A parallel-plate capacitor is constructed of two horizontal 17.2 cm-diameter circular plates. A 1.2g plastic bead, with a charge of - 5.2nC is suspended between the two plates by the force of the electric field between them.

What is the charge on the positive (upper) plate?

Given:

diameter, d = 0.172 m which means radius, r = 0.086 m

mass of bead = 1.2 g

= 0.0012 kg

charge of bead = - 5.2 nC

= - 5.2 * 10^-9 C

E = Q / (p*A)

where,

E = electric field

= F/Q

= (0.012 * 9.8) / 5.2 * 10^-9

= 2.26 * 10^7 N/C

p = permittivity constant

= 8.85 * 10^-12 C / (N*m^2)

A = area of capacitor.

= pi * r^2

= 0.093 m^2

Q = E*p*A

= 2.26 * 10^7 * 8.85 * 10^-12 * 0.093

= 1.86 * 10^-5 C