Sorry guys, but I can't resist weighing in on this topic one more time.
Variations on this thread (hole vs. electron conduction, direction of
current flow, etc.) pop up occasionally on the reflector and to me are
always interesting, so I hope you'll forgive me for pontificating a bit.
In science, there are often several ways to skin a cat. The various ways are
always equivalent (if the science is correct!), but their usefulness depends
on the application. Take something very simple, for example, such as the
reflection of light from a pane of glass. If you shine a beam of light onto
a pane of clear thin glass, roughly 4 percent of the light will be reflected
back toward the source and 96 percent will be transmitted through the pane,
no matter how clear the glass is. This effect has been known for hundreds of
years. In the 19th century, this effect was explained by using the language
of waves, with words like diffraction, interference, transmission,
reflectivity, index of refraction, and so forth. That language is similar to
what we use today in discussiing antennas and transmission lines. The
explanation works great.
In the early 20th century, however, it was discovered that light
isn't a wave but a particle (photon). Ditto for radio waves. You can prove
that light is a particle by reducing the intensity of a light beam and using
a photodector until you resolve individual photons. They arrive one at a
time, each one causing a click on your photodector when it arrives.. You can
do the same trick with radio waves, though it's harder. This was a shocking
discovery, because once you know that light is made of particles, and that
4% of them are reflected, then you've got a real puzzle on your hands. If
all photons are the same, then how do 4% of them decide whether or not to be
reflected? It's as if 4% of the photons say to themselves, "Hey, I think
I'll reflect off the glass instead of zipping through it." But if we have
identical photons, all interacting in exactly the same way with the pane of
glass, then all of them should behave exactly the same way. But that isn't
what happens. This puzzle drove people crazy in the early 20th century.
But then quantum mechanics came in and saved the day. When you work
out the details, it turns out that you can analyze light reflecting off a
pane of glass using the mathematics of quantum mechannics,which treats a
light beam as made up of particles, which we know it is. When you do this,
you get exactly the same answer as you do when you use the language of
reflection, transmission, refraction, interference, and so forth. You can
also use quantum mechanics to analyze radio waves on a transmission line, or
radiated off an antenna. But, we'd be crazy to do this. It's so much easier
to use our conventional wave explanation, even though we know there are
limits to how well that explanation applies to all situations. Today, we
know that the wave description of light and radio waves is exactly
equivalent to the quantum mechanical description for certain situations, but
it's not as generally valid.The quantum mecanical description is
horrendously complicated by comparison, even though it is always valid in
all situations.
Here's a more straightforward example. Consider a simple capacitor.
It is convenient to say that a capacitor passes RF current, with a reactance
that is inversely proportional to the frequency of the current. (Engineers
often approximate capacitors as short circuits and inductors as open
circuits, when looking at their impact on very fast pulses.) In all circuits
involving capacitors, therefore, we can treat the capacitor as an object
that passes RF current. It's a great way to describe the effect of a
capacitor on a circuit. However, in truth, no current actually goes through
a capacitor. There is no charge at all that flows through the capacitor
(assuming the capacitor is lossless). Instead, we know that the capacitor
merely stores charge on its plates, and the charging and discharging process
acts the same as if current flows through.it. The two descriptions are
equivalent. We can "pretend" that RF current flows through the capacitor and
we get the correct answer, even though we know it really doesn't flow
through it at all. In the same way, we can pretend that light or RF
radiation is a wave, even though we know it's a particle. The wave
desciption is much more convenient and easy to use than the more "correct"
particle description.
And the same situation applies to electron transport in a vacuum
tube or hole transport in a semiconductor junction. Whatever works and is
easiest to use is the best way to analyze the situation. In a cathode ray
tube, electrons go from the cathode to a fluorescent screen and are
deflected by a magnetic field. We would be nuts to try and articulate that
process in terms of current flowing from the fluorescent screen to the
cathode, although in principle we could do so. Similarly, in a vacuum tube,
if you're interested in how the space charge builds up, it's best to think
about negatively charged electrons. In a P-channel MOSFET, or a particle
accelerator (which accelerates protons), you'd best think about holes and
positive charges.
Now here's the key point. Whatever description we choose, we have to
be self consistent, or eventually we'll get hopelessly confused. So even
though we know plate current in a vacuum tube is carried by negative
electrons, electrical engineers always speak of the current in an operating
vacuum tube circuit as flowing from the plate to the cathode, i.e., from
positive to negative. They do that because the actual sign of the charged
particles (electrons) doesn't matter from the point of view of the circuit
explanation. If they described the current in the vacuum tube as flowing
from the cathode to the plate, then to be consistent they would have to
describe the current from the power supply as flowing into the positive
terminal of the power supply.. They could do that, and it wouldn't
technically be wrong, but it would be confusing, because circuit diagrams
are drawn with a sign convention in which current flows from a region of
high potential to a region of low potential. The little arrow in an NPN or
PNP transistor symbol points toward the direction of current flow (positive
to negative) as does the arrow in a diode symbol. It's just a convention,
and it could been chosen differently, but it's a universally accepted
convention. If someone bucks the convention, and does everything backwards,
it's not that they're wrong, but they're going to have a lot of trouble
communicating with other people without causing confusion. So the
convention, accepted by virtually all electrical engineers and scientists in
the world, is that current flows from positive to negative, no matter what
particular particle (proton, electron, hole, ion) carries the current.
Remember that electric currrent is a statistical quantity, like wind
velocity, or temperature, or entropy, or specific heat, that only makes
sense as the average of a large number of individual particle motions. Being
statistical means electric current cannot be used to analyze the sign or
direction of individual particles, just as temperature can't be used to
describe the motion of individual air molecules. So while it may seem
non-intuitive, individual electrons in a vacuum tube flow from the cathode
to the plate, but the universal convention is that current in a vacuum tube
flows the other.way. Sorry, but that's the way it is.
73,
Jim Garland W8ZR
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