Calculating Resistance of a Copper Pipe Based on Length and Diameter

[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 isn't right.

please help me where i am going wrong.

 

[Dick]

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]

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]

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

 

[GravityGirl]

ok ok...i will do that