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Homework Help: Thermodynamics problem, help calculate the energy transfer

  1. Jan 26, 2010 #1
    1. The problem statement, all variables and given/known data

    A mass of 5 kg of steam is contained within a piston cylinder assembly. The steam undergoes an expansion from its initial state with e1 = 3000 kJ/kg to its final state with e2 = 2500 kJ/kg. During the process there is a heat transfer of 80 kJ to the steam. Also, a paddle wheel transfers 10 kJ of work to the steam. Calculate the amount of energy transfer by work from the steam on the piston during the process.

    2. Relevant equations

    dE = del Q - del W

    3. The attempt at a solution

    Would this be correct?
    W = del Q - dE
    = (80kJ + 10kJ) - (2500 kJ/kg - 3000 kJ/kg)
    W = 590 kJ

    I don't believe this is correct but it's all I have been able to come up with so far. Any guidance is appreciated
  2. jcsd
  3. Jan 26, 2010 #2
    e1 and e2 are specific energies. Multiply them by the steam mass.

    You can not sum [kJ] and [kJ/kg].

    Watching the dimensions in formulas is a great way to verify your solution.
  4. Jan 26, 2010 #3
    sorry for my half asleep attempt...

    so after multiplying the steam mass buy each of the energies would I just take the difference?

    such that:

    m*e2 - m*e1

    (5kg)(2500 kJ/kg) - (5kg)(3000kJ/kg) = -2500 kJ
  5. Jan 26, 2010 #4
    Yes, you would.

    \Delta E = m(e_2 - e_1).

    By the way, the solution in the first post (after the correction) will give you the work done by the piston on the steam. But if I see it right the question is about the opposite value.
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