The space or region around a charge in which the effect of that charge can be felt is known as electric field.
SOURCE CHARGE:
The charge which create electric field around it is known as source charge.
TEST CHARGE:
The charge which experience the effect of source charge, is known as test charge.
ELECTRIC FIELD INTENSITY:
Force acting on a unit charge place in an electric field at a specified point as known as Electric field intensity. The intensity of an electric field at any point is determined by supposing a small positive charge is placed at that point, then finding the force on it and dividing by its charge to find the force per unit charge. field intensity is therefore defined by the relation given by
where E is the electric intensity, F is the force acting on the test charge and (q) is the magnitude of the test charge E is vector whose direction is the same as that of F and whose magnitude is measured in Newton per Coulomb in M.KS system. For simplicity, this must be kept in mind that the presence of test charge in electric field does not distort or otherwise influence the field.
To illustrate the above equation, suppose a point charge ' q ' is located at a certain point A in vacuum and is required to find the field intensity at a point ' p , which is 'r' meters from ' q ' as shown in the figure.
Let 'r ' be the position vector of P then the strength of field at point ' p' or the intensity 'E' of the field at 'P' will be determined by applying Coulomb's law of force. Thus
and dividing by (q) to find the force per unit charge, it is found that, the electric field intensity at ' P' is
It is obvious from the equation that the electric field intensity depends only upon the magnitude of the charge producing the field and the location of
' P'. Since q1 is positive and the test charge ( q) is always positive, the force is repulsive.
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