On Monday I’ll be giving my first Phys 119 lecture which, interestingly, starts by talking about and demonstrating tribo-electricity, which is, as I’m sure everyone knows, the name for the way that rubbing two different substances together causes one of them (sometimes) to pick up some charge.

A few reflections on this idea. The word “electron” comes from the greek word for amber because one of the earliest accounts of triboelectric is from Thales. So, in a way, electricity itself is named after the effect. Interesting how pedagogy often recapitulates history.

Interesting in another way, however, is this fact: we learn in elementary physics that opposite charges attract one another. However, when we rub two things together and one picks up charge, clearly “something” has overcome this effect, since the act of rubbing the things together has caused like charges to accumulate on one surface and left more like charges of the other sign on the other part. How do we explain this?

On the one hand, the explanation is simple: its entropy. When we put two dissimilar surfaces together there is typically an arrangement of electrons which has higher entropy where more of them move to one surface than another. You can hand wave this by just pretending the surfaces are grids of different sizes and counting the number of ways to distribute charges across the boundary, assuming the positive charges are fixed. For the more simulation minded, you can simulate free classical electrons at the boundary of two positive lattices of different sizes and see it play out.

On the other hand, its incredibly complicated and, in general, the tribo-electric relations between different materials are extremely difficult to calculate and are generally a matter of experimental fact, not theory.

There is something interesting here about the nature of physical explanations but this post is getting long already.

A demo of this is here.