2019Physical review. D/Physical review. D.Open access

Force on a point charge source of the classical electromagnetic field

Michael K.‐H. Kiessling

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Abstract

It is shown that a well-defined expression for the total electromagnetic force ${\mathbit{f}}^{\mathrm{em}}$ on a point charge source of the classical electromagnetic field can be extracted from the postulate of total momentum conservation whenever the classical electromagnetic field theory satisfies a handful of regularity conditions. Among these is the generic local integrability of the field momentum density over a neighborhood of the point charge. This disqualifies the textbook Maxwell-Lorentz field equations, while the Maxwell-Bopp-Land\'e-Thomas-Podolsky field equations qualify, and presumably so do the Maxwell-Born-Infeld field equations. Most importantly, when the usual relativistic relation between the velocity and the momentum of a point charge with bare rest mass ${m}_{\mathrm{b}}\ensuremath{\ne}0$ is postulated, Newton's law $\frac{\mathrm{d}}{\mathrm{dt}}\mathbit{p}=\mathbit{f}$ with $\mathbit{f}={\mathbit{f}}^{\mathrm{em}}$ becomes an integral equation for the point particle's acceleration; the infamous third-order time derivative of the position which plagues the Abraham-Lorentz-Dirac equation of motion does not show up. No infinite bare mass renormalization is invoked, and no ad hoc averaging of fields over a neighborhood of the point charge. The approach lays the rigorous microscopic foundations of classical electrodynamics with point charges.

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It is shown that a well-defined expression for the total electromagnetic force ${\mathbit{f}}^{\mathrm{em}}$ on a point charge source of the classical electromagnetic field can be extracted from the postulate of total momentum conservation whenever the classical electromagnetic field theory satisfies a handful of regularity conditions. Among these is the generic local integrability of the field momentum density over a neighborhood of the point charge. This disqualifies the textbook Maxwell-Lorentz field equations, while the Maxwell-Bopp-Land\'e-Thomas-Podolsky field equations qualify, and presumably so do the Maxwell-Born-Infeld field equations. Most importantly, when the usual relativistic relation between the velocity and the momentum of a point charge with bare rest mass ${m}_{\mathrm{b}}\ensuremath{\ne}0$ is postulated, Newton's law $\frac{\mathrm{d}}{\mathrm{dt}}\mathbit{p}=\mathbit{f}$ with $\mathbit{f}={\mathbit{f}}^{\mathrm{em}}$ becomes an integral equation for the point particle's acceleration; the infamous third-order time derivative of the position which plagues the Abraham-Lorentz-Dirac equation of motion does not show up. No infinite bare mass renormalization is invoked, and no ad hoc averaging of fields over a neighborhood of the point charge. The approach lays the rigorous microscopic foundations of classical electrodynamics with point charges.

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Available abstract

It is shown that a well-defined expression for the total electromagnetic force ${\mathbit{f}}^{\mathrm{em}}$ on a point charge source of the classical electromagnetic field can be extracted from the postulate of total momentum conservation whenever the classical electromagnetic field theory satisfies a handful of regularity conditions. Among these is the generic local integrability of the field momentum density over a neighborhood of the point charge. This disqualifies the textbook Maxwell-Lorentz field equations, while the Maxwell-Bopp-Land\'e-Thomas-Podolsky field equations qualify, and presumably so do the Maxwell-Born-Infeld field equations. Most importantly, when the usual relativistic relation between the velocity and the momentum of a point charge with bare rest mass ${m}_{\mathrm{b}}\ensuremath{\ne}0$ is postulated, Newton's law $\frac{\mathrm{d}}{\mathrm{dt}}\mathbit{p}=\mathbit{f}$ with $\mathbit{f}={\mathbit{f}}^{\mathrm{em}}$ becomes an integral equation for the point particle's acceleration; the infamous third-order time derivative of the position which plagues the Abraham-Lorentz-Dirac equation of motion does not show up. No infinite bare mass renormalization is invoked, and no ad hoc averaging of fields over a neighborhood of the point charge. The approach lays the rigorous microscopic foundations of classical electrodynamics with point charges.

Key concepts: Physics, Point particle, Classical electromagnetism, Charge (physics), Momentum (technical analysis), Electromagnetic field, Field (mathematics), Mathematical physics

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