Proving electrostatic analogy for root locus

Question

My teacher told us that there is a provable mathematical analogy between root locus and the lines of force generated by electric charges, where every pole can be associated to a positive charge and every zero can be associated to a negative charge.
This sometimes aids is the sketching of the locus when no other rule seems to be of any help (basically using the fact that poles are attracted to the zero that is closer to them, as positive charges do with negative charges).
However I haven't found references to this analogy on the net, thus I was wondering, out of curiosity, how such a relation could be proved: I'd be glad if you could give me some hint on that.
Thank you for your help in advance.