An electric dipole having each charge of magnitude $2 \mu \mathrm{C}$ is placed in an electric field of intensity $8 \times 10^{+4} \mathrm{~N} / \mathrm{C}$. If the maximum torque acting on the dipole is $4 \times 10^{-3} \mathrm{~N}-\mathrm{m}$, the length of the dipole is

Three concentric charged metallic spherical sheets $A, B$ and $C$ have radii $a, b, c$ potentials $V_A$, $\mathrm{V}_{\mathrm{B}}, \mathrm{V}_{\mathrm{C}}$ and charge densities $+\sigma,-\sigma$ and $+\sigma$ respectively. The value of potential $\mathrm{V}_{\mathrm{A}}$ is ( $\varepsilon_0=$ permittivity of free space)

Three point charges $+Q,+2 Q$ and $q$ are placed at the vertices of an equilateral triangle. The value of charge $q$ in terms of $Q$, so that electrical potential energy of the system is zero, is given by