# Help with resistance

#### FatoonsBaby71

Help with resistance!!

1. Homework Statement
Determine the resistance of a copper conductor 2m long with a circular cross section and a radius of 1mm at one end increasing linearly to a radius of 5mm at the other.

2. Homework Equations
R = ( 1 / 2pi * sigma * length ) * ln (b/a)

sigma = conductivity of copper 58 Mega

3. The Attempt at a Solution

Well the correct answer is 2.20mOHms
I use the equation above and I get 2.20 * 10^-9. I cant figure out why the order is off. Does anyone see something I am missing?

Thanks

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#### mgb_phys

Science Advisor
Homework Helper
Re: Help with resistance!!

Try converting everything to m
if you are off by a factor of 10^6 and a mm is 10^-3 of a metre and an area (ie a squared) is involved this should give you some clues

#### FatoonsBaby71

Re: Help with resistance!!

I was actually doing this from the start....because i changed the three and five to meters, I still get the -9 power answer? its not the ln part of the equation its the other part??

#### Doc Al

Mentor
Re: Help with resistance!!

Well the correct answer is 2.20mOHms
I use the equation above and I get 2.20 * 10^-9.
I get the same answer as you (and the equation is correct).

Are you sure that the correct answer isn't supposed to be in nΩ (nano Ohms) not mΩ?

#### greenday1260

Re: Help with resistance!!

hi. did u try:
R= (Resistivity)*[L/A]
1/Conductivity=Resistivity
L=length of wire
A= cross-sectional area of wire

#### greenday1260

Re: Help with resistance!!

Resistivity of Copper= 1.69 * 10^-8 Ohm*meter

#### Doc Al

Mentor
Re: Help with resistance!!

Resistivity of Copper= 1.69 * 10^-8 Ohm*meter
The above equation is in terms of conductivity, which is the inverse of resistivity.

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