Electronics Forums > Electric, Magnetic Field Intensity and Density

 Thread Tools Display Modes

# Electric, Magnetic Field Intensity and Density

Anand P. Paralkar
Guest
Posts: n/a

 04-07-2009, 09:22 PM
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

ggherold@gmail.com
Guest
Posts: n/a

 04-08-2009, 03:02 PM
On Apr 7, 7:16*pm, Tim Wescott <(E-Mail Removed)> 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

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Anand P. Paralkar Electronic Design 1 04-08-2009 03:02 PM Markus Mandl Electronic Components 14 11-08-2004 09:53 AM Markus Mandl Electronic Design 14 11-08-2004 09:53 AM