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Homework Help: How to find the radius of copper wire given resistivity values

  1. Nov 7, 2008 #1
    1. The problem statement, all variables and given/known data
    i need to find the radius of a copper wire given the following data:
    resistivity is 0.25 ohm meter
    length of wire is 200 m


    2. Relevant equations
    roe=RS/I
    roe=resistivity
    R= resistance of material
    S=cross sectional area
    I= length of specimen


    3. The attempt at a solution
    i was looking at the equation and noticed i dont have any value for the resistance of the copper, so i looked it up online and it said: 0.00000001678 ohm.m. since i have 200m of wire i multiplied it by 200. so into the equation with everything:
    0.25=3.356x10^-6 x s/200 meters
    rearranging for the cross section area:
    s= 21222410.9
    although it doesnt say in the question, i assume that the cross section area of copper wire is circular. so i use the area of a circle to find the radius:
    A= pi r^2
    r= 2599 m

    i dont know about you but this doesnt seem right, to have a wire with radius of 2.5 kilometers. im stuck at what to do, my only guess is that the resistance value is wrong and i know there is and equation for resistance:
    resistance= resistivity x length/area
    i have the resisivity and the length, but what would i use for area?
    help!
     
  2. jcsd
  3. Nov 7, 2008 #2

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    Yeah, 2.5 km for a radius doesn't make much sense. Neither does the question. The resistivity of copper is not 0.25 ohm*m. It's not even close. And if they give you the bulk resistivity and the length, that's just data, it's not a question. You can't determine radius from that. Can you post the question verbatim?
     
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