Consider 2 metals ,the smaller sphere has a radius of a carry +Q charges,what is the potential at the surface of the smaller sphere?
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It is usually assumed that the potential at infinity is zero. The field between the outer surface of the larger sphere and infinity is the same as for a point charge $Q$ in the center (without any spheres), so you can calculate the potential at the outer surface of the larger sphere. The potentials of the outer and inner surfaces of the larger sphere are the same. The field between the smaller and the larger spheres is the same as for a point charge $Q$ in the center (without any spheres), so you can calculate the difference of the potentials at the surface of the smaller sphere and the inner surface of the larger sphere.