If four charges $q_1=+1 \times 10^{-8} \mathrm{C}, q_2=-2 \times 10^{-8} \mathrm{C}$, $q_3=+3 \times 10^{-8} \mathrm{C}$ and $q_4=+2 \times 10^{-8} \mathrm{C}$ are kept at the four corners of a square of side 1 m , then the electric potential at the centre of the square is

The electric field intensity $E$ at a distance of 3 m from a uniform long straight wire of linear charge density $0.2 \mu \mathrm{~cm}^{-1}$ is

A point charge $q \mathrm{C}$ is placed at the centre of a cube of a side length $L$. Then, the electric flux linked with each face of the cube is

Three equal electric charges of each charge $q$ are placed at the vertices of an equilateral triangle of side of length $L$. Then, potential energy of the system is