# Homework Help: How to find the radius of copper wire given resistivity values

1. Nov 7, 2008

### subopolois

1. The problem statement, all variables and given/known data
i need to find the radius of a copper wire given the following data:
resistivity is 0.25 ohm meter
length of wire is 200 m

2. Relevant equations
roe=RS/I
roe=resistivity
R= resistance of material
S=cross sectional area
I= length of specimen

3. The attempt at a solution
i was looking at the equation and noticed i dont have any value for the resistance of the copper, so i looked it up online and it said: 0.00000001678 ohm.m. since i have 200m of wire i multiplied it by 200. so into the equation with everything:
0.25=3.356x10^-6 x s/200 meters
rearranging for the cross section area:
s= 21222410.9
although it doesnt say in the question, i assume that the cross section area of copper wire is circular. so i use the area of a circle to find the radius:
A= pi r^2
r= 2599 m

i dont know about you but this doesnt seem right, to have a wire with radius of 2.5 kilometers. im stuck at what to do, my only guess is that the resistance value is wrong and i know there is and equation for resistance:
resistance= resistivity x length/area
i have the resisivity and the length, but what would i use for area?
help!

2. Nov 7, 2008

### Dick

Yeah, 2.5 km for a radius doesn't make much sense. Neither does the question. The resistivity of copper is not 0.25 ohm*m. It's not even close. And if they give you the bulk resistivity and the length, that's just data, it's not a question. You can't determine radius from that. Can you post the question verbatim?