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Thermodynamics of heating water in a cylinder with piston

  1. Sep 19, 2014 #1
    1. The problem statement, all variables and given/known data
    Consider water contained in a cylinder at 25C with a frictionless piston with some weights on it. Initially the pressure inside the piston is 100kPa and then the water is heated such that the water does 290kJ/kg of work on the piston. Find the final temperature of the water.


    2. Relevant equations
    First Law: Ein-Eout=ΔEsystem


    3. The attempt at a solution
    Since heat is added and work in done by the system, by the first law,
    qin-wout=Δesystem=Δu=cv(T2-T1)

    q-290=cv(T2-T1)

    But how can I get q in order to calculate T2?

    Thank you
     
  2. jcsd
  3. Sep 19, 2014 #2

    BvU

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    You will need more equations than just the one under 2.
    cv requires v constant. How can the water do work if the piston doesn't move ?
     
  4. Sep 19, 2014 #3
    Can I assume the pressure is constant during the process?
     
  5. Sep 19, 2014 #4

    haruspex

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    Why would the pressure change? What determines the pressure in this set-up?
     
  6. Sep 19, 2014 #5
    The pressure is 100kPa in state1 but did not mention about the pressure about state2?
     
  7. Sep 19, 2014 #6

    DEvens

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    What would "a frictionless piston with some weights on it" mean about the pressure before and after?
     
  8. Sep 19, 2014 #7
    How can the pressure on the water be 100 kPa if the piston has weights on it? Isn't that just atmospheric pressure? Or is the 100 kPa gage pressure?

    Chet
     
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