Interesting article on the physics of static electricity. The previous thought on static electricity does not occur when the surface of two objects are of opposite charges come in contact and then are quickly separated (like glass and wool). Initially it was thought that the charge per unit of surface was typically very low. A new paper just published in science sheds light on the mechanism of static electricity: the surfaces are of opposite charge, yes, but only overall; within each surface, there is a mosaic of positive and negative charges, of nanoscopic dimensions (nano = 10-to-the-minus 9; therefore the charge changes polarity every 0.000000001 square meter). Also, the charge is way more powerful than initially thought. I love new looks into run-of-the-mill everyday physical processes we are all familiar with. Below is the abstract of the Science article.
The Mosaic of Surface Charge in Contact Electrification
H. T. Baytekin, A. Z. Patashinski, M. Branicki, B. Baytekin, S. Soh, B. A. Grzybowski*
+ Author Affiliations
Department of Chemistry and Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA.
↵*To whom correspondence should be addressed. E-mail: email@example.com
When dielectric materials are brought into contact and then separated, they develop static electricity. For centuries, it has been assumed that such contact charging derives from the spatially homogeneous material properties (along the material's surface) and that within a given pair of materials, one charges uniformly positively and the other negatively. We demonstrate that this picture of contact charging is incorrect. While each contact-electrified piece develops a net charge of either positive or negative polarity, each surface supports a random “mosaic” of oppositely charged regions of nanoscopic dimensions. These mosaics of surface charge have the same topological characteristics for different types of electrified dielectrics and accommodate significantly more charge per unit area than previously thought.
Wired reports on this paper, too, very nice article, I recommend reading it all.
By John Timmer, Ars Technica
For many of us, static electricity is one of the earliest encounters we have with electromagnetism, and it’s a staple of high school physics. Typically, it’s explained as a product of electrons transferred in one direction between unlike substances, like glass and wool, or a balloon and a cotton T-shirt (depending on whether the demo is in a high school class or a kids’ party). Different substances have a tendency to pick up either positive or negative charges, we’re often told, and the process doesn’t transfer a lot of charge, but it’s enough to cause a balloon to stick to the ceiling, or to give someone a shock on a cold, dry day.
Where to begin? The authors start about 2,500 years ago, noting that the study of static began with a Greek named Thales of Miletus, who generated it using amber and wool. But it wasn’t until last year that some of the authors of the new paper published a surprising result: contact electrification (as this phenomenon is known among its technically oriented fans) can occur between two sheets of the same substance, even when they’re simply allowed to lie flat against each other. “According to the conventional view of contact electrification,” they note, “this should not happen since the chemical potentials of the two surfaces/materials are identical and there is apparently no thermodynamic force to drive charge transfer.”
One possible explanation for this is that a material’s surface, instead of being uniform from the static perspective, is a mosaic of charge-donating and charge-receiving areas. To find out, they performed contact electrification using insulators (polycarbonate and other polymers), a semiconductor (silicon), and a conductor (aluminum). The charged surfaces were then scanned at very high resolution using Kelvin force microscopy, a variant of atomic force microscopy that is able to read the amount of charge in a surface.