We have known about the phenomenon of static electricity since at least the time of Aristotle. Aristotle credits fellow philosopher Thales of Miletus, who lived between 640 and 546 BCE, with the discovery that amber picks up pieces of dried grass after it has been rubbed with a cloth.
For a very long time, no real progress was made in finding out what it is, or how it works. Benjamin Franklin made a little headway in the area by rubbing wax and wool together, defining positive charge as the charge acquired by the rubbing wool and negative charge the charge associated with the wax that got rubbed.
While the world certainly appreciates all of Franklin’s rubbing, his understanding of the topic involved the exchange of fluids, which is not what’s really going on to produce the positive and negative charge. But now, thanks to a team modeling static charge at the nano scale, we finally know what’s going on, and why rubbing produces more static electricity than contact or rolling.
“For the first time, we are able to explain a mystery that nobody could before: why rubbing matters,” said Northwestern University’s Laurence Marks, who led the study, in a statement. “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. Just having different deformations – and therefore different charges – at the front and back of something sliding leads to current.”
The model produced by the team is related to “elastic shear”, where a material resists a sliding force. Think of sliding a plate across a table. When you stop pushing, the plate will stop quickly, and it is this resistance to sliding that causes the electrical charges to move.
“The model explains triboelectric currents during sliding that are a result of tangential forces breaking the contact symmetry,” the team explains in their paper. “Bound charges due to flexoelectricity, mean inner potential shifts, and deformation potentials are asymmetrically distributed and compensated by free charges that are involved in charge transfer. When combined with sliding motion, this leads to a current.”
Crucially, this model can be applied to different materials, as “the basic physics of electromechanical bound charges leading to triboelectricity apply generally, regardless of if they are generated by flexoelectricity, piezoelectricity, or other band bending.”
While you might associate static electricity mainly with getting zapped by a child who has just emerged dizzy from a bouncy castle, for others the problem can be a lot more serious, leading to industrial fires and even hindering dosing of medicines in powder form. With a better understanding of it, it could be possible to reduce these issues, or even find some fun new uses for static electricity.
“Static electricity affects life in both simple and profound ways,” Marks added. “Charging grains with static electricity has a major influence on how coffee beans are ground and taste. The Earth would probably not be a planet without a key step in the clumping of particles that form planets, which occurs because of the static electricity generated by colliding grains. It’s amazing how much of our lives are touched by static electricity and how much of the universe depends on it.”
And now we know why rubbing is so important to the process.
The study is published in Nano Letters.
