"Timo A. Nieminen" <timo@physics.uq.edu.au> wrote:
| On Sat, 18 Nov 2006, Sorcerer wrote:
|
| > "Timo A. Nieminen" <timo@physics.uq.edu.au> wrote:
| > | On Sat, 18 Nov 2006, Sorcerer wrote:
| > |
| > | > Refusal to discuss physics noted.
| >
| > [snip]
| > *****, *****, experiment over.
| > http://www.androcles01.pwp.blueyonder.co.uk/TrollExample.htm
|
| Hee hee!

Egads... not a question. How peculiar. I see Dork Van de merde has
trained you well. Excellent trolling, Nieminen. Pity you can't answer
any physics questions.
What limits the beam current in a vacuum, *****?

Btw, did you track down that paper by Bonnor, about gravitation and light?
It looked to me to be perfectly sensible, so didn't warrant further
comment. But then, I avoid GR, and you don't, so you might want to
discuss.

Btw, did you track down that paper by Bonnor, about gravitation and light?
It looked to me to be perfectly sensible, so didn't warrant further
comment. But then, I avoid GR, and you don't, so you might want to
discuss.

On Fri, 16 Nov 2006, xray4abc wrote:

Timo A. Nieminen wrote:

On Thu, 16 Nov 2006, xray4abc wrote:

jimp@specsol.spam.sux.com wrote:

xray4abc <lemhenyil@yahoo.ca> wrote:

<snip>

That's true.
Yet, still we did not answered the question.
If there is no electric field line(s) along the conductor or a
Lorentz force exerted INSIDE the conductor,
then how come that the electrons are drifting along the conductor ?
In order to sharpen the question, I have considered the case of
a zero resistance conductor.
In my opinion, it needs a little bit more than a traditional answer!
LL

From a circuit analyses viewpoint, the answer is trivial.

What you are really asking is why and how does current flow in a conductor.

Yes. I have always thought that the electric field created by
the power source was "pushing" the electrons.
It seems that there is something wrong with this picture of the
process.

What makes aircraft fly at a constant velocity? Thrust = drag. What makes
a spaceship travel at constant velocity? Thrust = drag must still be true,
but drag = 0, therefore, one can conclude what?

You're specifically considering the case of R=0. Why should any force be
needed to keep the electrons moving?

Of course, the electrons are never resting in a conductor.
No force is needed to keep them moving in a random manner.
But, we need an answer when is about their orderly movement
which implies either the action of a well defined force

OK, in the conductor, the surrounding matter exerts no force on the
electrons (the assumption of R=0), so any force that acts will act either
before or at the moment the electrons enter or leave:

or some other
way
of injecting/extracting electrons to/from the conductor in question.

Consider the power supply.

First, connect a non-ideal uniform conductor across
the terminals. You get an approximately uniform current in it. From Ohm's
law, J=sE, where s=conductivity, the electric field is uniform within the
conductor, and can be found from the terminal voltage: V=EL, where L is
the length of the conductor. The component of the Poynting vector directed
into the conductor brings in the energy that is lost due to the
resistance.

What happens in the power supply? That's more complicated, but you know
that the current in the power supply is equal to the current in the
conductor. You consider the power supply to be basically an electron pump.
Now, while the conductor is connected to the terminals, the electrons
aren't going to stop when they get to the -ve terminal, they'll keep going
into the conductor. If there is resistance, then you need an electric
field in the conductor to maintain that motion.

With R=0, you don't. The power supply simply injects the electrons in,
already moving, and they keep moving at that speed, and the power supply
allows them back in.

Now for a fun complication! If the conductor is curved, then the electrons
must be accelerated to go around the curves, even if they maintain a
constant speed. What would provide that force? Hint: ignore the power
supply, just consider the perfect conductor with a current in it and a
bend.

From a circuit analysis viewpoint, it really is trivial. From a Maxwell
viewpoint, it's less trivial. If you want it to be an educational problem,
figure out how Maxwell gives you the circuit theory picture. Just keep in
mind that circuit theory general treats wires as R=0, while in Maxwell,
their conductivity is not infinite.

I have started exactly from an example where it was shown that,
in a particular case, we can get U*I= S* E*H, that is the connection
between the Poynting vector and the electrical power dissipated
by a resistor. In the example, the power is supplied by an ideal
pair of conductors, from the source to the resistor.

The power isn't supplied by the ideal conductors. The power is supplied by
the source. If the conductors are ideal, the electric field in them is
zero. The electric field along the resistor can't be zero. How can that
be? How can we have: conductor (E=0), resistor (E non-zero), conductor
(E=0)? The ends of the resistor must be charged!

I think if you really want to go into more detail, you'll have to get into
some reading?

A good starting point would be:

N. W. Preyer, Surface charges and fields of simple circuits, American
Journal of Physics 68(11), 1002-1006 (2000).

The reference list of that paper will also point you to many other papers.
What might be especially useful for you is reference 3:
"Bruce A. Sherwood and Ruth W. Chabay, "A unified treatment of
electrostatics and circuits," URL http://cil.andrew.cmu.edu/emi."
for that www convenience.

--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html