Question

A vertical electric field of magnitude $4\times {10}^5\text{N/C}$ just prevents a water droplet of mass $1\times {10}^{-4}\text{kg}$ from falling. Find the charge on the droplet.

• A. $4.5\times {10}^{-9}\text{C}$
• B. $2.5\times {10}^{-9}\text{C}$
• C. $3.5\times {10}^{-9}\text{C}$
• D. $1.5\times {10}^{-9}\text{C}$

Answer 1

The correct option is B $2.5\times {10}^{-9}\text{C}$


For charge to stay in equilibrium electrostatic force should balanced the gravity.
Hence,
$qE=mg$
$\Rightarrow q=\frac{mg}{E}$
$\Rightarrow q=\frac{1\times {10}^{-4}\times 10}{4\times {10}^5}$
$\Rightarrow q=2.5\times {10}^{-9}C$