Calculate Copper Conductor Resistance with Varying Radius | 2m Length

[FatoonsBaby71]

Help with resistance!

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.

Homework Equations

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

sigma = conductivity of copper 58 Mega

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 can't figure out why the order is off. Does anyone see something I am missing?

Thanks

 

[mgb_phys]

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]

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]

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]

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

 

[greenday1260]

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

 

[Doc Al]

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

The above equation is in terms of conductivity, which is the inverse of resistivity.

 

[FatoonsBaby71]

No unfortunately the answer is in milli. I don't know why... I may have to stick to it though.

See...http://books.google.com/books?id=xV...y-XkCQ&sa=X&oi=book_result&resnum=1&ct=result


number 6.38

its so odd...i have no idea why

Thanks for your help again very much appreciated..