1. The problem statement, all variables and given/known data Suppose a strength that acts between two charges depends on the distance, 1/r^a, where 1) a>2 2)a<2 What will happen with a punctual charge, if it is put within a uniformly charged sphere. At the initial moment the punctual charge is in rest. 2. Relevant equations F is proportional to 1/r^a. 3. The attempt at a solution The solution says that for a> 2 the punctual charge q goes toward point O, that it is the center of the sphere, in case of same signals. and for a<2 the punctual charge goes toward point B that is in the surface of the sphere. And if the signals are opposite it happens the opposite too. I don't understand this. Why of going toward the center and the surface of the sphere?