Relativistic electromagnetism

This article is about a simplified presentation of electromagnetism, incorporating special relativity. For a more general article on the relationship between special relativity and electromagnetism, see Classical electromagnetism and special relativity. For a more rigorous discussion, see Covariant formulation of classical electromagnetism.

Relativistic electromagnetism is a modern teaching strategy for developing electromagnetic field theory from Coulomb's law and Lorentz transformations.

Electromechanics

After Maxwell proposed the differential equation model of the electromagnetic field in 1873, the mechanism of action of fields came into question, for instance in the Kelvin’s master class held at Johns Hopkins University in 1884 and commemorated a century later.[1]

The requirement that the equations remain consistent when viewed from various moving observers led to special relativity, a geometric theory of 4-space where intermediation is by light and radiation.[2] The spacetime geometry provided a context for technical description of electric technology, especially generators, motors, and lighting at first. The Coulomb force was generalized to the Lorentz force. For example, with this model transmission lines and power grids were developed and radio frequency communication explored.

An effort to mount a full-fledged electromechanics on a relativistic basis is seen in the work of Leigh Page, from the project outline in 1912[3] to his textbook Electrodynamics (1940)[4] The interplay (according to the differential equations) of electric and magnetic field as viewed over moving observers is examined. What is charge density in electrostatics becomes proper charge density[5][6][7] and generates a magnetic field for a moving observer.

A revival of interest in this method for education and training of electrical and electronics engineers broke out in the 1960s after Richard Feynman’s textbook.[8] Rosser’s book Classical Electromagnetism via Relativity was popular,[9] as was Anthony French’s treatment in his textbook[10] which illustrated diagrammatically the proper charge density. One author proclaimed, "Maxwell — Out of Newton, Coulomb and Einstein".[11]

The use of retarded potentials to describe electromagnetic fields from source-charges is an expression of relativistic electromagnetism.

Principle

The question of how an electric field in one inertial frame of reference looks in different reference frame moving with respect to the first is crucial to understanding fields created by moving sources. In the special case, the sources that create the field are at rest with respect to one of the reference frames. Given the electric field in the frame where the sources are at rest, one can ask: what is the electric field in some other frame?[12] Knowing the electric field at some point (in space and time) in the rest frame of the sources, and knowing the relative velocity of the two frames provided all the information needed to calculate the electric field at the same point in the other frame. In other words, the electric field in the other frame does not depend on the particular distribution of the source charges, only on the local value of the electric field in the first frame at that point. Thus, the electric field is a complete representation of the influence of the far-away charges.

Alternatively, introductory treatments of magnetism introduce the Biot–Savart law, which describes the magnetic field associated with an electric current. An observer at rest with respect to a system of static, free charges will see no magnetic field. However, a moving observer looking at the same set of charges does perceive a current, and thus a magnetic field. That is, the magnetic field is simply the electric field, as seen in a moving coordinate system.

See also

Notes and references

  1. ↑ Robert Kargon and Peter Achinstein (1987) Kelvin’s Baltimore Lectures and Modern Theoretical Physics: historical and philosophical perspectives, MIT Press ISBN 0-262-11117-9
  2. ↑ What led me more or less directly to the special theory of relativity was the conviction that the electromotive force acting on a body in motion in a magnetic field was nothing else but an electric field. Albert Einstein (1953) Shankland, R. S. (1964). "Michelson-Morley Experiment". American Journal of Physics. 32: 16–81. Bibcode:1964AmJPh..32...16S. doi:10.1119/1.1970063.
  3. ↑ If the principle of relativity had been enunciated before the date of Oersted’s discovery, the fundamental relations of electrodynamics could have been predicted on theoretical grounds as a direct consequence of the fundamental laws of electrostatics, extended so as to apply to charges relatively in motion as well as charges relatively at rest. Leigh Page (1912) "Derivation of the Fundamental Relations of Electrodynamics from those of Electrostatics", American Journal of Science 34: 57–68
  4. ↑ Leigh Page & Norman Ilsley Adams (1940) Electrodynamics, D. Van Nostrand Company
  5. ↑ Richard A. Mould (2001) Basic Relativity, §62 Lorentz force, Springer Science & Business Media ISBN 0387952101
  6. ↑ Derek F. Lawden (2012) An Introduction to Tensor Calculus: Relativity and Cosmology, page 74, Courier Corporation ISBN 0486132145
  7. ↑ Jack Vanderlinde (2006) Classical Electromagnetic Theory, § 11.1 The Four-potential and Coulomb’s Law, page 314, Springer Science & Business Media ISBN 1402027001
  8. ↑ Richard Feynman (1964) The Feynman Lectures on Physics, volume 2, chapter 13-6
  9. ↑ W.G.V. Rosser (1968) Classical Electromagnetism via Relativity, Plenum Press
  10. ↑ Anthony French (1968) Special Relativity, chapter 8, W. W. Norton & Company
  11. ↑ Jack R. Tessman (1966) "Maxwell - Out of Newton, Coulomb, and Einstein" American Journal of Physics 34:1048–55
  12. ↑ Edward M. Purcell (1965,85) Electricity and Magnetism: Berkeley Physics Course Volume 2, published by McGraw-Hill, 2nd ed.
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