Question

In the milikan experiment,and oil drop carries four changes and has a mass of 1.8*10^-12g.it is held almost at rest between two horizontal charged plates 1.8cm apart. What voltage must there be between the two charged plates?

Answer 1

The voltage must provide the field which creates the force can support the drop:

$$F=Eq=mg,\\ E=Vd,\\ Vdq=mg,\\\space\\ V=\dfrac {mg}{qd}=\dfrac {mg}{4ed}=156.04\text{ kV}.$$