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Thermodynamics - I am so frustrated!

  • Thread starter jmshipton
  • Start date
  • #1
2
0

Homework Statement



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

Homework Equations



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

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
 

Answers and Replies

  • #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:
  • #3
2
0
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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