Re: Displacement current-is there a more straight forward explanation anywhere?

Hello, George!

You asked: "Displacement current seems to be more of a mathematical current 
rather than the movement of electrons, q/t.Is displacement current the 
precursor for eletromagnetic radiation?"

Charges in motion give rise to the effect, but it is not the same as the 
magnetic field around a wire with a direct current flowing in it. The term 
"displacement current" was invented by Maxwell and I suppose it fit into his 
thinking patterns. Like you, it doesn't fit too well with mine.  What it 
really refers to is the creation of a magnetic field due to a changing 
electric field. This magnetic field "looks like" a current were present 
causing it (one magnetic field looking  more or less like another....) and 
so the fictional current was named. 

My old textbook (Berkeley Physics Course, vol. 2, Electricity and Magnetism) 
didn't like the term either (I quote "...the name has stuck although it no 
longer seems very appropriate."), and neither did a number of web sites. By 
the way, if this little note isn't to you liking, simply go to any good 
search engine and seach on the key phrase "displacement current" and you 
will find many, many explanations written by university professors 
trying to make the matter clear to their students. Perhaps one of their 
expositions will resonate better with you than what I have to say. Here are 
two URL's to get started:

    http://farside.ph.utexas.edu/~rfitzp/teaching/em1/lectures/node41.html

    http://scienceworld.wolfram.com/physics/DisplacementCurrent.html



Let's take the classic example of the capacitor.

   Electrons flow onto and off of the plates with alternating current, but 
none of them (assuming the capacitor doesn't leak) actually cross from one 
plate to the other. However, it is certainly true that the value of the 
electric field in the interplate gap is constantly changing. This changing 
field is certainly caused by the charges in motion on the plates (spreading 
out from and then gathering back together where the wire connects) and like 
any changing electric field, creates a magnetic field. To Maxwell, this 
behaved just as if a current were present.  So he added the term 
         
              1   dE
             --- ----
              c   dt

into his equation
                      4 (pi) J       1   dE
         Curl{B} = ------------  +  --- ----
                         c           c   dt

and gave it the name of "displacement current."

The term is extremely important.  In the absence of all charge or current, 
Maxwell's equations reduce to:
    
                    1   dE
        Curl{B} =  --- ----            Div{E} = Div{B} = 0 
                    c   dt

                     1   dB
        Curl{E} = - --- ----
                     c   dt

Note the symmetry! Note also that it allows a changing electric field to 
create a changing magnetic field which in turn makes a changing electric 
field and so on...propagating along at the speed of light. That is, 
basically, a description of the photon.