[SI-LIST] Re: Current Flow

  • From: "Cuchulain" <jlipsius@xxxxxxxxxxx>
  • To: <dp@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Sun, 7 Aug 2005 21:07:19 -0700

I meant to say ...

"but I'm not totally sure..."


----- Original Message ----- 
From: "Cuchulain" <jlipsius@xxxxxxxxxxx>
To: <dp@xxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Sent: Sunday, August 07, 2005 8:41 PM
Subject: [SI-LIST] Re: Current Flow


> I think it's about a transgalactic teletransporter prototype and supporting 
> instruments to measure charge flux
in any volume of the
> galaxy... but don't I'm totally sure
>   :-)
>
>
> ----- Original Message ----- 
> From: "Dimiter Popoff" <dp@xxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Sent: Friday, August 05, 2005 4:11 PM
> Subject: [SI-LIST] Re: Current Flow
>
>
> > Is'nt the physics about electricity taught in high school?
> > Electric field spreads at the speed of light, causing electron
> > motion, which we call current flow, which is responsible for
> > magnetic field, which while changing creates electric field etc. etc. ???
> >  Unless the thread is about teaching kids <16 how to design
> > boards, I don't quite get what this is about.
> >
> > Dimiter
> >
> > ------------------------------------------------------
> > Dimiter Popoff               Transgalactic Instruments
> >
> > http://www.tgi-sci.com
> > ------------------------------------------------------
> >
> >
> >
> >
> > -------Original Message-------
> > > From: steve weir <weirsi@xxxxxxxxxx>
> > > Subject: [SI-LIST] Re: Current Flow
> > > Sent: Aug 06 '05 01:46
> > >
> > >  Doug, as long as we take the view of electrons on one side of the path
> > >  moving at some finite velocity and as a consequence causing other 
> > > electrons
> > >  to later move along the return path, we are at an impasse with
> > >  reality. There is no such delay between signal and return. Both sides
> > >  propagate in unison because they are both the observable result of the
> > >  propagating wave front. Signal <=> Return. Return <=> Signal.
> > >
> > >  Regards,
> > >
> > >
> > >  Steve.
> > >
> > >  At 03:05 PM 8/5/2005 -0700, Doug Brooks wrote:
> > >  >At 02:18 PM 8/5/2005, you wrote:
> > >  >>Doug,
> > >  >>
> > >  >>In the fluid model, we would see current propagate down the signal
> > >  >>conductor and then later back in the return lead would we not?
> > >  >
> > >  >That's why I said I didn't think "fluid" was the best descriptor. As I
> > >  >tried to point out the first time, electrons start flowing down the 
> > > line,
> > >  >onto the "plates" of the distributed capacitance, repelling electrons
> > >  >(like charges repel) from the other "plates" of the distributed
> > >  >capacitance, and back, completing the loop. As the first "plates" charge
> > >  >up, the current flows past them and charges the next "plates". By the 
> > > time
> > >  >the current gets to the end if the line, all the "plates" are charged 
> > > up,
> > >  >and the flow looks like a DC flow would look. This is exactly what 
> > > Figure
> > >  >7-19 in Bogatin's book is describing. You describe this from the
> > >  >standpoint of "waves". I can equally well (no better, no worse) describe
> > >  >it as electron flow. I don't see a difference and I don't see a problem.
> > >  >
> > >  >The "fluid flow" model breaks down because we can't envision fluid
> > >  >crossing between the plates of a capacitor. But electron flow CAN cross
> > >  >the plates of a capacitor because of the property that "like charges 
> > > repel
> > >  >each other." Electrons don't physically cross the space between the
> > >  >plates, but they build up on one side and repel those on the other, so
> > >  >that the same number of electrons return to the source as left it.
> > >  >
> > >  >Doug
> > >  >
> > >  >
> > >  >
> > >  >>But in real life, we observe that current propagates in one polarity 
> > > from
> > >  >>the signal conductor portion of the wave guide, and simultaneously in 
> > > the
> > >  >>opposite polarity from the return conductor side of the wave guide. The
> > >  >>fluid flow model has problems both with time, and with the fact that 
> > > the
> > >  >>wave propagates down an infinitely long open transmission line just as
> > >  >>well as it does an end terminated line. In the open, or infinite length
> > >  >>line electrons never passed from one conductor to the other.
> > >  >>
> > >  >>How does a circular fluid flow analogy model this behavior? At the far
> > >  >>end of an open transmission line the conduction path is broken, the 
> > > fluid
> > >  >>has no contiguous path.
> > >  >>
> > >  >>We can agree that electrons in the conductors move in response to the
> > >  >>propagating fields, sic wave. But I have to reiterate that back at our
> > >  >>switch it is the fields interacting with the conductors that push on
> > >  >>those electrons you observe moving in the conductors. When the dv/dt
> > >  >>switches direction later in time, the charge will go the other way in
> > >  >>each conductor, but as far as charge between the two conductors: never
> > >  >>the twain shall meet.
> > >  >>
> > >  >>Regards,
> > >  >>
> > >  >>
> > >  >>Steve.
> > >  >>
