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Copper wire resistance

  1. Jun 11, 2007 #1
    A copper pipe has an inside diameter of 6.720 cm and an outside diameter of 8.000 cm. What length of this copper pipe will have a resistance of 8.05 × 10-3 W?

    so i am using the equation R=rho(L/A)

    A=pi(d2-d1)^2/4 which is 1.29E-4 m

    and for copper rho is 1.7E-8

    so 8.05E-3=(1.7E-8)(L/1.29E-4)

    then L=60.93 m

    but this isnt right.

    please help me where i am going wrong.
     
  2. jcsd
  3. Jun 11, 2007 #2

    Dick

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    Where did you get the funny area formula? Don't you want pi*r2^2-pi*r1^2 where r's are the radii?
     
  4. Jun 11, 2007 #3
    well i was just going to use the diameter...but i recalcualted te area using the radius instead and i came up with the same value
     
  5. Jun 11, 2007 #4

    Dick

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    But (d2-d1)^2 is not equal to d2^2-d1^2.
     
  6. Jun 11, 2007 #5
    so what did you get for the area...i still get 1.29E-4 m
     
  7. Jun 11, 2007 #6

    Dick

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    0.00147 m^2. Throw out that formula and subtract the area of the two circles!!!!!
     
  8. Jun 11, 2007 #7
    ok ok....i will do that
     
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