# Electric, Magnetic Field Intensity and Density

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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

Re: Electric, Magnetic Field Intensity and Density

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 Services
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Re: Electric, Magnetic Field Intensity and Density

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