# Vector solution differs mine

1. Jul 31, 2009

### electron2

the shown solution in the link is so different
http://i30.tinypic.com/35mfo94.gif

what those vectors represent
where are they
why they subtract them
i totally dont understand thing
did in a simpler way and got
a different result
where is the mistake

$$B=\frac{\mu _0 I}{4\pi}\int\frac{ds \hat{r}}{r^2}=\frac{\mu _0 I}{4\pi}\int_{0}^{\frac{2\pi}{3}}\frac{d\theta }{{R_1}^2}$$

2. Jul 31, 2009

### tiny-tim

Hi electron2!

It looks as if you're subtracting them because the first line is for W3, and the second line is for W4, so when you add them, they're in opposite directions, so one comes out negative.