• Support PF! Buy your school textbooks, materials and every day products via PF Here!

Help with resistance

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
 

mgb_phys

Science Advisor
Homework Helper
7,744
11
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
 
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
44,767
1,045
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?
 
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
 
Re: Help with resistance!!

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

Doc Al

Mentor
44,767
1,045
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.
 

Related Threads for: Help with resistance

Replies
1
Views
2K
  • Posted
Replies
2
Views
1K
  • Posted
Replies
1
Views
3K
Replies
1
Views
2K
Replies
2
Views
1K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top