[SI-LIST] Re: Current Flow
- From: "John Thomas" <jthomas@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 9 Aug 2005 08:44:41 -0500
We definitely didn't have this stuff in the high school I went to. We
had Physics I Light (non-calculus) and that was as far as we went there.
What I've seen in college is a different matter. I think most colleges
present the material; it's just a matter of how much the student
can/will absorb. I've had discussions on electromagnetics, signal
propogation and other subjects with students who took the material in
the same class I did and they can't recall it. I've also picked up
things I missed/forgot in similar conversations. =20
I think one instructor explained it well to me with this: most learning
occurs in the first and last 15 minutes of the course; everything in
between is mostly wasted time.
John
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Christopher.Jakubiec@xxxxxxxxxxxx
Sent: Tuesday, August 09, 2005 8:21 AM
To: dp@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Current Flow
Unfortunately a lot of high schools do not teach the physics of =3D
electricity in this manner. Even worse is that many universities in =3D
this country tend to dumb down (simplify) the physics of electricity and
=3D current flow so that the student only takes away a rudimentary
knowledge =3D of the subject, and not a real understanding.
That's just the way it is!
Chris
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
=3D On Behalf Of Dimiter Popoff
Sent: Friday, August 05, 2005 7:12 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Current Flow
Is'nt the physics about electricity taught in high school? Electric =3D
field spreads at the speed of light, causing electron motion, which we =
=3D
call current flow, which is responsible for magnetic field, which while
=3D changing creates electric field etc. etc. ??? Unless the thread is =
=3D
about teaching kids <16 how to design boards, I don't quite get what =3D
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
>=3D20
> Doug, as long as we take the view of electrons on one side of =
the=3D20
>path moving at some finite velocity and as a consequence causing=3D20 =
>other electrons to later move along the return path, we are at an=3D20 =
=20
>impasse with reality.=3DA0=3DA0There is no such delay between signal=20
>and=3D20 return.=3DA0=3DA0Both sides propagate in unison because they =
are=20
>both the =3D
> observable result of the propagating wave front.=3DA0=3DA0Signal =
<=3D3D> =3D
Return.=3DA0=3DA0
> Return <=3D3D> Signal.
> =3D20
> Regards,
> =3D20
> =3D20
> Steve.
> =3D20
> 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
> =3D
> >>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
> =3D
I
> >tried to point out the first time, electrons start flowing down the=20
> =3D
line,
> >onto the "plates" of the distributed capacitance, repelling =3D
electrons
> >(like charges repel) from the other "plates" of the distributed =20
> >capacitance, and back, completing the loop. As the first "plates" =3D
charge
> >up, the current flows past them and charges the next "plates". By =
=3D
the time
> >the current gets to the end if the line, all the "plates" are =3D
charged up,
> >and the flow looks like a DC flow would look. This is exactly what =
=3D
Figure
> >7-19 in Bogatin's book is describing. You describe =
this=3DA0=3DA0from =3D
the
> >standpoint of "waves". I can equally well (no better, no worse) =3D
describe
> >it as electron flow. I don't see a difference and I don't see a =3D
problem.
> >
> >The "fluid flow" model breaks down because we can't envision fluid =20
> >crossing between the plates of a capacitor. But electron flow CAN =3D
cross
> >the plates of a capacitor because of the property that "like charges
> =3D
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, =
=3D
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 =3D
polarity from
> >>the signal conductor portion of the wave guide, and simultaneously=20
> =3D
in the
> >>opposite polarity from the return conductor side of the wave =3D
guide.=3DA0=3DA0The
> >>fluid flow model has problems both with time, and with the fact =3D
that the
> >>wave propagates down an infinitely long open transmission line just
> =3D
as
> >>well as it does an end terminated line.=3DA0=3DA0In the open, or =
=3D
infinite length
> >>line electrons never passed from one conductor to the other.
> >>
> >>How does a circular fluid flow analogy model this =
behavior?=3DA0=3DA0At
> =3D
the far
> >>end of an open transmission line the conduction path is broken, the
> =3D
fluid
> >>has no contiguous path.
