# Thread: Finding the unknown using a formula help

1. ## Finding the unknown using a formula help

Hi,

I need help using a formula to find the length of an electrical conductor.

(A) The cross sectional area of the conductor is 2.5mm2 which I have been told : 2.5 x 10^-6 (power of minus 6).
(R) The resistance is 0.86 (in ohms).
(P) The resistivity of the conductor is 17.24 nanohm metres which is : 17.24 x 10^-9 (power of minus 9).
(L) The length is unknown.

The formula was R: PL/A, now that length is unknown I believe the formula changes to L: RA/P?

So I came up with 0.86 x (2.5 x 10^-6) / 17.24 x 10^-9

Which gives me 1.25 (this ain't the right answer), did I go wrong in formula transposing or something?

Thanks

2. ## Re: Finding the unknown using a formula help

Hey cliffgnome.

You need to careful about the brackets (you need 17.24 x 10^-9 in one single bracket).

By using the brackets in the right place we get (from R) the answer to be:

> 0.86*2.5/17.24*1000
[1] 124.71

since 10^-6 / 10^-9 = 10^3.