What is a Capacitor

   What is a Capacitor :-

• Capacitors are simple passive devices which are used to store 

   electricity. The capacitor has the ability or “capacity” to store 

    energy in the form of an electrical charge producing a potential 

    difference (Static Voltage) across its plates, much like a small 

     rechargeable battery.

   Capacitor Symbol :-

• A capacitor is formed from two conducting plates separated by 

air or by some form of a good insulating material such as waxed 

paper, mica, ceramic, plastic or some form of a liquid gel. The 

insulating layer between a capacitors plates is commonly called 

the Dielectric.

• Due to this insulating layer, the d.c. current can not flow through 

the capacitor but instead a voltage is produced across the plates in 

the form of an electrical charge.


• If a current i flows, positive change, q, will accumulate on 

the upper plate. To preserve charge neutrality, a balancing negative

charge will be present on the lower plate.



• Hence, there will be a potential energy difference (or 

voltage v) between the plates which is proportional to the charge q.

where,     A        is the area of the plates.

                d         is their separation.

              Ɛ0      is the permittivity of the 

                        insulating layer                           

                          ( Ɛ0 = 8.85 pF/m for  vacuum).

• The capacitance is given by the following 

 expression :

The capacitance is measured in Farads (F)

The charge q is hence given by the expression :

•  The conductive metal plates of a capacitor can be either 

square, circular or rectangular, or they can be of a cylindrical or 

spherical shape with the general shape, size and construction of 

a parallel plate capacitor depending on its application and 

voltage rating.

• When used in a direct current or DC circuit, a capacitor  

charges up to its supply voltage but blocks the flow of current 

through it because the dielectric. However, when a capacitor 

is connected to an alternating current or AC circuit, the flow 

of the current appears to pass straight through the capacitor 

with little or no resistance.

Types of Capacitors :-

• There are a large variety of different types of 

capacitor available in the market with their own set of 

characteristics and applications, from very small delicate 

trimming capacitors up to large power metal-can type 

capacitors used in high voltage power correction and 

smoothing circuits.

• Capacitor types are distinguished by the material used as 

the insulator.

• Let us now discuss a few common types of capacitor 

available.

Variable Capacitor Symbol :-

• Variable dielectric capacitors are multi-plate air-spaced 

types that have a set of fixed plates (the stator vanes) and 

a set of movable plates (the rotor vanes) which move in 

between the fixed plates.

• The position of the moving plates with respect to the 

fixed plates determines the overall capacitance value.

• The capacitance is generally at maximum when the two 

sets of plates are fully meshed together.

• High voltage type tuning capacitors have relatively large 

spacings or air-gaps between the plates with breakdown 

voltages reaching many thousands of volts.

Variable Capacitor Symbol :-

Film Capacitor :-

• Film Capacitors are the most commonly available of all 

   types of capacitors

• These capacitors have a relatively large family with the 

   difference being in their dielectric properties which 

   include  polyester (Mylar), polystyrene, polypropylene, 

   polycarbonate , metalised  paper, Teflon etc.

• Film type capacitors are available in capacitance ranges 

   from as small as 5pF to as large as 100uF .

• Film Capacitors which use polystyrene, polycarbonate or 

Teflon as their dielectrics are sometimes called “Plastic 

capacitors”. The main advantage of plastic film capacitors 

compared to impregnated-paper types is that they operate 

well under conditions of high temperature, have smaller 

tolerances, a very long service life and high reliability.

Film Capacitor :-