The general field consisting of superimposed electric and magnetic fields is called an electromagnetic field. While properties such as the potential energy stored in the field remain invariant of the observing reference frame, special relativity shows that different observers will actually see differing strength of the constituent electric and magnetic fields. The electric and magnetic components of an electromagnetic field are untitled in special relativity into an entity known as the electromagnetic field tensor.
The potential energy of an electromagnetic field consisting of an electric field E and a magnetic field B in cgs is
and in MKS,
where is the permittivity of free space and is the permeability of free space.
Electric Field, Electromagnetic Field Tensor, Magnetic Field
References
Jeffreys, H. and Jeffreys, B. S. "Motion of a Charged Particle in Electric and Magnetic Fields." §2.12 in Methods of Mathematical Physics, 3rd ed. Cambridge, England: Cambridge University Press, pp. 78-79, 1988.
