In a general way capacitance can be characterized as that property of circuit component (known as capacitor) in which energy is capable of being stored in an electric field. A significant and distinguishing feature of capacitance is that its influence in an electric circuit is manifested only when there exist a varying potential difference across the terminals of the circuit component.
The current-voltage relationship involving the capacitance component given as:
Above equation describes a situation in which the current through the capacitance component is proportional to the derivative of voltage across it.
Solving Eq. (1.11) for the voltage yields:
The proportionality constant C expresses the charge-storing property of the element and is called the capacitance of the component. Any circuit component showing the property of yielding a current which is directly proportional to the rate of change of the voltage across it terminals is called a capacitor.
A capacitor is basically meant to store electrons (or electrical energy), and release them whenever required. Capacitance is a measure of a capacitor’s ability to store charge and is measured in farads (F). Farad, the unit of capacitance is very large, so micro-farad (µF) or micro-micro-farad (µµF) is usually used 1 µ F = 1-6 F and 1 µ µ F, also called the Pico-farad (p F), = 10-12 F.
A capacitor offers low impedance to ac but very high impedance to dc. So capacitors are used to couple alternating voltage from one circuit to another circuit and at the same time to block dc voltage from reaching the next circuit. It is also employed as a bypass capacitor where it passes the ac through it without letting the dc to go through the circuit across which it is connected. A capacitor forms a tuned circuit in series or in parallel with an inductor.
A capacitor consists of two conducting plates, separated by an insulating material, called the dielectric. Capacitors, like resistors and inductors, can either be fixed or variable. Some of the commonly used fixed capacitors are mica, ceramic, paper, plastic-film and electrolytic capacitors.
Variable capacitors are mostly air-gang capacitors. An elementary variable capacitor consists of two sets of copper or aluminum plates (which may have the shape of half discs).
Each set is mounted on a common shaft, one set being fixed and the other, which interleaves with the former, being movable. The capacitance of the capacitor can be easily varied by varying the degree of interleaving, which is possible by rotation of movable plate shaft. Such variable-capacitance, air-capacitors, shown in Fig. 1.13 is widely used in radio work.
Symbols for fixed type and variable type capacitors are given in Figs. 1.14 (a) and 1.14 (b) respectively.
Capacitors employed in power supplies of electronic and radio equipment may have values ranging from a few micro-farads to several thousand micro-farads. The capacitors used in the tuning circuits of the TV receivers may be as small as 10-20 pF.