Abstract: When two conductor spheres are charged with the same charge, their interaction is an interesting problem for both research and teaching. Through the qualitative discussion and the numerical simulation, the condition for the two spheres with the same charge always keep repel is extended. Based on the relationship of the electric potential energy and the distance between two spheres, the general condition for two spheres are always repelling is given. It is found that the previous condition with exactly ratio of radius and charges is actually the sufficient condition. In more general conditions, two spheres with the same charges can keep repel all along.

Key words: conductor, electric potential energy, electric capacity coefficient, interaction