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Magnetic flux formula confusion

  1. Jul 16, 2008 #1
    Hello everyone,

    In the magnetic flux formula in our textbook it says Ø = BAcosø and somewhere else I saw it as Ø=BAsinø. I don't know which one to use. If F=BIL sinø
    why does magnetic flux have cos in it formula. If something is perpendicular don't we have to use sine. Any help would be apppreciated. Thanks:smile:
    Last edited: Jul 16, 2008
  2. jcsd
  3. Jul 16, 2008 #2
    formula [tex][tex]\phi[/tex] = BAcos[tex]\alpha[/tex][/tex] is magnetic flux
    it like [tex][tex]\phi[/tex]=[tex]\oint[/tex]EdA[/tex]
    formula [tex]F=ILBsin\alpha[/tex] was deduced from F=qvB or F=qvbsin[tex]\alpha[/tex]
    Last edited: Jul 16, 2008
  4. Jul 16, 2008 #3
    Those 2 formulas might both be fine, it just depends on how is the angle ø defined.

    They are however just special cases, when the area considered is flat and B is constant.

    The real definition of magnetic flux, which is valid for every surface and in every case, is:

    [tex]\Phi = \int_A B_n dA[/tex]

    which means that you have to sum (integral) the contributes of the perpendicular (normal) component of B in every point of the surface A.

    When B is constant everywhere (does not vary in different points of the surface) and A is flat, the integral goes away and the normal component of B is Bcosø if ø is defined as the angle between B and an axis perpendicular to A (or Bsinø if ø if the angle between B and the surface A).
  5. Jul 17, 2008 #4
    I got it. You are right it depend on the angle they give. Thanks a lot :smile:.
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