Given a solid metal sphere where all the positive charges are distributed evenly on the surface of the metal sphere. My textbook says that there is no electric field strength in the middle of the metal sphere because the charge = 0. However, my understanding is that wouldn't the center of the metal sphere be under the influence of the charges on the metal sphere surface, but because of the shape, all the electric field strength acts in opposite direction thus cancelling each other out. The second question is, when there is no electric field strength, the electric potential is zero , that would mean that potential is either maximum or minimum given, E = - dV/dr . So why is it maximum in the sphere, why not minimum? What is that explanation?