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Stuck on converting

  1. Feb 6, 2007 #1
    1. The problem statement, all variables and given/known data

    Suppose that you wish to fabricate a uniform wire out of 0.900 g of copper. If the wire is to have a resistance R = 0.700 ohm, and if all of the copper is to be used.

    (a) What will be the length of the wire? M

    2. Relevant equations

    R = pL/A

    R = .700 ohm and p = 1.7 x 10^-8

    3. The attempt at a solution

    first thing i did was input the given number

    .700 = (1.7x10^-8)L/A

    my main problem is finding A with the given weight of copper, but this is what i did.

    n = m/M = .900/63.54 = .0142 then,

    .0142(6.02x10^23) = 8.53 x 10^21

    this is where i get lost because to get A its A = DeltaQ/DeltaT but time was never a given and i believe its not needed.

    (8.53 x 10^21) x (1.6 x 10^-19) = 1364.8

    I know there are other ways to get "A" by A = DeltaV/R but Volts were never given.

    Any help on this would be great.
  2. jcsd
  3. Feb 6, 2007 #2


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    Homework Helper

    It looks like you are trying to find the number of electrons in the copper or something like that?

    In your formula R = pL/A, A is the cross sectional area of the wire. It's not the current, or anything else to do with Amperes.

    Start by finding the volume of the wire from the mass and the density of copper.
  4. Feb 6, 2007 #3
    okay that seem to be wrong from what I thought I was suppose to do. I am suppose to find the length of the wire but I dont know how and i guess i read the wrong things.

    How would i calculate the volume of the wire? I have no knowledge in chemisty so what would the density of copper be?
  5. Feb 6, 2007 #4
    Okay, i think i got it

    V = m/d

    V = .0009/8960 = .0000001 m^3
    Last edited: Feb 6, 2007
  6. Feb 6, 2007 #5


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    OK, now you know the volume, you have two unknown quantities, the area and the length, and two equations:

    volume = area * length
    R = pL/A
  7. Feb 6, 2007 #6
    Thanks for the help. I was finally able to get the answer
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