An electric dipole with dipole moment p⃗ is in a uniform electric field E⃗. A. Find all the orientation angles of the dipole measured counterclockwise from the electric field direction for which the torque on the dipole is zero. B. Which part of orientation in part (a) is stable and which is unstable?

A) When a dipole is placed in an electric field, it experiences a torque equal to the following

torque = p x E = p E sinθ, where θ is angle between direction of p and E.

It will be zero if θ = 0

or if both p and E are oriented in the same direction.

It is the stable orientation of dipole.

If θ = 180°,

Torque = 0

In this case both p and E are oriented in opposite direction.

It is the unstable orientation of the dipole because if we deflect the dipole by even small angle, it goes back to most stable orientation due to torque acting on it by electric field.