$(1)$ A thin, ring$-$shaped object of radius a holds a total charge Q distributed uniformly

around it$.$ Determine the electric field at a point M on its axis, a distance x from the center.

$(2)$ Charge is distributed uniformly over a thin circular disk of radius R$.$ The charge per unit

area $($C$/$m$2)$ is $.$ Calculate the electric field at a point P on the axis of the disk, a distance z

above its center. Hint: you can think of the disk as a set of concentric rings, then apply the

result of $(1)$ to each of these rings, and then sum over all the rings.