The Definition of Point Sources: Point Charge, Mass Point and Heat Point Source—A Generalized Sternberg Definition of Concentrated Forces

By considering the concept of point sources, such as point charge, mass point, and heat point source, we give a unified definition of these point sources with three conditions to judge what kind of distributed source in a small region can be taken as a point source by applying Sternberg's concept on concentrated forces. Comparing it with the traditional qualitative understanding of this concept, the definition gives a new condition with a very simple form. Counter-examples demonstrate that the traditional understanding of the point source is not sufficient and the new requirement should not be omitted.