A 0.10 g plastic bead is charged by the addition of 1.0 x 1010 excess electrons. What electric field (strength and direction) will cause the bead to hang suspended in the air?

Answer 1

The charge is Q = -10^10*e
e is the electron charge, 1.6*10^-19 C

You need an electric field pointed down to balance the weight of the drop. The strength of the field is given by
E*Q = M*g

E = M*g/Q newtons/coulomb