Electromagnetic Field

Electromagnetic field

A changing magnetic field always produces an electric field, and conversely, a changing electric field always produces a magnetic field. This interaction of electric and magnetic forces gives rise to a condition in space known as an electromagnetic field. The characteristics of an electromagnetic field are expressed mathematically by Maxwell's equation. See Electric field, Electromagnetic radiation, Electromagnetic wave

Electromagnetic Field

a special form of matter that mediates the interaction between charged particles (seeFIELD).

In a vacuum, an electromagnetic field is characterized by the electric field strength E and the magnetic flux density B, which determine the forces exerted by the field on stationary or moving charged particles. In addition to the vectors E and B, which are measured directly, an electromagnetic field may be characterized by a scalar potential φ and a vector potential A. The potentials are not uniquely determined, a gauge transformation being required.

In a medium, an electromagnetic field is also characterized by the following two auxiliary vector quantities: the magnetic field strength H and the electric flux density D (seeINDUCTION, ELECTRICAL AND MAGNETIC).

The behavior of electromagnetic fields is a subject of classical electrodynamics. In an arbitrary medium, the behavior of such fields is described by Maxwell’s equations. The equations make it possible to determine the fields as a function of the distributions of charge and current.

Microscopic electromagnetic fields, which are produced by individual elementary particles, are characterized by the strengths of the microscopic electric field (e) and the microscopic magnetic field (h). The mean values of the microscopic field strengths are related to the macroscopic characteristics of an electromagnetic field in the following way: ē = E and h̄ = B. Microscopic fields satisfy the Lorentz-Maxwell equations.

The electromagnetic field of stationary charged particles or of charged particles moving with constant velocity is inseparably bound to the particles. When particles are accelerated, their electromagnetic field is “torn away” and exists independently in the form of electromagnetic waves.

The generation of an electromagnetic field by an alternating magnetic field and the generation of a magnetic field by an AC electric field show that alternating electric and magnetic fields do not exist separately, that is, are not independent of one another. According to the theory of relativity, the components of the vectors that characterize an electromagnetic field form a single physical quantity called the electromagnetic field tensor. The components of the tensor are transformed upon a transition from one inertial reference frame to another, in accordance with the Lorentz transformations.

At high field frequencies, the quantum properties of an electromagnetic field become important. In this case, classical electrodynamics is not applicable and an electromagnetic field is described by quantum electrodynamics.

REFERENCES

Tamm, I. E. Osnovy teorii elektrichestva, 9th ed. Moscow, 1976.
Kalashnikov, S. G. Elektrichestvo, 4th ed. Moscow, 1977. (Obshchii kurs fiziki, vol. 2.)
Feynman, R., R. Leighton, and M. Sands. Feinmanovskie leklsii po fizike, vols. 5–7. Moscow, 1966–67.
Landau, L. D., and E. M. Lifshits. Teoriia polia, 6th ed. Moscow, 1973. (Teoreticheskaia fizika, vol. 2.)
Landau, L. D., and E. M. Lifshits. Elektrodinamika sploshnykh sred. Moscow, 1959.

G. IA. MIAKISHEV

electromagnetic field

[i¦lek·trō·mag′ned·ik ′fēld] (electromagnetism) An electric or magnetic field, or a combination of the two, as in an electromagnetic wave.

electromagnetic field

Public health
An invisible field of electromagnetic radiation on the spectrum of energetic particles that move as quanta (radiowaves, infrared, visible light, UV light and gamma radiation). EMFs are generated by moving electric charges that propagate outward from any object carrying an electric current, and result from:
(1) The electric field that pushes or pulls ions toward or away from the field; the electrical component of an EMF is blocked by most objects, from skin to concrete, and has a strength of 1 mV/m2, which is similar to the strength of the cells’ intrinsic electrical activity;
(2) The magnetic field that acts on moving particles, pushing them perpendicularly to their direction of motion, which passes through most matter without losing strength; the actual power generated by a magnetic field is a few milligauss (1% of the strength of the earth’s magnetic field).
Tumour cells exposed in vitro to extremely low electromagnetic fields (ELF) of 60-Hz electromagnetic radiation from electrical distribution systems (power lines, video display terminals, household appliances) have increased mitotic activity; some reports have suggested that ELF radiation is associated with a 1.5–2.5-fold increase in leukaemia, lymphomas and intracranial malignancy, especially in children living near either 765-kV power lines or 15-kV distribution lines. Although ELF increases ornithine decarboxylase activity or cell membrane resistance to spontaneous lysis, the relationship of ELF to malignancy is controversial and inconclusive.

electromagnetic field

Abbreviation: EMF
The region in space in which the photons produced by moving electric charges have an effect. EMFs can be produced by power lines, radio waves, and microwaves. The energy produced in an EMF increases as the frequency of the photons increases, and EMFs produced by very high frequency photons (e.g., xrays and gamma rays) are sufficiently energetic to induce cancer. See also: field

