Greetings allI have a question here and I need to know if im on the right track

A length of copper conductor 75m long, having an area of 1.5mm2 has a specific resistance of 1.67 x 10-8**ohm/m. **

What is the resistance?

R = pl / A

Hi,

You need to be consistant with your units when doing equations like this.

Resistance - ohms

Length - metres

Area - metres squared

Resistivity - ohm.metres (

**not ohms/metre!**)

If you don't do this then you will be way out - and it's more understandable if you go to the SI units rather than getting confused with mm etc.

Step 1 - Get your area in the correct units 1.5mm2 = 1.5 x 10-6m2

Now plug into your equation

R = pl/A

= 1.167 x 10-8 x 75/1.5 x 10-6

= 1.167 x10-2 x75/1.5

= 0.5835 ohms.

Remember to quote your units at the end!!! Better marks. Also write down any steps along the way. You can easily muck up the maths - but if everything else is written down you should get most of the marks available.

If you just plug the units into the equation you should be able to see how the formula works:-

R = pl/A

= ohm.m .m /m2

= ohms

As an aside you can rearranging to work out the units resistivity:-

R = pl/A

RA = pl

pl = RA

p = RA/l

= ohms . m . m /m

= ohms . m

Used that to help with my exams, as you can always work backwards to get to the formula if it wasn't given!

Good Luck

Dave