Re: Speed of electricity

jimp_at_specsol-spam-sux.com
Date: 12/22/04


Date: Wed, 22 Dec 2004 02:39:57 +0000 (UTC)

Franz Heymann <notfranz.heymann@btopenworld.com> wrote:

> "Edward Green" <spamspamspam3@netzero.com> wrote in message
> news:1103510196.242623.290190@f14g2000cwb.googlegroups.com...
> > Franz Heymann wrote:
> >
> > > > If you
> > > > had a simple circuit (ie a wire going from a terminal of a
> battery
> > > to a
> > > > light bulb to the other terminal), and lets say a mile of wire
> in
> > > > between the bulb and each terminal, how long would it take for
> the
> > > bulb
> > > > to light after completing the circuit?
> > >
> > > If your connection consists of a parallel wire transmission line
> > > suspended in air all the way, the speed of propagation of any
> change
> > > in PD at the input terminals would be very slightly less than c,
> the
> > > speed of light in vacuo.
> >
> > Just what would be the result of the thought experiment? I'm really
> > not sure.

> I am not talking about a thought experiment.
> Read up all about Lecher Lines. That is what we used to use in my
> youth for investigating the standing wave ratio in RF transmission
> systems.

> > Wouldn't there be effects related to the inductance and
> > capacitance of one mile of wire?

> The L and C per unit length of a cable are precisely what, in circuit
> langyuage, determines the propagation speed. Lo and behold, things
> conspire to make that precisely c for uniform cable, whether it is of
> the parallel line or the coaxial type. Maxwell's equations look
> after their own. {:-))

In practical terms, it is the insulator that determines the velocity
factor of cable, be it parallel line, coax, or anything else.

Open wire parallel lines that are mostly air with periodic bits of insulation
to maintain spacing have velocity factors in the range of 0.95 to 0.97.

Most coax has an insulator of either polyethylene, foamed polyethylene,
or Teflon with velocity factors of 0.66, 0.8, and 0.7 respectively.

There are air spaced coaxes where the insulator is plyethylene in periodic
lumps as opposed to being continuous down the line that have velocity factors
in the range of around 0.85.

> > Would it make any different that you
> > were trying to establish a DC current of 1 amp (say) rather than
> simply
> > sending a pulse down the wire and detecting it at the other end?

> When a switch is closed so as to establish a sudden pd (thereafter
> kept steady) at the input end, this front will propagate at a sped c
> >
> > Come to think of it, as a practical matter, even after the
> transients
> > had decayed

> Now that is handwaving. What transients are you talking about? It
> is precisely the transients (aka time-dependent pd's) which propagate
> at a speed c in a uniform cable

> -- however long that took -- I doubt your bulb would light
> > at all, because almost all the IR drop would be across the wire.

> Not if the bulb is a neon lamp. Once again, read up about Lecher
> lines.
> Radio hams use them (and the neon lamps) routinely when doing
> impedance matching in the transmission systems connecting their
> transmitters to their antennae.
> > So
> > the answer is likely "forever". _Some_ intelligence of the throw of
> > the knife switch would reach the other end on a time the order of
> D/c,
> > but I don't think the bulb would light. Am I wrong?

> Yes, unfortunately. {:-((

> Franz

You must be an ancient fart like me 'cause I haven't heard of anyone
using lecher lines to measure SWR in about 30 years.

-- 
Jim Pennino
Remove -spam-sux to reply.


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