A device which is used for storing electric charges is called capacitor. It consists of two parallel metal plates, separated by small distance. The medium between the two plates is air or a sheet of some insulator. This medium is known as dielectric.
When a charge is transferred to one of the plate say (A) due to electrostatic induction it would induce charge Q on the inner surface of the other plate B. It is commonly charged by connecting its plates for a while to the opposite terminals of a battery. In this way some electrons are transferred through the battery positive plate to the negative plate. Charge +Q and – Q appear on the plates. Mutual attraction between the charges keeps them bound on the inner surface of two plates and thus the charge remains stored in the capacitor even after removal of the battery.
Capacitance of a capacitor and its unit
The capability of a capacitor to store charges is called it capacitance. when a charge Q is transferred on one of the plates of a capacitor, the potential difference V between the plates also increases. In other words, the charge”Q “on the plate of a capacitor is directly proportional to the electric potential difference V between them i.e.
C = Q÷V
Where C is a constant called the capacitance of capacitor. The value of C depends upon the area of the plates, the distance between the plates and the medium (dielectric) between them.
The capacitance is thus defined as the ratio of magnitude of charge on either plate to the potential difference produced between them .The SI unit of capacitance is called farad.
Farad (F) is defined as “the capacity of that capacitor which stores a charge of 1 coulomb having the potential difference of l volt between the plates”.
Convenient sub-multiples of farad are:
1micro Farad=10-6 F
1Pico Farad = 10-12 F
The symbol of capacitor are given below .
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