1.11 Polar and Nonpolar Molecule

In many molecules, the centers of positive charges and of negative charges lie at the same point. Therefore, their dipole moment is zero. Such molecules are called non polar molecules. CO2 and CH4 are of this type.However, they can develop a dipole moment when an electric field is applied. In some molecules, the centers of negative charges and of positive charges do not coincide. Therefore they have a permanent electric dipole moment even in the absence of an electric field. Such molecules are called polar molecules. Water molecules, H2O is an example of this type.
1.11 a) Dipole in Uniform Field
  We have discussed polar molecule and non polar molecules. Consider an electric dipole of dipole moment p = 2aq kept in a uniform external electric field, inclined at an angle theta to the field direction.

                            Equal and opposite forces +qE and -qE act on the two charges. Hence the net force on the dipole is zero. But these two equal and opposite forces whose lines of action are different constitute a torque.

        torque = any one force * perpendicular distance(between the line of action of two forces)
1.11 (b) Dipole in a Non Uniform Field
   What happens if the field is not uniform? In that case, the net force will be non-zero. In addition there will, in general, be a torque on the system as before. Let us consider the simpler situations when dipole moment(p) is parallel to non uniform electric field. In this case, the net torque is zero, but theer is a net force on the dipole.