Hi all,
there is one thing that still needs to be said in this thread, regarding
the real flow of current. Unless somebody already said it and I missed
it, of course...
It's that while in most physical phenomena the electric charges moving
are electrons, and thus the physical flow is from negative to positive,
there are some situations in which current is carried by positively
charged particles, which move from positive to negative, that is, in the
"conventional" sense. This is the case for positive ions in gases or in
electrolytes.
For example, you might have a bulb with an easily ionizable gas inside,
and two electrodes. You apply a suficiently high voltage. Some gas
molecules near the anode will each give off one electron to the anode,
and thus become positively charged, since they now have one more proton
than electrons. These positively charged molecules will physically move
from the anode to the cathode, where each of them will capture one
electron from the cathode, becoming electrically neutral again. Then
they will slowly move back to the anode as neutral particles, carrying
no current, "pumped" by the stream of positive ions moving from anode to
cathode.
So in this system the physical current flow reverses at the electrodes:
From the electrodes outside, current is carried by negatively charged
electrons, flowing from more negative to more positive places. And from
the electrodes to the inside, current is carried by positively charged
ions, or you may think of current carried by protons attached to neutral
backpacks, moving from more positive to more negative places.
Since it's not very practical to think about "current" reversing at some
point of the circuit, just because the charge carriers have changed, it
was necessary to define a standard. And old Ben happened to pick the for
a definition the sense of flow that happens to be less common. Bad
luck... But not a real problem, as long as we know how it works, to a
level of detail that's sufficient the explain the phenomena we are
working with.
In a related, and more radio-tied theme, it would be fun to see the
ensuing discussions if another and more complex can of worms is opened:
The physically absurd model of RF power flowing from a transmitter to
the antenna on a transmission line, and part of it being reflected and
flowing back ON THE SAME LINE, without any interaction with the power
flowing forward! As a model it works great, is easy to imagine, and
practical, but physically it just makes no sense to have power flowing
in opposing directions on the same wires at the same time!
Manfred
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