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Stupid electromagnetics question

Discussion in 'Electrical Engineering' started by qwerty, May 10, 2007.

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  1. qwerty

    qwerty Guest

    I'm having some trouble getting my head around this:

    A current flowing through a (static) conductor produces a (static)
    magnetic field around it, as described by Ampere's Law.

    But the reverse appears not to be true; A magnetic field around a
    conductor doesn't induce current to it, unless the conductor is
    moving or the magnetic field changes (or both.)

    Isn't this a paradox?
  2. qwerty

    qwerty Guest

    But, Ampere's Law states that:

    So if we have B around a conductor, surely we have to have I as well.
    That's what the Law states. I know I'm missing something, but what do
    I miss?
  3. I wouldn't call it a paradox (or a stupid question, either). The
    situations are not symmetric - in the first, there is a static electric
    field as well, producing the current.

    You could say that, in the first case, the electric field driving the
    current produces the current and the magnetic field. You can get rid of
    the current in Ampere's law by substituting J = sigma E, where J is the
    current density and sigma is the conductivity.

    Another thing that stops it from being symmetric is that the current is
    moving electric charge, driven by an electric field. The symmetric case
    would be a magnetic current, composed of moving magnetic monopoles, being
    driven by the magnetic field and producing a static electric field. No
    magnetic monopoles, so we don't observer this.

    (Exercise for the reader: assume magnetic monopoles exist, and design a
    perpertual motion machine thereby.)

    You might be interested to know that sometimes magnetic currents are
    assumed to exist, to simplify calculations in problems where
    electromagnetic waves interact with objects (ie scattering).
  4. qwerty

    qwerty Guest

    I understand it now. If there's a magnetic field somewhere then that
    magnetic field is *always* caused by a current. So there's no way to
    create the magnetic field of a current-carrying conductor without
    using one.

  5. Yes, and AC current makes for continually moving flux. DC makes a
    standing field.
    To INDUCE (key term here), the flux MUST be in motion. The field
    doesn't have to "change" as you put it. It HAS TO MOVE, as in lines of
    flux must "cut" through the conductor, and the current induced in it will
    always be varying, not static.

  6. Permanent magnet.

  7. Add up a gazillion of 'em, and get a cumulative effect we call a field.

    Get a big planetoid sized orb full of iron and it will likely be
    magnetic to some degree. That's Gazillions to the Gazillionth power. :-]

  8. No. whenever it stops *moving*. It doesn't have to change strength,
    just position.

  9. WOW... one kook posting to another.

    Why don't you guys discuss it so we can all have a laugh?

  10. ESD damage can occur to chips due to electric fields, so yes, they DO
    have influence.
  11. ehsjr

    ehsjr Guest

    No. The logic is faulty. It is like
    stipulating that "all sandboxes contain sand" and
    because of the stipulation assuming that
    "all sand is in a sandbox".

    From Ampere's law you stipulate that a current through
    a wire results in a magnetic field around the wire.
    (That's the sandbox). You incorrectly assume, because
    of that stipulation, that a magnetic field around a wire
    must induce a current.

  12. Don Kelly

    Don Kelly Guest

    Ah but Heisenberg's cat thinks it is a litter box. To crap or not to crap,
    that is the question.

    Actually you put it very well but I couldn't resist :).
  13. ehsjr

    ehsjr Guest

    Is the cat's name Uncertain? :)
  14. qwerty

    qwerty Guest

    The Great Attractor
    The magnetic field of a permanent magnet is caused by currents inside
    the body of the magnet.

  15. It is caused by alignments of atoms in the lattice of the medium.

    They all spin in one plane.
  16. Depending on your point of view:)Actually your question is a very smart
    one.Maybe it's God's will (or a higher being's) that we can't get energy for
    free (there's no such thing as a free lunch).Because then we could place a
    piece of wire in Earth's magnetic field and get electricity,plenty and free,
    wouldn't we?
    But maybe He doesn't want to spoil us;-)So we have to rotate generators with
    prime movers, which on their turn need some fuel....Fuel costs money, and
    their supply is limited....Google for the perpetuum mobile, for an
  17. ? "Autymn D. C." <> ?????? ??? ??????
    ìÞíõìáI'm having some
    trouble getting my head around this:
    Fool, learn what induction is. There is already a current in such a
    field in other media, magnetic or Coriolis or otherwise. It's not
    much thouh.
    <end quote>
    Might be, but then, who am I, to tell right from wrong?
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