Dr Sam T. CarrInstitut fur Theorie der Kondensierten Materie
Tel: ++49 721 608-7006
This applet plots (in two dimensions) the electric field lines produced by an arbitrary system of point charges of values plus or minus one. Hopefully the user interface is fairly self explanatory.
Notes: This was intended as an aid to students learning electromagnetism for the first time. While the electric field at any point in space is trivial to calculate via Coulomb's law, it is far less trivial to plot this field as continuous lines, with the density of lines giving the magnitude of the field. While there is the odd glitch with my approach, the results seem to work pretty well.