In the previous section, we have described how charge accumulates at the terminals of an isolated battery. A capacitor,is a device that is specifically designed to be used for accumulating (i.e. storing) electrical charge. ( Note. Capacitors also have other characteristics that are particularly important in a.c. circuits.)
A capacitor consists of two conductive plates, separated by an insulator. (The insulator in a capacitor is given a special name, it is called a dielectric).
If we connect a capacitor to a battery, then there will be a brief flow of current, as charge accumulates on the capacitor plates. The current stops when the voltage between the plates increases to the same level as the battery voltage. Capacitors are designed with specific capacitance values, i.e. if we connect two different capacitors in turn to the same battery, (until they are fully charged), then the total charge stored will be different in each case. The capacitance (C) of a capacitor, is defined as the ratio of the charge stored (Q) to the voltage between the plates (V)
i.e. C = Q/V.
e.g. A capacitor that stores a more charge with the same voltage, has a larger capacitance.
In the next section we will discuss how a capacitor stores electrical charge, making use of the previous analogy between a battery and an air pump. Although a capacitor is a fairly simple component in terms of construction, there are several aspects to consider. Therefore we will consider individually how the area of the plates , their separation and the choice of dielectric all affect the capacitance of the capacitor.