Science > Physics > What's the fundamental reason for electric current?
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Science > Physics |
| User: |
"Anton81" |
| Date: |
02 Jan 2006 04:47:39 AM |
| Object: |
What's the fundamental reason for electric current? |
Hi!
I've finished a complete physics course at university, but I'm still not
sure about the microscopic, basic principle for electric current.
What happens microscopically when you attach a battery to a simple
resistance?
I'm confident with Maxwell's equations. So is there a field between the
poles before you attach the battery? Is there a field in the wire after you
do (I know Drude's model and calculation)? Does the magnetic field play a
role?
Would you consider the chemical potential somewhere? What would happen if
you attach your resistant wires to only one poles?
Anton
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| User: "Peter Christensen" |
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| Title: Re: What's the fundamental reason for electric current? |
02 Jan 2006 06:07:56 AM |
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"Anton81" <berrybear@gmx.net> skrev i en meddelelse
news:dpb0cc$1mg9$1@gwdu112.gwdg.de...
Hi!
I've finished a complete physics course at university, but I'm still not
sure about the microscopic, basic principle for electric current.
What happens microscopically when you attach a battery to a simple
resistance?
The electrons, which are moving through the resistor, are hitting the
material in the resistor. This causes a loss of velocity (and energy), which
is heating up the resistor. Adding impurities to the material of the
resistor will cause the resistance to increase.
In the electric field, the electrons are accelerating, and the velocity is
increasing. But the flow of electrons is kept down by these scattering
effects.
If You are using a socalled supercondoctor, there would be no scattering at
all in the material, and actually the resistance of the material would be
ZERO ohm. (But don't do that, because you would have a short circuit on the
battery without these limitations on the current.current.) :>)
On usual conductors, as most metals, the resistance is very low. You can
usually ignore it completely, when you are analysing a circuit.
I'm confident with Maxwell's equations. So is there a field between the
poles before you attach the battery? Is there a field in the wire after
you
do (I know Drude's model and calculation)? Does the magnetic field play a
role?
Not for resistors, but for coils it's important. But there is an electric
field over the resistor, if it's connected to the poles of the battery.
Would you consider the chemical potential somewhere? What would happen if
you attach your resistant wires to only one poles?
The potential of the whole resistor would be the same as the potential of
this pole of the battery. Without an electrical field over the resistor
itself, there would be no force to force the electrons through the resistor.
Then the current is obviously 0 ampere.
PC
Ps. Another thing: The conduction of socalled semiconductors is usually
limited by the amount of free carriers, rather than impurities of the
material. These are very interesting and very widely used, but it should not
be the subject here.
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| User: "Anton81" |
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| Title: Re: What's the fundamental reason for electric current? |
02 Jan 2006 06:38:51 AM |
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In the electric field, the electrons are accelerating, and the velocity is
increasing. But the flow of electrons is kept down by these scattering
effects.
Now how/when does the electric field build up? It is a local effect, so that
it cannot suddenly feel that the wires are connected. I prefer to think of
basics, such as "every electron has a field which combine linearly (and
eventually screen each other".
Does the magnetic field play a role?
Not for resistors, but for coils it's important. But there is an electric
field over the resistor, if it's connected to the poles of the battery.
I remember a calculation where we found the Poynting vector at the surface
of a conducting wire. It seemed to have some fundamental importance?!
Anton
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| User: "Peter Christensen" |
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| Title: Re: What's the fundamental reason for electric current? |
02 Jan 2006 08:19:18 AM |
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"Anton81" <berrybear@gmx.net> skrev i en meddelelse
news:dpb6sr$1q8d$1@gwdu112.gwdg.de...
In the electric field, the electrons are accelerating, and the velocity
is
increasing. But the flow of electrons is kept down by these scattering
effects.
Now how/when does the electric field build up? It is a local effect, so
that
it cannot suddenly feel that the wires are connected. I prefer to think of
basics, such as "every electron has a field which combine linearly (and
eventually screen each other".
The electric field will be over the whole resistor, from terminal to
terminal, as I understand it. But not before the wires are connected.
Does the magnetic field play a role?
Not for resistors, but for coils it's important. But there is an electric
field over the resistor, if it's connected to the poles of the battery.
I remember a calculation where we found the Poynting vector at the surface
of a conducting wire. It seemed to have some fundamental importance?!
