[EMAIL PROTECTED] wrote:
> And you both forgot that like water in a pipe, electrons will migrate to
> the side of the via in which the flow is occuring, further reducing the
> effective "trace width" of the via...
I can't tell if aj is joking or serious...
If we are going to use analogies to describe current flow, let's at least
use correct ones.
Current flow is the movement of electric charge through a medium. The
medium can be either conductive or non-conductive. If the medium is
conductive, then you have "conduction current", as in current flowing
through a wire. If the medium is non-conductive, you have "displacement
current", as in time-varying current flowing through the dielectric of a
capacitor.
Electric charge consists of positive or negative charges. At the atomic
level, a positive charge is an atom that is missing one or more electrons
from it's electron orbitals. The negative charge is a lone electron that
has broken away from the orbital it was in, and is free to travel. Where
does it go? It goes to the next electron-deficient atom it encounters. It
stays for a while, then hops to the next atom. This process (electron
drift) does not happen at the speed of light, as so many believe. What
does happen is that the electric field travels through the medium at nearly
the speed of light (or some significant fraction thereof). The actual
electrons only travel a few meters per second.
Here is the analogy that works: you remember that novelty item with 5
steel balls suspended in a row by threads? You pull back one of the steel
balls, let it go, and it swings back and hits the ball next to it. The
ball on the other end of the row swings out and then swings back, while the
3 inner balls stay stationary. The silly thing keeps doing that
"klak-klak" thing for a long time, until air resistance eventually robs it
of the kinetic energy. In this analogy, the 2 end balls are electrons, the
three middle balls are the conductor, and the kinetic energy is the
electric field. Even though the end balls (electrons) are moving at only a
some fraction of a meter per second, the speed at which the kinetic energy
(electric field) is conducted through the middle balls (conductor) is much
faster than that.
So, the electrons will not pile up on a particular region of a via. What
you may be thinking of is "skin effect". That happens when the electric
field of a flowing current cannot penetrate equally through the depth of a
conductor, and flows mostly on the surface of the conductor. This effect
becomes worse as you go higher in frequency. It results in a reduction of
the effective cross-sectional area of a conductor. If you want to think of
that as electron migration to the surface of the conductor, that is a valid
way to think of it. Maybe that's what you had in mind. Skin effect does
not occur at DC, and is negligible at low frequencies in most cases. It
does become a concern in high-power switching power supplies and
high-tension AC power transmission lines.
One last item: which way does current really flow?
Answer: it flows from where there are negative charges (electrons) to
where there are positive charges (atoms missing an electron). Therefore
current does not really flow from + to -, as we commonly analyze our
circuits. Current actually flows from - to +. So why does our stuff work?
Because unless you are working with the actual physics of semiconductors
and materials, it doesn't really matter which way the current flows, as
long as you choose one direction and stick with it. What most of us EE's
(myself included) use is called "convential flow", which is + to -, but is
incorrect theoretically. Some EE's use "electron flow", which is - to +,
and is correct theoretically.
Put that in your academic pipe and smoke it ;-)
Best regards,
Ivan Baggett
Bagotronix Inc.
website: www.bagotronix.com
[EMAIL PROTECTED] wrote:
And you both forgot that like water in a pipe, electrons will migrate to
the side of the via in which the flow is occuring, further reducing the
effective "trace width" of the via...
aj
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