Copper wire resistance

  • #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.
 

Answers and Replies

  • #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?
 
  • #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
 
  • #4
Dick
Science Advisor
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But (d2-d1)^2 is not equal to d2^2-d1^2.
 
  • #5
so what did you get for the area...i still get 1.29E-4 m
 
  • #6
Dick
Science Advisor
Homework Helper
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0.00147 m^2. Throw out that formula and subtract the area of the two circles!!!!!
 
  • #7
ok ok....i will do that
 

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