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Charging a capacitor

  1. May 9, 2015 #1
    I need some clarification on batteries and capacitors.

    Before a battery is connected to a capacitor, the capacitor is uncharged. When a battery is connected, an electron from one conductor moves toward the positive terminal of the battery, leaving this conductor positively charged. What causes the other conductor to become negatively charged? At this point, the positive terminal loses charge because it has an additional electron. The other conductor is still neutral at this point and something has to cause an electron from the negative terminal to move to the neutral conductor.
     
  2. jcsd
  3. May 9, 2015 #2

    Simon Bridge

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    The battery moves electrons from one side of the capacitor to the other...
    ... electrons get added to it.

    ... of the battery? The + terminal loses electrons, they get transferred to the - terminal.

    The neg terminal is negatively charged, so it repels electrons.
    Like charges on a conductor - which is the neg terminal, the wire, and the capacitor plate, all together - try to get as far away from each other as possible.
     
  4. May 9, 2015 #3
    Doesn't something prevent the electrons from moving from the positive terminal to the negative terminal? If not, how would the two terminals have opposite charges to begin with?
     
  5. May 9, 2015 #4

    Simon Bridge

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    The battery uses a chemical energy store to move charges from the positive side to the negative side.
    The exact mechanism varies from battery to battery

    Think of the electrons being like water and the battery is a pump moving water from a low reservoir to a high one. What prevents the water from flowing back down?
     
  6. May 9, 2015 #5
    A battery is not a gun that can just shoot electrons. It is important that you understand that for every single electron that comes out of the negative terminal of the battery, an electron must go into the positive terminal at the same time (at least for an ideal battery). If you were to look at a sketch the electric field lines for this circuit, you would see why. The positive end of the battery pulls electrons just as much as the negative end pushes electrons. Remember, there are always electrons at all parts of the circuit, since everything is made of atoms which contain both protons and electrons.

    When a battery is connected to a cap, electrons are added to one plate, making that plate negative, and removed from the other plate, making that plate positive. These happen concurrently.
     
  7. May 10, 2015 #6

    davenn

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    yes, and to follow on from that, to the OP, the voltage source doesnt charge a capacitor, it energises a capacitor
    The net charge in a capacitor is always zero

    Dave
     
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