(17%) Problem 4: As shown in the diagram, two insulating spheres have a center-to- center separation of d - 6.36 em. The first insulating sphere, with a radius of ri = 1:82 cm. has a total charge of gi - -13.5 nC. The second insulating sphere, with a radius of re - 2.24 cm, has a total charge of or = + 18.1 BC, Assume that, on each sphere, its charge is uniformly distributed over its surface. What is the magnitude, in newtons per coulomb, of the electric field at the midpoint between the centers of the spheres? N/C