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Science Forum Index » Physics - Electromagnetic Forum » Current in a wire: electric field velocity...
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| Angelo... |
 Posted: Thu May 22, 2008 3:53 am |
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Hi all,
I mysef wondered why the electric field velocity
is a fraction of c. I thik I understand why the drift
velocity is so low (some mm/s or lower) because
of electrons being scattered by the positive ions in
the metal lattice, but I cannot reconcile this mechanism
for explainig (to myself) the electric field propagation
between the two extremes of the wire.
Any enlightening is much appreciated.
Regards
Angelo |
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| operator jay... |
| Posted: Thu May 22, 2008 7:46 pm |
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Guest
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The speed of light in a medium (conductor) is less than the speed of
light in vacuum. The speed of the electric field propagation in a
conductor is less than the speed of light in a vacuum but I think it
is equal to the speed of light in the conductor.
I have seen it suggested that the reason for the slow down of the
speed of light in media is because the photons are absorbed by the
atoms (bumping orbiting electrons up to higher states) then re-emitted
by the atoms. So, in a strange way, the reason for the slow down of
the propagation of fields may have some similarity to the reason for
the slow drift velocity.
While the drift velocity is way slow due to 'scattering', electrons
would not move at light speed regardless (being massive). The
electrons do get some pretty good speed (10^6 m/s) even though there
is so little room for acceleration between collisions.
"Angelo" <patrik56 at (no spam) libero.it> wrote in message
news:212368a6-908d-48ea-a5f1-b4e39f88ece5 at (no spam) 26g2000hsk.googlegroups.com...
Quote: Hi all,
I mysef wondered why the electric field velocity
is a fraction of c. I thik I understand why the drift
velocity is so low (some mm/s or lower) because
of electrons being scattered by the positive ions in
the metal lattice, but I cannot reconcile this mechanism
for explainig (to myself) the electric field propagation
between the two extremes of the wire.
Any enlightening is much appreciated.
Regards
Angelo |
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| ... |
| Posted: Sat May 24, 2008 10:51 am |
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Guest
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Angelo wrote:
Quote: I mysef wondered why the electric field velocity
is a fraction of c. I thik I understand why the drift
velocity is so low (some mm/s or lower) because
of electrons being scattered by the positive ions in
the metal lattice, but I cannot reconcile this mechanism
for explainig (to myself) the electric field propagation
between the two extremes of the wire.
Any enlightening is much appreciated.
Firstly, it doesn't really make sense to talk about
"electric field velocity". Do you mean the velocity
of electromagnetic waves?
The velocity of electromagnetic waves along a wire
is _not_ the same as the electron drift velocity.
For a single wire, the EM waves move along the
wire at c. In a coax, somewhat slower, firstly,
because a coax cable is a hollow waveguide, and
secondly, because it isn't free space (or air, which
is close enough often enough), but an insulating
dielectric.
You could think about the drift velocity as being a
local response to the local electric field, which is
the local effect of EM waves moving along the wire
(or coax, or whatever) at c, very close to c, or at
some (large) fraction of c determined by the
geometry of the conductors and the dielectric
constant of the insulation.
Feel free to ask more.
--
Timo |
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| Angelo... |
| Posted: Mon May 26, 2008 5:42 am |
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On 24 Mag, 22:51, t... at (no spam) physics.uq.edu.au wrote:
Thank you Timo for this sound answer,
and sorry for the delay of this mine.
Quote: Angelo wrote:
I mysef wondered why the electric field velocity
is a fraction of c. I thik I understand why the drift
velocity is so low (some mm/s or lower) because
of electrons being scattered by the positive ions in
the metal lattice, but I cannot reconcile this mechanism
for explainig (to myself) the electric field propagation
between the two extremes of the wire.
Any enlightening is much appreciated.
Firstly, it doesn't really make sense to talk about
Here I should have forget some basic concepts.
If possible, could you give me, if possible ( I know
that not always it is possible), a hint about why
"it doesn't really make sense"?
Quote: "electric field velocity". Do you mean the velocity
of electromagnetic waves?
Most probably. I had in mind a DC regime, so a
constant current, and a constant magnetic field.
Quote: The velocity of electromagnetic waves along a wire
is _not_ the same as the electron drift velocity.
yes, of course, I said so above.
Quote: For a single wire, the EM waves move along the
wire at c. In a coax, somewhat slower, firstly,
because a coax cable is a hollow waveguide, and
secondly, because it isn't free space (or air, which
is close enough often enough), but an insulating
dielectric.
OK, I understand.
Quote: You could think about the drift velocity as being a
local response to the local electric field, which is
the local effect of EM waves moving along the wire
(or coax, or whatever) at c, very close to c, or at
some (large) fraction of c determined by the
geometry of the conductors and the dielectric
constant of the insulation.
Feel free to ask more.
You've cleared up my main problems!
Regards
Angelo |
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