Electric Dipole :
- The system formed by two equal and opposite charges separated by a small distance is called an electric dipole.
- The torque on a dipole in an electric field is zero both in a stable as well as in an unstable equilibrium.
- The potential energy of a dipole in an electric field is minimum in a stable equilibrium and maximum in an unstable equilibrium.
- Electric dipole moment is the product of the magnitude of the either charge (q) of the dipole and the separation (2a) between the charges i.e., p = q (2a)
- Dipole moment is vector quantity whose unit is coulomb-meter (Cm).
- The maximum value of electric field is at the surface of the shell of the charge as Cell as the cloud of charge.
- A dipole moment
in an electric field 
experiences a torque, given by
- (a) The torque is the minimum for θ = 0º or 180º, i.e.
when θ = 0º or 180º. In this position, the dipole is said to be in equilibrium.
(b) The equilibrium of the dipole is stable for θ = 0 and it is unstable for θ= 180º.
(c) The torque is the maximum for 
(d) As e increases from 0º to 180º, first increases, becomes maximum and then decreases to zero.
- (i) In a uniform electric field, a dipole experiences only the torque, i.e. the net force on the dipole is zero.
(ii) In a uniform electric field, a dipole experiences only rotatory motion and no translatory motion.
(iii) In a non-uniform electric field, a dipole experiences both torque as well as the net force.
(iv) In a non-uniform electric field, a dipole experiences both rotatory as well as translatory motion.
- The electrostatic potential energy of a dipole of moment
in uniform electric field 
is :
(i) For θ = 0º, we have Up = - pE
It is the minimum value of the potential energy and the dipole is in stable equilibrium.
(ii) For θ = 90º, we have Up = 0
(iii) For θ = 180º, we have UP = - pE
