Two non-conducting infinitely long straight wires with uniform linear charge densities ${\lambda }_1$ and ${\lambda }_2$ are arranged along $X$ and $Y$-axis respectively. Their point of intersection is at origin. One of the field lines due to these wires in $X-Y$ plane is a straight line passing through the origin. The angle made by this line with $X$-axis is

Two charged metallic spheres of same size repel each other by a force $F$. They are now touched with each other and are then separated to same initial distance. Now the force of repulsion is $F\text{'}$. Which of following are possible ?

In which of the following figures, electric field at point $O$ is non-zero?

Which of the four particles contribute to the electric field at point $P$ on the surface?

Which of the four particles contribute to the net electric flux through the closed surface?

A few electric field lines for a system of two charges $Q_1$ and $Q_2$ fixed at two different points on the $x$-axis are shown in the figure. These lines suggest that