An electron and a positron enter a uniform electric field $E$ perpendicular to it with equal speeds at the same time. The distance of separation between them in the direction of the field after a time ' $t$ ' is

( $\frac{e}{m}$ is specific charge of electron)

A charge $q$ is placed at the centre ' $O$ ' of a circle of radius $R$ and two other charges $q$ and $q$ are placed at the ends of the diameter $A B$ of the circle. The work done to move the charge at point $B$ along the circumference of the circle to a point $C$ as shown in the figure is

For any fixed distance, the electromagnetic force between two protons is $10^n$ times of the gravitational force between them. Then, $n=$

A thin spherical shell of radius $R$ and surface charge density $\sigma$ is placed in a cube of side $5 R$ with their centers coinciding. The electric flux through one face of the cube is $\left(\varepsilon_0=\right.$ Permittivity of free space $)$