In the previous section, we have described how charge accumulates at the terminals of a battery. A capacitor,is a device that is specifically designed 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, until the voltage between them, is the same as the battery voltage. (The charge accumulated on the capacitor plates, will typically be significantly greater, than the amount that would accumulate at the battery terminals alone.) Capacitors are designed with specific capacities, i.e. if we connect two different capacitors in turn to the same battery, until they are fully charged, then the amount of charge stored will be different. 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 larger amount of charge at 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 affect the properties of the capacitor.
