Everything about Electromagnetic Field totally explained
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.
Structure of the electromagnetic field
The electromagnetic field may be viewed in two distinct ways.
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
of a photon to its frequency
through the equation:
»
where
is
Planck's constant, named in honour of
Max Planck, and
is the frequency of the photon . For example, in the
photoelectric effect —the emission of electrons from metallic surfaces by electromagnetic radiation— it's 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.
The feedback loop can be summarized in a list, including phenomena belonging to each part of the loop:
- charges generate fields
- the fields interact with each other
- fields act upon charges
- Lorentz force: force due to electromagnetic field
- electric force: same direction as electric field
- magnetic force: perpendicular both to magnetic field and to velocity of charge ()
- charges move
Phenomena in the list are marked with a star (
) if they consist of magnetic fields and moving charges which can be reduced by suitable
Lorentz transformations to electric fields and static charges. This means that the magnetic field ends up being (conceptually) reduced to an appendage of the electric field, for example something which interacts with reality only indirectly through the electric field.
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're often written as
James Clerk Maxwell was the first to obtain this relationship by his completion of
Maxwell's equations with the addition of a
displacement current term to
Ampère's circuital law.
Relation to and comparison with other physical fields
Being one of the four fundamental forces of nature, it's useful to compare the electromagnetic field with the
gravitational,
strong and
weak fields. The word 'force' is sometimes replaced by 'interaction'.
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's only one type (no
negative masses), or, in more colloquial terms, 'gravity is always attractive'.
The relative strengths and ranges of the four interactions and other information are tabulated below:
Applications
Properties of the electromagnetic field are exploited in many areas of industry. The use of electromagnetic radiation is seen in various disciplines. For example,
X-rays are high frequency electromagnetic radiation and are used in
radio astronomy,
radiography in medicine and
radiometry in telecommunications. Other medical applications include
laser therapy, which is an example of
photomedicine.
Applications of lasers are found in military devices such as
laser-guided bombs, as well as more down to earth devices such as
barcode readers and
CD players. Something as simple as a relay in any electrical device uses an electromagnetic field to engage or to disengage the two different states of output (ie, when electricity isn't applied, the metal strip will connect output A and B, but if electricity is applied, an electromagnetic field will be created and the metal strip will connect output A and C).
Health and safety
The potential health effects of the very low frequency EMFs surrounding power lines and electrical devices are the subject of on-going research and a significant amount of public debate. In workplace environments, where EMF exposures can be up to 10,000 times greater than the average, the National Institute for Occupational Safety and Health (NIOSH) has issued some cautionary advisories but stresses that the data is currently too limited to draw good conclusions.
The potential effects of electromagnetic fields on human health vary widely depending on the frequency and intensity of the fields. For more information on the health effects due to specific parts of the
electromagnetic spectrum, see the following articles: -
Static electric fields: see Electric shock
Static magnetic fields: see MRI/Safety for one of the few applications in which magnetic fields are strong enough to have safety implications
Extremely low frequency (ELF): see Power lines/health concerns
Radio frequency (RF): see Electromagnetic radiation and health
Light: see Laser safety
Ultraviolet (UV): see Sunburn
Gamma rays: see Gamma rayFurther Information
Get more info on 'Electromagnetic Field'.
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