# Re: [SI-LIST] : Speed of electron & Speed of signal on the trace

From: [email protected]
Date: Thu Mar 15 2001 - 00:43:11 PST

Selva,

this is a common question, which is not taught at school.

The answer is simple: a generator excites an electromagnetic wave at the source
side of the transmission line. The electromagnetic
waves follow the wires of the line and propagate at the speed of light in the
medium. However, there is more! As the electromagnetic
waves propagate along the wires of the line (or lines) the electromagnetic waves
are scattered by the wires. The wires are good conductors
and they will react to the propagating fields and currents/charges will be
set-up on (and in) the surface of the conductors. The reason for
this is that the boundary condition at conductors need to be satisfied: the
tangential electric field at the conductors is zero (for perfect
conductors - PECs). So the incoming or incident field produces a scattered
field, which is generated by surface currents/charges in order
to fulfil the boundary conditions. This process continues indefinitely:
propagate-scatter-propagate- ....
So the overall picture is this: as the electromagnetic fields propagate along
the transmission line they build up currents at the speed of
which they propagate along the line, it is not that the currents speed along the
line at high speeds! Actually the speed of DC current is, as
you quote, a fraction of cm/s or even mm/s. The speed of current depends on the
how the charges propagate through the lattice of the
conductor and this is rather slow!

The misconception here is the electrical model of the transmission line: it uses
capacitance and inductance to represent the wave
propagation. However, the only reason that this works is that the LC circuit
model is similar in appearance to the electromagnetic wave
model, that is, the differential equations of the LC model are similar to the
differential equations of the electromagnetic wave propagation.
Unfortunately, this link is not explained in most courses on electromagnetic
wave propagation.

I have a nice analogy (thanks to prof. Catrysse of the KHBO Oostende). The
electromagnetic fields are like sound, the transmission line
is like a pipe in which the sound waves travel. The currents are like vibrations
of the pipe wall. The sound waves travel in the pipe and
make the pipe wall jiggle as they pass.

Hope this helps.

Regards,

Jan Vercammen
Agfa-Gevaert NV

[email protected]@[email protected] on 03/15/2001 06:02:56 AM

Sent by: [email protected]

To: [email protected]@INTERNET
cc:
Subject: [SI-LIST] : Speed of electron & Speed of signal on the trace
Hello,

I read one of the articles by Dr.Eric Bogatin of Bogatin enterprises.

He has mentioned that the signal moves down the trace/line at the speed of light
in the dielectric medium typically about

6 inches/nsec?

But there is a question why it moves so fast and is not the speed of the
electrons which is closer to 1cm/sec.

regards,

Selva

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