# 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

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  