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Question 2: Explain the concept of electric field and hence define electric field intensity. Discuss the direction as well the unit of E.


Electric Field

The concept of electric field was introduced by Michael Faraday. He said that the charge ‘q’ produces an electric field surrounding it and when another charge is brought in that region, it exerts a force (attractive or repulsive) F on it. Moreover, the force is exerted in a spherical region, i-e, in all directions.
The region around a charge in which an electric test charge would experience a force is called the electric field of that charge.

Electrostatic force between two electrical charges is either repulsive or attractive, depending upon the nature of charges. This means that the electrostatic force is a directed quantity. Therefore, electric field is a vector field having magnitude and direction at every point of the field. We can determine the strength (i-e, magnitude) and direction at a certain point of the field by placing a unit positive test charge at that point. The strength of the field is found by the force experienced by the test charge at that point. The direction in which the unit positive test charge moves or tends to move is the direction of the electric field at that point.

Electric Field Intensity

It is defined as Electric Intensity of an electric field , at any point in the electric field, is the force per unit positive test charge placed at that point.




This is important to say the test charge is so small that it doesn’t change the original field due to the source charge.
The SI unit of electric field intensity is newton/coulomb (NC-1) or volt/meter.

Calculation of E and its direction


Consider an isolated point charge q. An electric field exists around this charge and we want to find an expression for the intensity of electric field around this charge as an example.
Imagine a small point charge q0 placed at a distance r from q. Coulomb’s force on q0 due to the field of q is,


Electric field intensity is defined as, force per unit charge, and in this case,


Put the value of force,


Cancelling q0 and rearranging in vector form


Here shows the direction of intensity.
It should be noted that the direction of electric intensity is along the direction of the electrostatic force. Also it is directed radially outward or inward depending upon the nature of the charges in question.

Unit of


Unit of E can be easily calculated from the equation for the calculation of E. Its unit is unit of force divided unit of charge. Therefore, unit of E is NC-1.