How Friction Causes That Familiar Static Electrical Shock

An explanation has been offered for static electricity, the charge built up when insulators are rubbed together, potentially illuminating one of the oldest mysteries in physics.

The first recorded description of static electricity comes from Thales of Miletus 2,600 years ago, who picked up leaves with charged amber, although cave people probably noticed the effects of running tree sap through furs millennia before. Yet in all that time, no one has been able to explain why rubbing things together creates sparks (no not like that, this is physics). Finally, the mystery may have ended, with it all being about the bending of microscopic protrusions.

We’ve all had that painful zap when touching something metal after walking in socks over carpet and seen the sparks jump when brushing fur in the dark. Part of the explanation for this phenomena was provided when physicists realized some materials have a greater affinity for positive charges and others for negative. Friction between them sees electrons move from one to the other, causing a charge build-up known as triboelectricity that is eventually neutralized.

This may have been enough for generations of physics students, but some, including Professor Laurence Marks of the Northwestern University, wanted more. On a basic level, they have wanted to know what caused these affinities. “It has become clear that rubbing induces static charging in all insulators – not just fur,” Marks said in a statement. “However, this is more or less where the scientific consensus ended.” Proposed explanations have included local heating and quantum mechanical effects.

In Physics Review Letters, (pre-print on arXiv.org) Marks attributes static charges to the fact no material is perfectly smooth, and the parts of the surface that stick out above the rest get bent by the force of rubbing them against another object. As a result of what is known as the “flexoelectric effect”, a charge gradient forms along these protrusions when they are placed under strain.

Marks and his co-authors demonstrate that the additional charge at the bent ends can produce a potential difference of up to 10 volts along such micro-protrusions. This is large enough to create the macroscopic charge we witness, producing the various party tricks static electricity makes possible.

“Our finding suggests that triboelectricity, flexoelectricity and friction are inextricably linked,” Marks said.

Static electricity can have much more important consequences than a slight shock when grabbing a door handle. It can start fires and disrupt sensitive equipment, but it is also harnessed in laser printers. It’s possible Marks’ identification of the causes, if verified in future studies, will lead to more sophisticated methods to prevent charge build-up when we don’t want it and control it when we do, although gloves are still probably the best way to avoid being shocked.

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