单词	electromagnetic field
释义	electromagnetic field electromagnetic field n. The field of force associated with electric charge in motion, having both electric and magnetic components and containing a definite amount of electromagnetic energy. electromagnetic field n (General Physics) a field of force associated with a moving electric charge equivalent to an electric field and a magnetic field at right angles to each other and to the direction of propagation Electromagnetic Field Electromagnetic field A changing magnetic field always produces an electric field, and conversely, a changing electric field always produces a magnetic field. This interaction of electric and magnetic forces gives rise to a condition in space known as an electromagnetic field. The characteristics of an electromagnetic field are expressed mathematically by Maxwell's equation. See Electric field, Electromagnetic radiation, Electromagnetic wave Electromagnetic Field a special form of matter that mediates the interaction between charged particles (seeFIELD). In a vacuum, an electromagnetic field is characterized by the electric field strength E and the magnetic flux density B, which determine the forces exerted by the field on stationary or moving charged particles. In addition to the vectors E and B, which are measured directly, an electromagnetic field may be characterized by a scalar potential φ and a vector potential A. The potentials are not uniquely determined, a gauge transformation being required. In a medium, an electromagnetic field is also characterized by the following two auxiliary vector quantities: the magnetic field strength H and the electric flux density D (seeINDUCTION, ELECTRICAL AND MAGNETIC). The behavior of electromagnetic fields is a subject of classical electrodynamics. In an arbitrary medium, the behavior of such fields is described by Maxwell’s equations. The equations make it possible to determine the fields as a function of the distributions of charge and current. Microscopic electromagnetic fields, which are produced by individual elementary particles, are characterized by the strengths of the microscopic electric field (e) and the microscopic magnetic field (h). The mean values of the microscopic field strengths are related to the macroscopic characteristics of an electromagnetic field in the following way: ē = E and h̄ = B. Microscopic fields satisfy the Lorentz-Maxwell equations. The electromagnetic field of stationary charged particles or of charged particles moving with constant velocity is inseparably bound to the particles. When particles are accelerated, their electromagnetic field is “torn away” and exists independently in the form of electromagnetic waves. The generation of an electromagnetic field by an alternating magnetic field and the generation of a magnetic field by an AC electric field show that alternating electric and magnetic fields do not exist separately, that is, are not independent of one another. According to the theory of relativity, the components of the vectors that characterize an electromagnetic field form a single physical quantity called the electromagnetic field tensor. The components of the tensor are transformed upon a transition from one inertial reference frame to another, in accordance with the Lorentz transformations. At high field frequencies, the quantum properties of an electromagnetic field become important. In this case, classical electrodynamics is not applicable and an electromagnetic field is described by quantum electrodynamics. REFERENCES Tamm, I. E. Osnovy teorii elektrichestva, 9th ed. Moscow, 1976. Kalashnikov, S. G. Elektrichestvo, 4th ed. Moscow, 1977. (Obshchii kurs fiziki, vol. 2.) Feynman, R., R. Leighton, and M. Sands. Feinmanovskie leklsii po fizike, vols. 5–7. Moscow, 1966–67. Landau, L. D., and E. M. Lifshits. Teoriia polia, 6th ed. Moscow, 1973. (Teoreticheskaia fizika, vol. 2.) Landau, L. D., and E. M. Lifshits. Elektrodinamika sploshnykh sred. Moscow, 1959. G. IA. MIAKISHEV electromagnetic field [i¦lek·trō·mag′ned·ik ′fēld] (electromagnetism) An electric or magnetic field, or a combination of the two, as in an electromagnetic wave. electromagnetic field electromagnetic field Public health An invisible field of electromagnetic radiation on the spectrum of energetic particles that move as quanta (radiowaves, infrared, visible light, UV light and gamma radiation). EMFs are generated by moving electric charges that propagate outward from any object carrying an electric current, and result from: (1) The electric field that pushes or pulls ions toward or away from the field; the electrical component of an EMF is blocked by most objects, from skin to concrete, and has a strength of 1 mV/m2, which is similar to the strength of the cells’ intrinsic electrical activity; (2) The magnetic field that acts on moving particles, pushing them perpendicularly to their direction of motion, which passes through most matter without losing strength; the actual power generated by a magnetic field is a few milligauss (1% of the strength of the earth’s magnetic field). Tumour cells exposed in vitro to extremely low electromagnetic fields (ELF) of 60-Hz electromagnetic radiation from electrical distribution systems (power lines, video display terminals, household appliances) have increased mitotic activity; some reports have suggested that ELF radiation is associated with a 1.5–2.5-fold increase in leukaemia, lymphomas and intracranial malignancy, especially in children living near either 765-kV power lines or 15-kV distribution lines. Although ELF increases ornithine decarboxylase activity or cell membrane resistance to spontaneous lysis, the relationship of ELF to malignancy is controversial and inconclusive. electromagnetic field Abbreviation: EMF The region in space in which the photons produced by moving electric charges have an effect. EMFs can be produced by power lines, radio waves, and microwaves. The energy produced in an EMF increases as the frequency of the photons increases, and EMFs produced by very high frequency photons (e.g., xrays and gamma rays) are sufficiently energetic to induce cancer. See also: field AcronymsSeeEMFThesaurusSeefield
随便看	gyrostabilised gyrostabiliser gyrostabilisers gyrostabilized gyro stabilized platform gyrostabilizer gyrostabilizers gyrostat gyrostatic gyrostatically gyrostatics gyro subsystem gyrosyn compass gyro system misalignment evaluation test gyrotonic expansion system gyro topple gyro transfer table system gyrotron gyro tumble gyrous gyrovague gyrovagues gyro wander gyro wheel gyro-while-drilling