Hi, I'm trying to understand point charges along a uniformly charged line with a charge per unit length. This is a screenshot of what i'm trying to understand. http://i.imgur.com/s1vX7.jpg I am not sure where the term r/R is coming from. I realise that the r term in sigma*dx/4pi*enot should change as you go down the x axis but I don't get where the factor r/R comes from. I feel like i'm missing something. This is an example page from the book "Electromagnetism" by John C. Slater and Nathaniel Frank.