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Do magnetic poles of an object have to be perpendicular to the object's surface?

  1. Apr 4, 2012 #1
    Topic. If I have an iron shaped like a bar magnet placed flat on the floor, can the poles of the magnet be pointing anywhere else other than 90 degrees and 180 degrees?
     
  2. jcsd
  3. Apr 4, 2012 #2

    mfb

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    They can. But I would expect that the field you can get with unusual orientations is a bit weaker, as the magnetic field gets an odd shape.
     
  4. Apr 4, 2012 #3

    K^2

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    The poles don't point. They have no direction. Just a location.
     
  5. Apr 4, 2012 #4
    Magnetic flux lines are always normal to the surface at a pole.
     
  6. Apr 4, 2012 #5
    The poles can be located anywhere on an object depending on how you magnetize it. By the way, the concept of macroscopic "poles" is a loose conceptual entity that helps visualize thing. There is no exact pole location - a single point in space - where a little physical thing called a pole sits. Rather, a material can have a magnetization throughout its extent. If the magnetization is fairly uniform and the object's shape is fairly simple, then it looks like all the field lines are created by two poles on opposite sides of the object.

    For example, four loops of current-carrying wire produce the magnetic field shown below. Where exactly would you say is the location of the poles?

    220px-VFPt_quadrupole_coils_1.svg.png
     
  7. Apr 4, 2012 #6
    A "magnetic dipole" is, as described above, a vague and ambiguous term. Indeed, magnetic dipoles do not even exist in nature (div(B) = 0, always). In a general sense, though, all magnetic field lines are not always normal to the current-carrying surface.
    This follows directly from the Biot-Savart law, in the general case of a surface current:

    B(r) = [itex]\frac{\mu}{4\pi}[/itex] [itex]\int[/itex] [itex]\frac{K(\acute{r}) χ \hat{r}}{r^2}[/itex]d[itex]\hat{\tau}[/itex]

    where K([itex]\acute{r}[/itex]) is the surface current density,
    and [itex]\hat{r}[/itex] is the vector extending from the source to the point r

    We note that the direction of the magnetic field will be given by the cross product between a vector pointing in the direction of the current and a vector pointing towards the point. Ergo, the magnetic field lines must always be perpendicular to the direction of current, but may not be perpendicular to the surface itself.
    In your particular case, the field lines will always be perpendicular to the outer edges ("dipoles") of the bar.
    Hope this helped. :3
     
  8. Apr 4, 2012 #7
    Thanks to everyone who's helped. My question has been adequately answered :)
     
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