BUILDING AND WORKING WITH ELECTROSTATIC EQUIPMENT

Abstract

When two materials with different affinities for positive and negative charges are rubbed together or separated, one may end up with an excess of negative charge and the other with an excess of positive charge, giving the materials net negative and positive charges. This will occur to some extent with any two materials, but the results may not be obvious unless the materials are good insulators. This process is called triboelectric charging,. The actual mechanisms of frictional or contact charging are complex and not fully understood, depending on the nature of the materials, surface impurities, moisture, etc. and may involve both motion of electrons and ions. The contact between dissimilar surfaces seems to be the important feature, with rubbing acting to increase the number of contact points.(See Cross, 1987, and Loeb, 1953, Horn and Smith, Science, 256(5055),362, Apr 17, 1992). Several experimenters have established lists of materials and their relative affinities for negative and positive charge when rubbed together. Such a list is called a triboelectric series. The example here is adapted from Graf (1964, p 37.) Once students have a reliable method for determining the sign of a charge (see Laboratory Activity Five) the construction of such a series using materials found in the classroom and the home can be an interesting independent investigation.