[SI-LIST] Re: Return Path
- From: Doug Brooks <doug@xxxxxxxxxx>
- To: weirsi@xxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Fri, 05 Aug 2005 12:51:47 -0700
A couple of people have interpreted my statement re "flow" of electrons as
meaning electron drift. Let's kill that right now.
One electron in = one electron out is the flow of electrons. One electron
in = SAME electron out is electron drift --- not at all the same thing.
Certainly I don't argue against Maxwell's equations. But I don't argue
against the fundamental definition of one amp of current either --- the
flow of one coulomb of charge (6.25 x 10^18 electrons) across a surface in
one second. I spend a lot of time with engineers (and technicians) who
never took Maxwell's equations and didn't understand them if they did. My
goal has been to take our difficult SI concepts and explain them in terms
that these "poor" people can understand. To suggest that you can't explain
what happens during planar transitions without Maxwell's equations (I
believe) is simply wrong. To say that the classical description of current
can't explain the difference between DC and high frequency is also (I
believe) flat wrong. To say that one description is "more accurate" than
the other --- well I suggest that depends a lot on whose working with them!
And while people have been misled by seminar leaders teaching without the
benefit of Maxwell's equations, we all know seminar leaders whose ability
to mislead wasn't one bit hampered by a thorough knowledge of Maxwell's
equations!
Don't sell these more basic principles short when it comes to understanding
what is happening on circuit boards. They can very effectively explain what
is happening, and why one design approach may be more effective than
another depending on the important design considerations. Especially for
all those board designers who have no knowledge of Maxwell and wave theory.
Doug
At 12:01 PM 8/5/2005, steve weir wrote:
>Doug, well I am going to argue vehemently that until someone repeals
>Maxwell that the wave description is fundamentally more accurate than the
>fluid analogy. The E/M fields cause the electron drift in those
>wires. From the time you closed the switch the changing E/M field that
>resulted propagated outward. Marconi found a useful purpose for that
>phenomenon.
>
>The fluid analogy is certainly easy to understand, but what is the point
>when it is so misleading? I can't tell you how many times otherwise
>intelligent engineers that I have known have been thrown off understanding
>PCB wave guides, because they were intent on following the DC current loop
>of the fluid analogy.
>
>Teaching the fluid analogy requires that we later break that teaching when
>we want to explain what happens at significant frequencies. Consider for
>instance visualization of return current ( which is the original subject
>matter ) when we transition planes in a PCB. If we think about it as a
>fluid model we are easily misled into searching out a conduction path. For
>ready examples of this mass confusion, just look at some of the discussions
>on splitting-up grounds in the wrong ways for the wrong reasons, with the
>wrong results. But if we simply consider waves to begin with, then the
>behavior is easy enough to intuit out.
>
>Eric does a very nice job in his book explaining signal propagation that
>does not rely on the fluid analogy. I think his approach is very accessible.
>
>Regards,
>
>
>Steve
>At 11:21 AM 8/5/2005 -0700, Doug Brooks wrote:
> >With all due respect, Steve, if I have a battery connected to a transistor
> >through a switch, I can turn the transistor "on" and "off" with the
> >switch. That is easy to explain using the electron flow concept (which I
> >hesitate to call an analogy, it in fact describes the physics involved).
> >
> >Is your description more complete AND also easier to understand?
> >
> >And if it is the frequency with which I "flip" the switch that bothers
> >you, that simply means that some of the parameters that were not an issue
> >with slow "flipping" (inductance and capacitance, for example) start
> >becoming more of an issue with faster "flipping!" But the basic nature of
> >what is happening (in particular where the electrons are flowing) is not
> >changing, just speeding up. (How the electrons are flowing is speeding up,
> >the electrons themselves, of course, don't change speed!)
> >
> >Doug
> >
> >
> >
> >At 10:45 AM 8/5/2005, steve weir wrote:
> >>Doug, I have some real heartburn with some of those representations,
> >>particularly the fluid analogy that speaks of current as the flow of
> >>electrons. When I grew up current was defined as time variation of
> >>electric flux. When an E/M field impinges a chunk of metal the resulting
> >>interaction concentrates the field forming a wave guide. All practical
> >>wave guides leak, be they a microstrip over a plane, a stripline, or
> >>whatever. Some, like a good semirigid coax leak only a little tiny
> >>bit. When they leak too much creating excessive disturbance in nearby wave
> >>guides, we have cross talk problems. I hope that this is what you were
> >>trying to convey.
> >>
> >>Regards,
> >>
> >>
> >>Steve.
> >>
> >>A
> >
> >Check out UltraCAD's new presentation videos and new skin effect
> >calculator at http://www.ultracad.com
> >
>
>
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