Static electricity was first noted back in 600 B.C. However, scientists have long found it difficult to explain the phenomenon of electricity generation through rubbing. Researchers made significant progress in 2019 by identifying nanoscale surface changes as a key factor. They now assert that when an object slides, electrical charges accumulate differently on its front and back sides, which leads to electric currents.
Anyone who’s ever stroked a cat or dragged their feet on a carpet knows that rubbing two objects together can generate static electricity. Yet, understanding precisely how this happens has puzzled scientists for over two thousand years.
Recently, scientists from Northwestern University have shed light on the mechanics involved in this phenomenon.
The researchers discovered that when an object moves, the forces acting on its front and back sections differ. This disparity results in varying amounts of electrical charge accumulating on these sections, creating a current that results in a tiny electric shock.
This research was published on September 17 in the journal Nano Letters.
“For the first time, we can provide a clear explanation for a mystery that has baffled people for centuries: the significance of rubbing,” stated Laurence Marks, the study’s lead researcher from Northwestern. “Others have attempted to explain it, but often relied on unsubstantiated assumptions. Our findings offer a straightforward answer: different surface deformations and corresponding charges at the front and back of a sliding object create a current.”
Marks, who specializes in surface structures, is a professor emeritus of materials science and engineering at Northwestern’s McCormick School of Engineering. The first author of the paper, Karl Olson, is a Ph.D. student in Marks’ research group.
The ancient Greek philosopher Thales of Miletus was the first to document the relationship between rubbing and static electricity back in 600 B.C. After he rubbed amber with animal fur, he observed that the fur could attract dust particles.
“Since that time, we’ve recognized that rubbing causes static charging not only in fur but in all insulators,” Marks mentioned. “Yet, that was essentially where the scientific understanding stalled.”
Marks and his group began piecing together this puzzle in 2019. They published findings in Physical Review Letters that revealed how rubbing two different materials would bend tiny bumps on their surfaces, generating voltages as a result of these deformations.
“In 2019, we had a glimpse of the underlying mechanism, but like seeds, it required time to develop,” Marks explained. “Now it has flourished into a new model capable of calculating electrical currents. Our findings matched very well with experimental data across various scenarios.”
The core of this new model revolves around a concept known as “elastic shear.” This term describes the resistance a material offers to sliding forces. For instance, when someone pushes a plate across a table, the plate pushes back against this force. Once the pushing stops, the plate immediately halts movement due to this friction, which in turn drives the movement of electrical charges.
“There is a deep connection between sliding and shear,” Marks clarified.
While static electricity can sometimes produce amusing events, such as hair standing on end after sliding down a play structure, it can also pose serious risks. Sparks generated by static electricity are known to cause industrial fires and explosions. Moreover, it can disrupt the accurate dosing of powdered medications. Gaining insights into how static electricity functions could lead researchers to develop solutions for these challenges.
“Static electricity plays a role in our lives in both simple and significant ways,” said Marks. “For example, static charges can greatly affect how coffee beans are ground and their resulting flavor. Additionally, the Earth may not exist in its current form without the static electricity involved in the particle clumping that forms planets. The impact of static electricity on our lives and even on the universe is remarkable.”
The research titled “What puts the ‘tribo’ in triboelectricity” received support from Northwestern Engineering.
