The electromagnetic field can be viewed as the combination of an electric field and a magnetic field. The electric field is produced by stationary charges, and the magnetic field by moving charges (currents); these two are often described as the sources of the field. The way in which charges and currents interact with the electromagnetic field is described by Maxwell's equations and the Lorentz force law.It is further classified as near field and far field.
The near field and far field and the transition zone are
regions of time-varying electromagnetic field around any object that serves as
a source for the field. The different terms for these regions describe the way
characteristics of an electromagnetic (EM) field change with distance from the
charges and currents in the object that are the sources of the changing EM
field. The more distant parts of the far-field are identified with classical
electromagnetic radiation.
The basic reason an EM field changes in character with
distance from its source is that Maxwell's equations prescribe different
behaviors for each of the two source-terms of electric fields and also the two
source-terms for magnetic fields. Electric fields produced by charge
distributions have a different character than those produced by changing
magnetic fields. Similarly, Maxwell's equations show a differing behavior for
the magnetic fields produced by electric currents, versus magnetic fields
produced by changing electric fields. For these reasons, in the region very
close to currents and charge-separations, the EM field is dominated by electric
and magnetic components produced directly by currents and charge-separations,
and these effects together produce the EM "near field." However, at
distances far from charge-separations and currents, the EM field becomes
dominated by the electric and magnetic fields indirectly produced by the change
in the other type of field, and thus the EM field is no longer affected (or
much affected) by the charges and currents at the EM source. This more distant
part of the EM field is the "radiative" field or
"far-field," and it is the familiar type of electromagnetic radiation
seen in "free space," far from any EM field sources (origins).
The far-field thus includes radio waves and microwaves
several wavelengths from most types of antennas etc.