The electromagnetic field is a physical field produced by electrically charged objects. It affects the behaviour of charged objects in the vicinity of the field.
The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction. It is one of the four fundamental forces of nature (the others are gravitation, the weak interaction, and the strong interaction).
The 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.
From a classical
point of view, the electromagnetic field can be regarded as a smooth,
continuous field, propagated in a wavelike manner, whereas from a quantum mechanical point of view, the field can be viewed as being composed of photons.
Continuous structure
Classically, electric and magnetic fields are thought of as being
produced by smooth motions of charged objects. For example, oscillating
charges produce electric and magnetic fields that may be viewed in a
‘smooth’, continuous, wavelike manner. In this case, energy is viewed
as being transferred continuously through the electromagnetic field
between any two locations. For instance, the metal atoms in a radio transmitter
appear to transfer energy continuously. This view is useful to a
certain extent (radiation of low frequency), but problems are found at
high frequencies (see ultraviolet catastrophe). This problem leads to another view.
Discrete structure
The electromagnetic field may be thought of in a more ‘coarse’ way.
Experiments reveal that electromagnetic energy transfer is better
described as being carried away in ‘packets’ or ‘chunks’ called photons with a fixed frequency. Planck’s relation links the energy E of a photon to its frequency ν through the equation:
where h is Planck’s constant, named in honour of Max Planck, and ν is the frequency of the photon . For example, in the photoelectric effect
—the emission of electrons from metallic surfaces by electromagnetic
radiation— it is found that increasing the intensity of the incident
radiation has no effect, and that only the frequency of the radiation
is relevant in ejecting electrons.
This quantum picture of the electromagnetic field has proved very successful, giving rise to quantum electrodynamics, a quantum field theory describing the interaction of electromagnetic radiation with charged matter.
Dynamics of the electromagnetic field
In the past, electrically charged objects were thought to produce
two types of field associated with their charge property. An electric
field is produced when the charge is stationary with respect to an
observer measuring the properties of the charge and a magnetic field
(as well as an electric field) is produced when the charge moves
(creating an electric current) with respect to this observer. Over
time, it was realized that the electric and magnetic fields are better
thought of as two parts of a greater whole — the electromagnetic field.
Once this electromagnetic field has been produced from a given
charge distribution, other charged objects in this field will
experience a force (in a similar way that planets experience a force in
the gravitational field of the Sun). If these other charges and
currents are comparable in size to the sources producing the above
electromagnetic field, then a new net electromagnetic field will be
produced. Thus, the electromagnetic field may be viewed as a dynamic
entity that causes other charges and currents to move, and which is
also affected by them. These interactions are described by Maxwell’s equations and the Lorentz force law.
The electromagnetic field as a feedback loop
The behavior of the electromagnetic field can be resolved into four
different parts of a loop: (1) the electric and magnetic fields are
generated by electric charges, (2) the electric and magnetic fields
interact only with each other, (3) the electric and magnetic fields
produce forces on electric charges, (4) the electric charges move in
space.
Mathematical description
There are different mathematical ways of representing the
electromagnetic field. The first one views the electric and magnetic
fields as three-dimensional vector fields.
These vector fields each have a value defined at every point of space
and time and are thus often regarded as functions of the space and time
coordinates. As such, they are often written as 
(electric field) and 
(magnetic field).
If only the electric field (
) is non-zero, and is constant in time, the field is said to be an electrostatic field. Similarly, if only the magnetic field (
) is non-zero and is constant in time, the field is said to be a magnetostatic field.
However, if either the electric or magnetic field has a
time-dependence, then both fields must be considered together as a
coupled electromagnetic field using Maxwell’s equations[1].
Electromagnetic and gravitational fields
Sources of electromagnetic fields consist of two types of charge
– positive and negative. This contrasts with the sources of the
gravitational field, which are masses. Masses are sometimes described
as gravitational charges, the important feature of them being that there is only one type (no negative masses), or, in more colloquial terms, ‘gravity is always attractive’.
X-rays are high frequency electromagnetic radiation!
The image above, the disappearing formula, is not a bad quality .gif. I manipulated the image to create a picture that you can see but you cannot fully discern. This is how i imagine science.
più che quelle due equazioni d’onda per essere concisi è più suggestiva questa, altrimenti dalle quattro div D, rot E, div B, rot H, non si scappa, anche se penso sia una questione di opinioni.
From wikipedia, slightly edited…
I was speaking with Martin House few days ago about magnetic instruments and he was explaining me you never understand everything but you find ways to use what you recognise as a pattern. We know about electromagnetic fields , but we cannot define where it is, where is the magnetic field?
I need to know more.