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Electric, Magnetic Field Intensity and Density

Discussion in 'Electronic Basics' started by Anand P. Paralkar, Apr 7, 2009.

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  1. Hi,

    I am a reading a book on electromagnetism and I read the following
    relations:

    1. For electric intensity and field density:

    The flux density D is *independant* of the medium whereas the
    intensity E is *dependant* on the medium. This is
    captured by the equation:

    D = eE; where e is the permittivity of the medium.

    2. For magnetic intensity and field density:

    The flux density B is *dependant* on the medium whereas the intensity
    H is *independant* of the medium. This is
    captured by the equation:

    B = uH; where u is the magnetic permeability of the medium.

    I would like to know:

    Q1. Why this difference in approach between the electric and magnetic
    fields. Why can't we have
    both the intensties to be independant (or dependant) on the medium
    or vice-versa?

    Q2. Why have two distinct notions (for both electric and magnetic field):
    1. Intensity vector and, 2. Flux density
    vector? (Especially when both the notions indicate the same
    thing - the intensity of the field at a point.)

    Thanks,
    Anand
     
  2. Guest

    Indeed it wasn't until recently (1960's?) that we came to understand
    that it is the B field that is fundamental and not the H field

    http://en.wikipedia.org/wiki/Magnetic_field#cite_note-0

    Thanks to Purcell (and Feynman?) was were teaching the intro physics
    course.

    The Feynman Lectures on Physics (volume 2.) does a nice job of making
    this distinction. Bottom line is that D and H are sometimes useful
    for 'book keeping' but the physics is in E and P, and B and M.

    George Herold
     
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