ELECTRIC-FIELD LINE DIAGRAMS DONT WORK

Electric fields produced by coplanar point charges have often been rep resented by field line diagrams that depict two-dimensional slices of the three-dimensional field. Serious problems with these ''conventiona l'' field line diagrams (CFLDs) have been overlooked. Two of these pro blems, ''equatorial clumping'' and ''false monopole moment,'' occur be cause a two-dimensional slice lacks information vital to the accurate representation of an inherently three-dimensional field. Equatorial cl umping causes most CFLDs to exhibit unphysical behavior such as irregu lar spacing between field lines terminating on negative charges. CFLDs can also mistakenly indicate that a neutral charge distribution has a significant monopole moment. Such phenomena make the visual estimatio n of local field strengths impossible and render CFLDs of little utili ty for representing three-dimensional fields. While these ''projection '' problems can be avoided by using two-dimensional field line diagram s to represent two-dimensional (1/r) electric fields, or by using thre e-dimensional field line diagrams to represent three-dimensional field s, other forms of distortion generally remain. (C) 1996 American Assoc iation of Physics Teachers.