Asolid metallic sphere of radius a carries total charge q. no other charges are nearby. the electric field just outside its surface is keq/a2 radially outward. at this close point, the uniformly charged surface of the sphere looks exactly like a uniform flat sheet of charge. what is the electric field strength inside the metallic sphere at r = a/2? use σ = q/4 π a2.

You have two photos of a person walking. one shows the person at the corner of third and main streets, the other shows the person at the corner of tenth and main streets. there are lampposts at every corner in this town, and the first picture shows it to be 10: 32: 00 exactly. the second picture shows it to be 10: 49: 30. you know three facts: (1) all of the clocks are synchronized; (2) there are exactly 12 equal-sized blocks per kilometer in this town; and (3) the streets that cross main in this area are numbered consecutively, with no interruptions. what is the person’s average speed in kilometers per hour?