Talk:Coulomb's law

Even if this is correct, can someone clarify what "small volume" means? On one hand, the implication seems to be, the smaller the volume of the two substances/objects/particles, the more correctly the Law will predict the forces acting on them. So perhaps we could reformulate Coulomb's Law as a statement along the lines of "as the limit of the particles' volumes goes to 0..." But if this is so, how are we to think about the lowest levels of matter, where everything seems to be quantized, not continuous?

Finally, I don't suppose there's a handy expression for how accurate one can expect Coulomb's Law (or other such laws) to be, given the size of the substances involved? How does the day-to-day engineering question of "Is Coulomb's Law accurate enough for my particular use?" get answered? (Or perhaps Coulomb's law, by itself, isn't useful in a day-to-day sense....)