A ring-shaped conductor with radius $a=2.50 \mathrm{cm}$ has a total positive charge $Q=+0.125 \mathrm{nC}$ uniformly distributed around it, as shown in Fig. $21.23 .$ The center of the ring is at the
origin of coordinates $O .$ (a) What is the electric field (magnitude and direction) at point $P,$ which is on the $x$ -axis at $x=40.0 \mathrm{cm}$ ? (b) A point charge $q=-2.50 \mu C$ is placed at the point $P$ described in part (a). What are the magnitude and direction of the force exerted by the charge $q$ on the ring?