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