Discharging a capacitor Why should a capacitor discharge? Imagine the capacitor with all its charges is kept inside a bag and tied at two open ends (open-circuit). The positive charges on one plate are attracted strongly to the electrons on the other plate, and the electric field is strong and uniform between the opposite charges on the plates. The opposite charges on either plate seem to hold each other tightly. The field is not so strong to cause the electrons to cross the gap and reunite with the positive charges. There is a fringe field at both ends ready to burst the bag, but the open circuit does not allow charge flow in the external circuit. The electric fields of surface charges which move onto the wires from both the plates and the fringe field will, during the initial transient when the wires are attached to the plates, combine to make the resultant electric field in the wires, zero. The fringe field is different from the field between the capacitor plates. When the charged capacitor is short-circuited using a wire, the effect is to open the tied ends of the bag and the charges are released. The fringe field causes the initial crucial release of charges for further flow of the charges….and development of surface charges that will enable the charges to flow around the wire, and neutralize each other. After all the charges on the plates are neutralized, the circuit attains a state of static equilibrium, a natural relaxed state of equilibrium and we say the capacitor is discharged. Electrostatics and circuits belong to one science not two. To learn how a capacitor charges and discharges and the origin and role of the fringe field in the circuit process it is instructive to understand Current, the conduction process and Voltage at the fundamental level as in the following two videos: i. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-REsWdd76qxc.html and ii. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-8BQM_xw2Rfo.html It is not possible in this post to discuss in more detail the formation of the fringe field when a capacitor is charged and discharged. The last frame References in video #1 lists textbooks 3 and 4 which discuss these topics in more detail using a unified approach and provide an intuitive understanding of discharging a capacitor with the help of sequential diagrams. Charging a capacitor "The voltage across a capacitor cannot change instantaneously" is a statement one finds often in textbooks on circuit theory which discuss the application of a step voltage to an RC circuit. Most students memorise and apply this in circuits without understanding the physical processes involved. It will be useful to learn the operation of charging a capacitor using a unified approach to electrostatics and circuits. It is not possible in this post to discuss the charging of an uncharged capacitor. During the first few nanoseconds after switch ON, while the surface charges arrange themselves, there is no electric field E_cap and fringe field because there is no initial charge on its plates; it is as though the capacitor was not there - as though there were a continuous wire with no break in it. The last frame References in video #1 lists two textbooks 3 and 4 which discuss in detail with a series of diagrams the physical processes in charging a capacitor without and with a resistor (RC circuit).
