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Capacitors : concepts

  1. Mar 30, 2013 #1
    I have several questions about capacitor's concepts.
    Q1)What does battery really do in charging a capacitor?
    Q2)Uncharged capacitor doesn't mean the plates don't have any charges?
    Q3)During charging,the developing electric field between the plates does have any effect in charging?
    Q4)When charging is progressed,battery has to do more and more work until the potential difference becomes same.Is this right?

    Energy stored in a capacitor of capacitance C:Let small work done dW to move a charge dq from one plate to another plate.Then at an instant let V be the potential difference of the capacitor and q be the charge in the plates.
    => dW=V.dq =>dW=q/C.dq
    =>Integrating the above equation by q=0 to q=Q,then we get W=(Q^2*C)/2
    I don't understand this derivation. Does a charge actually move from one plate to another within capacitor?

    There is another derivation which also I cannot understand.It is about moving a plate by a small distance and calculating work.But I don't understand why someone moves that plate to move by about a small distance.Also it says electric field acting on one plate is σ/2ε and not σ/ε.Because one plate is held stationary and the other is moving .So it experiences attractive force of the other plate and it's own attraction to the other plate.And while determining the force, the latter term should be used right?

    I am sorry but I am pretty weak in Physics.
  2. jcsd
  3. Mar 30, 2013 #2


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    Staff: Mentor

    It leads to net charges (of same magnitude and opposite sign) on both plates of the capacitor.
    Uncharged capacitors do not have (net) charges on their plates (to a very good approximation).
    Depends on your view of "have any effect". The electric field is related to the voltage in the capacitor, which can influence the charging process.
    During the whole charging process, energy is transferred to the capacitor.
    Not through the gap (in an ideal capacitor), but you can consider this as an option. Charge can move through cables if you connect both sides.
    Why not?
    That is the same attraction.
  4. Mar 30, 2013 #3
    It pulls charges out from one plate and pushes same type of charges onto the other plate.

    A neutral plate has an equal amount of plus and minus charge, we often call that an uncharged plate. If two batteries have pushed the same minus charge onto both plates, we might say the capacitor is charged, or we might just as well say the capacitor is uncharged.

    When some charge has been pushed onto a plate, pushing more charge requires more force, and therefore more energy.

    To charge an almost empty capacitor a battery could use a very small voltage, if batteries could adjust their voltage.

    One way to charge a capacitor is to take a charge from a plate and put it onto other plate.

    Other way is to take a charge from a plate and store it somewhere, and take a charge from somewhere else and put that onto the other plate.

    Well that just sounds all wrong.
    Last edited: Mar 30, 2013
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