A uniformly charged disk like the disk in Fig. 21.25 has radius 2.50 cm and carries a total charge of 7.0 * 10-12 C. (a) Find the electric field (magnitude and direction) on the x-axis at x = 20.0 cm.

(b) Show that for x W R, Eq. (21.11) becomes E = Q>4pP0 x2, where Q is the total charge on the disk.

(c) Is the magnitude of the electric field you calculated in part (a) larger or smaller than the electric field 20.0 cm from a point charge that has the same total charge as this disk? In terms of the approximation used in part (b) to derive E = Q>4pP0 x2 for a point charge from Eq. (21.11), explain why this is so. (d) What is the percent difference between the electric fields produced by the finite disk and by a point charge with the same charge at x = 20.0 cm and at x = 10.0 cm?