Electrical – Basic Capacitor Discharge


I found this illustration from the SparkFun website on how a capacitor charges and discharges, but I have some difficulties with it (and capacitors in general):
enter image description here

  1. What initially causes the capacitor to accumulate a charge difference? I would expect the current to see this as an open circuit, given that no current flows through the dielectric to ground, and therefore there would be no initial flow.
  2. When we discuss a build-up of positive charge on one of the plates, we are saying the valence electrons have been repelled by the negative charge of the the other plate. Maybe this is a chicken/egg situation, but shouldn't the electrons on the negative side of the plate be repelled by the same force (even more since it's closer) as the positive side, inhibiting the charge buildup from occuring in the first place?
  3. In the picture below, won't the discharge reverse the direction of current? That is, it's going to flow from the negatively charged plate, across the LED, resistor, gate, and then to the positively charged plate?
  4. When the capacitor discharges, the current seems to want to flow to redistribute/equalize charge across the circuit. This is different than just flowing to ground (or the positive terminal on the battery), as I am used to seeing. If when the gate closed to allow discharge there was also a path to the positive terminal on the battery, would this change anything?

Best Answer

  1. Current does flow through the capacitor, through the dielectric. But only while it's charging.

  2. This discussion of capacitors may help. The charge buildup does repel further charge buildup. That's why the rate of current flow decreases as the capacitor charges. Work must be done on the charge to move it - the integral of that work is the energy stored in the capacitor.

  3. Yes. The animation is very bad at showing this.

  4. Yes, current will tend to flow so as to equalise the potential on both sides. Neither is necessarily related to ground. That's how a Cockroft-Walton multiplier works.