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Homework Help: Resistance, ohms

  1. Feb 28, 2010 #1
    1. The problem statement, all variables and given/known data
    A 1 mile long copper wire has a resistance of 68 ohms. What will be its new resistance when doubling it over and using it as "one" wire



    2. Relevant equations?
    R=V/current



    3. The attempt at a solution unable to attempt, I know that when it is cut in half R=34 ohms
     
  2. jcsd
  3. Feb 28, 2010 #2

    Doc Al

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    Staff: Mentor

    OK. More generally, how does resistance depend on length and cross-sectional area?
     
  4. Feb 28, 2010 #3
    inverse square rule?
     
  5. Feb 28, 2010 #4

    Doc Al

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    Staff: Mentor

    Last edited by a moderator: Apr 24, 2017
  6. Feb 28, 2010 #5
    the correct answer is 17 ohms...double width, 1/2 length =1/4 resistance?
     
  7. Feb 28, 2010 #6

    Doc Al

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    Staff: Mentor

    That's right. As that site explains, the resistance is inversely proportional to the cross-sectional area (which gives one factor 1/2) and directly proportional to the length (which gives a second factor of 1/2).
     
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