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Homework Help: Question for the resistance of the wire

  1. May 18, 2016 #1
    • Member advised to use the homework template for posts in the homework sections of PF.
    I think about many times for few days, but I still cant find out the correct answer! Maybe my steps are somethings wrong, please advise me! Thanks!

    A copper rod, 0.4m long and 4.0mm in diameter, has a resistance of 550μΩ at 20℃. Calculate the resistivity of copper at that temperature. If the rod is drawn out into a wire having a uniform diameter of 0.8mm, calculate the resistance of the wire when its temperature is 60℃. Assume the resistivity to be unchanged and the temperature coefficient of resistance of copper to be 0.00426/℃.

    Correct answer : 0.0173 x 10^-6 ohm m, 0.398ohm

    My steps as follows:

    the resistivity of copper : 0.0173 x 10^-6 ohm m ( I've got it, no problem!)

    the resistance of the wire :

    Area : π x [(0.8/2)x10^-3]^2

    R0 = Pl/A =[ 0.0173 x 10^-6 x 0.4] / 3.14 x 1.6 x 10^-7 =0.01377ohm

    R1 /R2= (1+0.00426x20) / (1+0.00426x60)

    R1/0.01377ohm = 0.8652

    R1 = 0.1184ohm (wrong answer!!)
  2. jcsd
  3. May 18, 2016 #2


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    The equation R0 = ρL/A is correct but you forgot about L changing when you substituted the numbers.

    L is no longer 0.4m
  4. May 18, 2016 #3
    I don't know what is the value of changing L and how to calculate it, please advise me!
    Last edited: May 18, 2016
  5. May 18, 2016 #4


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    If the diameter is reduced the length will increase. Assume the volume of copper is constant.
  6. May 18, 2016 #5
    I got it, thanks so much!
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