Electric field intensity and electric field is also related to the electrostatics.
According to Coulomb’s law if a unit positive charge q0 (call it
the test charge) is brought near a charge q (call it the field
charge) placed in space, the charge q0 will experience a force
The value of this force would depend upon the distance
between the two charges. If the charge q0is moved away from
q this force would decrease till at a certain distance the force
would practically reduce to zero. Now the charge q is out of
the influence of charge q.
The region of space surrounding the charge q0 in which it
exerts a force on the charge q0 is known as electric field of the
charge q. Thus the electric field of a charge is defined as :
The electric field is a region around a charge in which it exerts
electrostatic force on another charges.
The strength of electric field at any
point in space is known as electric field intensity.
In order to find the value of electric intensity at a point in the
field, of charge +q, we place a test charge q0 at that point
(Fig. 13.13). If F is the force acting on the test charge qo, the
electric field intensity would be given by
Thus the electric field intensity at any point is defined as the
force acting on a unit positive charge placed at that point.
SI unit of electric intensity is NC-1.
If the electric field due to a given arrangement of charges is
known at some point, the force on any particle with charge q
placed at that point can be calculated by using the formula
Electric intensity being a force is a vector quantity. Its
direction is the same as that of the force acting on the
positive test charge. If the test charge is free to move, it will
always move in the direction of electric intensity.
The direction of electric field intensity in an electric field can
also be represented by drawing lines. These lines are known
as electric lines of force. These lines were introduced by
Michael Faraday. The field lines are imaginary lines around a
field charge with an arrow head indicating the direction of
force. Field lines always move away from positive charge but
toward negative charge. The spacing between the field lines
shows the strength of electric field (Fig 13.14).
Electric Field Lines
Field lines are related to the electric field intensity in any
region of space in the following way:
1. The electric field intensity is tangent to the electric field
lines at each point (Fig. 13.15-a).
2. The number of lines per unit area through a surface
perpendicular to the lines is proportional to the electric field
strength in a given region (Fig. 13.15-b).
3. Electric field is strong when the field lines are close
together and weak when the lines are far apart. No two field
lines cross each other.
Electric field lines for an isolated positive and negative point
charges are shown below: