Question

In Millikan's oil drop experiment, what is viscous force acting on an uncharged drop of radius $2.0 \times 10^{-5} \,m$ and density $1.2 \times 10^{3} \,kgm ^{-3}$ ? Take viscosity of liquid $=1.8 \times 10^{-5} \,Nsm ^{-2}$. (Neglect buoyancy due to air).

• A. $3.8 \times 10^{-11}\, N$
• B. $3.9 \times 10^{-10} \,N$
• C. $1.8 \times 10^{-10}\, N$
• D. $5.8 \times 10^{-10} \,N$

Answer 1

Answer: (B)

Viscous force $=$ Weight
$=\rho \times\left(\frac{4}{3} \pi r ^{3}\right) g$
$=3.9 \times 10^{-10}$