• A capacitor is a wonderful device which conducts current eventhough it contains insulating material between its two conducting plate. Yes, you read it correct. However there are certain exceptions, which we shall see when we study this wonderful device in detail.
  • The goal of this section is to understand the concept of capacitor and capacitance in detail.

To understand the concept of capacitor and capacitance, consider a conducting sphere as shown in the figure below.

Suppose we add some positive charge on the sphere, then the electric field will be established around the sphere.

Conducting sphere establishes an electric field

Suppose we go on adding positive charge to the sphere, then electric field around the sphere will also start to increase accordingly. In this process, at some stage, the electric field around the sphere will become sufficiently strong to ionize the air particles around the sphere(In simple words, it means that the molecules of air will split into positive ions and electrons). Take a look at the figure shown below.

molecule in electric field.

The free electron will eventually cause the air to lose its insulating property (air now contains a free electron). This phenomenon is called dielectric breakdown. Now due to conducting nature of air, the sphere will not be able to store any additional charge as the additional charge on the sphere will leak into the air. During this entire process. “The ratio of charge(Q) on the capacitor and the potential(V) on the surface of sphere remains constant. This ratio is called capacitance(C) of capacitor.

The ratio of charge (Q) on the sphere and the potential (V) on the surface of sphere remains constant. This ratio is called capacitance (C) of capacitor.

C=Q/V

In other words, capacitance indicates how much charge a capacitor can hold per unit voltage. Capacitor is measured in Farads. One Farad is considered to be very large value of capacitor.  Capacitors used in low power devices are usually in the μF or nF range, or even pF range.

If we want to increase the capacity to store charges, then we can proceed as follows.

Continuing with our discussion, let us ask ourself – “Can we increase the capacity of sphere to store charges?”. The answer is yes, we can. How? This question can be answered if know what exactly caused the charge to leak from the capacitor (sphere). As discussed previously, we know that high electric field around the sphere and potential on its surface caused air molecules to break down. These free electron broken down from the air molecue caused the charge on the capacitor to leak into the air. Thus if we want to increase the capacity (capacitance) to store the charges, we need to decrease the electric field around the sphere.

To decrease the electric field near the sphere which is positively charged,we bring another identical sphere, negatively charged, around the first sphere.This will lead to decrease in the potential on the surface of the first sphere and the electric field near it.

In this case also, the ratio of charge (Q) on the potential difference (V) between the two spheres remains constant. This ratio is called capacitance of this system of two spheres.

Definition of capacitor :

Having learned the concept of capacitance, let us see how a capacitor can be defined. A capacitor can be defined as “A device formed by two conductors insulated from each other, which stores energy in the form of electrostatic fields“.