Written by Willy McAllister.


Where we’re headed

The variables q1 and q2 represent an arbitrary amount of charge. They could be elementary charges (electron or proton charge), or some other value.

A “point” charge is the idea that some amount of charge is concentrated in a tiny location, at a single point. We like this idea because it makes it perfectly clear how distance should be measured. You can measure the distance between two point charges using a ruler, and everyone will agree with how you did it.

The definition of distance becomes less clear if you try to specify the distance say between two charged cannonballs. Do you measure between their nearest points? centers? farthest points? all points in each cannonball pairwise? Coulomb himself realized that for the purposes of measuring electric force, all the charge spread out on a cannonball could be considered to exist at its center point.

So for Coulomb’s Law we say there is such a thing as a point charge. We end up with one number for the amount of charge, and one number for the distance. This concept gives us the simplest form for Coulomb’s Law.

There are a number of classic electrostatic problems where we analyze charge that is not concentrated at a point. The math is pretty clever. https://www.khanacademy.org/science/electrical-engineering/ee-electrostatics/ee-fields-potential-voltage/a/ee-electric-field-near-a-line-of-charge

On my own web site I wrote an article about Charge that may help you get the idea. http://spinningnumbers.org/a/charge.html