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Dc current flow

  1. Sep 10, 2015 #1
    I have a DC source. then I connect a capacitor from the +ve to ground.
    if all connection are made from ordinary , copper for example, wires then electrons flow from negative to positive terminals.So electrons from the negative side of the capacitor would flow to the positive.

    I wonder what would happen if the wire from the +ve to the capacitor positive plate would be made out of a chemical or substance in which only ions can flow or the positive charge carriers?
  2. jcsd
  3. Sep 10, 2015 #2


    Staff: Mentor

    Then you would have positive ions flowing from positive to negative.
  4. Sep 10, 2015 #3


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    This needs a little basic clarification and without getting into deep physics

    OK this I suspect is the circuit setup you are referring to .....

    cap across battery.GIF

    now firstly current ( electrons) don't flow through a capacitor. The circuit symbol is a good representation of this
    see how there is a gap between the plates of the capacitor ?
    Electrons ( charge) moving from the 0V ( negative 0 terminal of the battery flow into the plate of the capacitor connected to B
    This will repel an equal number of electrons off the plate connected to A and they will flow ( move) to the battery positive terminal
    This will happen in a brief time and when the voltage across the terminals of the capacitor equals that of the battery,
    there will be no more current ( electron) flow

    hope that helps :smile:

  5. Sep 11, 2015 #4
    well thanks for the interest , I appreciate, i have know for some years that current doesnt go through a capacitor, I was wondering more about some other things like, DC unlike AC has a net energy flow in one direction, well if you reverse it then the other but I suppose one at a time.
    Now even though I mention ions and electrons as charge carriers the actual force is carried by the fields (electric, magnetic)
    lets take your picture , davenn, as an example , as I said the the wire from battery ground to capacitor negative plate is copper , but the wire from batery positive to capacitor positive is such that ions are the carriers in that wire.
    Now as you said once connected to the battery the electrons rush towards the negative plate , now the negative charge from the positive plate should rush to the battery's positive terminal but it can't because in that pathway the carriers are ions and surely + charges wont rush back to other + charges.So does in this situation the ions move from the battery + to the capacitor positive plate ?

    or maybe an even simpler and better example, take a wire with certain resistance connected across a battery, if that wire is copper electrons from negative flow into the positive terminal and form current.
    Now in the middloe of that wire cut it in half and insert a piece of a salt water tube for example or something similar, the connection is back once again but how do the charge carriers flow now? the electrons in the copper still flow from negative to positive , but the ions flow the other way , how can there be still current if the ions flow the opposite direction of electrons , dont they collide and cancel the current ?
  6. Sep 13, 2015 #5
    bump :)
  7. Sep 13, 2015 #6


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    Staff: Mentor

    Your salt solution is electrically neutral so contains both positive ions and negative ions. The positive ions migrate towards the negative electrode where they accept electrons and become neutral; the negative ions migrate towards the positive electrode, and here they give up their electrons. This transportation of electrons from one electrode to the other is seen by the external circuit as an electric current.
  8. Sep 13, 2015 #7
    Ok salt was my first guess , technically I wanted to make the example of having an ordinary copper wire but one part ot if replaced by a conductor were only the positive charge carriers can move and form current (I suppose there are such conductors)
    then I wanted to ask what happens in such a situation ?

    I suppose the electrons from the negative terminal start their way up to the positive in the copper wire but then they encounter the positive charge carrier wire , were the positive charges are going down the opposite way of electrons , so what happens ? how can there be a net current formed in thios situation if the charges creating that current go head on from opposite directions?
  9. Sep 13, 2015 #8


    Staff: Mentor

    The sign of the charge carriers is almost always completely irrelevant. When you close the circuit in davenn's drawing current goes clockwise. It doesn't matter a bit whether the clockwise current is due to positive charges moving clockwise or negative charges moving counter clockwise or a mix of both.
  10. Sep 14, 2015 #9
    I do believe it works the way you just said, but then it's interesting , because normally an electric field has field lines as we all have seen in the pictures that illustrate that they go out from a positive charge and in to a negative charge. so that alike charges repel and opposite ones attract.

    Since this circuit is powered by a DC source (battery) I assume the field along the length of the conductor from negative to positive battery terminals is in one net direction?
    If so then i'm a little puzzled to see why oppositely charged particles all somehow follow the same path for a given field pointing in a given direction?
  11. Sep 14, 2015 #10


    Staff: Mentor

    I have no idea what you mean by this. Negative charge carriers go counter-clockwise (in the davenn figure), positive charge carriers go clockwise. What do you mean that they follow the same path?
    Last edited: Sep 14, 2015
  12. Sep 14, 2015 #11
    exactly this is what puzzles me.the battery sets up the electric field which points from the positive side into the negative side/terminal of the battery, I hope this is correct , so the field lines go a certain direction , (for test particles they go from the positive into the negative one) so if I placed a test charge in this field of the battery and wire connecting terminals , the test charge would either want to attract/repel one terminal or the other based on whether its positive or negative,
    so in a wire negative electrons travel towards positive terminal while in the section were the wire has only positive charge carriers the charges travel the opposite direction from + to -.

    this is the part that puzzles me how come the overall current, as looked from the perspective of the batery's both terminals being connected , flow in one direction even though parts of it flow the opposite ?
  13. Sep 14, 2015 #12


    Staff: Mentor

    You must distinguish "current" from "flow of charge carriers". In the circuit the current is always clockwise. The sign of the charge carriers is completely irrelevant. A clockwise current can be formed by counter-clockwise movement of negative charge carriers. A clockwise current can also be formed by clockwise movement of positive charge carriers. A clockwise current can also be formed by the simultaneous counter-clockwise movement of negative charge carriers and clockwise movement of positive charge carriers (this is the usual case in an electrolyte). In all three cases, for almost all practical purposes, the sign of the carriers is irrelevant, and only the current is relevant.
  14. Sep 14, 2015 #13
    that is interesting, I thought that this clockwise / counterclockwise current going on at the same time was only a phenomenon found in chemical batteries and oher chemical electric power sources, so it seems it might as well go on on a DC circuit if the conditions are like the ones i mentioned in my previous example.
  15. Sep 14, 2015 #14


    Staff: Mentor

    Any time you have electrical current flowing through an animal or any other biological medium you will have motion of both positive and negative ions in opposite directions at the same time. This type of conduction is quite common.
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