I know people might think this is an unworthy question, but I want to know anyway. Does anyone know why "r" was chosen as distance in this formula? I know many formulas and most of them follow the same variable usage for measurement of distance ("s"). The letter "r" in my experience is mostly used as radius. Being that they are both measures of length, it seems out of place to use "r" if your talking about point charges in, say, a spherical shape because it may be confusing to someone who doesn't use the formula all the time. If the variables didn't have a generally set and known usage then equations would be unnecessarily hard to remember. Why veer from the norm on equations like this, when I don't see any other "s" that might be used already in this particular equation?