Scientists at Northwestern University may have figured out why walking on carpet in your socks, petting your furry friend, or rubbing a balloon on your hair creates static electricity. In a new study, they say they’ve found that rubbing can create tiny deformations on an object’s surface that allows the phenomenon to occur.
People have known about the existence of static electricity for millennia, with the first recorded observation of it thought to have been made by Greek philosopher Thales of Mileus in 600 B.C. (Thales noticed that fur would attract dust right after it was rubbed with amber). Since then, we’ve learned that lots of things can cause static electricity and that it can be advantageous to animals, such as ticks that use static electricity to extend their host-grappling range. But scientists have remained in the dark about many of the basics behind static electricity, particularly why rubbing often induces it—at least, perhaps, until now.
“For the first time, we are able to explain a mystery that nobody could before: why rubbing matters,” said lead researcher Laurence Marks, Professor Emeritus of Materials Science and Engineering at Northwestern, in a statement from the university. “People have tried, but they could not explain experimental results without making assumptions that were not justified or justifiable. We now can, and the answer is surprisingly simple.”
Most people have seen the classic balloon trick, in which rubbing a balloon on your head creates static electricity that makes hair stand on end. The basic principle of the trick is that rubbing two objects with very different physical properties causes one object (our hair, in this example) to lose electrons and become positively charged and the other object to become negatively charged (gaining electrons). When the objects then meet again, the difference in charge causes them to be attracted to each other and for electrons to rapidly move from one to the other. This rapid movement of electrons also explains why rubbing our feet along our carpet and then touching a metal doorknob can cause a small shock. But rubbing pieces of the same material can also generate static charge, and previous research seems to have debunked a common explanation for why this can happen (the argument was that this charge could be created by rubbing pieces of the same materials with two different sizes).
In this new study, published last month in the journal Nano Letters, Marks and his team say they’ve found another major mechanism to explain why rubbing can cause static charging. In 2019, they published a study finding that the mere act of rubbing two materials together can cause small deformations on the surfaces of the objects, which create voltage. But they’ve now worked out exactly how rubbing can lead to static charging, which is influenced by the existence of elastic shear, or a material’s ability to resist sliding when moving along a surface (this is why we eventually stop sliding on a floor even when wearing socks). They argue that the increasing friction caused by elastic shear means that the front and back of an object can have different deformation that carry opposing charges, which then allows static electricity to happen, akin to how the difference in air pressure above and below a plane’s wing causes lift.
“In 2019, we had the seed of what was going on. However, like all seeds, it needed time to grow,” Marks said. “Now, it has blossomed. We developed a new model that calculates electrical current. The values for the current for a range of different cases were in good agreement with experimental results.”
Other researchers will have to verify the results, and while this hypothesis may help explain many kinds of static electricity, including the kind produced by same-material rubbing, it wouldn’t explain every instance. Still, it’s oddly comforting to know that science has plenty of mundane mysteries left to solve in trying to understand the world around us—it means that there’s always something new left to learn.