> >>
> >>We can agree that electrons in the conductors move in response to =
=3D
the
> >>propagating fields, sic wave.=3DA0=3DA0But I have to reiterate that =
=3D
back at our
> >>switch it is the fields interacting with the conductors that push =
=3D
on
> >>those electrons you observe moving in the conductors.=3DA0=3DA0When =
the
> =3D
dv/dt
> >>switches direction later in time, the charge will go the other way=20
> =3D
in
> >>each conductor, but as far as charge between the two =3D
conductors:=3DA0=3DA0never
> >>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=20
> =3D
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 =3D
focus
> >>>should be at the point of the switch.
> >>>
> >>>2. I have introduced the problem in some of my transmission line =
=3D
classes
> >>>that deal with point 2. Assume that (a) there is a propagation =3D
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 =3D
mutually
> >>>exclusive conditions? The answer is no! The current flows down the
> >>>transmission line from one side to the other through the =3D
distributed
> >>>capacitance (as suggested in Bogatin's Figure 7-19). This is a =3D
current
> >>>flow (i.e. electron flow) picture. If you want to call it a wave =
=3D
flow,
> >>>well that's fine. But you can also describe it as current =3D
(electrons)
> >>>flowing to the point of the distributed capacitance, repelling =3D
charge
> >>>away from the other side of the capacitance back to the beginning=20
> =3D
of the
> >>>line, charging the capacitance up (with electrons) along the way.=20
> =3D
At the
> >>>steady state, current (electrons) is flowing in the DC loop we =3D
would
> >>>expect.=3DA0=3DA0If we don't have a transmission line ---- well, =
we =3D
always have
> >>>a transmission line of sorts. The question is whether it's ideal =
=3D
or REAL
> >>>crummy. There is always a characteristic impedance, even if it is=20
> =3D
only
> >>>that of air.
> >>>
> >>>So the "fluid" analogy (I don't think that's the best descriptor)=20
> =3D
can
> >>>deal with this issue perfectly fine. Likewise, it can deal with =
=3D
the
> >>>crosstalk coupling issue equally as well. (I don't have a figure =
=3D
like
> >>>Eric's in my book, but there is a very detailed illustration of =
=3D
how
> >>>crosstalk coupling works in my book that doesn't need Maxwell and=20
> =3D
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 =3D
some
> >>>>point and the matching return current closing that path.
> >>>>
> >>>>If we take the switch example you offered, one might imagine a =
=3D
couple
> >>>>of different cases:
> >>>>
> >>>>a. The switch is located very close to one terminal of the =3D
battery and
> >>>>a say 300m wire connects it to the other through some load =3D
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=20
> =3D
us
> >>>>about each case?=3DA0=3DA0Where does each model show the =
propagation=20
> =3D
beginning
> >>>>and ending?=3DA0=3DA0How accurate is each?=3DA0=3DA0Which model =
can =3D
explain behavior
> >>>>from virtual DC to any frequency we like?=3DA0=3DA0I don't think =
it's
> =3D
the fluid
> >>>>analogy.
> >>>>
> >>>>On a PCB with switching I/Os the time and distance scales have =
=3D
changed
> >>>>but not the behavior.=3DA0=3DA0What we have is in essence case b =
from
> >>>>above.=3DA0=3DA0The wave emanates from the switches in our ICs, =
not =3D
from the
> >>>>power supply.=3DA0=3DA0The wave model makes this clear, as it =
does =3D
the
> >>>>propagation path.=3DA0=3DA0The wave model makes clear the =
critical =3D
point that
> >>>>the return and forward currents propagate together.=3DA0=3DA0The =
=3D
fluid analogy
> >>>>with its unidirectional emphasis fails us badly.
> >>>>
> >>>>Where has the fluid analogy brought us?=3DA0=3DA0How many times =
have=20
> =3D
you seen
> >>>>people talk about bone-headed ideas like the PCB planes or bypass
> >>>>capacitors supplying current to high speed edges, when the entire=20
> =3D
edge
> >>>>has completed long before the wave front through power pins can =
=3D
reach
> >>>>significant charge in the planes, much less even reach the PWB =
=3D
bypass
> >>>>caps?=3DA0=3DA0Yet this kind of junk mythology sadly makes its =
way =3D
into books
> >>>>and other publications on a regular basis.=3DA0=3DA0I don't like =
it =3D
one bit.
