# Maxwell's equations and Lorentz force

In a previous question I was told that the Lorentz force can compute the force on charges in one system A from electric and magnetic fields from another system B, but A cannot be the same system as B because then self interaction terms would create infinitely huge energies. (Please let me know if it is possible despite the infinities)

So how is the Lorentz force law used in e.g. network models of a circuit? What parts gets to generate electric/magnetic fields and what parts gets to have charges?

• If you apply the potential of a point particle to itself you get infinite repulsive energy. In a circuit you are dealing with finite charge and current densities. Only if you consider charges and currents to reside on infinitesimally thin plates or wires, or in points, infinities appear. – my2cts Jun 16 '18 at 9:31
• But the finite charge is in turn made up of smaller elements, all the way down to point particles in classical theories at least. Say I was simulating each particle, how would it be then. – Emil Jun 16 '18 at 9:34
• Also, not sure how you mean one can avoid the divergences if one speaks of densitites either. – Emil Jun 16 '18 at 9:40
• To create an infinite charge density requires an infinite energy. There is no free lunch. When you describe a circuit, as your question suggests, there is no reason to go there. – my2cts Jun 16 '18 at 9:46
• Why not? Do one have bounds on how much error one gets for disregarding self interaction or something? (Which I assume is done in what you describe because you avoid it somehow?) – Emil Jun 16 '18 at 13:18