Electric field intensity:
Electric field is a region of space around a charge or a system of charges which extends upto infinity within which other charged particles experience electrostatic force.
The electric field strength or field intensity at a point in an electric field is the force acting on a test charge (unit positive charge) placed at that point, without disturbing the source charge or the configuration of source charges producing the electric field.
If F is the electrostatic force experienced by a positive test charge q0 at a point, then the electric field intensity at that point is given by 
= 
.
Therefore, if a static charge q is placed at that point in the electric field of intensity E, then the electrostatic force experienced by the charge 
= q .
Electric field intensity due to a point charge:
Let us consider O is the origin where a point charge q is placed. To find the electric field intensity at point P, at a distance r from O, a test charge q0 is placed at P. According to Coulomb’s law, the force exerted on q0 by q is = 
where r̂ is a unit vector directed from O to P.
If is the electric field created by q at P, then by definition, = 
= 
.
is directed radially outwards at P If q is positive, and if q is negative, is directed towards q.
Properties of electric field lines:
(i) Field line is a continuous and smooth curve originating from a positive charge and ending on a negative charge.
(ii) Two field lines never intersect each other because at the point of intersection we can draw two tangents which indicate two directions of electric field. As a vector contains single direction, so is not possible.
(iii) The electric field within the conductor is zero.
(iv) The electric field just outside the surface of the conductor is perpendicular to its surface.
(v) Due to the repulsion between two like charges electric field lines exert a lateral pressure on each other.
(vi) Due to attraction between two unlike charges electric field lines exert longitudinal tension in the field lines.
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