Static Electricity

 At 8:30 this morning I was checking my email when Maya texted me: "Static electricity question: when two objects are rubbed together, which one gains electrons, and which one loses electrons? Is there a way to know in advance which will happen?"

I was stumped, and replied, "The short answer is: I don't know." 

But why didn't I know? This is elementary-school science as well as a party trick. Rubbing balloons on one's sweater to hang them on the wall as decorations, shuffling on a carpeted floor and extending one's hand to an unwitting victim, picking up little bits of paper with a charged plastic pen barrel... Phenomena like these have been observed for ages, so why is that the limit of my knowledge?

I had a vague memory of having thought about this before, but the thought obviously fled. I was sitting at my laptop, though, so I pursued it. I found a Wikipedia article on Triboelectric Effect, a term I never encountered before, but which refers to static electricity by rubbing. It contained some lists, called triboelectric series, of materials that tend to become more or less charged, both positively and negatively, but those lists were inconsistent. It linked to a 2019 paper from the publication Nature Communications that offered material scientists a reproducible mechanism for creating a standard series. Relying in that way on experiment instead of some well-understood law suggested to me that we still don't really understand this! That's pretty astonishing after millennia (literally) of observation. If all we have is empirical lists that means it is unpredictable... unpredictable in the sense that we cannot - in Maya's words - "know in advance which will happen."

I found another 2019 paper, this one in Nature Reviews: Chemistry, titled "Long-standing and unresolved issues in triboelectric charging." The authors explicitly discuss the problems of a field and a phenomenon which has been so well known for so long, and yet which remains poorly understood. I'll let their opening paragraph speak for itself:

Nearly a century ago, Peter Shaw remarked in an article on triboelectric charging that “This class of research is simple-seeming. But those who have spent time on the subject will allow that it is very baffling; those who have not done so will at least remember that despite great efforts by physicists the subject has not yet passed the pioneer stage”. Since this article appeared, the work of several generations of determined scientists has left the subject only slightly better understood, and this modest progress is countered by the emergence of new baffling behaviours. Indeed, as Mark Twain famously described in a different context, researchers have “thrown much darkness on this subject, and it is probable that, if they continue, we shall soon know nothing at all about it.”

I'll give you a moment to chuckle. 

Among the anomalies they mention:

• Identical materials be rubbed together to produce opposite electric charges
• Items with the same charge can be made to attract
• Objects' relative position in the series changes when their shape changes
• Different methods of creating series yield different series

I find this confusion exciting. Physicists who study elementary particles love to talk about a "theory of everything," as if one more generation of more powerful accelerators will answer all our questions. But in the real world everything is much more complex. I have always preferred the view of the British biologist J.B.S. Haldane who wrote: "The universe is not only queerer than we suppose, but queerer than we can suppose." [emphasis mine] The authors of the paper on unresolved issues agree with me and I will let them have the last words:

We find that the simple game of rubbing a balloon on one’s hair provides delight on different levels. A child is delighted by the surprising tendency of the charged balloon to stick to a wall. As scientists, we are delighted by the surprise that this simple game may defy precise measurement, and, perhaps, the charging of a balloon cannot be precisely measured.

[Again, emphasis mine.]