A charge $q$ is placed at the centre of face of a regular cube of side $\ell$. Find the electric flux passing through the cube.

Use Gauss's Law to determine the electric field at$a.$

The "Gaussian Surface" for an infinitely large charged plate is pillbox of surface area A and length $2a$ centred about the origin. The plate has positive surface charge density $\sigma$.

Find the dipole-moment of system

Two molecule each with permanent dipole moment $\overrightarrow{p}$ are aligned as shown. Calculate the force between the molecules if they are separated by a distance that is large compared with the dimension of the dipoles.
[Hint : The relation $(1+x{)}^{-2}\cong 1-2x+3x^2-\dots \dots$ for small $x$ is useful].