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Appendix B - Forces, Torques, and Responses of Localized Charge Densities

Published online by Cambridge University Press:  16 October 2026

Jene A. Golovchenko
Affiliation:
Harvard University, Massachusetts
Edlyn V. Levine
Affiliation:
Harvard University, Massachusetts
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Summary

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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