Two horizontal parallel conducting plates are kept at a separation `d = 1.5 xx 10^(-2) m` apart one above the other in air as shown in figure. The upper plate is maintained at a positive potential of `1.5 kV`while the other plate is earthed which maintains it at zero potential. Calculate the number of electrons which must be attached to a small oil drop of mass `m =4.9 xx 10^(-15) kg` between the plates to maintain it at rest. Consider density of air is negligible in comparison with that ofoil. If the potential of above plate is suddenly changed to `-l .5kV`, what will be the initial acceleration of the charged drop? Also calculate the terminal velocity of the drop if its radius is `r= 5.0 xx 10^(-6)m` and the coefficient of viscosity of air is `eta = 1.8 x 10--5 N-s//m^(2) [3,2g, 5.7 xx 10^(-5) m//s^(2)]`