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Homework Help: Archived Thermodynamics problem, calculate energy transfer?

  1. Jan 25, 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. Mar 25, 2016 #2
    Determining ΔE
    [tex]\Delta \hat{E} = 2500 \frac{kJ}{kg} - 3000 \frac{kJ}{kg} = -500 \frac{kJ}{kg}[/tex]
    [tex]\Delta E = 5 \ kg \left( - 500 \frac{kJ}{kg} \right) = -2500 \ kJ[/tex]
    Now, applying an energy balance on the cylinder (1st Law of Thermodynamics)
    [tex]\Delta E = Q + W_s - W[/tex]
    Where Ws is the work input by the paddle wheel and W is the work output on the piston. Finally
    [tex]W = Q + W_s - \Delta E = 80 \ kJ + 10 \ kJ - (-2500 \ kJ) = 2590 \ kJ[/tex]
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