> >>>>
> >>>>For my money, I find the fluid analogy terribly misleading, and a
> >>>>source of much misunderstanding.=3DA0=3DA0One doesn't need to be =
able=20
> =3D
to derive
> >>>>Maxwell to understand wave propagation.=3DA0=3DA0I think that as =
=3D
Eric's book
> >>>>demonstrates, most SI concepts are not that difficult to =20
> >>>>understand.=3DA0=3DA0Even a dummy like me gets them from time to =
=3D
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 =20
> >>>>>electrons as meaning electron drift. Let's kill that right now.
> >>>>>
> >>>>>One electron in =3D3D one electron out is the flow of electrons. =
=3D
One
> >>>>>electron in =3D3D SAME electron out is electron drift --- not at =
=3D
all the
> >>>>>same thing.
> >>>>>
> >>>>>Certainly I don't argue against Maxwell's equations. But I don't
> =3D
argue
> >>>>>against the fundamental definition of one amp of current either=20
> =3D
---
> >>>>>the flow of one coulomb=3DA0=3DA0of charge (6.25 x 10^18 =
electrons)=20
> =3D
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 =3D
understand
> >>>>>them if they did. My goal has been to take our difficult SI =3D
concepts
> >>>>>and explain them in terms that these "poor" people can =3D
understand. To
> >>>>>suggest that you can't explain what happens during planar =3D
transitions
> >>>>>without Maxwell's equations (I believe) is simply wrong. To say=20
> =3D
that
> >>>>>the classical description of current can't explain the =3D
difference
> >>>>>between DC and high frequency is also (I believe) flat wrong. To
> =3D
say
> >>>>>that one description is "more accurate" than the other --- well=20
> =3D
I
> >>>>>suggest that depends a lot on whose working with them! And while
> >>>>>people have been misled by seminar leaders teaching without the =20
> >>>>>benefit of Maxwell's equations, we all know seminar leaders =3D
whose
> >>>>>ability to mislead wasn't one bit hampered by a thorough =3D
knowledge of
> >>>>>Maxwell's equations!
> >>>>>
> >>>>>Don't sell these more basic principles short when it comes to =20
> >>>>>understanding what is happening on circuit boards. They can very
> >>>>>effectively explain what is happening, and why one design =3D
approach may
> >>>>>be more effective than another depending on the important design
> >>>>>considerations. Especially for all those board designers who =3D
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 =
=3D
repeals
> >>>>>>Maxwell that the wave description is fundamentally more =3D
accurate than the
> >>>>>>fluid analogy.=3DA0=3DA0The E/M fields cause the electron drift =
in=20
> =3D
those
> >>>>>>wires.=3DA0=3DA0From the time you closed the switch the =
changing =3D
E/M field that
> >>>>>>resulted propagated outward.=3DA0=3DA0Marconi found a useful =
=3D
purpose for that
> >>>>>>phenomenon.
> >>>>>>
> >>>>>>The fluid analogy is certainly easy to understand, but what is=20
> =3D
the point
> >>>>>>when it is so misleading?=3DA0=3DA0I can't tell you how many =
times=20
> =3D
otherwise
> >>>>>>intelligent engineers that I have known have been thrown off =20
> >>>>>>understanding >>>>>>PCB wave guides, because they were intent=20
> on following the DC =3D
current
> >>>>>>loop
> >>>>>>of the fluid analogy.
> >>>>>>
> >>>>>>Teaching the fluid analogy requires that we later break that =
=3D
teaching
> >>>>>>when
> >>>>>>we want to explain what happens at significant =3D
frequencies.=3DA0=3DA0Consider for
> >>>>>>instance visualization of return current ( which is the =3D
original subject
> >>>>>>matter ) when we transition planes in a PCB.=3DA0=3DA0If we =
think =3D
about it as a
> >>>>>>fluid model we are easily misled into searching out a =3D
conduction
> >>>>>>path.=3DA0=3DA0For
> >>>>>>ready examples of this mass confusion, just look at some of the
> >>>>>>discussions >>>>>>on splitting-up grounds in the wrong ways for
> the wrong =3D
reasons, with the
> >>>>>>wrong results.=3DA0=3DA0But if we simply consider waves to =
begin =3D
with, then the
> >>>>>>behavior is easy enough to intuit out.