> > >  >>At 02:12 PM 8/5/2005 -0700, Doug Brooks wrote:
> > >  >>>(I have changed the subject line to better represent what I think we 
> > > are
> > >  >>>talking about.)
> > >  >>>
> > >  >>>You raise an excellent example. Let me deal with the two points.
> > >  >>>
> > >  >>>1. I'm not sure I understand what you are getting at here. The focus
> > >  >>>should be at the point of the switch.
> > >  >>>
> > >  >>>2. I have introduced the problem in some of my transmission line 
> > > classes
> > >  >>>that deal with point 2. Assume that (a) there is a propagation time 
> > > for
> > >  >>>a signal, (b) current (i.e. electrons) flows in a closed loop, (c)
> > >  >>>current is constant everywhere in that loop ---- aren't these mutually
> > >  >>>exclusive conditions? The answer is no! The current flows down the
> > >  >>>transmission line from one side to the other through the distributed
> > >  >>>capacitance (as suggested in Bogatin's Figure 7-19). This is a current
> > >  >>>flow (i.e. electron flow) picture. If you want to call it a wave flow,
> > >  >>>well that's fine. But you can also describe it as current (electrons)
> > >  >>>flowing to the point of the distributed capacitance, repelling charge
> > >  >>>away from the other side of the capacitance back to the beginning of 
> > > the
> > >  >>>line, charging the capacitance up (with electrons) along the way. At 
> > > the
> > >  >>>steady state, current (electrons) is flowing in the DC loop we would
> > >  >>>expect. If we don't have a transmission line ---- well, we always have
> > >  >>>a transmission line of sorts. The question is whether it's ideal or 
> > > REAL
> > >  >>>crummy. There is always a characteristic impedance, even if it is only
> > >  >>>that of air.
> > >  >>>
> > >  >>>So the "fluid" analogy (I don't think that's the best descriptor) can
> > >  >>>deal with this issue perfectly fine. Likewise, it can deal with the
> > >  >>>crosstalk coupling issue equally as well. (I don't have a figure like
> > >  >>>Eric's in my book, but there is a very detailed illustration of how
> > >  >>>crosstalk coupling works in my book that doesn't need Maxwell and wave
> > >  >>>theory to understand.)
> > >  >>>
> > >  >>>So I don't see the difficulty here.
> > >  >>>
> > >  >>>Doug
> > >  >>>
> > >  >>>
> > >  >>>
> > >  >>>
> > >  >>>
> > >  >>>At 01:18 PM 8/5/2005, steve weir wrote:
> > >  >>>>Doug, in the fluid model, there are two misleading elements:
> > >  >>>>
> > >  >>>>1. The focus is on the source of EMF, sic the battery,
> > >  >>>>2. It implies a time lag between the foward current starting from 
> > > some
> > >  >>>>point and the matching return current closing that path.
> > >  >>>>
> > >  >>>>If we take the switch example you offered, one might imagine a couple
> > >  >>>>of different cases:
> > >  >>>>
> > >  >>>>a. The switch is located very close to one terminal of the battery 
> > > and
> > >  >>>>a say 300m wire connects it to the other through some load resistor.
> > >  >>>>b. The switch is located at the end of two 300m wires back to the
> > >  >>>>battery through some load resistor.
> > >  >>>>
> > >  >>>>Now, what will each the fluid analogy, and wave propagation tell us
> > >  >>>>about each case? Where does each model show the propagation beginning
> > >  >>>>and ending? How accurate is each? Which model can explain behavior
> > >  >>>>from virtual DC to any frequency we like? I don't think it's the 
> > > fluid
> > >  >>>>analogy.
> > >  >>>>
> > >  >>>>On a PCB with switching I/Os the time and distance scales have 
> > > changed
> > >  >>>>but not the behavior. What we have is in essence case b from
> > >  >>>>above. The wave emanates from the switches in our ICs, not from the
> > >  >>>>power supply. The wave model makes this clear, as it does the
> > >  >>>>propagation path. The wave model makes clear the critical point that
> > >  >>>>the return and forward currents propagate together. The fluid analogy
> > >  >>>>with its unidirectional emphasis fails us badly.
> > >  >>>>
> > >  >>>>Where has the fluid analogy brought us? How many times have you seen
> > >  >>>>people talk about bone-headed ideas like the PCB planes or bypass
> > >  >>>>capacitors supplying current to high speed edges, when the entire 
> > > edge
> > >  >>>>has completed long before the wave front through power pins can reach
> > >  >>>>significant charge in the planes, much less even reach the PWB bypass
> > >  >>>>caps? Yet this kind of junk mythology sadly makes its way into books
> > >  >>>>and other publications on a regular basis. I don't like it one bit.
> > >  >>>>
> > >  >>>>For my money, I find the fluid analogy terribly misleading, and a
> > >  >>>>source of much misunderstanding. One doesn't need to be able to 
> > > derive
> > >  >>>>Maxwell to understand wave propagation. I think that as Eric's book
> > >  >>>>demonstrates, most SI concepts are not that difficult to
> > >  >>>>understand. Even a dummy like me gets them from time to time.
> > >  >>>>
> > >  >>>>Regards,
> > >  >>>>
> > >  >>>>
> > >  >>>>Steve.
> > >  >>>>At 12:51 PM 8/5/2005 -0700, Doug Brooks wrote:
> > >  >>>>>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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