Principle of a capacitor

Last updated on April 21st, 2022 at 02:45 pm

The principle of a capacitor is based on two physical events. These are as follows:

1 ) The capacitance of a conductor increases by a small amount when another uncharged conductor is placed near it.
2 ) The capacitance of a conductor increases greatly when an earth-connected conductor is placed near it.

A capacitor is a device for storing a large quantity of electric charge.

A conductor can store a small amount of charge and hence it possesses a small capacitance. To store a large amount of charge, two (or more) conductors are arranged to form a capacitor. Thus, a capacitor possesses a large capacitance to store charge, and that too in a small space.

Detailed discussion:

The capacitance of a conductor increases by a small amount when another uncharged conductor is placed near it.

consider an insulated metal plate A. Suppose that it is given a positive charge, till its potential becomes maximum.
At this point, the plate won’t hold any more charge over it. If any more charge is given to the metal plate A, it will leak to the surroundings.
Now, let’s place another metallic plate B near plate A.
due to induction, the negative charge will be induced on the nearer face of plate B and the positive charge will be induced on the farther face of plate B.
The potential of plate A gets lowered due to the induced negative charge on plate B and a bit raised due to the induced positive charge on plate B.
Since the metal plates have a finite thickness, the induced negative charge is nearer to plate A than the induced positive charge on plate B is.
therefore, on the whole, the potential of plate A gets lowered by a very small amount.
In order to make the potential of the metal plate A again the same, an additional small amount of charge has to be given to plate A.
It means the capacitance of a conductor increases by a small amount when another uncharged conductor is placed near it.

The capacitance of a conductor increases greatly when an earth-connected conductor is placed near it.

Now connect the metal plate B to the earth as shown.
The induced positive charge on plate B gets earthed immediately, as it is repelled by the positive charge on plate A.
However, the induced negative charge on plate B will stay on it. It doesn’t get earthed as it is attracted by the positive charge on plate A.
Thus the metal plate B will have only the negative charge on it after being connected to the earth.
Due to the induced negative charge on plate B, the potential of plate A will get lowered by a large amount.
In order to raise the potential of plate A again to its initial level, a large amount of charge has to be given to plate A.
Thus, it can be said that the capacitance of a conductor increases greatly when an earth-connected conductor is placed near it.
This forms the principle of a capacitor and such an arrangement of 2 metal plates is called a capacitor.

Principle of a capacitor – step by step