Re: Catt anomaly?

[William Beaty <billb@ESKIMO.COM>]

On Mon, 26 Nov 2001, Brian Whatcott wrote:

> He asks effectively: when a battery is connected to one end of a delay
> line of this kind - where does the charge in the final capacitor come from?
> Though electromagnetic transmission is inherently relativistic, I expect
> one could provide a satifying description of an energy transfer between
> magnetic
>   and electric modes, without taking oneself too seriously?

Yeah, Mr. Catt makes a classic error: he seems to imagine that conductors
are like "empty pipes", and in order to transmit a pulse along the "pipe"
at high velocity, he thinks that the "water" must move just as fast as the
pulse.  Yet conductors behave as "full pipes."  Push a piston into the end
of a water-filled pipe, and the compression wave moves faster than the
water.


The stuff about infinite mass and empty pipes aside, there is one
interesting aspect to the problem.  When a DC supply is suddenly connected
to a transmission line, what is the drift velocity of the charges at the
leading edge of the pulse?  Because of the skin effect, velocity of
carriers should be very high.  As a sharp-edged pulse propagates along,
initially only the electrons very near the conductor surface will
participate in the current.  The counductor will behave as if it's cross
sectional area is greatly reduced, and so the drift velocity of charges
must be greatly increased.  (I think you'd need a pulse with picosecond
rise time in order to see this effect become significant.)



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