Write equation of electric field by system of $\mathrm{'n'}$ charges.

The intensity of the electric field required to keep a water drop of radius ${10^{ - 5}}\, cm$ just suspended in air when charged with one electron is approximately

Two point charges $( + Q)$ and $( - 2Q)$ are fixed on the $X-$axis at positions $a$ and $2a$ from origin respectively. At what positions on the axis, the resultant electric field is zero

What is the magnitude of a point charge due to which the electric field $30\,cm$ away has the magnitude $2\,newton/coulomb$ $[1/4\pi {\varepsilon _0} = 9 \times {10^9}\,N{m^2}/{C^2}]$

A cube of side $'a'$ has point charges $+Q$ located at each of its vertices except at the origin where the charge is $- Q$. The electric field at the centre of cube is

Two identical point charges are placed at a separation of $d$. $P$ is a point on the line joining the charges, at a distance $x$ from any one charge. The field at $P$ is $E$, $E$ is plotted against $x$ for values of $x$ from close to zero to slightly less than $d$. Which of the following represents the resulting curve