hi guys,(adsbygoogle = window.adsbygoogle || []).push({});

I know this sound strange but it did happen. I was working on a question asking about calculating the resistance of a resistors, using different material one is copper and another is aluminium. I got the right answer, and wrote down the formula and steps that I used. But then I only didn't put in the values that I actually used. Just formula, steps and answer. Now when I come back to look at it again, I couldn't get the same answer that I originally correct. Either could I back track how I did it, so I hope someone can point it out to me, how I did it in the first place, and did I do it right then.

1. The problem statement, all variables and given/known data

a 20mm long piece of copper wire (sigma = 5.8*10^7 (ohm.m)^-1) with a diameter of 0.01mm is used as a resistor. you want to design a resistor with the same resistance but the only wire avaiable to you is an aluminium wire (sigma = 3.4*10^7)ohm.m)^-1) with diameter 0.02 mm. calculate the length for the piece of aluminium wire for which the two resistances are the same

2. Relevant equations

R = pL/A

3. The attempt at a solution

the steps that I wrote down original listed below, and I haven't put in the fitted values then, but as I tries to put in all the values in again, I didn't get a consistence result.

Rcu = pL/A

= L/ (sigma* A)

= (L/sigma) (4/ pi d^2)

=4.39ohm

Values I used to reattempt to get answer:

L = 20mm = 0.02m

sigma = 5.8*10^7

d = (0.01*10-3) ^2

Rcu = (0.02/(5.8*10^7)) * (4/ (pi*(.01*10^-3)^2

Why......?

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Resistance of resistors

**Physics Forums | Science Articles, Homework Help, Discussion**