.P30 Consider a thin plastic rod bent into a semicircular arc of radius R with center at the origin (Figure 15.57). The rod carries a uniformly distributed negative charge - Q. -Q R X Figure 15.57 (a) Determine the electric field E at the origin contributed by the rod. Include carefully labeled diagrams, and be sure to check your result. (b) An ion with charge -2e and mass M is placed at rest at the origin. After a very short time At the ion has moved only a very short distance but has acquired some momentum p. Calculate p