Electrodynamics
Electrodynamics is the study of the forces between electric charges and currents, and of the electromagnetic fields and radiation they produce. It is the branch of physics in which the deepest disagreements catalogued on this wiki are concentrated.
The standard account
The modern theory descends from a rapid sequence of nineteenth-century discoveries: Oersted's 1820 observation that a current deflects a compass needle; Ampère's experimental and mathematical analysis of the force between current-carrying conductors (1820–1827); Faraday's discovery of electromagnetic induction in 1831; and Maxwell's synthesis, published in 1865 and elaborated in the Treatise on Electricity and Magnetism of 1873, which showed that the equations of the field admit wave solutions travelling at the measured ratio of electromagnetic to electrostatic units — a speed indistinguishable from that of light. Hertz produced and detected such waves in 1887.
The form in which Maxwell's theory is now taught is not Maxwell's own. Oliver Heaviside, Hertz and Gibbs recast twenty quaternion equations into the four vector equations universally called Maxwell's Equations; the force on a charge was given its modern statement as the Lorentz Force, F = q(E + v × B). Combined with Special Relativity and later with quantum mechanics as quantum electrodynamics, this framework produces the most precisely tested predictions in physics: the electron's anomalous magnetic moment agrees with measurement to better than one part in a billion.
Competing force laws
A genuine and often-forgotten historical point is that Maxwell's was not the only nineteenth-century electrodynamics. Wilhelm Weber proposed in 1846 a force law between charges that depends on their relative velocity and relative acceleration, an action-at-a-distance theory from which he and Kohlrausch extracted the constant c in 1856 — before Maxwell's wave theory. Ampère's original force law between current elements contains a longitudinal component, a force along the direction of current flow, which the Grassmann–Lorentz form used today does not. For closed circuits the two give identical results, so the difference only shows up in configurations where a circuit element is effectively open, or where a conductor is under internal stress.
On this wiki
Category:Electrodynamics holds over three hundred pages, and the Ampère-versus-Lorentz question runs through many of them.
- Peter Graneau and Neal Graneau argued from exploding-wire and railgun experiments that longitudinal Ampère tension is real and measurable — see The Graneau Experiments, Ampere Repulsion and Graneau's Exploding Wires and the book Ampere-Neumann Electrodynamics of Metals.
- Panos Pappas performed the "Ampère bridge" experiments with Moyssides; Thomas E Phipps examined that claim in Ampere's Original Force Law Compared with the Moyssides-Pappas Results and, in Ampere Tension and Newton's Laws (Apeiron, 1993), argued that the experiment as performed was indecisive and proposed a modification that would be crucial.
- Paul Wesley developed a Weber-type electrodynamics with an absolute frame in Weber Electrodynamics with Fields, Waves, and Absolute Space; Charles William Lucas built a universal force law for finite-size elastic particles in Weber's Force Law for Realistic Finite-Size Elastic Particles.
- Andre K T Assis has been the principal modern expositor of Weber's programme — see Webers Electrodynamics and Webers Planetary Model of the Atom.
- Ampere Electrodynamics and Electrodynamics from Ampere to Einstein survey the historical dispute; Thomas E Phipps called for a return to the laboratory in Electrodynamics: Rebirth of an Experimental Science?.
Not everyone here takes the Ampère side. Several papers argue that the observed effects follow from Coulomb's Law together with propagation delay and relativistic corrections, without any longitudinal force — the wiki carries both cases, and readers should read them against each other rather than assume the category speaks with one voice.