Published online by Cambridge University Press: 16 October 2026
This appendix derives the Lorentz force and torque on localized charge and current distributions in terms of their multipole moments. Starting from the general Lorentz force density, it introduces a comoving reference point X(t), rewrites the charge and current in a particle frame, and expands the external fields about that point. This yields a multipole expansion of the total force in terms of the net charge, electric dipole moment, charge quadrupole tensor, and magnetic dipole moment, and identifies the leading contributions, q0 E(X,t), (p⋅∇)E, and ∇(μ⋅B). The appendix defines the electric and magnetic dipole moments and discusses when higher-order multipoles can be neglected for slowly varying fields. It then interprets the force law as an effective equation of motion for a localized system carrying internal structure. A final section derives the torques on electric and magnetic dipoles, τE = p × E and τB = μ × B, summarizing how localized systems respond translationally and rotationally to external electromagnetic fields.
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