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 V/cm
• B. 260 N/C
• C. 130 V/cm
• D. 130 N/C

Answer 1

Answer: (B)

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