Question

A parallel-plate capacitor is constructed of two horizontal 10.0-cm-diameter circular plates. A 1.0 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. a. Which plate, the upper or the lower, is positively charged? b. What is the charge on the positive plate?

Answer 1

answer) a) upper plate is positively charged because to balance the -6 nC charge, there is gravitational force pulling the charge down and there is an electric force pulling upward

b) we know the formula

Q=_oAE...............1)

=8.85*10^-12C/Nm²

A=pir², r=5cm=0.05m

E=F/q=mg/q=0.001 kg*9.8/6*10^-9C=1.63*10⁶N/C

so using eqn 1 we have

Q=8.85*10^-12*3.14*0.05²*1.63*10⁶=1.1*10^-7C

so the answer is 1.1*10^-7C or 1.13*10^-7C