If the electric flux entering and leaving an enclosed surface area are ' $\phi_1$ ' and ' $\phi_2$ ' respectively, the electric charge inside the surface will be ( $\varepsilon_0=$ permittivity of free space)

A simple pendulum has mass 2 gram and charge $2 \mu \mathrm{C}$. In a uniform horizontal electric field of intensity $1000 \mathrm{~V} / \mathrm{m}$, pendulum is at rest. At equilibrium, the angle made by the pendulum with the vertical is ( $\mathrm{g}=$ acceleration due to gravity $=10 \mathrm{~m} / \mathrm{s}^2$ )

The charges are arranged at the four corners of square $A B C D$ of side ' $d$ ' as shown in figure. The work required to put this arrangement together is given by

Three charges $Q,-2 q$ and $-2 q$ are placed at the vertices of an isosceles right-angled triangle as shown in figure. The net electrostatic potential energy is zero if $Q$ is equal to