Homework Help: Cant uderstand some parts this another problem

1. Aug 28, 2009

nabliat

here is the problem and solution:
http://i30.tinypic.com/2w2mnap.jpg

why they divide into two cases?

then they find a magnetic field

but we dont have any electricity flowing??

the i recognized the formula for a magnetic field from a wire
but when we find a the formula from the biotsavart law
the oly direction is dlX$$\hat{r}$$

there is no $$\vec{\theta}$$ there
because thus in Cartesian coordinates the direction always changes

how they get this theta direction thing??