Electric, Magnetic Field Intensity and Density

Apr 07, 2009 2 Replies
A

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.



  1. 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



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T

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 http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
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G

e

dium.

c

the

he same

d 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

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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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