What is the principle of capacitor?

A small device used to store huge amount of electric charge in a small room is called capacitor.

Take an insulated metal plate A. Charge the plate to its maximum potential. Now take another insulated plate B. Take the plate B nearer to plate A. You will observe that negative charge will be produce on the plate near to plate A and the same amount of positive charge will be produced on the other side of plate B.

Now the plate B will start affecting the plate A slowly. The negative charge will start decreasing the electric potential of plate A. But positive charge helps in increasing the potential. But the effect of negative charge is much more than that of the positive because the negative side of plate is near to the plate A. So potential of A will start decreasing and it can be charged again to raise its potential to maximum.

Result: From the above discussion the result is this that the charge carrying capacity of a conductor can be increased by bringing an uncharged conductor in its nearby area. It is shown in the figure above.

Lets take an another discussion over it. Now in this case connect the plate B to earth. All the positive charge present on plate B will go into the earth. So, only negative charge will remain on the plate B. So the electric potential of plate A will become less to a greater extent. So as a result A will want much more charge to gain its lost potential due to the effect of the negative charge present on plate B.

Note: By taking an uncharged conductor near an insulated conductor, capacitance of the insulated conductor can be increased to a larger amount.

Capacitors are of many types:
Parallel plate capacitors are those in which conductors used are simple parallel plates.

Spherical conductors are those in which spherical conductors are used.

Third types of conductors are those in which conductors used are of cylindrical type.

Now you will be thinking that how to find out the Capacity of charge storage inside a capacitor. Here is your formula.
C=Q/V.

It can be calculated by dividing the charge (Q) present on the plates of conductor by the potential (V) of the conductor. The common word used in our daily life for capacitors is Condensers which we use in most of our electric appliances. Their main purpose is to provide the initial start to the device in which it is installed.