What is called Gaussian surface ?

If atmospheric electric field is approximately $150 \,volt / m$ and radius of the earth is $6400 \,km$, then the total charge on the earth's surface is .......... coulomb

The electric field in a region of space is given by, $\overrightarrow E  = {E_0}\hat i + 2{E_0}\hat j$ where $E_0\, = 100\, N/C$. The flux of the field through a circular surface of radius $0.02\, m$ parallel to the $Y-Z$ plane is nearly

What is the net flux of the uniform electric field of $E =3 \times 10^{3} i\; N / C $ through a cube of side $20\; cm$ oriented so that its faces are parallel to the coordinate planes?

A charge $Q\;\mu C$ is placed at the centre of a cube, the flux coming out from any surfaces will be

Consider an electric field $\vec{E}=E_0 \hat{x}$, where $E_0$ is a constant. The flux through the shaded area (as shown in the figure) due to this field is