The expression for an electric field is given by $\vec{E} = 4000 x^2 \hat{i} \text{ V/m}$. The electric flux through the cube of side $20 \text{ cm}$ when placed in the electric field (as shown in the figure) is $......... \text{ V cm}$.

What is called a Gaussian surface?

Assertion: Four point charges $q_1, q_2, q_3$ and $q_4$ are as shown in the figure. The flux over the shown Gaussian surface depends only on charges $q_1$ and $q_2$.
Reason: Electric field at all points on the Gaussian surface depends only on charges $q_1$ and $q_2$.

The figure shows the electric field lines. The spacing between the lines is parallel to the paper at every point. If the magnitude of the field at $A$ is $40 \ N/C$,then the approximate magnitude of the field at $B$ is ....... $N/C$.

The flat base of a hemisphere of radius $a$ with no charge inside it lies in a horizontal plane. $A$ uniform electric field $\vec{E}$ is applied at an angle $\frac{\pi}{4}$ with the vertical direction. The electric flux through the curved surface of the hemisphere is

The electric field in the region is $\vec{E} = a\hat{i} + b\hat{j}$ where $a$ and $b$ are constants. The net electric flux passing through a square area of side $l$ parallel to the $Y-Z$ plane is