The D (displacement) vector has only the free charges as source. I heard it is better to work with D than E, because E can be due to both free and induced charge, and we cannot easily know the bound charge distribution(?). But it seems that we can know the polarization P however...from which we can derive the bound charges.(first the egg or the chicken). I also read that the D field makes Gauss law easier to use. How? Q: What is the story? Can we not just use B and E since they are the primary fields that a charge would actually experience? In summary: * E_net is due to both free and bound charges. * D is only due to free charges. * The electric bound charges can be derived from P (and vice versa). * The electric permittivity can be derived from P OR from the bound charges(and vice-versa). * It is said that it is HARDER to know the bound sources than P or the permittivity. Not sure why.... * Maxwell equation can account for materials and only use B and E, IF we stick into them an expression for P(and M), OR just the bound charges,OR the material permittivity. * Why bother with extra constitutive eqns, when we can just work with E and B, if we just insert P and M (or rho or epsilon) in M.E.? * The constitutive eqns still demand knowledge of P and M (or,again, the bound rho or permittivity). * Maybe Maxwell eqns then become too difficult to solve computationally with E and B... * Using D (and H) we treat the material as if it was vacuum. We get D and then value of E_net at every point, adjusted by the local permittivity.... *D has field lines that behave differently from the field lines of E in materials (dielectric,ferroelectric...). (Maxwell came up with D...because they did not know about atoms and P. D=(eps_0)E_net *P, i.e., D always intrinsically takes P and bound charges into account....) thanks!