When various materials are touched together, which become positive
and which become negative? Ah, you want the...
The above list is from NATURE'S ELECTRICITY, p63
by Charles K Adams (c)1987 Tab Books, #2769
* I don't know if I believe that "air" really is in this position. Electrification via airflow usually involves collisions with dust, snow, or condensed water suspended in the air. - Bill B.WARNING: the order of the above list is different in different books. Polarities are dependant upon atomically-thin surface layers, so don't be suprised if your particular examples of a material are not in the right place should you make your own list. It is best to choose two materials which are far apart on the list, then you can be fairly sure that their polarity will not be backwards from what you expect.
Note: the term "frictional electrification" is misleading. Separation of
charges is not accomplished by friction. Surface charge imbalance comes
from the same source 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 (such
as ionic bonds) will tend to leave imbalanced charges behind. Other
effects are important too, and 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. 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, this 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." Use these terms and avoid giving the idea that the mechanism for the electrification is CAUSED by friction. I also like to tell people that "surface charging is caused by peeling," since the scotchtape demo works so well, and since "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 together rather than simply touched. The order of the series also changes when surfaces of differing roughness are rubbed together. Even IDENTICAL substances can generate a charge-imbalance if one surface is rough and the other one smooth. 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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