Question

A tiny spherical oil drop carrying a net charge q is balance in still air with a vertical uniform electric field of strength $\frac{8L\pi }{7}\times 10^{5}V{m}^{-1}$. When the field is switched off, the drop, observed to fall with terminal velocity $2\times 10^{-3}{ms}^{-1}giveng=9.8{ms}^{-2}$, viscosity of the air =$1.8\times 10^{-5}Ns{m}^{-2}$ and the density of oil =900 kg$m^{-3}$, the magnitude of q is

• A. $1.6\times 10^{-19}C$
• B. $3.2\times 10^{-19}C$
• C. $4.8\times 10^{-19}C$
• D. $8.0\times 10^{-19}C$

Answer 1

Answer: (A)

$1.6\times 10^{-19}C$

A tiny spherical oil drop carrying a net charge $=q$

the balance is still air with a vertical uniform electric field of strength $=\frac{8L\pi }{7}\times {10}^{5}V/m$ the field is switched off the drop observed to fall with terminal velocity $=2\times {10}^{-3}m/s$

$g=9.8m/s^2$

Viscosity of the air $=1.8\times {10}^{-5}Ns{m}^{-2}$

The density of oil $=900kg/m^3$

$q=1.6\times {10}^{-19}C$ because it is universal charge.