What is the torque acting on a dipole in the following two cases?
When it is held in the direction of E

Explanation

Torque acting on an electric dipole placed in uniform electric field is `vecpi= vec xx vecE`.
Special cases
When the dipole is held in the direction of `vecE`, i.e.,
`theta - 0°`, then `pi = p.E sin 0^(@) implies pi = 0`
In this case, torque acting on the dipole is zero and the dipole is said to be in stable . equilibrium. However, when the dipole is held in the direction opposite to `vecE`, the torque would turn the dipole through 180°. The dipole will be in unstable equilibrium.