Electric, Magnetic Field Intensity and Density

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

  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
    Anand P. Paralkar, Apr 7, 2009
    #1
  2. Anand P. Paralkar

    Guest

    On Apr 7, 7:16 pm, Tim Wescott <> wrote:
    > Anand P. Paralkar wrote:
    > > 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) onthe
    > > 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

    >
    > A1:
    >
    > I believe that the various symbols and equations were in use before
    > James Clerk Maxwell unified the treatment of electricity and magnetism
    > -- without understanding that they were dealing with essentially the
    > same thing, Faraday and Ampere had no reason to coordinate their efforts
    > with Henry and Gauss.
    >
    > Now it's too late to change.
    >
    > A2:
    >
    > Because the intensity and flux density are different things, and as long
    > as you are interested in calculating the fields in the presence of more
    > than just a vacuum, you need to know how the permeability and
    > permitivity affect the behavior of the fields.
    >
    > --
    >
    > Tim Wescott
    > Wescott Design Serviceshttp://www.wescottdesign.com
    >
    > Do you need to implement control loops in software?
    > "Applied Control Theory for Embedded Systems" was written for you.
    > See details athttp://www.wescottdesign.com/actfes/actfes.html- Hide quoted text -
    >
    > - Show quoted text -


    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
    , Apr 8, 2009
    #2

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