Shaji asked “An electric dipole placed in an external electric field experiences a torque.why?”
If the axis of electric dipole makes an angle worth the electric lines of force, it will experience a torque.
What is torque?
The turning effect of a force is called torque. When an electric dipole is placed in an electric field making an angle between the axis of the dipole and the direction of electric field, each of the charges constituting the dipole experiences a force qE in the opposite directions, but not along the same line.
Such a pair of forces of equal magnitude but opposite direction not acting along the same line constitute a couple of forces and has a turning effect. (Torque)