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From: Michele D'Amico
Date: 20 Nov 2002
Time: 02:45:53
Remote Name: 131.175.123.77
Thanks David, you're correct; and we can go further: if we add the fifth Maxwell equation (i.e. the one that relates the divergence of the corrent density to the time derivative of charge) we can eventually express the fields as a function of charges alone. But having said that, I don't feel like saying "since we don't need the current density, then it doesn't exist". In fact, we never use charges to calculate radiation; why ? Simply because they are very difficult to evaluate; on the contrary, current density is much simpler, sometimes even "intuitive". What I am trying to say is that if it is sometimes very difficult to separate the cause and the effect, very often it is extremely difficult to separate primary and secondary causes - or sources, if you prefer(i.e., for example, impressed (primary) and conduction (secondary) current densities on antennas, or charges (primary) and current density (secondary))... leaving the "existence" of things alone. All the best, Michele