> >>>>>>
> >>>>>>Eric does a very nice job in his book explaining signal =3D
propagation that
> >>>>>>does not rely on the fluid analogy.=3DA0=3DA0I think his =
approach =3D
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
> =3D
a
> >>>>>> transistor
> >>>>>> >through a switch, I can turn the transistor "on" and "off" =
=3D
with the
> >>>>>> >switch. That is easy to explain using the electron flow =3D
concept
> >>>>>> (which I
> >>>>>> >hesitate to call an analogy, it in fact describes the physics
> >>>>>> involved).
> >>>>>> >
> >>>>>> >Is your description more complete AND also easier to =3D
understand?
> >>>>>> >
> >>>>>> >And if it is the frequency with which I "flip" the switch =3D
that bothers
> >>>>>> >you, that simply means that some of the parameters that were=20
> =3D
not an
> >>>>>> issue
> >>>>>> >with slow "flipping" (inductance and capacitance, for =3D
example) start
> >>>>>> >becoming more of an issue with faster "flipping!" But the =3D
basic
> >>>>>> nature of
> >>>>>> >what is happening (in particular where the electrons are =3D
flowing)
> >>>>>> is not
> >>>>>> >changing, just speeding up. (How the electrons are flowing is
> >>>>>> speeding up, >>>>>> >the electrons themselves, of course,=20
> don't change speed!) >>>>>> > >>>>>> >Doug >>>>>> > >>>>>> > =20
> >>>>>> > >>>>>> >At 10:45 AM 8/5/2005, steve weir wrote:
> >>>>>> >>Doug, I have some real heartburn with some of those =3D
representations,
> >>>>>> >>particularly the fluid analogy that speaks of current as the
> =3D
flow of
> >>>>>> >>electrons.=3DA0=3DA0When I grew up current was defined as =
time =3D
variation of
> >>>>>> >>electric flux.=3DA0=3DA0When an E/M field=3DA0=3DA0impinges =
a chunk=20
> =3D
of metal the
> >>>>>> resulting
> >>>>>> >>interaction concentrates the field forming a wave =3D
guide.=3DA0=3DA0 All
> >>>>>> practical
> >>>>>> >>wave guides leak, be they a microstrip over a plane, a =3D
stripline,=3DA0=3DA0or
> >>>>>> >>whatever.=3DA0=3DA0Some, like a good semirigid coax leak =
only a=20
> =3D
little tiny
> >>>>>> >>bit.=3DA0=3DA0When they leak too much creating excessive =3D
disturbance in
> >>>>>> nearby wave
> >>>>>> >>guides, we have cross talk problems.=3DA0=3DA0I hope that =
this =3D
is what you
> >>>>>> were
> >>>>>> >>trying to convey.
> >>>>>> >>
> >>>>>> >>Regards,
> >>>>>> >>
> >>>>>> >>
> >>>>>> >>Steve.
> >>>>>> >>
> >>>>>> >>A
> >>>>>> >
> >>>>>> >Check out UltraCAD's new presentation videos and new skin =3D
effect
> >>>>>> >calculator at http://www.ultracad.com >>>>>> > >>>>>> =20
> >>>>>> =3D
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>>>>>>=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3D=
A0=3DA0=3DA0 =3D
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>>>>>>=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3D=
A0=3DA0=3DA0 =3D
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>>>>>>=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3D=
A0=3DA0=3DA0 =3D
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A0=3DA0=3DA0 =3D
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> =3D
>>>>>___________________________________________________________________
>>>>>_=3D
________-
> >>>>>Check out UltraCAD's new presentation videos and new skin effect
> >>>>>calculator at http://www.ultracad.com >>> =3D
>>>_____________________________________________________________________
>>>_=3D
______-
> >>>Check out UltraCAD's new presentation videos and new skin effect
> >>>calculator at http://www.ultracad.com
> >>
> >>
> >
> =3D
>_______________________________________________________________________
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-------Original Message-------
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Other related posts:
