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Ampere's law

  1. Apr 12, 2014 #1
    Dear all,

    I am trying use Ampere's law and came across a problem which is a result of my lack of understanding.

    [itex]\oint[/itex] B.dl=uI
    My lecturer derived the above equation by defining a point in the middle of a potato shaped loop. Where he stated that dl in this case is just rdθ. (r is the line joining dl to the point defined in the middle). And using B=[itex]\frac{uI}{2πr}[/itex] he integrated it with respect to rdθ from θ=0 to 2π to get [itex]\oint[/itex] B.dl=uI.

    Now, I am trying to do the same for a circular loop. Obviously I got the same answer uI. But its wrong as it should be [itex]\frac{uI}{2r}[/itex]. The problem is that I am using the same approch he used for the general shape but not getting the same answer. I am using B=[itex]\frac{uI}{2πr}[/itex] where I should actually use B=[itex]\frac{uI}{4πr^{2}}[/itex]

    Could someone explain why can't I do that?

    P.s sorry if this is messy. It is my first time.
  2. jcsd
  3. Apr 12, 2014 #2
    I think I got it guys. Mods can close this.
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