Illustration Electric Potential Graph (inside / outside) - Hollow sphere

Electric potential $$\varphi$$ of a hollow sphere as a function of distance $$r$$. The hollow sphere has the radius $$R$$.
• Outside ($$r \gt R$$) of the hollow sphere, the potential $$\varphi$$ is proportional to $$1/r$$ as for a point charge.
• Inside ($$r \lt R$$), however, the electric potential $$\varphi$$ is constant.