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Help with resistance

  1. Mar 28, 2009 #1
    Help with resistance!!

    1. The problem statement, all variables and given/known data
    Determine the resistance of a copper conductor 2m long with a circular cross section and a radius of 1mm at one end increasing linearly to a radius of 5mm at the other.

    2. Relevant equations
    R = ( 1 / 2pi * sigma * length ) * ln (b/a)

    sigma = conductivity of copper 58 Mega

    3. The attempt at a solution

    Well the correct answer is 2.20mOHms
    I use the equation above and I get 2.20 * 10^-9. I cant figure out why the order is off. Does anyone see something I am missing?

    Thanks
     
  2. jcsd
  3. Mar 28, 2009 #2

    mgb_phys

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    Science Advisor
    Homework Helper

    Re: Help with resistance!!

    Try converting everything to m
    if you are off by a factor of 10^6 and a mm is 10^-3 of a metre and an area (ie a squared) is involved this should give you some clues
     
  4. Mar 29, 2009 #3
    Re: Help with resistance!!

    I was actually doing this from the start....because i changed the three and five to meters, I still get the -9 power answer? its not the ln part of the equation its the other part??
     
  5. Mar 29, 2009 #4

    Doc Al

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    Staff: Mentor

    Re: Help with resistance!!

    I get the same answer as you (and the equation is correct).

    Are you sure that the correct answer isn't supposed to be in nΩ (nano Ohms) not mΩ?
     
  6. Mar 29, 2009 #5
    Re: Help with resistance!!

    hi. did u try:
    R= (Resistivity)*[L/A]
    1/Conductivity=Resistivity
    L=length of wire
    A= cross-sectional area of wire
     
  7. Mar 29, 2009 #6
    Re: Help with resistance!!

    Resistivity of Copper= 1.69 * 10^-8 Ohm*meter
     
  8. Mar 29, 2009 #7

    Doc Al

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    Staff: Mentor

    Re: Help with resistance!!

    The above equation is in terms of conductivity, which is the inverse of resistivity.
     
  9. Mar 29, 2009 #8
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