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Homework Help: Question about heat transfer

  1. Nov 24, 2008 #1
    1. The problem statement, all variables and given/known data

    Hello, first question here,

    A copper rod and an aluminium rod are in thermal equilibrium with each other. Copper rod = 50cm long and the aluminium rod is 20cm. Radius of pair is 8cm.
    Copper end is heated (and maintained) at 80C , other end of pairing is 20C. k of copper is 390 and k aluminium is 210. Calculate the rate of heat flow through the pairing.

    2. Relevant equations

    Q/t = kA.(temperature difference, theta)/l

    3. The attempt at a solution


    Thanks for any help!
     
  2. jcsd
  3. Nov 24, 2008 #2

    mgb_phys

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    That's not much of an attempt at a solution.
     
  4. Nov 24, 2008 #3
    I think this is the answer, not 100%,

    delta Q = delta T / SUM ( R )

    => DQ = DT / R1+R2....+...R0

    R1 = 1 / k1A , R2 = 1 / k2A ,....., R0 = 1 / k0A
     
  5. Nov 24, 2008 #4

    mgb_phys

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    You should explain what you have done.
    What do you know about the heat flow rate in both materials?
     
  6. Nov 24, 2008 #5
    I just need the equation, i am not going to derive everything from first principles, you can see what i have done,

    Dt / sum of R where, R is as stated.

    I am not William Thomson attempting to experimentally derive with this conclusion, i just have asked for a verification of the above formula.
     
  7. Nov 24, 2008 #6

    mgb_phys

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    Yes the formula is correct - it's pasted out of wikipedia.
    The point of this site is to explain physics and help people understand it.
     
  8. Nov 24, 2008 #7
    Thanks for your help!
     
  9. Nov 26, 2008 #8
    Or should it be this,

    = DQ = DT/ (L1/k1A + L2/k2A) where L = length of the rod. A = cross sectional area of rod.

    i had,

    DQ = DT / (1/k1A + 1/k2A)

    Is it dependent on the length?
     
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