Question

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

• A. 260 volt $/cm$
• B. 260 newton/coulomb
• C. 130 volt $/cm$
• D. 130 newton/coulomb

Answer 1

Answer: (B)

For balance, $mg=eE\Rightarrow E=\frac{mg}{e}$
Also, $m=\frac{4}{3}\pi r^{3}d=\frac{4}{3}\times \frac{22}{7}\times {\left(10^{-7}\right)}^3\times 1000kg$
$\Rightarrow E=\frac{(4/3)\times (22/7)\times {\left(10^{-7}\right)}^3\times 1000\times 10}{1.6\times 10^{-19}}=260N/C$
$(g=10$ newton $/kg,e=1.6\times 10^{-19}$ coulomb $)$