Electrical Engineering ⇒ Topic : Capacitance

The ability of a capacitor to store charge is known as its capacitance. It has been found experimentally that charge Q stored in a capacitor is directly proportional to the p.d. across it i.e.

The constant C is called the capacitance of the capacitor. Hence capacitance of a capacitor can be defined as under :

The ratio of charge on capacitor plates to the p.d. across the plates is called capacitance of the capacitor

Unit of capacitance

We know that : C = Q/V

The SI unit of charge is 1 coulomb and that of voltage is 1 volt. Therefore, the SI unit of capacitance is one coulomb/volt which is also called farad (Symbol F) in honour of Michael Faraday.

                            1 farad = 1 coulomb/volt

A capacitor is said to have a capacitance of 1 farad if a charge of 1 coulomb accumulates on each plate when a p.d. of 1 volt is applied across the plates.

Thus if a charge of 0.1C accumulates on each plate of a capacitor when a p.d. of 10V is applied across its plates, then capacitance of the capacitor = 0.1/10 = 0.01 F. The farad is an extremely large unit of capacitance. Practical capacitors have capacitances of the order of microfarad (μf) and micro-microfarad (μff) or picofarad (pF).

                                     μf = 10^-6F ; 1 μff (or 1 pF) = 10^-12 F

Capacitance is defined as the property of a capacitor to store charge. It is also defined as the amount of charge required in creating a unit p.d. between its plates.

If Q coulomb of charge is given to one of the two plates of a capacitor and p.d. of V volt is established between the plates, it can be written as:

                                                                                  ............... (1)

where C is its capacitance.

From Eq. (1), the unit of capacitance is coulomb/volt which is also known as farad. Farad is for honour of Michael Faraday.

So one farad is defined as the capacitance of a capacitor that requires a charge of one coulomb to create a p.d. of one volt between its plates.

The smaller units of capacitance are as follows:

The property of a capacitor to store electrical energy when its plates are at different potentials is called its capacitance. The unit of capacitance is Farad (symbol F).

   Consider a pair of conductors A and B as shown in Fig. , which are initially uncharged. If a charge q is removed from A and placed on B, an electric field will be established between A and B since A is positively charged and B is negatively charged. Because of the electric field, a potential difference (V) now exists between the two conductors. The ratio of the charge (q) and this potential difference (V), that is q/V is known as the capacitance between the two conductors and is represented as

C is expressed in Farads, if q is Coulombs and V is in volts.

   The capacitance depends upon the physical dimensions of the system of bodies (in this case, two conductors) and the permittivity of the medium in which the bodies are placed.

                                        Electric field between two charged bodies (conductors) A and B