Initially, we have 34rρg = qE (Since viscous force = 0) After switching off the field, we have 34πr3ρg = 6πqrvr where VT is the terminal speed and μ is the coefficient of viscosity. Therefore, qE = 6πμrvT ⇒ q =
[(81×3.14)/7]×1056×3.14×18×10−5×r×2×10−3
From the equation 34πr3ρg = 6πrμvT, we get r2 = 2ρg9μvT =
2×900×9.89×1.8×10−5×2×10−3
= 0.18 × 10−10 Therefore, r = 0.42×10−5m and q = 0.78 × 10−18 = 8 × 10−19C