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 (g = 10newton / kg, e$=1.6\times {10}^{-19}$ coulomb)

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

Answer 1

The correct option is B 260 newton / coulomb

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