# cant uderstand certain parts this another problem..

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#### transgalactic

##### Banned
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??

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