Electromagnetic Radiation from a Neutralized Polarized Sphere with Two Conserved Currents for One Charge History

Abstract

Can a source radiate when its total charge density vanishes identically? Consider a uniformly polarized sphere coated with free surface charge that cancels the bound surface charge at every point and time. Then $\rho_{\mathrm{tot}}=0$, so every electric charge multipole vanishes. Continuity determines only $\nabla\cdot\mathbf{J}$, which allows the same charge history to be supported by different conserved currents. A compensating interior current gives $\mathbf{J}_{\mathrm{tot}}=\mathbf{0}$ and produces no $\mathbf{E}$ or $\mathbf{B}$ at any frequency. The minimum-norm tangential sheet current instead leaves $\mathbf{J}_{\mathrm{tot}}$ nonzero and divergence-free. Its radiation-zone field is exact in $kR$ and proportional to $j_2(kR)$. At long wavelength the radiated power is suppressed by $(kR)^4/100$, and it vanishes exactly at the positive roots of $j_2$. The same calculation gives the interior field and a closed-form energy balance. The average work supplied by driving equals the radiated power and falls to zero at those roots even though interior fields remain. For comparison, the bare sphere has the factor $3j_1(kR)/(kR)$ and is silent at the roots of $j_1$.