An electric dipole of dipole moment $\vec{P}$ is placed parallel to a uniform electric field of intensity $\vec{E}$. On rotating it through $180^{\circ}$,the amount of work done is . . . . . . .

Point $P$ lies on the axis of a dipole. If the dipole is rotated by $90^o$ anticlockwise,the electric field vector $\vec{E}$ at $P$ will rotate by

Two point masses,each of mass $m$ and charges $+q$ and $-q$,are attached to the ends of a massless rod of length $L$. This assembly is placed in a uniform electric field of intensity $E$ at a small angle $\theta$ with the field. How much time will it take for the rod to become parallel to the electric field?

Two opposite and equal charges of $4 \times 10^{-8} \ C$ are placed at a distance of $2 \times 10^{-2} \ cm$ to form a dipole. If the dipole is placed in an external electric field of $4 \times 10^8 \ N/C$,calculate the maximum torque and the work done to rotate it by $180^{\circ}$.

Write the equation for the electric field produced by an electric dipole at a point on its axial line,given the condition $r >> a$.

Determine the electric dipole moment of the system of three charges,placed on the vertices of an equilateral triangle,as shown in the figure.