It has been already discussed that electric field is represented by electric lines of force. The strength of the electric field is different at different points and is called field strength or field intensity or electric intensity. The strength or intensity of an electric field at a point depends upon the concentration of electric lines of force at that point, that is, the point where the electric lines of force are spaced closer and the field strength is higher at that point, and vice versa.
Mathematically, field strength at a point in an electric field is determined by the force acting on a unit positive charge placed at that point. Obviously, this force decreases as we go away from a charged body and ultimately diminishes to zero. Hence, electric intensity or field strength at a point in an electric field is the force acting on a unit positive charge placed at that point.
Electric intensity at a point, 
where Q = charge in coulomb placed at the given point.
F = force in newton acting on Q coulombs of charge.
Consider a body, charged at +Q coulomb of charge. It is required to determine electric intensity at point P at a distance of d metre away from the charged body. Place a unit positive charges ( +1 C) at point P, as shown in Figure 3.8. Then, the force acting on this unit positive charge will be the electric intensity at that point.
Fig. 3.8 Electric intensity at a point near a charged body
The direction of electric intensity is determined by Coulomb’s first law. In this case, it is radially away from the charged body. However, for negative charge (i.e., − Q), its direction would have been radially towards the charged body
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