Re: Velocity of Electrons

skeptic wrote:
> Thanks for your answer Ayudin, however you raise several new issues I
> had not considered.

I apologize for delaying my reply a few days, but the the time in
between has given me the chance to consult (ie meet him on his way to
the physics building and chat for a few minutes in the middle of the
street) one of my physics professors in regards to this question.

It appears there are quite a lot of things that are to be considered in
this question. The fact that we're dealing with charged particles,
electric fields, and magnetic fields (these three interract with each
other over and over again) and fluid dynamics(?) makes this a pretty
complex question.

I've been told that your question is akin to those in
"Magnetohydrodynamics" (this is your cue to google it ;)). Apparently
this branch of science is also related to the efforts made in controled
fusion (ie trying to contain the plasma with a magnetic field, and
probably other attempts). Another keyword is "dynamo" which means (as
far as I could understand it) the behaviour of conductive fluids to
create currents and magnetic fields when stirred (example: earth's
magnetic field created by moving of molten iron in the mantle(?)),
although I doubt this would be a good word to google.

First let me correct a few of my errors. You can probably adjust the
electron drift velocity by adjusting the flow against it, and even stop
it. Though this would mean you'd be making positively charged particles
flow in the opposite direction to that the elecrons would have been
drifting, in essence causing a current in the same direction as that of
the current the electrons would have caused. So you can stop (on
avarage..) the electron drift, but probably not the current.

> If the free electrons in the conductor are not interacting with the Hg
> atoms but are moving because of "...an electric field which is
> accelerating the electrons", then how is this different from electrons
> being accelerated in a vacuum? Wouldn't the electrons continue to be
> accelerated throughout the length of the conductor?

Not quite, I forgot some of my electro-magnetics classes and gave you
the wrong impression. As Zigoteau mentioned the electrons bounce back
and forth off the ions (so they are actually interracting with the Hg+
ions), however because of the general effect of the electric field they
are biased towards one direction, so on avarage they drift in that
direction (direction of the current or whatever it's called). Forcing
the Hg+ ions to move in the opposite direction could probably offset
the bias caused by the electric field (when they bounce they'd get more
momentum towards the other direction) reducing net electron drift to
zero, but as I've said that doesn't change the direction of the
current.

> I also had not considered the effect of the Hg+ ions on the magnetic
> field. If we imagine that both the free electrons and the ions are
> influenced equally by the motion of the Hg, then shouldn't the change
> in the magnetic field caused by the movement of the ions exactly cancel
> the change in the field caused by additional movement of the electrons
> due to the flow of the mercury? Thus no change in magnetic field would
> be apparent.

You make a good point here, although I misunderstood it at first and
had to read over it again. I think you're right, although I'll need to
do some more thinking/consulting. In any case this seems to agree with
what I said about the current above. Ah of course there's also the
dynamo effect to consider I think.

But this actually gave rise to an even more outrageous question in my
mind:
Let's go back to the point where you were creating a current thru the
mercury without forcing any flows in it. Now let's say that the Hg+
ions are as free to move as the free electrons (after all this is a
fluid, the atoms aren't nailed down, although I don't know what the
intermoleculer forces would have to say on this ;), gotta find my high
school chemistry books). Let's say the positive end of the battery is
connected to the left end of the tube section in which we want the
current, and let's say the negative end is connected to the right end.
Our electric field points towards the right, and let's assume it's
uniform. This electric field would not only affect the free electrons
but also the Hg+ ions (they aren't nailed down like the ions in a solid
metal are). In effect the free electrons, and the Hg+ ions would be
accelerated in opposite directions, but should gain the same amount of
momentum (again in opposite directions) resulting in collisions in
which the partipicants would have equal but opposite momentums (on
avarage). This would result in net drift velocity of 0 for both the
ions and the electrons, ie no current.

But I'm pretty sure that if you apply enough voltage you could get a
current across liquid Hg. So what am I missing here? Maybe the
collisions don't have to be elastic and they emit photons? That'd
actually explain stuff like neon-lamps, but this is pure speculation on
my part.

Aydin

.

Relevant Pages

  • Re: Velocity of Electrons
    ... I think you used mercury with those assumptions apart from ... > hasn't been my strong side), then the magnetic field should only depend ... > backwards by increasing the flow, after all electrons don't travel by ... > considering electrons as seperate from the ions of the fluid, ...
    (sci.physics)
  • Re: Is fusion a dead horse?
    ... exceeds fusion power -- or the electrons are cold, in which case the power of electron ... drag on the ions far exceeds fusion power. ... But even slight lateral velocity will mean the ions miss this peak. ...
    (sci.energy)
  • Re: Is fusion a dead horse?
    ... This is a key point, of course, and (as Tom points out) wants more experimental verification. ... exceeds fusion power -- or the electrons are cold, in which case the power of electron ... drag on the ions far exceeds fusion power. ...
    (sci.energy)
  • Re: The electron shell model is in challenge
    ... Such ions are routinely used in experiments. ... Well, you *are* an idiot. ... A google search for "all electron model" yields only 14 results. ... The "shell model" has nothing to do with "manipulating electrons ...
    (sci.physics)
  • Re: OT - Global Warming Revisited
    ... > The ions in the lattice are vibrating. ... The ions at the hot end of a piece of metal ... [Note the electrons have been left out of picture 1.5 to keep it clear.] ... And, because they are free to move through the lattice, they are able ...
    (alt.machines.cnc)