Resistivity equations

[Gary Tollison]

Hi folks.
I'm getting ready for some upcoming exams and I find myself tripping up over resistivity equations. I've been working through example questions, but this one confuses me.

Example:
Find the CSA of a single copper cable 500 metres long, which carries a current of 2.5A, and has voltage drop of 3.58V, resistivity of copper being 1.72x10(-8) ohm/m.

any help would be great!
Thanks

Gary

 

[1Justin]

@Gary. Try to visualize it this way if it helps:
A 500M length of 1M^2 cable (wow that's a big fat one) would be 500*1.72e-8 = 8.6e-6 ohms.
Your 500M cable is 1.43 ohms. Hence as a fraction of 1M^2, the CSA is simply one over the other. = 8.6e-6/1.43. - And that's in square metres.
Since there are 1000*1000 square millimetres in a square metre, multiply the answer by 1000000. QED

 

[COOKIET123]

use ohms law to work out Ohms. V/I

I then used the formula and put a Value for CSA in that is not known yet.

I will, give you a clue, It is less than 10mm2.

I expect there is any easier way though.

 

[COOKIET123]

is it 6 my answer was 6.005586592

i did 17.2x10^-9x500 /1.432

the reason its 17.2x10^-9 is because my question is the same but it isnt 10^-8 its 10^-9