The figure shows four charges $q_1, q_2, q_3$ and $q_4$ fixed in space. The total flux of the electric field through a closed surface $S$,due to all charges $q_1, q_2, q_3$ and $q_4$ is:

$A$ point charge is distributed in a small volume. The electric flux through a spherical surface of radius $10 \ cm$ enclosing the charge is $20 \ Vm$. What is the electric flux through a concentric spherical surface of radius $20 \ cm$?

The spatial distribution of the electric field due to charges $A$ and $B$ is shown in the figure. Which one of the following statements is correct?

$A$ point charge $+Q$ is placed just outside an imaginary hemispherical surface of radius $R$ as shown in the figure. Which of the following statements is/are correct?
$[A]$ The electric flux passing through the curved surface of the hemisphere is $-\frac{Q}{2 \varepsilon_0}\left(1-\frac{1}{\sqrt{2}}\right)$
$[B]$ Total flux through the curved and the flat surfaces is $\frac{Q}{\varepsilon_0}$
$[C]$ The component of the electric field normal to the flat surface is constant over the surface
$[D]$ The circumference of the flat surface is an equipotential

For a closed surface $\oint \vec{E} \cdot d\vec{s} = 0$,then:

Electric field in a region is given by $\overrightarrow{E} = a \hat{i} + b \hat{j}$,where $a$ and $b$ are constants. The net flux passing through a square area of side $l$ parallel to the $y-z$ plane is