Q: In Millikan method of determining the charge of an electron, the terminal velocities of oil drop in the presence and in the absence of an electric field are x cm/s upwards and y cm/s downwards respectively. Find the ratio of electric force to gravitational force on the oil drop. (Neglect Buoyancy)
Sol: In gravitational field, weight = viscous force
W = 6 π η r y … (i)
In electric field,
Electric force = weight + viscous force
q E = W + 6 π η r x …(ii)
Substitute (i) in (ii) then
q E = 6π η r (y + x) …(iii)
(Electric force)/(Gravitational force) = (x + y)/y.