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magnetic flux question

Discussion in 'Misc Electronics' started by Ken Williams, Jun 16, 2009.

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  1. Ken Williams

    Ken Williams Guest

    can someone tell me if this is true or false?

    magnetic flux more easily travels through ferrite/soft iron then it does
    air. so a coil with an air core is less "magnetically conductive" then a
    coil with an iron core.

  2. Bob Myers

    Bob Myers Guest

    Well, you *might* say it that way, but it's not quite correct.
    For one thing, it treats magnetic flux as something which "travels
    through" a medium, presumably in a manner similar to an
    electric current, and that's just not really so.

    The difference between having a coil with a ferrite or iron core
    and one with an air core is the *permeability* of the core material.
    Technically, there are two "magnetic fields" covered by electromagnetic
    theory, known as the "B field" and the "H field," and the permeability
    of the medium is the relationship between these two. That by itself
    is about as clear as mud; another way to say it is that permeability is
    what determines how strongly magnetized a given material will be
    when a magnetic field is passing through it. (For instance, an air-core
    coil wth a given current might be a fairly weak magnet, but put an
    iron core into the same coil an pass the same current through it, and
    you get a much stronger magnet. The difference is the much greater
    permeability of the iron.)

    Yet another way to look at this is that permeability is a hugely
    important factor in determining inductance, which is simply the
    ability to store energy in the form of a magnetic field (and is therefore
    analogous to capacitance, which has to do with the storage of energy
    in an electric field). If the magnetic field produced by a coil of wire
    is established within a material of high permeability (again, as in the
    case of an iron or ferrite core), you see a much greater inductance.
    (And so "permeability" has much the same role in inductors as
    "permittivity" - more commonly seen in the form of the "dielectric
    constant" - has in capacitors.)

    Does any of that help?

    Bob M.
  3. Ken Williams

    Ken Williams Guest

    Thanks for your answer. can you confirm the following then?

    I always thought a b-field was magnetism and an a-field was the
    electro-field which traveled with the b-field (when applicable) but at
    right angles to it.

    so then inductance is the storage of a b-field (magnetic "work") only,
    where capacitance is the storage of an a-field (electrical "work") only,

    and so a coil when charged/induced at its max is only storing a
    "magnetic potential", no electrical potential will exist at this time.
    when the magnetic field collapses back through the coil then a larger
    a-field will then be realized. right?

    also, what is an h-field in common sense terms?

  4. Jamie

    Jamie Guest

    Reluctance maybe what your looking for.
  5. Bob Myers

    Bob Myers Guest

    There is no field I've seen commonly referred to as the "a" field. The
    "magnetic" fields are commonly referred to as the "B" and "H"
    fields (again, related by the permeability of the material in which
    the fields are established), while the "electric" fields are the "E"
    and "D" (which are similarly related by the permittivity of the
    material). The classic "electric" and "magnetic" fields are the
    E and B, respectively, while the D and H are related to these
    but you don't really need to worry about them right now. If
    you get into a course on electromagnetic waves, etc., then you're
    going to get into the differences, but not until then.
    More correctly, "inductance" is that property of a component
    or conductor whereby *energy* is stored in the form of a magnetic
    field, and "capacitance" is that property whereby energy is stored
    in an electric field.
    I'm not sure what you're trying to say here, exactly, but let me
    take a whack at what I THINK it might be.

    If you're running a direct current through an ideal coil (no
    resistance), then a magnetic field is created by the coil and
    there is a certain amount of energy in that field. There is no
    voltage drop ("electric potential") across the coil in this case,
    just a current through it. If, however, you attempt to change
    the current (including trying to shut it off), then energy comes
    back out of the field and creates a potential (voltage) across
    the coil in such a way as to oppose the change. (If, for instance,
    you try to open a switch which is carrying a sizable current through
    a large inductor, you can get a HUGE voltage and the switch
    will "arc over" as the inductor "tries" to maintain the current
    through it.)

    This is where things get a bit complex. To a physicist, the
    B-field is the "magnetic field," while the H-field is the
    *magnetizing* field; from an engineer, you'd commonly
    hear these called the "magnetic flux density" and the
    "magnetic field strength," respectively. About the best way
    I can show a "common sense" difference is to go back to
    an example I used in the last post - if you have an air-core
    coil you're trying to use as an electromagnet, you will find that
    that putting an iron core in increases the magnetic strength
    significantly. The B-field is the same in both cases, but the H-field
    has gone up with the iron core (due to the increased permeability).

    Bob M.
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