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Thermodynamics piston-cylinder closed system

  1. Feb 12, 2017 #1
    1. The problem statement, all variables and given/known data
    An insulated piston-cylinder device contains 5 L of saturated liquid water at a constant pressure of 175 kPa. Water is stirred by a paddle wheel while a current of 8 A flows for 45 min through a resistor placed in the water, If one-half of the liquid is evaporated during this constant pressure process and the paddle -wheel work amounts to 400 kJ, determine the voltage of the source. Also, show the process on a P-v diagram with respect to saturation lines.

    ElBL8rY.jpg

    2. Relevant equations
    ΔE = Ein - Eout
    Welectric = VIΔt
    if ΔP = 0, ΔH = m(h2 - h1)
    ΔH = ΔU + Wboundary


    3. The attempt at a solution
    V = 5 L
    m = 5 kg (by conversion)
    P = 175 kPa
    I = 8 A
    Δt = 45*60 sec = 2700

    Assuming that KE, and PE = 0,
    ΔE = ΔU

    Also since it is insulated we are assuming that the heat lost is 0

    We + Wsh - Wboundary = ΔU
    We + Wsh = ΔU + Wboundary
    We + Wsh = ΔH
    We + Wsh = m(h2 - h1)

    Here is where I get lost. I know that we can find h1 from the saturated water tables
    h1 = 487.01 kj/kg (at Psat = 175 kPa)
    I'm confused as to where the second enthalpy value comes from since our state has constant P.

    Also the P-v should look something like this I believe:

    Bg1l7tO.jpg
     

    Attached Files:

  2. jcsd
  3. Feb 13, 2017 #2
    If you are using the saturation tables, you can get ##h_2## using the lever rule, or equivalently, as ##h_2=h_1+\Delta h_{vap}/2##.

    Regarding the P-V diagram, your result is correct.
     
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