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The moving magnet and conductor problem is a famous thought experiment, originating in the 19th century, concerning the intersection of classical electromagnetism and special relativity. In it, the current in a conductor moving with constant velocity, ''v'', with respect to a magnet is calculated in the frame of reference of the magnet and in the frame of reference of the conductor. The observable quantity in the experiment, the current, is the same in either case, in accordance with the basic ''principle of relativity'', which states: "Only ''relative'' motion is observable; there is no absolute standard of rest".〔The ''Laws of Physics'' are the same in all inertial frames.〕 However, according to Maxwell's equations, the charges in the conductor experience a magnetic force in the frame of the magnet and an electric force in the frame of the conductor. The same phenomenon would seem to have two different descriptions depending on the frame of reference of the observer. This problem, along with the Fizeau experiment, the aberration of light, and more indirectly the negative aether drift tests such as the Michelson–Morley experiment, formed the basis of Einstein's development of the theory of relativity. ==Introduction== Einstein's 1905 paper that introduced the world to relativity opens with a description of the magnet/conductor problem.() An overriding requirement on the descriptions in different frameworks is that they be consistent. Consistency is an issue because Newtonian mechanics predicts one transformation (so-called Galilean invariance) for the ''forces'' that drive the charges and cause the current, while electrodynamics as expressed by Maxwell's equations predicts that the ''fields'' that give rise to these forces transform differently (according to Lorentz invariance). Observations of the aberration of light, culminating in the Michelson–Morley experiment, established the validity of Lorentz invariance, and the development of special relativity resolved the resulting disagreement with Newtonian mechanics. Special relativity revised the transformation of forces in moving reference frames to be consistent with Lorentz invariance. The details of these transformations are discussed below. In addition to consistency, it would be nice to consolidate the descriptions so they appear to be frame-independent. A clue to a framework-independent description is the observation that magnetic fields in one reference frame become electric fields in another frame. Likewise, the solenoidal portion of electric fields (the portion that is not originated by electric charges) becomes a magnetic field in another frame: that is, the solenoidal electric fields and magnetic fields are aspects of the same thing.〔There are ''two'' constituents of electric field: a solenoidal field (or ''incompressible field'') and a conservative field (or ''irrotational field''). The first is transformable to a magnetic field by changing the frame of reference, the second originates in electric charge, and transforms always into an electric field, albeit of different magnitude.〕 That means the paradox of different descriptions may be only semantic. A description that uses scalar and vector potentials φ and ''A'' instead of ''B'' and ''E'' avoids the semantical trap. A Lorentz-invariant four vector ''A''α = (φ / ''c'', ''A'' ) replaces E and B〔The symbol ''c'' represents the speed of light in free space.〕 and provides a frame-independent description (albeit less visceral than the E– B–description).〔However, φ and ''A'' are not completely disentangled, so the two types of ''E''-field are not separated completely. See Jackson (''From Lorenz to Coulomb and other explicit gauge transformations'' ) The author stresses that ''Lorenz'' is ''not'' a typo.〕 An alternative unification of descriptions is to think of the physical entity as the electromagnetic field tensor, as described later on. This tensor contains both E and B fields as components, and has the same form in all frames of reference. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Moving magnet and conductor problem」の詳細全文を読む スポンサード リンク
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