Question

An electric dipole is held in a uniform electric field . (1) show that the net force acting on it is zero. (2) the dipole is aligned parellel to the field/find the work done in rotating it through the angle of 180 degree .

Answer 1

1) the forces experienced by the 2 charges constituting the electric dipole when placed in an uniform external electric field are equal and opposite in nature, the net force on the dipole is zero.

2) angle of rotation = 180
than
torque = pE sin180  = 0

No torque act on the dipole when the moment of electric dipole is parallel to the electric field. 

Answer 2

ii) When dipole is aligned parallel to the electric field. i.e., θ1= 00 and suppose we have to set it an angle θ with −→E i.e., θ2 = 1800.

As the potential energy for a dipole in a uniform electric field is given by :-

P.E.= W= -pE (cos θ2 - θ1) = -pE(cos1800 - cos 00 ) = 2pE