From: Mike LaBonte ([email protected])
Date: Thu Mar 15 2001 - 06:20:57 PST
In school I did have one professor who explained the difference between
the slow electron drift rate and the fast signal propagation rate. The
simple explanation he used: Imagine a large number of electrons are lined
up in a queue to enter a cinema. It will be quite a while before the last
electron in line reaches the cinema. But it can shove the electron in front
of it, which will shove the electron in front of it, and pretty soon the
presence of that last electron will be known at the front of the line.
Of course electrons "shove" by means of nuclear force. But the point is
that a particular electron does not have to arrive at the destination to
convey a signal.
Mike LaBonte
[email protected] wrote:
>
> 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
>
> Please respond to [email protected]@INTERNET
>
> 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.
>
> Expecting your comments.
>
> regards,
>
> Selva
>
> ------------------------------------------------------------------------
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