# Copper wire resistance

#### GravityGirl

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.

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#### Dick

Homework Helper
Where did you get the funny area formula? Don't you want pi*r2^2-pi*r1^2 where r's are the radii?

#### GravityGirl

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

#### Dick

Homework Helper
But (d2-d1)^2 is not equal to d2^2-d1^2.

#### GravityGirl

so what did you get for the area...i still get 1.29E-4 m

#### Dick

Homework Helper
0.00147 m^2. Throw out that formula and subtract the area of the two circles!!!!!

#### GravityGirl

ok ok....i will do that

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