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Tuesday, February 4, 2014

Inductive Reactance and Special Relativity


I'd been meaning to share these stories with you after I read your book a couple years ago but I never got to it.  I thought you might enjoy them, from an "engineering perspective", I guess.

One of the courses I had to take for my undergrad was an engineering physics type class.  I loved it.  I think a lot of hams seem to have more curiosity about the physics of electronics than regular non-ham engineers, at least that's how it's always seemed to me.  Anyway, I'm sending you a snapshot of the relativistic length contraction figure in the book "Concepts of Modern Physics", 4th Ed by Arthur Beiser.  I thought you'd enjoy it as it is almost identical to what you mentioned in Soldersmoke (from your "Atoms to Amperes" book I think).

Hopefully there's enough resolution there to make it out.  Basically, when you flow current in the same direction in both wires, they attract.  That's because the electrons see effectively many more positively charged nuclei from the other wire than they do other electrons due to the nuclei distances being compressed by the Lorentz-Fitzgerald contraction (later refined by Einstein). 

When I first saw this, in my early 20s, I was completely floored!  Nowhere had I ever learned anything like this from the ham license manuals or even my basic physics course.  The implications were also very profound -- magnetism was nothing more than electrostatic attraction, the attraction between charges.  The "electromagnetic" force was really just an electric force.  Relative motion between charges gives the illusion of "magnetism". 

Much later, I listened to some of the old Feynmann lectures.  In them at one point he adamantly proclaimed that there's only the electric force between charges, and there is no magnetic force!  I still find this confusing.  Recently I brought this up to a university RF engineering professor.  I wondered why we dealt with Maxwell's equations when in reality the magnetic field is an illusion.  The "real" formulas come from Feynmann's theory of quantum electrodynamics!  His reply was something along the lines of Maxwell's equations being a solution of quantum theory that worked well for our purposes.  To be honest, I didn't really understand his reply and I'm still skeptical!  I think his point was that the QED calculations are overly complicated and unnecessary for most problems we deal with, things like patterns from an antenna.  I don't think Maxwell's equations appropriately describe things like lasers though, which are more quantum in nature with the coherent beam.

FYI, most engineering students I ran across had only passing curiosity for these things.  Only in graduate school did I start to find people curious enough to really try to understand "what lies beneath" some of this stuff, mainly this physics.  Honestly not even everyone in grad school was all that captivated.  As you've said before, there's a lot of "turn the crank" mentality in engineering where you wade through mathematics to get answers, not always thinking about the physics.  It's even worse in the digital world, where everything gets boiled down to computer code! 

One more quick thing.  I talked to a physics prof once, asking him if there was any research happening in his department
focused on electromagnetics and radio waves, etc.  His reply: "radio waves are nothing more than the result of accelerating electrons".  Period!  Discussion over.  In other words, that's ancient history.  Engineers are still very much involved with new technologies involving antennas and amplifiers, etc.  But as far as the physicists are concerned, I get the impression that our whole field is pretty ho-hum.  But he was right about accelerating electrons, I also found out later.  And it doesn't have to be electrons.  Anything carrying charge undergoing acceleration will emit photons.  That's another crazy situation that I only more recently learned.

Hope that was entertaining!

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  1. Interesting stuff, Bill. However, if "magnetism" is an illusion, how do magnetic loop antennas work? Supposedly they are much lower noise than other types of antennas, ostensibly because the "magnetic" half of electromagnetism is inherently free of atmospheric electrical noise.

    Dave, KF7JAF

  2. A lot of food for thought there - including current and attraction. I'll re-read it and ponder.

    Having got a problem with a n-channel mosfet at the moment I'm wondering where the magic smoke comes into physics. HI HI HI


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