Capacitor

Electrical devices that are constructed of two metal plates separated by an insulating material, called a dielectric, are known as capacitors (Figure 10a). Schematic symbols shown in Figures 10b and 10c apply to all capacitors.

Capacitor and Symbols

Figure 10 Capacitor and Symbols

The two conductor plates of the capacitor, shown in Figure 11a, are electrically neutral, because there are as many positive as negative charges on each plate. The capacitor, therefore, has no charge.

Capacitor Principle

Now, we connect a battery across the plates (Figure 11b). When the switch is closed (Figure 11c), the negative charges on Plate A are attracted to the positive side of the battery, while the positive charges on Plate B are attracted to the negative side of the battery. This movement of charges will continue until the difference in charge between Plate A and Plate B is equal to the voltage of the battery. This is now a “charged capacitor.” Capacitors store energy as an electric field between the two plates.

Charging a Capacitor

Figure 11 Charging a Capacitor

Because very few of the charges can cross between the plates, the capacitor will remain in the charged state even if the battery is removed. Because the charges on the opposing plates are attracted by one another, they will tend to oppose any changes in charge. In this manner, a capacitor will oppose any change in voltage felt across it.

If we place a conductor across the plates, electrons will find a path back to Plate A, and the charges will be neutralized again. This is now a “discharged” capacitor (Figure 12).

Discharging a Capacitor

Figure 12 : Discharging a Capacitor

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Capacitor

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