Ionic motion


by somasimple
Tags: electric field, ion
somasimple
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#1
Jul29-08, 09:57 AM
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Hi All,

A stupid question for sure (sorry).

Does a moving ion, since it carries an electric field, creates some kind of electricity ? (This concept remains unclear for me)
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tiny-tim
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#2
Jul29-08, 06:07 PM
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Hi somasimple!

A moving ion is electricity (just like a moving electron).
Astronuc
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#3
Jul29-08, 07:24 PM
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A moving ion (+) creates a time varying electric field in relation to other charges. The current goes in the direction of the ion's velocity. The current (by convention) is opposite an electrons velocity.

At the same energy, the speed of the electron is much greater than an ion because the electron's mass is much smaller than protons and neutrons.

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Jul29-08, 11:44 PM
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Ionic motion


Thanks for these replies.
Since an ion obeys to Coulomb's Law, I suppose that in some conditions (a ion + attracted i.e.)
there is an acceleration (?), does that means the current augments as the acceleration?
Defennder
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#5
Jul30-08, 05:09 AM
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If you're talking about electrons in wire, then it's inaccurate to say that it accelerates all the time because the electrons tend to collide with the lattice structure and each collision would reduce its speed to zero. But since the field is present, it then accelerates again until it collides and loses its speed. The time spent in between collisions is known as the mean free time and there is a particular expression for electron mobility given in terms of the mean electron drift speed. This is why you see current is a function of an average drift velocity instead of acceleration.
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Jul30-08, 05:22 AM
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Thanks, but I'm talking about ions in a solute that are attracted elsewhere by an opposite charge (atoms instead of electrons).
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Jul30-08, 06:20 AM
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here is a visual example :
http://www.somasimple.com/flash_anims/three_forces.swf

Of course, we know that in a real situation there is friction, viscosity.... (and of course, ions interact with the other ones).
Defennder
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Jul30-08, 11:29 AM
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Well if you are dealing with a liquid solution of charged ions, then if you were to consider the steady state case only, there would be no current after some time because all the ions would have settled into a net equilibrium position within the solution. Unless you apply an electric field through the liquid, and the resulting ions would be polarised by the field.
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Jul30-08, 11:39 AM
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I understand and it's the reponse I waited for.
The flash animation was of course unreal but shows two states at time t1 and t2 and t2 ids far from an equilibrium.
Defennder
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Jul30-08, 11:55 AM
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I don't understand what you're asking. The animation you linked shows three charged particles being attracted to a central charge, so I don't know how this is related to your case of a solution of charged ions. Current usually refers to the net motion of a lot of charged particles, not the movement of a few individual ions. In the case of a solution of charged ions, the current would be zero because the individual movements of the ions are haphazard and random in the absence of an ordering electric field.
somasimple
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Jul31-08, 12:22 AM
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Sorry, I wasn't clear.

I understand that the current is the expression of an average of motion when ions do not move in a random way.

But, if some ions are attracted in a same direction (forget the movie), their motion may be different and their speed, too? (coulomb). Of course the resultant current is a computation of of fields.
Defennder
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Jul31-08, 02:30 AM
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Well yes, if you have a net movement of charged particles then you would have a current. The net movement should really be called the average drift velocity because of brownian motion. The individual speeds of the ions are different and they differ based on their local conditions. It's their net average speed which we call drift. There's something called the Drude model in statistical physics (or is it solid state?) which you can derive Ohm's law regarding current density and magnitude of electric field.

http://en.wikipedia.org/wiki/Drude_model
Unfortunately I don't know much about this if this is what you're asking.
somasimple
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#13
Jul31-08, 04:31 AM
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You're a man!


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