Consider an electric dipole in a uniform electric field which is due to some other source.The dipole moment ( ) makes an angle θ with the electric field () as shown in Figure.
The forces on charges +q and -q are qE and -qE respectively.
The magnitude of net force is given by,
Fnet = qE – qE = 0
Note:The net force on a dipole in a uniform electric field is zero.
These two equal and opposite forces acting on the dipole constitute a couple. This couple tends to rotate the dipole in the clockwise direction and hence tries to align the dipole along the direction of the electric field.
Torque acting on the dipole, (τ)
= Magnitude of either force x Perpendicular distance between two forces
= qE x BC —— (1)
In triangle ABC,
The direction of torque is given by right hand screw and cork rule.
The dipole experiences a net force if It is placed in a non-uniform electric field.