Re: transistors: so confusing!!


"Dorian McIntire" <dorianmc@xxxxxxxxxxxxx> wrote in message
news:T_KGf.10427$697.1831@xxxxxxxxxxxxxxxxxxxxxxxxx
While I don't completely agree with Claude's post I can shed some light on
one concept you took issue with; the idea of a charge moving with no
applied voltage. It is indeed possible for a charge to move with no
voltage. Superconductivity is one situation where charges move with no
applied voltage. Granted, a voltage is initially required to get the
charges moving but the voltage is not required to keep charges moving at a
constant rate in a superconducting material.



Technically if there is an electric field then there is charge moving then
there is work being done. Obviously there are electrons "moving" all the
time without a "field"(well, theres always a field though) but we don't
consider a voltage moving them. The reason, Ig uess, is that there is no
mean free movement and/or the voltages are extremely small as to be
inconsequential.

About the SC material. The problem with that is that one cannot do any work
with it(hence no voltage) else one has a perpetual motion device. If you
try to measure the current flowing the electrons will stop flowing(or
decrease depending on how much work was used to get them to move). Its
similar to the uncertainty principle. So we can only talk about it in a
theoretical way.


One special case of this is an electron moving at some constant velocity
in a vacuum. Voltage is initially required to accelerate the electron but
momentum will carry it on at a constant velocity if no extraneous fields
are allowed to act on the charge.


Yes, but you are forgetting that voltage is basicaly the work/energy
supplied or given up by the moving charge. The electron has energy(you said
momentum) so technically it has a voltage. Voltage is an abstract definition
as is an electric field. As far as nature is concerned theres no such thing
as a E field or voltage. So you say that when we apply a voltage to
accelerate an electron and then remove that voltage it is gone... but it is
not. We have just transfered it to the electron(as far as you can transfer
voltage(really transfering energy)).

I think the problem is that you are thinking of voltage in macroscopic terms
then applying it to the microscopic. (sorta like wave-particle duality).
I.e., if you could just look at one electron at the microscopic level then
you wouldn't be able to see a "voltage" directly. If you were watching the
electron and you saw it move then you could hypothesize that there was a
voltage applied to it by some electric field. But you wouldn't be able to
really "see" the voltage.

Basicaly these are mathematical concepts applied to natural so we have to be
careful what we really been by these ideas. Although I can see what you
mean when we "apply" a voltage then stop "applying" it but you have to ask
yourself "What does it really mean to "apply" a voltage?". Then you might
realize that a voltage is really looking at the boundry conditions of some
even... i.e., measuring the energy at one point in time then at another
point in time and if they are different then something must have
happened(i.e., a "voltage").

For an analogy I could create a "field" that discribes how people move. We
could think of people as point charges and then plot there velecity vectors.
We would see areas in the field that look like there are "paths"(like a
highway with all the cars going in one direction or in an elevator,
etc...)... We could then hypothesize there is some force that makes the
"people" go in that common direction. In some locations it would be
disorganized or random and we could say there is no field there. We could
come up with some way to measure this "field" and create a whole theory
around it. In actuality there is probably not a field though and it is just
a "tool"/"concept" used to help us understand why the people seem to be
moving as they do. We could say "These people over here are experiencing a
force that makes them move along this path" and "These people here have some
forces that are holding them together"(such as at a football game), etc...

Hell, who knows ;) Basicaly the definition of voltage is independent of
what happens between two measurements so we can't really talk about what is
going between them(just like a particle as voltage is a measurement on
particles)... If we do talk about them we end up arriving at the problems we
do.





This may seem to be minor point to some, but physical concepts such as
momentum, inertia and energy are just as important in electronics as they
are in mechanical systems.


True, but they are not well defined physical terms. They are mathematical
concepts applied to reality. Any time you start to dive into this stuff all
kinds of little problems start creeping up and then you have to start using
the quantum mechanical definition of these terms to get around these
problems... but then even stranger things start happening. I do agree that
it helps and is usually a good idea but you gotta be careful in how you use
them too. Our knowledge about reality is very limited and is only decent
approximation. We could actually be way off and just by happenstance happen
to have a model that approximates it well. That is, our model's axioms could
be completely wrong in that the "real" model has totally different axioms
but it turns out that the end results(large theorems) happen to agree with
each other. I have a feeling this is what is going on and our concepts of
reality is way off(but in some sense parallel to reality). This is why we
have so many little problems in the sciences today that don't make much
sense and it seems that we keep on having to modify them to get it to work
and introduce things that make less and less sense. I doubt thats how
reality works but it could be. My evidence is it seems everyone we find a
problem in nuclear physics it seems we have to create/find a new particle to
make everything right again... It could be this way or we are just "forcing"
it to work.


Anyways,


Dorian


<snip>


Jon


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