Question

. A large parallel plate capacitor is made out of two circular plates that are 20.0 cm in diameter. They are arranged so that one is a short distance above the other. A small glass bead has a mass of 0.100 g and carries a charge of +5.00 nC. The glass bead hovers between the two plates of the parallel plate capacitor. (a) Which plate, upper or lower, is positively charged? (b) What is the charge on the positive plate?

Answer 1

A.

The lower plate is positively charged.

Because to hover midway, the gravitational pull must be canceled by opposite force and that comes from repulsion of the lower plate. As the lower plate repel, it must contain the same sign as the charge it repels.

B.

The electric field between the plates can be calculated using

Let us assume the distance between the plates is d.

So, the charge on the plate is 4.45 x 10^-8 C.