A tiny spherical oil drop carrying a net charge is balanced in still air with a…

PHYSICS · Single correct · CBSE · Physics · Mechanical Properties of Fluids · Mechanical Properties of Fluids

Problem

A tiny spherical oil drop carrying a net charge

q

is balanced in still air with a vertical uniform electric field of strength

(81\pi/7) \times 10^5\,\text{V m}^{-1}

. When the field is switched off, the drop is observed to fall with terminal velocity

2 \times 10^{-3}\,\text{m s}^{-1}

. Given

g = 9.8\,\text{m s}^{-2}

, viscosity of the air is

1.8 \times 10^{-5}\,\text{N s m}^{-2}

, and the density of oil is

900\,\text{kg m}^{-3}

, the magnitude of

q

is (IIT-JEE, 2010) (1)

1.6 \times 10^{-19}\,\text{C}

(2)

8.0 \times 10^{-19}\,\text{C}

(3)

4.8 \times 10^{-19}\,\text{C}

(4)

8.0 \times 10^{-10}\,\text{C}