Consider a permanent dipole of dipole moment p in a uniform external field E, as shown in Fig. 1.22. (By permanent dipole, we mean that p exists irrespective of E; it has not been induced by E.) There is a force `qE` on q and a force -qE on -q. The net force on the dipole is zero, since E is uniform. However, the charges are separated, so the forces act at different points, resulting in a torque on the dipole. When the net force is zero, the torque (couple) is independent of the origin. Its magnitude equals the magnitude of each force multiplied by the arm of the couple (perpendicular distance between the two anti-parallel forces).

Magnitude of torque = `q Exx 2asintheta`
`= q a E sintheta`

Its direction is normal to the plane of the paper, coming out of it. The magnitude of `p - E` is also p E sinθ and its direction is normal to the paper, coming out of it. Thus

`tau = pxxE`

This torque will tend to align the dipole with the field E. When p is aligned with E, the torque is zero.
Figure is self-explanatory. It is easily seen that when p is parallel to E, the dipole has a net force in the direction of increasing field. When p is anti-parallel to E, the net force on the dipole is in the direction of decreasing field. In general, the force depends on the orientation of p with respect to E.