1. The problem statement, all variables and given/known data Here is an interesting reading to my question. http://coraifeartaigh.wordpress.com/2010/02/27/what-is-a-field/ The author wrote: The reason he said each charge will experience a different force is assuming different r (and not a constant r)? Another question is to understand the purpose of a field (in mathematics and physics). So a field is use to describe the effect of a test particle that places in space? Any help is appreciated! Thank you.