Further, gravity differs from electromagnetism in that positive masses attract other positive masses whereas same charges in electromagnetism repel each other. Its quantum counterpart is one of the four fundamental forces of nature (the others are gravitation, weak interaction and strong interaction. E The exposure of office workers to fields generated by computers, monitors, etc. low-energy radiation that comes from the interaction of electric and magnetic fields. With the advent of special relativity, physical laws became susceptible to the formalism of tensors. {\displaystyle (\varepsilon )} ρ These interactions are described by Maxwell's equations and the Lorentz force law.

B 0 ), The field can be viewed as the combination of an electric field and a magnetic field. non-quantum) field produced by moving electric charges. 2 also called electromagnetic field.発音を聞く 例文帳に追加 「electromagnetic field（電磁場）」とも呼ばれる。 - PDQ®がん用語辞書 英語版 となっている ( (

These equations are derived from Maxwell's equations. EMR consists of the radiations in the electromagnetic spectrum, including radio waves, microwave, infrared, visible light, ultraviolet light, X-rays, and gamma rays. {\displaystyle u} An electromagnetic field (also EM field) is a classical (i.e. ] There are different mathematical ways of representing the electromagnetic field. Changing magnetic dipole fields (i.e., magnetic near-fields) are used commercially for many types of magnetic induction devices. と sources include power lines, electric appliances, radio waves, microwaves, and others. Maxwell's equations relate (a) the presence and movement of charged particles with (b) the generation of fields. c 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 Ampere's circuital law. 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. ρ {\displaystyle \mu _{0}}

The units used above are the standard SI units. 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. See also near-field communication. 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.

Magnetic fields are created when electric current flows: the greater the current, the stronger the magnetic field. Copyright ©2004-2020 Translational Research Informatics Center. This is the principle behind the electric generator. Likewise, any new motion of a charge in a region that seemed previously to contain only a magnetic field, will show that the space now contains an electric field as well, which will be found to produces an additional Lorentz force upon the moving charge. p If only the electric field (E) is non-zero, and is constant in time, the field is said to be an electrostatic field. An electromagnetic field, sometimes referred to as an EM field, is generated when charged particles, such as electron s, are accelerated.

のとき、これらの等式は次の公式に帰着される。, また、 {\displaystyle \sigma } All Rights Reserved, Copyright © Japan Science and Technology Agency. はマクスウェルの応力テンソルである[注釈 1]。, Griffiths ではマクスウェルの応力テンソルを反対の符号に定義しているが、ここではランダウ&リフシッツ「場の古典論」での定義に従った。, https://ja.wikipedia.org/w/index.php?title=電磁場&oldid=75894460. [4], The electromagnetic field may be thought of in a more 'coarse' way.

Copyright (C) 1994- Nichigai Associates, Inc., All rights reserved. Once this electromagnetic field has been produced from a given charge distribution, other charged or magnetised objects in this field may experience a force. Maxwell's equations take the form of an electromagnetic wave in a volume of space not containing charges or currents (free space) – that is, where

This contrasts with the sources of the gravitational field, which are masses. μ B The tangential components of the electric and magnetic fields as they relate on the boundary of two media are as follows:[11]. B E However the general case of a static EM field with both electric and magnetic components present, is the case that appears to most observers. [1] It is the field described by classical electrodynamics and is the classical counterpart to the quantized electromagnetic field tensor in quantum electrodynamics.

{\displaystyle \mathbf {B} '} [2] The force created by the electric field is much stronger than the force created by the magnetic field.

Maxwell's equations can be written in tensor form, generally viewed by physicists as a more elegant means of expressing physical laws. electromagnetic field (複数形 electromagnetic fields) (physics) The field of force that is due to the interaction of electric and magnetic forces of charged bodies If the exposure cannot be determined upon manufacturers' information, comparisons with similar systems or analytical calculations, measurements have to be accomplished. An electromagnetic field (also EM field) is a classical (i.e. NIOSH has issued some cautionary advisories but stresses that the data are currently too limited to draw good conclusions. The electromagnetic field may be viewed in two distinct ways: a continuous structure or a discrete structure. 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. / ) また、これらの物理量は電磁ポテンシャルによっても記述され、ラグランジュ形式などで扱う場合は電磁ポテンシャルが基本的な物理量として扱われる。このような場合には電磁ポテンシャルを指して電磁場という事もある。CGS単位系では電場と磁場は同一の物理次元を持つが、MKSA単位系では c B

This discussion ignores the radiation reaction force. はポインティング・ベクトルである。その保存則として次の連続の式が成り立つ。, 従ってポインティングベクトルは電磁場の運動量密度を表すと同時に、電磁場のエネルギー流速密度をも表している。また [ The electromagnetic field propagates at the speed of light (in fact, this field can be identified as light) and interacts with charges and currents. In the past, electrically charged objects were thought to produce two different, unrelated types of field associated with their charge property. {\displaystyle [E]=c[B]}

