“Note: the term "frictional electrification" is misleading. Separation of charges is not accomplished by friction. Surface charge imbalance comes from the same place that friction does: adhesion on the molecular level. Surfaces stick together because chemical bonds form. When surfaces in contact are separated, the bonds rupture, and any asymmetrical bonds will tend to leave imbalanced charges behind. Other effects are important too. This subject is not very well studied in science. Go search for info about surface charge, contact electrification, and Atomic Force Microscopes. Since most materials are not flat enough or flexible enough to attain large-area contact, most materials don't become very strongly electrified when simply touched together then separated. For example, when fur is used on plastic it helps greatly if you drag the filaments of hair across the plastic surface. Among other things, it acts to increase the total contact area. Some surfaces, such as adhesive tape or plastic sheets, CAN attain intimate contact over a large area, and DO exhibit strong charging when they are simply touched to another surface and pulled away. This phenomenon is called "contact charging" or "electrification by contact," and it's better to use these terms and avoid giving the idea that the mechanism for the electrification is CAUSED by friction. I also like to say that "surface charging is caused by peeling," since the scotch-tape demo works so well, and because "peeling" always implies a preexisting intimate contact between surfaces. Caveat: contact electrification is not well understood. Friction DOES play a part. For example, the ordering of the triboelectric series is different when surfaces are rubbed rather than simply touched. The order also changes when surfaces of differing roughness are rubbed together, and IDENTICAL substances can even generate a charge-imbalance if one surface is rough. This is probably a major reason why different references give different ordering of the series: the experiment must guarantee that no rubbing occurs, otherwise results will vary from trial to trial. “ |
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