An oil drop of 12 excess electrons is held stationary under a constant electric field of $ 2.55 \times 10^4 Vm^{–1} $ . If the density of the oil is $ 1.26 gm/cm^3$ then the radius of the drop is
As the drop is stationary , weight of drop = force due to electric field $ { 4 \over 3 } \pi r^3 \rho g = neE $ So, $ r^3 = { 3neE \over 4 \pi \rho g } $ now find out r