I have modified the example of electrofied oil (ex 15.4) to show what happens when the charge on the oil drop changes. The electric force (Fe) on a negative charged oil drop will be upward while the gravitational force (Fg) is downward. If Fg is greater than Fe the drop will have negative acceleration and will fall. If there is enough charge on the particle so that the electric force exceeds the gravitational force the particle will have positive acceleration and will rise. For the example below I started with a charge of 3 electron on the oil drop and there is essentially no acceleration. With two electron charges there will be less electric force and the particle accelerates slowly downward. With 1 electron charge the drop has even larger negative acceleration. If the drop contains no charge at all it will fall at the acceleration of gravity (as it should in a vacuum). So the conclusion is that the drop will have the greatest negative acceleration (but not as much as g) when the charge on it is the least it can be. The least it can be is 1.6x10^-19 C. Try working this problem when the drop contains 4 electron charges, i.e., 4 x 1.6x10^-19 C.