Electric Field and its intensity

electric field and its intensity

An electric field is defined as any region around a charge in which an electric test charge would experience an electric force.

The concept of field theory was introduced by Michael Faraday. He stated that the charge q produces an electric field in the space surrounding it and when a charge qo is brought in its field then it exerts a force F on it. The electric field around a charge is like a sphere with in which other charges are influenced by it.

The existence of electric field can be proved by bringing a test charge qo into its field.


The force F experienced by a test charge qo , is given by

               F= qo E.

 It is a vector field so it is characterized by strength and direction at every point in space. The strength and direction of it can be determined by placing a unit positive test charge in that field. The strength of the field at a point in space determines the amount of the force that a charge will experience if it is placed at that point. The direction in which this unit positive test charge move or tends to move is the direction of electric field. 

Electric Field Intensity 

A single vector quantity containing information about the field strength and its direction at that point is denoted by E and is known as electric field intensity. Thus the field strength is the magnitude of the force expressed by the unit positive test charge placed at that point.


lf unit positive test charge qo experience a force F due to the electric field of charge q. Then the intensity of E at any point is the force per unit positive test charge placed at that point.


The electric field intensity

                 Ē= F/qo


    F =qoĒ

Electric field strength is also a vector quantity. The test charge is so small that it does not distort the original field due to the primary source. The SI unit of E is newton per coulomb (N/C) or volt per metre (V/m).

In order to find out the field intensity due to a point charge q. A  positive test charge q, is placed at a distance r from point charge q .Then by coulomb’s law the force experienced by a unit positive test charge qo due  to the field of a point charge.

        F = kqqo / r2

As r` is the unit vector directed from charge q to qo, so in vector form

F = kqqor`/r2

so the E is given by 

     Ē = F/q = k(qqo)r`/qor2


The strength of the field is proportional to the magnitude of the source charge. Its strength decrease as the teat charge moves away from source charge q.

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