$A$ point charge $q$ is placed on the center of the flat circular base of a hemispherical surface as shown in the figure. The net flux of the electric field through the hemispherical surface is closest to:

Four closed surfaces $S_1, S_2, S_3$ and $S_4$ together with charges $+q, -q$ and $-2q$ are shown. Through which one of the surfaces is the net flux zero?

$A$ long straight wire of radius $1 \, mm$ has a uniform charge distribution. The charge per $cm$ length of the wire is $Q \, Coulombs$. $A$ cylinder of radius $50 \, cm$ and length $1 \, m$ encloses the wire symmetrically. The total flux passing through the surface of the cylinder is ..........

$A$ closed surface passes through a spherical conductor as shown in the figure. If a negative charge $-Q$ is placed at point $P$,what will be the nature of the electric flux coming out of the closed surface?

$Assertion (A):$ $A$ charge $q$ is placed at a height $h/4$ above the center of a square of side $b$. The flux associated with the square is independent of the side length $b$.
$Reason (R):$ Gauss's law is independent of the size of the Gaussian surface.

Careful measurement of the electric field at the surface of a black box indicates that the net outward flux through the surface of the box is $8.0 \times 10^{3} \; N m^{2} / C$.
$(a)$ What is the net charge inside the box?
$(b)$ If the net outward flux through the surface of the box were zero,could you conclude that there were no charges inside the box? Why or why not?