Geometric invariance from outer surfaces: Laplace-governed magnetization in the high-permeability limit

Abstract

The magnetization of bodies in static fields is a textbook topic in electrodynamics, governed by Laplace equations with interface continuity (transmission) conditions. In the infinite-permeability limit, textbooks emphasize the quasi-equipotential interior and normality of the external field at the boundary, but leave the exterior largely uncharacterized. Here we identify a singular property that has not been explicitly stated in the existing literature: in this limit, the entire external magnetic response, including the external field distribution and all multipole moments, is determined solely by the outer surface geometry, independent of internal structure or deformation. Numerical simulations confirm this limiting property is well approximated under finite high-permeability conditions, thereby providing a theoretical basis for the lightweight design of magnetic devices such as flux concentrators. Since analogous Laplace transmission problems arise across physics, including heat conduction, electrostatic polarization, and acoustic scattering, this geometric invariance exhibits cross-disciplinary universality. Together with the quasi-equipotential property, it provides a complementary and essentially complete characterization of Laplace transmission problems in the infinite-permeability limit.