>> If a negative charge flows left, then a positive charge appears to
>> flow right. Yes, only the electron "physically" moves.
"That's halfways right, but I still just don't see the need of even
talking about the motion of positive charges. Talking about motion of
negatively charged electrons is enough in electronics.
Why only halfways right? Because it works only for a continual flow of
charges. If a continuous current flows, with a certain number of
trillions of electrons passing each spot of the wire in every second,
then it works fine to think in terms of the same number of positive
charges moving in the other direction. But if a single electron moves
through space, in a tube, and takes a while to arrive, how do you make
the equivalent with "conventional" current flow? You make a positive
quant jump out of the plate, and arrive at the cathode A LITTLE WHILE
EARLIER?....
Manfred"
I mostly agree with you, Manfred. Electric current is defined as the time
derivative of the charge (J=dq/dt, where J is the current density and q is
the charge density). This definition only makes sense in the continuum
regime, when the discrete nature of the charged particles can be neglected.
If it is important to consider individual charges, then the concept of
current breaks down and one speaks instead of pulses of charged particles.
Issues such as the time-of-flight of the electrons in a vacuum tube, shot
noise, and other effects caused by charge fluctuations have to be considered
separately and render invalid the usefulness of a generalized current flow.
An analogous situation is with air pressure and wind speed. These are useful
concepts when discussing the weather. However, if the density of air is low
enough then the motion of individual air molecules becomes important and the
concept of pressure is no longer useful. Same thing with electrons and
current flow.
Two minor points. The "flow" of current is a useful concept when visualizing
circuits, but in AC circuits there actually is no flow of current along a
wire. The conduction electrons in a wire just slosh back an forth a fraction
of a micron and never go anywhere. In DC circuits, there is a flow, although
the distance traveled is very small and the speed of the charge flow is very
very slow -- tiny fractions of a mm per second -- hardly the speed of light,
which many people presume!
73,
Jim W8ZR
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