Electric field strength due to a point charge of $5\,\mu C$ at a distance of $80\, cm$ from the charge is

Four charges $q, 2q, -4q$ and $2q$ are placed in order at the four corners of a square of side $b$. The net field at the centre of the square is

Four charges are placed on corners of a square as shown in figure having side of $5\,cm$. If $Q$ is one microcoulomb, then electric field intensity at centre will be

A charged oil drop is suspended in a uniform field of $3 \times$ $10^{4} V / m$ so that it neither falls nor rises. The charge on the drop will be $.....\times 10^{-18}\; C$

(take the mass of the charge $=9.9 \times 10^{-15} kg$ and $g=10 m / s ^{2}$ )

A thin semi-circular ring ofradius $r$ has a positive charge $q$ distributed uniformly over it. The net field $\vec E$ at the centre $O$ is

A vertical electric field of magnitude $4.9 \times 10^{5} N / C$ just prevents a water droplet of a mass $0.1\, g$ from falling. The value of charge on the droplet will be  ........ $\times 10^{-9} \;C$ $\left(\right.$ Given $\left.g =9.8 m / s ^{2}\right)$