The Electric field at a point is-

If there were only one type of charge in the universe, then

Refer to the arrangement of charges in the figure and a Gaussian surface of radius $\mathrm{R}$ with charge $\mathrm{Q}$ at the centre, then

A positive charge $\mathrm{Q}$ is uniformly distributed along a circular ring of radius $\mathrm{R}$. A small test charge $\mathrm{q}$ is placed at the centre of the ring . Then