Far-field effects (EMR) in the quantum picture of radiation, are represented by ordinary photons. {\displaystyle \mathbf {B} } Electromagnetic Fields (2nd Edition), Roald K. Wangsness, Wiley, 1986. {\displaystyle \mathbf {p} }

E

If current does flow, the strength of the magnetic field will vary with power consumption but … An electric field can be produced also by a changing magnetic field. {\displaystyle (\mu )} For example, oscillating charges produce variations in electric and magnetic fields that may be viewed in a 'smooth', continuous, wavelike fashion. non-quantum) field produced by moving electric charges. Experiments reveal that in some circumstances electromagnetic energy transfer is better described as being carried in the form of packets called quanta (in this case, photons) with a fixed frequency. Its quantum counterpart is one of the four fundamental forces of nature (the others are gravitation, weak … In the vector field formalism, these are: where E Observers who see only an electric or magnetic field component of a static EM field, have the other (electric or magnetic) component suppressed, due to the special case of the immobile state of the charges that produce the EM field in that case. The angle of refraction of an electric field between media is related to the permittivity Since electromagnetic fields may influence passive or active implants of workers, it is essential to consider the exposure at their workplaces separately in the risk assessment.[14]. It also gives rise to quantum optics, which is different from quantum electrodynamics in that the matter itself is modelled using quantum mechanics rather than quantum field theory. The potential effects of electromagnetic fields on human health vary widely depending on the frequency and intensity of the fields.

2 is the charge density, which can (and often does) depend on time and position, Inside other materials which possess more complex responses to electromagnetic fields, these terms are often represented by complex numbers, or tensors. 2 When the velocity of a charged particle changes, an EM field is produced.

All Rights Reserved. For more information on the health effects due to specific electromagnetic phenomena and parts of the electromagnetic spectrum, see the following articles: Electric and magnetic fields produced by moving charged objects, Reciprocal behavior of electric and magnetic fields, Behavior of the fields in the absence of charges or currents, Relation to and comparison with other physical fields, Static E and M fields and static EM fields, Time-varying EM fields in Maxwell’s equations. 概念として用いる場合は、電場の強度と電束密度、あるいは磁場の強度と磁束密度を明確に区別せずに用いるが、物理量として用いる場合は電場の強度と磁束密度の組であることが多い。

Being one of the four fundamental forces of nature, it is useful to compare the electromagnetic field with the gravitational, strong and weak fields.

These include motors and electrical transformers at low frequencies, and devices such as metal detectors and MRI scanner coils at higher frequencies. [13], Employees working at electrical equipment and installations can always be assumed to be exposed to electromagnetic fields. It is found that increasing the intensity of the incident radiation (so long as one remains in the linear regime) increases only the number of electrons ejected, and has almost no effect on the energy distribution of their ejection.

Schaum's outline of theory and problems of electromagnetics(2nd Edition), Joseph A. Edminister, McGraw-Hill, 1995. I It is the field described by classical electrodynamics and is the classical counterpart to the quantized electromagnetic field tensor in quantum electrodynamics. This quantum picture of the electromagnetic field (which treats it as analogous to harmonic oscillators) has proven very successful, giving rise to quantum electrodynamics, a quantum field theory describing the interaction of electromagnetic radiation with charged matter. {\displaystyle v/c\ll 1}

That is, a pure static electric field will show the familiar magnetic field associated with a current, in any frame of reference where the charge moves. The first one views the electric and magnetic fields as three-dimensional vector fields. c {\displaystyle \mathbf {S} } ′ Field and Wave Electromagnetics (2nd Edition), David K. Cheng, Prentice Hall, 1989. (

{\displaystyle c} of each medium: The angle of refraction of a magnetic field between media is related to the permeability An electric field will exist even when there is no current flowing. An electromagnetic field (also EM field) is a classical (i.e. This type of dipole field near sources is called an electromagnetic near-field. Similarly, if only the magnetic field (B) is non-zero and is constant in time, the field is said to be a magnetostatic field. Under these conditions, the electric and magnetic fields satisfy the electromagnetic wave equation:[12]. は光速)。, 電磁場のふるまいは、マクスウェルの方程式、あるいは量子電磁力学(QED)によって記述される。マクスウェルの方程式を解いて、電磁場のふるまいについて解析することを電磁場解析と言う。, 電場と磁場はローレンツ変換により互いに移り合う。座標系 O で電場 Faraday's Law may be stated roughly as 'a changing magnetic field creates an electric field'. An EM field that varies in time has two “causes” in Maxwell's equations.

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