CBSE Class 12-science Answered

Let's consider that an electric dipole is placed in an external electric field.

When an electric dipole is placed in an external uniform electric field, both the charges experience force F = qE. Since both charges have the same magnitude and are in the same electric field E, the electric force F = qE on the positive and negative charges which make the dipole are equal in magnitude and opposite in direction. Thus, there is no net force acting on the electric dipole as a whole.

However, if the axis of the dipole makes an angle  with the direction of, then we observe that the lines of action of the two forces do not coincide. In fact they are separated by a distance d as shown

Therefore, although there is no net force acting on the dipole, there is net torque acting about its centre.

By definition, the magnitude of this torque is given by

Here p is the magnitude of the dipole moment.

Or simply,

Since this is a cross product,  is perpendicular to the plane containing the vectors  and.

This torque will tend to align the dipole with the field.