Yes, it can be very important for circuits with very high frequencies. But
in those cases, one would usually talk about wave-guides rather than
conductors. (I think about conductors rather than wave-guides, when you talk
about batteries.) At high frequencies, these electro-magnetic fields are
very important, as you say. I wouldn't like to describe the theories for
wave-guides right here, as it's very complicated, But the wave-guide model
is also very common, and for example something as 'simple' as the cable from
your antenna to your TV would probably be modelled as a wave-guide rather
than a conductor.
If the frequency is high (maybe in MHz) these electromagnetic effects are
beginning to be important, and one would have to think about a wave-guide
rather than a conducting cable. (As I said, i was thinking about normal
conductors (DC) when you said batteries.)
I'm quite sure, that you have been studying theories for wave-guides, if you
think about Poynting vectors.
PC
-I hope, that I'm not confusing you even more ... :>)
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| User: "Anton81" |
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| Title: Re: What's the fundamental reason for electric current? |
02 Jan 2006 08:54:20 AM |
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I also thought of electrical current as electrons driving themselves due to
ionised poles.
But then we did this Poynting vector calculation for a DC wire and the power
matched the power consumed in the wire. Surprise!
It seemed to have some fundamental importance and I didn't know how to merge
the mechanical and the waveguide picture in order to understand the
cross-over region.
The difference would be in the speed of propagation of cause. In the
mechanical picture the electron in the middle region would start moving
quite later after all the others have accelerated.
In the el.mag. wave picture the field would spread almost instantly and
accelerate all electron at once.
What's correct?
Anton
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| User: "Peter Christensen" |
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| Title: Re: What's the fundamental reason for electric current? |
02 Jan 2006 09:37:57 AM |
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"Anton81" <berrybear@gmx.net> skrev i en meddelelse
news:dpbeqs$1v4h$1@gwdu112.gwdg.de...
I also thought of electrical current as electrons driving themselves due to
ionised poles.
But then we did this Poynting vector calculation for a DC wire and the
power
matched the power consumed in the wire. Surprise!
It seemed to have some fundamental importance and I didn't know how to
merge
the mechanical and the waveguide picture in order to understand the
cross-over region.
A DC-wire (or quasi-DC-wire) should not be able to 'loose' power to an
electromagnetic field..?
PC
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| User: "Anton81" |
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| Title: Re: What's the fundamental reason for electric current? |
02 Jan 2006 10:10:21 AM |
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A DC-wire (or quasi-DC-wire) should not be able to 'loose' power to an
electromagnetic field..?
Something must happen. The calculation I mean can be found for example on
http://facstaff.morehouse.edu/~cmoore/Phy254ElectromagneticEnergy
Example 2.12
Anton
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| User: "Der alte Hexenmeister" |
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| Title: Re: What's the fundamental reason for electric current? |
02 Jan 2006 12:18:02 PM |
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"Anton81" <berrybear@gmx.net> wrote in message
news:dpb0cc$1mg9$1@gwdu112.gwdg.de...
Hi!
I've finished a complete physics course at university, but I'm still not
sure about the microscopic, basic principle for electric current.
What happens microscopically when you attach a battery to a simple
resistance?
I'm confident with Maxwell's equations. So is there a field between the
poles before you attach the battery?
Yes.
Is there a field in the wire after you
do
Yes.
(I know Drude's model and calculation)? Does the magnetic field play a
role?
Is there a magnetic field between the poles of a horseshoe magnet?
Does the keeper change it?
Is there an electric field between the poles of a battery?
Does the conductor change it?
Would you consider the chemical potential somewhere?
No.
Der alte Hexenmeister.
What would happen if
you attach your resistant wires to only one poles?
Anton
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| User: "srp" |
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| Title: Re: What's the fundamental reason for electric current? |
02 Jan 2006 08:03:27 AM |
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Anton81 a écrit :
Hi!
I've finished a complete physics course at university, but I'm still not
sure about the microscopic, basic principle for electric current.
What happens microscopically when you attach a battery to a simple
resistance?
I'm confident with Maxwell's equations. So is there a field between the
poles before you attach the battery? Is there a field in the wire after you
do (I know Drude's model and calculation)? Does the magnetic field play a
role?
Would you consider the chemical potential somewhere? What would happen if
you attach your resistant wires to only one poles?
Thinking with fields is fine when you want to visualize how test
particles for example will behave in them, but for electric current
in a wire, the charged particle interaction view makes it easier
to visualize what is mechanically occurring.
