## Monday, May 4, 2015

### What is capacitor? How capacitor stores charge? How big is Farad?

Capacitor is used to store the energy but the process how capacitor stores energy will be discussed in this article. Capacitor is also known as condenser. It is used both in electrical and electronic circuits and they are available in smaller sizes from Picofarad to KVAr. Capacitors have wide applications in electrical systems such as for starting of single phase motors. Capacitors are also used for providing reactive power so as to improve power factor which will leads to lower electricity bill and lower burden on Generating station.

Capacitors are also used in :-
1.      Oscillation circuits
2.      They are used to smooth rectified AC Outputs.
4.      Time delay circuits
5.      Electrical filters.

Units of capacitor are Farad. It is symbolized by Letter “F”. A Farad is defined as “the capacitance when a potential difference of one volt appears across the plates when charged with one coulomb.”

Capacitors used in day to day activity are Mircofarad i.e. 10-6

Capacitor Symbol:-

Capacitor symbols are as shown below:-

You Know how big is the Farad is??

Let’s have an idea how big size of plates are required to get a value of just one Farad with a reasonable plate separation of just say 1mm operating in a vacuum. If we rearranging the equation for capacitance above this would give us a plate area of:
C= Q/V= permittivity, ( ε )X Area of plates
Distance between Plates
For air as dielectric medium Value of Permittivity is = 8.85pF/m
Put the values into formula we get:-

A = (C)d ÷ 8.85pF/m = (1 x 0.001) ÷ 8.85×10-12 = 112,994,350 m2
or 113 million m2 which would be equivalent to a plate of more than 10 kilometres x 10 kilometres (over 6 miles) square. That Value is very huge and is practically not possible.

How Capacitor store Charge??

Two Parallel Plates form a capacitor. When capacitor is connected to DC supply then one side of plate is get positive charged and other get negative charges depending upon polarity of DC supply. I.e. how they store the charge i.e. store energy. If plates are kept on charge in air beyond a certain limit then plates will start getting discharged by spark getting produced due to excessive charge.To increase the amount of stored charge a dielectric medium i.e. a non conducting medium placed between plates. This medium also act as spark blocker .

Types of Dielectric Medium:-
§  Some of dielectric material used in commercial capacitors :-
1.      Paper
2.      Plastic film
3.      Mica
4.      Glass
5.      Ceramic
6.      Air.
The electrical energy is stored in the capacitor in the form of electrostatic field.
C = Q/V
Here
C = Capacitance in F
Q = Charge in Coulomb
V = Voltage applied across the capacitor in Volts
The capacitor current equation is given below
I = C.dV/dt
C =  Capacitance

dV/dt = Change in voltage across the capacitor
I is known as Displacement current. It is not the current flows through the capacitor.

There are so many types of capacitors available depending upon the type of insulation material used as dielectric for knowing about types of capacitors please visit link below:-