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Electricity & Magnetism: Circuits and the movement of charge

  1. Aug 13, 2008 #1
    Hi,

    So my former knowledge of electricity and its movements made me understand that only electrons move from the negative electrode to the positive and then through the wires back into the battery. But as I am reading from my textbook (about capacitors) I am starting to feel that even protons are forced to move through the circuit until the charge is distributed properly. Is this true or did I just confuse myself? An explanation would be wonderful. Thank you in advance.
     
  2. jcsd
  3. Aug 13, 2008 #2

    Dale

    Staff: Mentor

    In metal the charge carriers are electrons. The protons are not free to move around because they are physically "stuck" in the lattice.

    In the capacitors, where you see + charges, that is really a deficit of electrons i.e. when an electron moves away from a region there is a net positive charge left behind.
     
  4. Aug 13, 2008 #3
    Alright, so I guess I got a bit confused after all. Thanks for clarifying that dalespam.
     
  5. Aug 13, 2008 #4
    Ok I have one more question that I wish to be explained. When we are talking about a battery, and when we discuss a positive charge being moved towards the positive plate - are we discussing protons or actual atoms that are positive ions? That is lost their electrons (which I understand are being used to power the current).
     
  6. Aug 13, 2008 #5

    Dale

    Staff: Mentor

    I don't know much about battery chemistry. I guess in a lot of batteries there is some metal part where the charge carriers are electrons and some electrolyte part where the charge carriers are ions of one sort or another.
     
  7. Aug 14, 2008 #6
    In the battery you have two different groups of ions separated physically. On one side a group of positive ions, on the other side a group of negative ions. Those groups are connected to a conductor (the terminals). When another conductor comes and connects the two terminals then you get current as there is a difference in electromotive force (voltage?)

    I don't remember what actually happens from there. Negative ions give up electrons and electrons move to the positive?
     
  8. Aug 14, 2008 #7

    Defennder

    User Avatar
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    I believe the answer can be found here:
    http://en.wikipedia.org/wiki/Electrochemical_cell

    It works by electrochemical reactions and potentials. There isn't any positive ions moving which contributes to the current. Now, I do not know if there are batteries which make use of positive ions as well as electrons for current flow, but if you're talking about semiconductors, then the current contribution is also due to the flow of holes which may be considered positively charged ions.
     
  9. Aug 15, 2008 #8
    Electrons are the only particles that move in current. Remember, it is the PROTONS that determine elements. If you add/subtract a proton from an element you have an entirely different element. The way a battery works is one side is at a defficit of electrons (+) and the other has a surplus (-). The flow of electrons from positive to negitive is what does the work in a circut.
     
  10. Aug 15, 2008 #9

    Dale

    Staff: Mentor

    This is only true for currents in metal. In other conductors other things may be charge carriers, e.g. dissolved ions in an electrolyte or ionized particles in a plasma.
     
  11. Aug 15, 2008 #10
    Ah, good catch.
     
  12. Aug 29, 2008 #11
    Ooohhh...explain how that works. Interesting...
     
  13. Aug 29, 2008 #12

    Dale

    Staff: Mentor

    A metal is a conductor because, in response to an applied e-field, electrons will experience a force and move. An electrolyte is a conductor because, in response to an applied e-field, dissolved ions will experience a force and move. Rubber is an insulator because, in response to an applied e-field, the charges inside the rubber experience a force but are so tightly bound that they cannot move.
     
  14. Aug 30, 2008 #13
    Okay, so say you have a soln of NaCl. What provides the e-field? I've been out of college for a bit and need refreshed on these things. Thanks for everyone's input.
     
  15. Aug 30, 2008 #14
    You can think of the charge as flowing from positive to negative. If you start with a neutrals charge at both end of the wire 0 ------ 0 and move to a state +2 -------- -2 then how do you know that a positive chargge of +2 has not flowed right to left along the wire? Some old books on EM might say that you can 'think of' positive charge flowing right to left. But what's 'really' happening is that electrons are flowing left to right, i.e., the negative charge is moving.

    In the NaCl solution example, do you really mean "what provides the E-field?" Or do you mean, "how is the NaCl solution affected by the field?" Any power source can provide the E-field - a battery a nuclear power station, wind turbine...

    In solution, the Na atoms are positive ions and the Cl atoms are negative ions. That is, each Na atom has lost one electron which has been picked up by a Cl atom. So the Na atoms move in one direction when affected by the field, the Cl ions in the other.

    Why did Na lose an electron? Look up 'Bohr's model of the atom'. Quantum gurus will tell you it's an 'incomplete model', but it suffices for basic chemistry! Because Na only has one electron in its outer shell it's easily dislodged. Because Cl wants only one electron to complete its shell it easily picks up an electron.

    Remembering back through the decades, I seem to remember that Asimov's "Understanding Physics" was very good on this basic stuff.
     
  16. Aug 30, 2008 #15
    Thank you so much!
     
  17. Sep 3, 2008 #16
    consider this question:
    a 4v battery is in parallel with 12 and 2 ohm resistor. Find power used by 2 ohm resitor.

    Now you wonder what is the problem'
    well relation between battery and resitor is given but relation between two resitors is not given .I mean to say it is not specified whether two resistor are in series or parallel.

    This is same as we say two line segments AB and CD are parallel to line XY. Now AB and CD may be two distinct line or may be parts of same line segement.

    I wan't opinon of y'all on this ASAP and answer.I lost a competition due this ques. but i think what i think is right ???
    Guys Help!!!!!!!!
     
  18. Sep 3, 2008 #17

    Defennder

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    It is given. Your statement of the problem indicates so.
     
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