Introduction of the State Quantities of the Electromagnetic Field in Vacuo
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The magnetic field is the physical system that exists in the space regions in which ponderomotive actions (forces or torques) of magnetic nature can be exerted upon magnetized bodies or on bodies carrying macroscopic electric currents. The magnetic field exists around magnetized bodies as well as in their interior and represents, in any reference frame, as a component of the electromagnetic field, namely the magnetic field. The magnetic field can be produced by various causes: Electric charges in motion, electric currents, variation with time of the electric field strength, variation with time of the electric polarization, and electrified bodies in motion relatively to the considered reference frame. The electromagnetic field is a physical system that exists in the space regions, in which ponderomotive actions of electric nature as well as of magnetic nature are exerted upon bodies that are electrified, magnetized or crossed by electric currents.
2.7. TRANSFORMATION RELATION OF THE VOLUME DENSITY OF THE FREE (TRUE) ELECTRIC CHARGE Let K o and K1 be two inertial reference frames. In any reference frame, the volume density of the true electric charge will be expressed in terms of the volume density of the true electric charge in the reference frame in which the electric charges are at rest. We suppose that the electric charge is at rest in the reference frame K1 . At the same time, we take in view that in accordance with the non-correlative definition of the electric charge (of the Sub-section 1.6.2), the magnitude of the true electric charge is invariant relatively to the change of the inertial reference frame by another one. Then, the conservation law of the true electric charge of a volume element, in the two reference, frames K o and K1 , can be expressed as follows: U vo d vo
U v1 d v1 ,
(2.37)
and according to relation (2.10), it results that: U vo
D1o U v1 ,
(2.38)
which is the transformation relation of the volume density of the true (free) electric charge. The transformation relation of the volume density of the polarization electric charge has the same form.
