Ion flow and current

Discussion in 'Physics' started by jpanhalt, Feb 22, 2010.

  1. jpanhalt

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    Any comments?

    John
     
  2. Wendy

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    So I wonder how the current got there.

    The other thought is, discounting chemical reactions, the remaining majority current is electrons.

    There are other electrochemical types of current (notably batteries), but when it hits a wire or conventional conductor it is all electrons.

    Lets take an example that isn't quite as obvious though, water and table salt. There is electrolysis going on there, but there is also straight forward conduction. Does this mean most of the current is not electrons? I've never really thought about it, my instinct says it is electrons too. I remember my physics class in college measuring the electric field in diluted water.

    Pure water is not a conductor, not even a poor one. I've serviced high power lasers that used DI water to cool xenon flash tubes that were continuously fired with 350VDC at quite a few amps, all the current went through the xenon tube, not the water.
     
  3. Wendy

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    Just googling around...

    http://everyday-chemistry.suite101.com/article.cfm/does_distilled_water_conduct_electricity

    Wikipedia's take on the subject...
    http://en.wikipedia.org/wiki/Ion_flow

    One paragraph I found interesting, and am not sure I believe, in the article...

    The problem I have with this statement is the atoms of water are bound down in ice, nothing can move. Since ions tend to be the molecules this seems contradictory.
     
  4. jpanhalt

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    I am sure most contributors to the thread from which that quote came ("delete my account " thread) will recognize it. It piqued my interest from the standpoint of definitions. That is, what is the most accepted definition of electrical current flow?

    Is it restricted only to electrons in a conductor or does it include movement of holes in semiconductors and ions in other systems (e.g., electrophoresis and batteries)? Where do action potentials and nerve impulses fall?

    Calling all pedantics. :D

    John

    Bill, We cross posted. John
     
  5. Wendy

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    I do not consider the hole movement anything but analogy, a way to understand what is happening inside a transistor that humans can make sense of. They are not particles, but the lack of a specific particle, the electron.

    I get the feeling some folks think that electricity changes when it enters the body. I disagree with that point of view.

    There are extremes in physics that doesn't involve electrons, but during everyday use, with one or two exceptions, we never see them.

    The exceptions is battery chemistry and plating chemistry, which resemble in a lot of ways. In the end though, it is still electrons in and electrons out, which some complicated physics in between.

    Positrons are a form of antimatter, wonder if the OP knew that? They are not naturally occuring particles either, but have to be manufactured on demand and used before they are consumed.

    So ions is it. I'm interested in examples that might come up during this thread.
     
  6. t06afre

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    A simple way of measuring skin conductance is to use a low voltage DC source. Two electrodes are placed on the skin, and the resulting current is measured. The current that flow in the electrode leads is the same current that flow in the skin and body tissue.
     
  7. loosewire

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    Is that calling for ratch a second chance I f he want it.do you get a
    second chance.
     
  8. beenthere

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    From some years building electrofishing equipment, I can say that placing electrodes in impure water and applying a voltage will promote quite a bit of ionic conduction. I had to place a bit of protection circuitry in the control box to limit current at 15 amps.

    It works in a vacuum, too. That is how mass spectrometry works. Or even isotopic separation as in the calutrons at Oak Ridge.

    Any time an electrically unbalanced atom exists, an electric field will cause it to move.

    In cells, there is a pump mechanism - http://en.wikipedia.org/wiki/Na+/K+-ATPase - that maintains a potential difference across a cell membrane. I have little experience with animal cells, but plant cells exhibit on the order of 50 mv potential difference intra - extracelular.
     
    Last edited: Feb 22, 2010
  9. Wendy

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    I used to work on helium leak testers, which is a dedicated mass spectrometer. Between the filament, the target, and the magnet I thought is was pretty obvious how they worked.

    The quote is a bit out of context though, ultimately it is the wires that deliver the current.

    As I understand it ionic conduction is still electron flow, as in the case of salt water. There are two mechanism happening as I understand it, one is the ions themselves moving to their respective electrodes, and the second is the electrons moving from ion to ion. Do I have it wrong?
     
  10. studiot

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    Without trying to delve deeper into what charge is

    Physics theory as we know it recognises two polarities of charge viz positive and negative.

    Charge is always associated with matter (ie particles with mass) As far as I know there are no known particles of zero rest mass with a charge.

    Negative charges are associated with low mass particle, the most common being the electron.

    Positive charges are associated with high mass charges the most common being the proton.

    Current is an averaging term which describes the action of large numbers of charged particles over time.

    So it is no suprise in our low energy world that current is associated with electrons not protons.

    However most of the rest of the universe is made of stars. These are high energy worlds where the energy to move massive particles is in plentiful supply. Obviously electrons are also moved in stars but cosmic physics allows for proton currents in plasmas.

    It is this asymetry of mass in charge carriers which makes one sort or the other more important depending upon the local environment.

    Positive holes or negative holes are a convenient fiction or model. They have no physical reality and do not actually carry charge, they just simplify the charge accounting.
     
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