Electric Field and Electric Field Intensity

Electric Field : Electric field due to a charge is the space around the test charge in which it experiences a force. The presence of an electric field around a charge cannot be detected unless another charge is brought towards it.

Electric Field Intensity (E) : Electric field at a point is measured in terms of electric field intensity. Electric field intensity at a point, in an electric field is defined as the force experienced by a unit positive charge kept at that point.

                                                       or 

the electric field intensity due to a charge q at a point in space may be defined as the force that a unit positive charge would experience if placed at that point.

The charge q, which is producing the electric field, is called a source charge and the charge qo, which tests the effect of a source charge, is called a test charge.

E = F/qo 

It is a vector quantity. The unit of electric field intensity is N/C or V/m.

Electric field due to a point charge : Let q be the point charge placed at O in air (Fig.). A test charge qo is placed at P at a distance r from q. According to Coulomb’s law, the force acting on qo due to q is

F = k q qo / r^2    where k=1/4πεo

Fig. Electric field due to a point charge

The electric field at a point P is, by definition, the force per unit test charge.

E = F/qo = k q / r^2 

E = (1/4πεo) q / r^2 

The direction of E is along the line joining O and P, pointing away from q, if q is positive and towards q, if q is negative.