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Re: [Amps] "Conventional" current flow and Physics 101

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Subject: Re: [Amps] "Conventional" current flow and Physics 101
From: "Jim Garland" <>
Date: Tue, 1 Nov 2016 16:40:33 -0600
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Ron and Bill and any others interested in this thread, sorry to keep
disagreeing with you gentlemen, but I believe your skepticism about
"conventional" current is based on a misunderstanding of the concept. It has
nothing to do with a "mistake" about the sign of the electron.  Let me try
to explain, starting with where I think we agree.

Yes, electric current as understood by scientists and engineers is a defined
quantity, and in that respect it is "imaginary."  It is a mathematical
abstraction which turns out to be hugely important to understanding the laws
of nature and the operation of electronic circuits. In some ways "current"
is analogous to "temperature," or "wind," or "pressure," which also are
defined abstract quantities. This is what I meant when I said that electric
current is a "statistical" quantity. An electric current is only defined in
the context of the average flow of large numbers of charge carriers. "Wind"
is a similar concept, so let me use that as an example.

When a wind blows through a window, we know the wind is made up of a huge
number of air molecules. These molecules move at a speed of hundreds of feet
per second, and they travel in all directions, colliding and bouncing off
each other every micron or so. Even if the air is completely still, the
molecules are moving and colliding, and exchanging kinetic energy. This is
the thermal motion of the air, and the air temperature is a measure of how
frenetic that motion is. If the air is nearly a vacuum, however, with only a
few air molecules, then it doesn't make sense to speak of wind, or air
temperature, or air pressure, because those quantities have no meaning
unless there is a large enough number of molecules to have an averaging

It's the same way with electric current. The beam current in a CRT, for
example, is only defined in terms of the average behavior of large numbers
of electrons. Those electrons travel from the CRT cathode to the phosphor
screen, where they are collected and returned to the power supply. The beam
current can be measured by an ammeter in the cathode lead, but one can't
describe the motion of individual electrons by that current. So yes,
individual elections are emitted by the cathode and strike the phosphor
screen after a transit time. However, the current flows in the reverse
direction, because the current (a) only pertains to the average motion of
zillions of elections, and (b) the direction of the current is arbitrarily
defined to be the direction of positively charged carriers, even if the
actual carriers are negatively charged electrons. It's the same with plate
current in a vacuum tube. The current flows inside the tube from plate to
cathode, even though the electrons move in the opposite direction. By the
way, the technical term for the motion of individual electrons in a vacuum
tube, or CRT, is called "ballistic motion," which means that the electrons
are moving without colliding with anything during their trajectory.

So what's the point of defining a quantity that seems so counterintuitive?
Turns out there are very important reasons. First, electric current doesn't
depend on the sign of the carriers, and that's a good thing. It would be
horribly confusing if we had to keep track of electrons, holes, protons (as
in a particle accelerator), ions (as in a battery), solar wind (a mix of all
kinds of charged particles), every time we wanted to describe something's
electrical properties. There are just as many positive charges in the
universe as negative charges (so far as we know), so there's no reason to
give preference to one polarity over the other. Intuitively, the positive
sign convention makes good sense. An electric field from a positively
charged particle radiates outwards from the particle, which seems more
natural than having electric field lines converge in on the particle from
infinity. The forces on moving charges created by a magnetic field can be
described by a so-called "right hand rule," which is easy to remember since
most people are right-handed. Similarly the direction of the magnetic field
lines created by an electric current is described by another
right-hand-rule. And for ham operators, it's very intuitive to think of
current as flowing out of the positive terminal of a battery or B+ supply
and returning to ground and eventually back into the negative terminal.
Kirchhoff's law (which states that the steady state current flowing into a
node (junction) has to leave the node at the same rate) is at the heart of
all circuit theory. That law depends on current flow being independent of
the sign of the underlying charge carriers. There are many other reasons why
"conventional" current is an essential concept.

Part of the confusion about current flow versus charge motion is actually
the fault of the US Army. Back in WWII days, army training manuals often
confused the two concepts, ultimately misleading generations of electronic
techs. Some of the drawings in those days showed current flowing out of the
negative terminal of a battery through a resistor and into the positive
terminal. That's true for electrons, but not for the electric current! 

Jim W8ZR

> -----Original Message-----
> From: Amps [] On Behalf Of Ron Youvan
> Sent: Friday, October 28, 2016 04:46 PM
> To: amps
> Subject: Re: [Amps] "Conventional" current flow
>   Jim W8ZR Garland wrote in part:
> >> Concur with Bill W6WRT re "positive vs negative" convention in
> >> describing
> > current.  The
> >> convention we use now is left over from at least a hundred years ago
> >> when
> > a scientist
> >> guessed wrong about the polarity of the electron.  To me, thinking in
> > terms of the direction
> >> of electron movement is far more clear.
> >> -Gene WB8WKU
> > Sorry to disagree with you and Bill, Gene, but the standard convention
> > for electric current makes a lot of sense, and it's not just that
> > somebody guessed wrong a century ago about the polarity of electrons.
> > The important concept is that electric current is a statistical
> > concept. Electric current is sometimes carried by positive charges,
> > sometimes by negative charges, but the sign of the charge carrier is
> > an entirely separate issue from the direction of current flow.  In a
> > vacuum tube, plate current flows into the plate and out of the
> > cathode, even though the motion of the electrons in the tube envelope
> > is in the reverse direction.  Similarly, current flows into the
> > collector of an NPN transistor and out the emitter (hence the words
> > collector and emitter), even though transistor current may be carried
> > by holes.  Similarly, current flows out the positive terminal of a
> > battery and returns into the negative terminal, no matter whether the
> > charge of the ions in the battery are positively or negative. It would
> > be a nightmare to keep track of current flow, if the direction of flow
> > depended on the sign of the underlying charge carriers.  You might
> > have situations where current flows out of a B+ supply to the plate of
> > a tube, and then is annihilated by current flowing in the opposite
direction from within the
> tube, coming from the cathode.
>    Bull!  Conventional current flow was a wrong guess.
>    I'm sorry I must say ...
>    Please explain to us how the CURRENT flows up the ultor lead to the
side of the bell of
> the cathode ray tube, runs in and around the aquadag inside the bell and
at the right place
> jumps off and strikes that precise spot on the screen (that is based on
the magnet field that
> the current is yet to pass though) then is shoots through that varying
magnet field (when it is
> at the exactly correct magnitude) at high velocity at a tiny hole in the
end of the electron
> gun, (which under no circumstance fails to go through) and lands ONLY on
the only hot
> thing at the bottom of the gun.
> Yea sure!
>    The flow of holes through conducting materials is the RESULT of the
motion of
> ELECTRONS.  To say other wise makes me say BULL.
>    Only some (older) larger universities and a few manufacturing companies
(GE and a few
> others) hang on to the obsolete conventional current flow that should be
forgotten by
> everyone.  Why?  Because electrons are what IS MOVING.  The flow of
electron is
> electricity.
> (and lighting)
> --
>    Ron  KA4INM - Youvan's corollary:
>                  Every action results in unwanted side effects.
> _______________________________________________
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