83. Capacitors

Not only conductors, but also conductor systems have electrical capacitance. The greatest practical interest concerning electrical capacitance represents capacitors.

The capacitor can be named system of two conductors at which after a charge of conductors by electricity of equal charges of opposite sign power lines beginning on one conductor, end on another.

The simplest flat capacitor consists of two parallel plates located at a small distance from each other (fig. 1, a). If the plates' charges are equal in quantity and opposite in sign, then almost all power lines of the electric field are concentrated inside the capacitor. At the spherical capacitor, consisting of two concentric spheres (fig. 1, b), all field is concentrated inside the capacitor.

To charge the capacitor, its plate must be connected to opposite lines of voltage source, for example to the battery poles. It is also possible to connect one plate to a voltage source and ground the other plate. In this case, the charging on the grounded plate will remain the opposite sign and the same as the charging on the other plate. The same amount of charge will go into the ground.

By capacitor charge we mean the absolute quantity of the charge of one of the plates.

Capacitor capacitance is the ratio of capacitor charge \(Q\) to potential difference \(V\) between its plates

\( C \,= \,\frac{Q}{\Delta{V}} \) (8-29)

The capacitance of the capacitor does not depend on the quantity of its charge. It is determined by the geometry of the capacitor and the dielectric permittivity \(\varepsilon\) of the dielectric that fills it.

Capacitors can have huge capacitance with small geometric dimensions (see section 84). In addition, the capacitance of a capacitor is very little dependent on other charged bodies that are located nearby. The external field hardly penetrates into the capacitor (electrostatic protection) and thus does not influence the potential difference between the capacitor plates. These two properties are most important for practical applications of capacitors.

Their application in radio engineering will be described in Book Four.