1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
    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?
  2. jcsd
  3. Nov 7, 2008 #2


    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?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook