000 kj and the heat transferred to the surroundings from the tank is 3000 kj. The external work done by the system is ?

    2. Relevant equations

    dU = Q-W

    3. The attempt at a solution

    Work input = -9000 kj
    heat given to the surroundings = -3000 kj

    The answer given is Zero, but no explanation has been given .
    Is it zero because there is no movement of the boundary in this case?
  2. jcsd
  3. Aug 12, 2014 #2

    Andrew Mason

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    Welcome to PF Idris Sayed!

    The air is in a tank, which I assume is a fixed volume. Does the volume of air change? If not, how can the system do work?

  4. Aug 12, 2014 #3
    Thank you sir :)

    There is no mention of the change of volume of air in the question. So if the answer is Zero, then volume is fixed has to be assumed I guess in this case.
  5. Aug 13, 2014 #4

    Andrew Mason

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    I think it is the other way around. If the air is in a tank, its volume does not change. External work can only be done if there is a change in volume (i.e. a pressure applied over a change in volume = force x distance). Since there is no change in volume, no external work is done by the air.

  6. Aug 13, 2014 #5
    Thank you sir , This cleared my doubt :)
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