# Thread: cant uderstand certain parts this another problem..

1. ## cant uderstand certain parts this another problem..

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 $\hat{r}$

2. It goes without saying that there is a current flowing through the wire.