4. A parallel-plate capacitor is constructed from two horizontal 12.0-cm-diameter circular plates. A 0.50 g plastic bead, with a charge of -6.0 nC, is suspended between the two plates by the force of the electric field between them. (Think about what condition must be met in order for this to be the case.) (a) Which plate - the upper or the lower - is positively charged? How do you know? (b) What is the charge on the positive plate