Halliday, Resnick & Walker — Electric Fields: Problem 28

28 A circular plastic disk with radius \(R = 2.00 \text{ cm}\) has a uniformly distributed charge \(Q = +(2.00 \times 10^6)e\) on one face. A circular ring of width \(40 \text{ \(\mu\)m}\) is centered on that face, with the center of that width at radius \(r = 0.50 \text{ cm}\). In coulombs, what charge is contained within the width of the ring?