Re: displacement current

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

On Sat, 21 Feb 1998, Mark Shapiro wrote:

>       I also point out that the magnetic field generated by the
> "displacement current" between the plates of the capacitor is different in
> an important way from the magnetic fields that they have dealt with
> before.  It is easy to show that it has a 1/r spatial dependence, so it is
> a radiation field.  Previously, they have seen that a localized,
> time-varying electric field produces an electric field with a 1/r spatial
> dependence.  This paves the way for the discussion of e.m. waves.

Mark,

I can't make sense of this; what am I missing?  Doesn't the first 
magnetic field most students see--that of a long straight current 
carrying wire--have a "1/r" dependence?  Doesn't the magnetic field 
between the plates of a "charging" (sorry Bill) parallel circular 
plate capacitor (as if such things were actually used) in fact have 
an "r" dependence? And why should "1/r" necessarily imply a radiation 
field anyway?

John
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