Think of your battery poles as follows: the positive pole, is connected
to plates whose atoms have been stripped of some electrons and so are
a pool of positive ions. The negative pole is connected to plates whose
atoms are overloaded with extra electrons and constitute a pool of
negative ions.
The wire you connect both poles with is typically made of copper
whose atoms do not hold very strongly some surface electrons.
As you connect both poles with the wire, easy to move electron in
the wire touching the positive pole will be sucked into the large
pool of positive ions and electrons further away in the wire will
follow suit to fill the gaps caused by the previously sucked in
electrons. The "current" starts flowing. If you put a resistance
of sort in the circuit, this will regulate the flow. At some
point, resistance or not, all easy flowing electrons from the
negative pool will have moved to the positive pool.
Battery "empty". You will need to recharge.
In very pedestrian terms, this is what happens microscopically
as current flows in wires between the poles of a battery.
André Michaud
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| User: "srp" |
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| Title: Re: What's the fundamental reason for electric current? |
06 Jan 2006 11:21:37 AM |
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Anton81 a écrit :
Hi!
I've finished a complete physics course at university, but I'm still not
sure about the microscopic, basic principle for electric current.
What happens microscopically when you attach a battery to a simple
resistance?
Think of your battery poles as follows: the positive pole, is connected
to plates whose atoms have been stripped of some electrons and so are
a pool of positive ions. The negative pole is connected to plates whose
atoms are overloaded with extra electrons and constitute a pool of
negative ions.
The wire you connect both poles with is typically made of copper
whose atoms do not hold very strongly some surface electrons.
As you connect both poles with the wire, easy to move electron in
the wire touching the positive pole will be sucked into the large
pool of positive ions and electrons further away in the wire will
follow suit to fill the gaps caused by the previously sucked in
electrons. The "current" starts flowing. If you put a resistance
of sort in the circuit, this will regulate the flow. At some
point, resistance or not, all easy flowing electrons from the
negative pool will have moved to the positive pool.
Battery "empty". You will need to recharge.
In very pedestrian terms, this is what happens microscopically
as current flows in wires between the poles of a battery.
I'm confident with Maxwell's equations. So is there a field between the
poles before you attach the battery?
No, since no current is flowing
Is there a field in the wire after you do (I know Drude's model and
calculation)?
Yes. As electrons start moving on the wire, their velocity vector
align in the direction of motion, which causes their electric vectors
to align radially, perpendicular to the direction of motion, and which
by the same token causes their magnetic fields to align circularly
around the wire, perpendicular to both electic field and direction of
motion, all of this in perfect sync with Maxwell's theory.
Does the magnetic field play a role?
The discrete magnetic fields of the aligned electrons and of their
energy of motion simply align.
Would you consider the chemical potential somewhere? What would happen if
you attach your resistant wires to only one poles?
Nothing.
André Michaud
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| User: "" |
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| Title: Re: What's the fundamental reason for electric current? |
07 Jan 2006 09:46:36 PM |
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srp wrote:
Anton81 a =E9crit :
Hi!
I've finished a complete physics course at university, but I'm still not
sure about the microscopic, basic principle for electric current.
What happens microscopically when you attach a battery to a simple
resistance?
Think of your battery poles as follows: the positive pole, is connected
to plates whose atoms have been stripped of some electrons and so are
a pool of positive ions. The negative pole is connected to plates whose
atoms are overloaded with extra electrons and constitute a pool of
negative ions.
..=2E....
I'm confident with Maxwell's equations. So is there a field between the
poles before you attach the battery?
No, since no current is flowing
Thanks for Your nice explanation, Andr=E9.
I tried the following:
I put into the hole for the life-wire of ac-current a circuit-tester.
It doesn't light up. When i put my finger at the other end of the
tester, it lights up, indicating "there are electrons standing by,
ready to work for me". Then i replaced my person by a piece of metall,
some hundred gramms, insulating it with a towel from my hands. Again
the tester lights up.
Is this explained by electrons beeing pushed into the lump of metall
and pulled out again, many times per second?
Friendly greetings
Hero
PS This is of course a tiny step in the quest for an explanation of
the nature of the "charge" of an electron.
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| User: "srp" |
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| Title: Re: What's the fundamental reason for electric current? |
09 Jan 2006 03:36:18 PM |
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a écrit :
srp wrote:
Anton81 a écrit :
Hi!
