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How electrons flow in Capacitor Circuit

  1. May 7, 2014 #1
    Suppose we have an un-charged parallel plate capacitor separated by some distance (say d). As the capacitor is not charged, the plates are in neutral position. Means, it has equal no. of electrons and protons. So, Potential difference across the plates is 0 because energy possessed by 1 C of charge is same on both plates.


    Now when we connect the capacitor with a battery, having potential difference ##3V##, then the left plate of the capacitor will acquire positive charge as electron will be attracted towards the positive terminal of the battery and hence pushed by negative terminal of the battery towards the right plate of the capacitor. Meanwhile as electrons passes through the battery, 1 C of charge will acquire 3 J of energy. When whole electrons are settled on the right plate of the capacitor, we will have a potential difference of 3V at the terminals of the capacitor because each coulomb of charge has gained 3 J of energy.


    Now my doubt is

    1) Does my above understanding is right or it has some misconceptions
    2) As we connect a capacitor to the battery using conducting wire, Conducting wires also has electrons and protons. Do we have to consider them also. In all online Lectures & articles, No body has considered the electrons and protons in connecting wires.
  2. jcsd
  3. May 7, 2014 #2
    1) yes except that you missed a factor of 1/2. The energy is

    E = (1/2) Q V.

    2)Wire's do have a small capacitance that may be considered negligible in most situations just as they do have a small resistance that may also be considered negligible in most situations.
  4. May 7, 2014 #3


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    Staff: Mentor

    In the final result, no. The electrons in the wire simply move around the circuit. The part of the wire that they "vacate" at one end is filled by the electrons that leave one capacitor plate. The electrons that "spill out" of the other end are the ones that enter the other plate.

    For a crude analogy, think of a pipe full of water. When you push some water in at one end, some water spills out of the other end, and the pipe remains full.
  5. May 7, 2014 #4
    You mean connecting wires will also have electrons and protons just like the plate in the capacitor. But no. of electrons and protons they have is very less (negligible) as compared to the plate and hence can be ignored.
  6. May 7, 2014 #5
    The question is not how many electrons are in the wire. That's irrelevant. The question is whether or not the wire builds up an excess (or deficit) of electrons. when a voltage is applied. this charge build up is negligible (most of the time, though there are pesky exceptions).
  7. May 7, 2014 #6


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    Science Advisor

    The units (VCJ) here are expressed in charge and charge separation so I think that's what's important here. The charged particles in the wire (free electrons) move when first connected to the capacitor to neutralize the difference of potential on the wire from the two terminals of the battery generated from it's electrochemically based charge separation process. As the capacitors charge separation increases (the rate of change between plates is not infinitely fast or infinitely large ) between the plates the Potential difference increases at the capacitor reducing the difference of potential on the wire until the battery and capacitor are at the same potential and the wire is neutral IRT the battery terminal. The wire (assuming a low impedance) is just a 'carrier' of charge via the slowly moving (low drift speed) huge number of free electrons in it.
  8. May 7, 2014 #7
    There are no protons, except the ones bound into nuclei. Unless the material used is a plasma or a very exotic ionic conductor, the positive charges are fixed ions forming a crystal lattice.
  9. May 7, 2014 #8
    You might also have wholes in the valence band. That's a very important factor in dopped semi conductors type p.
  10. May 7, 2014 #9
    Holes and protons are two entirely different entities. Also the wiring of a capacitor is a conductor. Conductors usually have excess electrons, e.g. metals or P doped Si. IF the wire is B doped Si then the charge carriers are holes. Only one type of carrier occurs in a conducting wire.
  11. May 7, 2014 #10
    You said "the positive charges are fixed ions forming a crystal lattice" That's why I pointed out that some positive charges are not fixed.
  12. May 8, 2014 #11
    You are correct. In a semiconductor device the wires may be implemented by p++ doped silicon.
    In the original question hoever the positive charges are fixed ions.
  13. May 9, 2014 #12
    I think I have some misconceptions about positive charge and protons. When an electron leaves a place, we say it creates a hole which poses positive charge. Whats the other name of this positive charge.
  14. May 9, 2014 #13
    The charge comes from the protons but the protons don't move while the holes do. That's why we call them holes - not protons.
  15. May 12, 2014 #14
    Holes are missing electrons, so what really moves are the remaining electrons. No positive charge is moving. Nevertheless, the holes can be describes as positive particles with mass, energy, mobility, spin, just like the electrons themselves. Imagine a round dinner table layer out with one fork missing. If the dinner guest left to the hole passes his fork to the right, the hole moves to the left. Repeat this and each fork moves one step to the right. An alternative interpretation is that a fork hole moves around the table.
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