EMRFD Message Archive 12657

Message Date From Subject
12657 2016-04-17 07:43:05 Chris Howard w0ep magnetic lines of force
I find the idea of a magnetic field to be very interesting.

Electric is one thing, the movement of electrons in a conductor.
That makes sense. Magnetic is like magic.

In one of the older books I have, I think Terman, it talks
about inductance in terms of the number of "lines" of magnetic
field. I don't think I have a grasp on that. Are the lines
like some kind of standing waves?

(I may have asked about this before. I come back to this
subject periodically, it seems really mysterious to me)
12658 2016-04-17 08:07:57 Alberto I2PHD Re: magnetic lines of force
12659 2016-04-18 07:38:38 Bob Re: magnetic lines of force
Magnetism is a direct consequence of moving charge together with the special relativistic length contraction.  In this reference: http://scitation.aip.org/content/aapt/journal/tpt/45/3/10.1119/1.2709673  the magnitude of the magnetic field is calculated from moving charges and special relativity.  The article is written at a level that can taught/understood in a college freshman level course, or earlier if you have a general understanding of special relativity.  

The beauty of this approach is that the magical nature of magnetism is removed (although replaced by another magical property: special relativity).  It is interesting and surprising (at least to me) that magnetism does not require electrons moving at relativistic speeds (ie: fractions of the speed of light) but is actually a consequence of the "drift rate" of electrons in a wire (on the order of cm/s).  Unfortunately, the article referenced is behind a firewall.  Perhaps a colleague or library might have access; it is well worth the read.

12660 2016-04-18 08:19:03 Andy Talbot Re: magnetic lines of force
I have also always felt uneasy referring to "lines" of magnetic flux.    Surely there are an infinite number of them.

But as Alberto says, they are just a convention
12661 2016-04-18 09:07:28 Russell Shaw Re: magnetic lines of force
12662 2016-04-18 13:25:15 Alberto I2PHD Re: magnetic lines of force
12663 2016-04-18 13:59:48 aa0zz Re: magnetic lines of force
As with gravity and many other mysteries in our universe, I think we know more about what magnetic forces "do" than what they "are".  
-Craig, AA0ZZ
12669 2016-04-19 05:53:09 Bob Re: magnetic lines of force
Thanks! That is the article!  
12670 2016-04-19 06:01:33 Bob Re: magnetic lines of force
RE: Changing approach to teaching electromagnetism in a conceptually oriented 
introductory physics course

I have not read this one yet, but it looks like the same approach.  Frankly, I find this approach much more appealing than referring to magnetism as a magical additional force.  This approach is concordant with the search for a GUT (grand unifying theory).
12671 2016-04-19 06:06:56 Bob Re: magnetic lines of force
Hi Craig,

I think that general relativity gives as a much better understanding (at least geometrically) to gravity than our current electromagnetic view.  The papers cited at least allow the electric and magnetic fields to reconciled geometrically.  

The topic that continues to confound me is the rotating field of a polarized electromagnetic wave.  I have a rote understanding of the one field lagging behind the other, but especially when we consider that the magnetic field is a fictitious field generated via the special relativistic contraction ("Fitzgerald Contraction"), I find it difficult to image how this could relate to a circularly polarized EM field.  This is of course relevant to our understand of L and R handed polarization using helical antennas.