I've finished a complete physics course at university, but I'm still not
sure about the microscopic, basic principle for electric current.
What happens microscopically when you attach a battery to a simple
resistance?
Think of your battery poles as follows: the positive pole, is connected
to plates whose atoms have been stripped of some electrons and so are
a pool of positive ions. The negative pole is connected to plates whose
atoms are overloaded with extra electrons and constitute a pool of
negative ions.
......
I'm confident with Maxwell's equations. So is there a field between the
poles before you attach the battery?
No, since no current is flowing
Thanks for Your nice explanation, André.
I tried the following:
I put into the hole for the life-wire of ac-current a circuit-tester.
It doesn't light up. When i put my finger at the other end of the
tester, it lights up, indicating "there are electrons standing by,
ready to work for me". Then i replaced my person by a piece of metall,
some hundred gramms, insulating it with a towel from my hands. Again
the tester lights up.
Is this explained by electrons beeing pushed into the lump of metall
and pulled out again, many times per second?
This very minimal info to really assess, but yes, the electrons flow
will move to and fro right through your piece of metal according to
the frequency of the ac-current (50 or 60 Hz depending on where you
are located).
If you experiment with line AC current, please be carefull. Simple
insulation with a towel seems clearly insufficient. I suggest not
having your hands anywhere near live wires.
Friendly greetings Hero
PS This is of course a tiny step in the quest for an explanation of
the nature of the "charge" of an electron.
Yes. Promising field for exploration since "charge" has not been
understood.
André Michaud
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| User: "Autymn D. C." |
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| Title: Re: What's the fundamental reason for electric current? |
16 Jan 2006 08:45:37 PM |
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Didn't I already tell the foreigners this?
carefull (n.) -> careful (a.)
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| User: "" |
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| Title: Re: What's the fundamental reason for electric current? |
16 Jan 2006 09:50:18 PM |
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Didn't I already tell the foreigners this?
carefull (n.) -> careful (a.)
******************************
Who are "the foreigners" when Usenet reaches the entire globe?????
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| User: "" |
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| Title: Re: What's the fundamental reason for electric current? |
10 Jan 2006 04:33:19 AM |
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srp wrote:
Hero.van.Jindelt@gmx.de a =E9crit :
I tried the following:
I put into the hole for the life-wire of ac-current a circuit-tester.
It doesn't light up. When i put my finger at the other end of the
tester, it lights up, indicating "there are electrons standing by,
ready to work for me". Then i replaced my person by a piece of metall,
some hundred gramms, insulating it with a towel from my hands. Again
the tester lights up.
Is this explained by electrons beeing pushed into the lump of metall
and pulled out again, many times per second?
This very minimal info to really assess, but yes, the electrons flow
will move to and fro right through your piece of metal according to
the frequency of the ac-current (50 or 60 Hz depending on where you
are located).
Thanks.
If you experiment with line AC current, please be carefull. Simple
insulation with a towel seems clearly insufficient. I suggest not
having your hands anywhere near live wires.
That's true. But You should touch with the piece of metall only the
end, where You are allowed to place Your finger. There's a bulb inside
the tester, with high resistence, limiting the flow of "charges".
Electricity is lightning put into a wire, so one has to be sure about
what he's doing.
Thanks Andr=E9=20
Hero
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| User: "" |
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| Title: Re: What's the fundamental reason for electric current? |
08 Jan 2006 10:12:44 AM |
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srp wrote:
Anton81 a =E9crit :
Hi!
I've finished a complete physics course at university, but I'm still not
sure about the microscopic, basic principle for electric current.
What happens microscopically when you attach a battery to a simple
resistance?
Think of your battery poles as follows: the positive pole, is connected
to plates whose atoms have been stripped of some electrons and so are
a pool of positive ions. The negative pole is connected to plates whose
atoms are overloaded with extra electrons and constitute a pool of
negative ions.
..=2E.....
I'm confident with Maxwell's equations. So is there a field between the
poles before you attach the battery?
No, since no current is flowing
Thanks for Your nice explanation, Andr=E9.
I have done this: I put a circuit-tester into the hole of a socket for
the life-wire of an ac-current and it didn't light up. Then i put my
finger against the other end of the tester and now it indicated with
light, that there are electrons "standing by, ready to work for me".
