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Magnetic field lines

  1. Oct 14, 2005 #1
    Hi, I have a question about magnetic field lines in a motor-like situation. Suppose I have two horizontal bar magnets, with one north end parallel to x=-X and one south end parallel to x=X, as here:

    -- --
    | |
    | |
    N | | S
    | |
    | |
    -- --

    (forget the 'as here', the board is stripping out my nice spacing...)

    If I have one or more current-carrying wires in the intervening space (not near the poles), can I still be sure that the magnetic field line from (-X,y) will end at (X,y)? In my model, this isn't happening, but I'm not sure if this is due to numerical error, some other error in the model, or whether it's actually expected real-life behaviour. I've made every correction I can think of to the line integration, and the lines are still warping significantly.

    Last edited: Oct 14, 2005
  2. jcsd
  3. Oct 24, 2005 #2


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    Staff Emeritus
    Science Advisor

    Are you describing two parallel bar magnets

    [N====S] [N====S] or [N====S] [S====N],

    or is it more like
    N . . S
    | . . |
    | . . |
    S . . N

    and then how are the electrical wires oriented between the bar magnets, perpendicular or parallel? And are the electrical wires effectively infinite or are they looped?

    The bar magnets will establish a static magnetic field.

    Is there current in the electrical wires? Is it DC or AC?

    A DC current will induce a static magnetic field, where as AC will induce a time varying magnetic field and voltage.

    Also, since both magnetic fields are vector fields, they are additive (vector addition).
  4. Oct 25, 2005 #3
    The magnets are collinear as in your first picture, with S facing N. The wires are looped, but we're ignoring that, so consider them as two infinitely long wires with oppositely facing currents. (incidentally, I think I know the answer to my question: the fields don't return to the same place, but are drawn up or down)
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