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Homework Help: Cant uderstand some parts this another problem

  1. Aug 28, 2009 #1
    here is the problem and solution:

    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[tex]\hat{r}[/tex]

    there is no [tex]\vec{\theta}[/tex] there
    because thus in Cartesian coordinates the direction always changes

    how they get this theta direction thing??
  2. jcsd
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