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when will a dipole be in stable and unstable ?

If the axis of the dipole makes an angle ? with the field, a torque , ? = PE acts on the dipole. The dipole rotates. When its axis becomes parallel to the field , ? = 0. But as it acquires an angular momentem, it continues to rotates. Again a torque acts on it and brings it back to the equilibrium position. Thus the dipole oscillates.

We have learnt that when an electric dipole is placed in a uniform electric field, equal but oppositively directed forces act on the two charges of the dipole and they constitute a couple. The net force on the dipole is zero.

So, the dipole experiences only rotatory motion and no translatory motion. If we place the dipole in a non-uniform field, the forces acting on the two charges of the dipole differ from each other. As a result, the dipole experiences both a torque as well as a force. So both rotatory as well as translatory motions may take place.