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Capacitors

Discussion in 'General Electronics Discussion' started by Ricardo7890, Sep 3, 2012.

  1. Ricardo7890

    Ricardo7890

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    Jul 17, 2012
    This will probably seem like a stupid question to those who know more about these things but why does a capacitor with bigger capacitance take longer to charge.Capacitance is a measure of how much of a charge differential it takes to create a certain voltage across its plates right. if this true then a thing with higher capacitance could store the same voltage as something with lower capacitance, but with less charge. so if i apply a voltage to two capacitors wouldn't the one with the higher capacitance "fill up" first. my logic for this:
    1. a capacitor stops charging when it adopts the voltage of a source
    2.This voltage is created by the charges of electrons that are stored in the plates, which can be measured in coulombs
    3. a capacitor with a high capacitance will need less coulombs to create a certain voltage than one with lower capacitance
    4. Then a high capacitance capacitor will stop conducting quicker than one with less capacitance because at the same rate of amperes the higher one will create the same voltage quicker
    i know from experience that this isn't true, but my model for how capacitors work doesn't agree with it, could someone tell me where i'm wrong
     
  2. Timescope

    Timescope

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    Aug 30, 2012
    A capacitor is like a container that stores water. Imagine two containers of the same height, one "thin" and the other "fat". If you use a hose to fill them, the thin one will fill up faster than the fat one.
    Also, the thin one will hold a smaller volume of water (lower capacitance) than the fat one (higher capacitance). In this analogy, water represents the charge, the flow of water per second represents the current and the height represents the voltage.
    Timescope.
     
  3. wingnut

    wingnut

    233
    8
    Aug 9, 2012
    C = Q/V ie how much charge can be squeezed into a capacitor if you apply a certain amount of pressure (V). A bigger cap will allow more charge in for the same V.

    The idea in your mind that capacitance is "charge differential" makes no sense to me, and that this somehow creates voltage, is putting the cart before the horse. There already is voltage (pressure) because without pressure you cannot blow up a balloon (capacitor). I guess a big capacitor would be like an easy to inflate balloon, and a small cap like a super hard to blow up balloon.
     
  4. Ricardo7890

    Ricardo7890

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    Jul 17, 2012
    i don't think that comparing a capacitor to an empty bucket is the right thing if electrons are like water particles. a capacitor has the same amount of charge particles charged as uncharged, When voltage adds one electron to a plate it's electrical field pushes another electron away from the other plate across the dielectric so for a while its like a complete circuit, but for every electron that's added to one plate, the other plate loses and equal amount, so the capacitor as a whole has the same number of charges, just in different parts, so i don't see how an empty/ full bucket analogy applies
     
  5. john monks

    john monks

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    Mar 9, 2012
    The bucket analysis is weak for the reason you state.
    1. The capacitor always has the source voltage unless you have a resistor or some else between the source and the capacitor.
    2. Is essentially correct. The number of coulombs is directly proportional to the voltage.
    3. Exact opposite. A low capacity capacitor needs more voltage for the same amount of charge than a high capacity capacitor.
    4. Exact opposite. A high capacity capacitor will take longer to reach a particular voltage that a low capacity capacitor will.

    Remember this. A one farad capacitor starting from zero volts will take one second at one amp to charge to one volt. That is voltage = coulombs divide by capacity in Farads.
    A coulomb is a number of electrons. It is 6.24 X 10^18 electrons.
    One coulomb of charge comes from a current of one ampere for one second.
    Analyze this and you never be confused again.
     
    Last edited: Sep 3, 2012
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,222
    2,696
    Jan 21, 2010
    I sometimes explain a capacitor like a tank with two pipes on either side and a rubber diaphragm. It's always full of water, but if you pump water in one side, exactly the same amount flows out of the other side.

    At a given pressure, the water will get pumped in quickly at first, but will slow down until it eventually stops (the pressure can no longer overcome the springyness of the diaphragm.

    The larger the tank, the longer this takes, and the more water that gets pumped into one side.

    It's better than the bucket analogy in a number of respects, but the bucket analogy explains the question you asked. :)
     
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