Current through / energy storage in capacitors.

Discussion in 'General Electronics Chat' started by josebencosme, Jul 6, 2008.

  1. josebencosme

    Thread Starter New Member

    Jul 6, 2008
    As far as I understand, dielectrics do not allow the flow of charged particles. How, then, does the path of current from + to - (and viceversa) get completed in a capacitor?

    Also, in the All About Circuits e-book I read "The measure of a capacitor's ability to store energy for a given amount of voltage drop is called capacitance". In a previous paragraph, I read that, when faced with a voltage drop, the capacitor releases (rather than store) energy.

    Would someone on this forum be so kind to answer either or both of these questions for me please?
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Look at energy storage in a capacitor as the amount of charge it can hold. When that charge is drawn off through a resistance, then the quantity of charge present decreases.

    Capacitors do not pass current. If they did, they would not block DC. But when you drive charge on one plate, you may also force charge off the other. With an AV voltage pushing charge on and off, the capacitor appears to allow current flow.
  3. Ratch

    New Member

    Mar 20, 2007

    You are correct. Dielectrics should not leak leak charge carriers. The capacitor works by allowing a voltage to unbalance the charge on its two plates. No matter how much voltage you put across the capacitor, its net charge is always zero. For that reason, I never liked the phrase "charging a capacitor". I think "energizing a capacitor" is more appropriate. While the capacitor is being imbalanced, the charge carriers (electrons) are flowing onto one plate and away from the other plate until the back voltage of the capacitor is equal to the energizing voltage. Therefore the charge carriers do not flow through the dielectric. When the energizing voltage drops, the capacitor will push and attract the charge carriers in the opposite direction so as to drain its stored energy.

    Although the capacitance can be calculated by E=(CE^2)/2, capacitance is defined by how much of a charge imbalance that occurs when a voltage is impressed across it. C=Q/V. It takes energy to force charge carriers together. So in a energized capacitor, you have a number of charge carriers with potential energy. The voltage is the energy density of the charge. Ratch