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Homework Help: Thermodynamics - I am so frustrated!

  1. Feb 19, 2007 #1
    1. The problem statement, all variables and given/known data

    Water @ 100kPa and 127 C
    Heat added electrically 700kJ/kg
    Find final temp using:
    a. water table
    b. ideal-gas table
    c. constant specific heat

    2. Relevant equations

    Cp = a + bT + cT^2 + dT^3
    the constants are also listed in the table

    3. The attempt at a solution

    by using the superhead water table to find u1 at 2547.618 kJ/kg, I added the 700 kJ/kg and then interpilated to get a final temp of 568 C. This seems correct.

    I don't see how one can use the ideal gas tables, and I am at a loss as how to use the constant specific heat to find the final temp. Please advise as how to compute! Thanks
     
  2. jcsd
  3. Feb 21, 2007 #2
    (ugh...I may not be so much help after all, I seem to have forgotten a lot

    but maybe it's helpful anyway...I'm pretty sure about the stuff I have below, it just doesn't agree with your answer for a, and I don't know why not)


    let's see if the other methods agree with your answer for a)

    let's try c)

    a little of the ol'

    Q = mCp(T2-T1)

    we don't have the mass...but we don't need it either since q = Q/m

    (they gave us the heat in kJ/kg)

    q = Cp(T2-T1)

    so, you have q, you can look up Cp from the tables...which makes me wonder if I'm really doing part b) hmm...probably, and for part c you have to calculate Cp using the a, b, c, d constants from another table? anyways: you know the initial temperature (T1)

    be careful with your units...and you should have a nice looking answer
     
    Last edited: Feb 21, 2007
  4. Feb 21, 2007 #3
    Well, I see then how c) makes sense but like you say, it is WAY off a).

    Thanks for the reply tho, I appreciate it bud.
     
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