A pendulum bob of mass $30.7 \times {10^{ - 6}}\,kg$ and carrying a charge $2 \times {10^{ - 8}}\,C$ is at rest in a horizontal uniform electric field of $20000\, V/m$. The tension in the thread of the pendulum is $(g = 9.8\,m/{s^2})$

A charged particle is suspended in equilibrium in a uniform vertical electric field of intensity $20000\, V/m$. If mass of the particle is $9.6 \times {10^{ - 16}}\,kg$, the charge on it and excess number of electrons on the particle are respectively $(g = 10\,m/{s^2})$

The insulation property of air breaks down at $E = 3 \times {10^6}$ $volt\,/\,metre$. The maximum charge that can be given to a sphere of diameter $5\,m$ is approximately (in coulombs)

An oil drop of radius $2\, mm$ with a density $3\, g$ $cm ^{-3}$ is held stationary under a constant electric field $3.55 \times 10^{5}\, V\, m ^{-1}$ in the Millikan's oil drop experiment. What is the number of excess electrons that the oil drop will possess ? (consider $\left. g =9.81\, m / s ^{2}\right)$

The electric field in a region is radially outward and at a point is given by $E=250 \,r V / m$ (where $r$ is the distance of the point from origin). Calculate the charge contained in a sphere of radius $20 \,cm$ centred at the origin ......... $C$

Deutron and $\alpha - $ particle are put $1\,\mathop A\limits^o $ apart in air. Magnitude of intensity of electric field due to deutron at $\alpha - $ particle is