[SI-LIST] Current Flow through a capacitor
- From: "Eric Bogatin" <eric@xxxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 8 Aug 2005 07:26:41 -0500
Hi folks-
I have been out of town the last week, so missed some of the details on the
discussion on current flow. At the risk of providing a redundant
recommendation, I think a lot of the basis of the confusion is rooted in our
intuition about how current flows through a capacitor.
After all, as Scott pointed out, you have no DC connection between the two
plates, so how can the fluid model work? How do electrons get from one
conductor to the other when they are separated by an insulating dielectric?
How do we feed our intuition to think about how current flows through a
capacitor?
In addition to having this answered in my book, in feature articles you can
download from my web site, and in one of my columns, also available from my
web site, I put together an online lecture: OLL-004, Current through a
Capacitor, all of which are available for free viewing from my web site,
www.BeTheSignal.com <http://www.bethesignal.com/> . I invite you all to
check them out.
--eric
**************************************
Dr. Eric Bogatin
www.BeTheSignal.com
Signal Integrity on-demand training
26235 w 110th terr
Olathe, KS 66061
v: 913-393-1305
f: 913-393-0929
c:913-424-4333
e:eric@xxxxxxxxxxxxxxx
Signal Integrity-Simplified
Prentice Hall, 2004
****************************************
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
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
- » [SI-LIST] Current Flow through a capacitor
