When the switch is closed, as in Figure 3, there is a momentary surge of current through the conductor to and from the plates of the capacitor. When the current reaches the negative plate of the capacitor, it is stopped by the dielectric.
The surge of electric current to the capacitor induces a counter electromotive force in the conductor and the plates. This counter electromotive force is call reactance. When reactance has reached a level equal to the voltage of the battery, the capacitor is fully charged. There is no further flow of current. When the capacitor is fully charged, the switch may be opened and the capacitor will retain its charge (Figure 4). Because of the difference of charges on the plates there is a source of potential energy in the capacitor. The energy stored is the energy that was required to charge the capacitor.
The lines of force between the plates of the capacitor represent an electric force field (see Figures 3 and 4). This electric force field exists because of the unequal charges, positive and negative, on the inside surfaces of the plates. Current cannot flow through the electrostatic field because of the dielectric insulator. In other words, the difference in potential between the plates induces within the dielectric an electrostatic field that retains the charge.
Discharging of a Capacitor The charged capacitor shown in Figure 4 is now a source of potential energy. This potential energy is now available for its intended electronic application. If the switch is closed, as in Figure 5, current will immediately begin to flow through from the negative plate to the positive plate. The capacitor is discharging.
The charged capacitor is the source of voltage for the current flow. The current will cease flowing when the charges of the two plates are again equal, meaning that the capacitor is completely discharged.
We have simulated the charging and discharging of a capacitor in our Capacitor Interactive Java Tutorial.
Interactive Java Tutorial
Capacitor Discover how a capacitor is charged and discharged.
Dielectric Materials The dielectric material in a capacitor prevents the flow of current between its plates. It also serves as a medium to support the electrostatic force of a charged capacitor. A variety of materials are used for dielectrics as shown in the chart below.
Dielectric materials are rated based upon their ability to support electrostatic forces in terms of a number called a dielectric constant. The ability of the dielectric to support electrostatic forces is directly proportional to the dielectric constant. A vacuum is the standard by which other dielectrics are rated. The dielectric constant of a vacuum is 1. You can see from the chart that there is very little difference in the dielectric constant of a vacuum and air. Therefore, air is often referred to as having a dielectric constant of 1.
Material Dielectric Constant Vacuum 1.0 Air 1.00059 Polystyrene 2.5 Paper 3.5 Mica 5.4 Flint glass 9.9 Methyl alcohol 35 Glycerin 56.2 Pure water 81