# Electric, Magnetic Field Intensity and Density

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

1. ### Anand P. ParalkarGuest

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

2. ### 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