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Homework Help: Resistance and fabrication

  1. Sep 19, 2010 #1

    J89

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    1. The problem statement, all variables and given/known data
    Suppose that you wish to fabricate a uniform wire out of 1.00 g of copper. Assume the wire has a resistance R = 0.600 , and all of the copper is used.

    What will be the diameter of the wire?




    2. Relevant equations
    A = V/l
    R = sqrt(A/pi)



    3. The attempt at a solution
    For the volume I have gotten 1.12e-7 ( (1*10e-3)/8.92e3)=8.92e3 is the density of copper and it's 1 g of copper. and for the length, I have gotten 1.988.

    For the diameter..
    I divided the volume/length to get the area = 1.12e-7/1.988 = 5.63e-8
    Then I used the formula r = sqrt(A/pi) = sqrt(5.63e-8/pi) = 1.34e-4
    so diameter = 2 *r = 1.34e-4*2 = 2.68e-4, but I keep getting it wrong! Can someone explain what I'm doing wrong??
     
  2. jcsd
  3. Sep 19, 2010 #2

    Gokul43201

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    How did you get that?
     
  4. Sep 19, 2010 #3

    J89

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    The resistivity of copper is 1.7e-8. The resistance is .600 ohms. The volume of copper is 1.12e-7 (1 kg copper/density of copper) = 1e-3/8.92e3 = 1.12e-7. So..

    I/A = R/P =

    .600/(1.7e-8) = 3.53e7 m..then multiply it by the volume of copper = 3.53e7 * (1.12e-7) = 3.9525 and then take the sqrt of that to get the length = 1.988, which is correct according to the problem I'm doing.
     
    Last edited: Sep 19, 2010
  5. Sep 19, 2010 #4

    Gokul43201

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    That looks good, as does the rest of it. Could it be that you need more sig figs? How do you know you answer is wrong?
     
  6. Sep 20, 2010 #5

    J89

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    The length is right. I know my diameter is wrong because everytime I put it in the hw box, it keeps marking it wrong, and I think this is the correct way to do it..I did a similar problem to this one, and that was marked right..don't know, what I am doing wrong for the diameter..can someone help me?
     
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