Now i replaced my person with a piece of metall some hundred gramms,
insulating it from my hands with a towel, again the tester did light
up.
Is this due to electrons pushing into the metall and beeing pulled out
again many times a second ?
Friendly greetings
Hero
PS. This of course a small step in the quest for an explanation of the
nature of the "charge" of an electron.
.
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| User: "PD" |
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| Title: Re: What's the fundamental reason for electric current? |
04 Jan 2006 08:05:30 AM |
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Anton81 wrote:
Hi!
I've finished a complete physics course at university, but I'm still not
sure about the microscopic, basic principle for electric current.
Mobile negative carriers go from low potential to high potential, and
mobile positive carriers go from high to low potential.
What happens microscopically when you attach a battery to a simple
resistance?
Resistivity is essentially the inverse of conductivity. Conducting
leads have resistance, too, but less than in a resistor, and the
converse is true for resistors. So let's say our resistor is a
conductor with a finite resistivity/conductivity. Attaching the battery
forces one end of the conductor to have a higher potential than the
other, creating a field in the conductor. Thus charges flow, as
mentioned above.
I'm confident with Maxwell's equations. So is there a field between the
poles before you attach the battery?
Yes.
Is there a field in the wire after you
do (I know Drude's model and calculation)?
Yes, see above.
Does the magnetic field play a
role?
Some of the energy delivered by the battery is spent creating the
magnetic field that surrounds the conductor with current in it, yes.
It's typically smaller than the amount of energy lost to resistance
(heat).
Would you consider the chemical potential somewhere?
Yes, inside the battery. That's what generates the potential difference
between the poles.
What would happen if
you attach your resistant wires to only one poles?
There would be a brief transient while charges redistributed in the
conductor, but would quickly settle down to zero current. If attached
to the positive pole, most of the excess positive charge on the
conductor would reside on the surface at the far tip, away from the
pole.
PD
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| User: "Autymn D. C." |
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| Title: Re: What's the fundamental reason for electric current? |
04 Jan 2006 07:44:38 AM |
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There are fields everywhere except at equilibrial regions. And when is
there not a magnetic field? DC is an depletionally-iterative
Doppler-laden half-wave happening.
-Aut
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| User: "rusty" |
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| Title: Re: What's the fundamental reason for electric current? |
04 Jan 2006 08:49:38 AM |
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Autymn D. C. wrote:
There are fields everywhere except at equilibrial regions. And when is
there not a magnetic field? DC is an depletionally-iterative
Doppler-laden half-wave happening.
an ??? I think you should ask your doctor to change your prescription.
--
rusty
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| User: "Autymn D. C." |
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| Title: Re: What's the fundamental reason for electric current? |
16 Jan 2006 08:37:33 PM |
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rusty wrote:
Autymn D. C. wrote:
There are fields everywhere except at equilibrial regions. And when is
there not a magnetic field? DC is an depletionally-iterative
Doppler-laden half-wave happening.
an ??? I think you should ask your doctor to change your prescription.
I slipped depletionally- in at the last moment.
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| User: "Autymn D. C." |
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| Title: Re: What's the fundamental reason for electric current? |
16 Jan 2006 08:39:17 PM |
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rusty wrote:
Autymn D. C. wrote:
There are fields everywhere except at equilibrial regions. And when is
there not a magnetic field? DC is an depletionally-iterative
Doppler-laden half-wave happening.
an ??? I think you should ask your doctor to change your prescription.
Oh, you failed my credibility test:
<http://groups.google.com/group/sci.physics/browse_frm/thread/10b82e2222cc6105/82c95a1dff6f9ded#82c95a1dff6f9ded>.
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| User: "rusty" |
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| Title: Re: What's the fundamental reason for electric current? |
17 Jan 2006 06:01:34 AM |
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Autymn D. C. wrote:
rusty wrote:
Autymn D. C. wrote:
There are fields everywhere except at equilibrial regions. And when is
there not a magnetic field? DC is an depletionally-iterative
Doppler-laden half-wave happening.
an ??? I think you should ask your doctor to change your prescription.
Oh, you failed my credibility test:
<http://groups.google.com/group/sci.physics/browse_frm/thread/10b82e2222cc6105/82c95a1dff6f9ded#82c95a1dff6f9ded>.
I am deeply flattered. I think, however, this does not excuse your
deplorable grammatical error. Incidentally, did you ever pass any kind of
graduate qualifying exam?
--
rusty
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