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07-SEPTEMBER-2008 03:17:44 - Electromagnetic field Electromagnetism Electricity · Magnetism Electrostatics · Electric charge · Coulomb's law · Electric field · Electric flux · Gauss' law · Electric potential · Electrostatic induction · Electric dipole moment · Magnetostatics · Ampère's law · Electric current · Magnetic field · Magnetic flux · Biot-Savart law · Magnetic dipole moment · Gauss's law for magnetism · Electrodynamics · Free space · Lorentz force law · EMF · Electromagnetic induction · Faraday's law · Displacement current · Maxwell's equations · EM field · Electromagnetic radiation · Liénard-Wiechert Potentials · Maxwell tensor · Eddy current · Electrical Network · Electrical conduction · Electrical resistance · Capacitance · Inductance · Impedance · Resonant cavities · Waveguides · Covariant formulation · Electromagnetic tensor · EM Stress-energy tensor · Four-current · Four-potential · Scientists · Ampere · Coulomb · Faraday · Heaviside · Henry · Hertz · Lorentz · Maxwell · Tesla · Weber · This box: view talk 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 perspective, the electromagnetic field can be regarded as a smooth, continuous field, propagated in a wavelike manner; whereas, from a quantum mechanical perspective, the field is seen as quantised, being composed of individual photons. Contents 1 Structure of the electromagnetic field 1.1 Continuous structure 1.2 Discrete structure 2 Dynamics of the electromagnetic field 3 The electromagnetic field as a feedback loop 4 Mathematical description 5 Properties of the field 5.1 Reciprocal behaviour of electric and magnetic fields 5.2 Light as an electromagnetic disturbance 6 Relation to and comparison with other physical fields 6.1 Electromagnetic and gravitational fields 7 Applications 8 Health and safety 9 See also 10 References 11 External links 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 E of a photon to its frequency ν through the equation: E= \, h \, \nu 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. The feedback loop can be summarized in a list, including phenomena belonging to each part of the loop: charges generate fields Gauss's law Coulomb's law: charges generate electric fields Ampère's law: currents generate magnetic fields \star the fields interact with each other displacement current: changing electric field acts like a current, generating 'vortex' curl of magnetic field Faraday induction: changing magnetic field induces negative vortex of electric field Lenz's law: negative feedback loop between electric and magnetic fields Maxwell-Hertz equations: simplified version of Maxwell's equations electromagnetic wave equation 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 \star charges move continuity equation: current is movement of charges Phenomena in the list are marked with a star \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, i.e. something which interacts with reality only indirectly through the electric field. Mathematical description Main article: Mathematical descriptions of the electromagnetic field 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 \mathbfEx, y, z, t electric field and \mathbfBx, y, z, t magnetic field. If only the electric field \mathbfE is non-zero, and is constant in time, the field is said to be an electrostatic field. Similarly, if only the magnetic field \mathbf B 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 equations1. With the advent of special relativity, physical laws became susceptible to the formalism of tensors. Maxwell's equations can be written in tensor form, generally viewed by physicists as a more elegant means of expressing physical laws. The behaviour of electric and magnetic fields, whether in cases of electrostatics, magnetostatics, or electrodynamics electromagnetic fields, is governed in a vacuum by Maxwell's equations. In the vector field formalism, these are: \nabla \cdot \mathbfE = \frac\rho\varepsilon_0 Gauss' law \nabla \cdot \mathbfB = 0 Gauss' law for magnetism \nabla \times \mathbfE = -\frac \partial \mathbfB\partial t Faraday's law \nabla \times \mathbfB = \mu_0 \mathbfJ + \mu_0\varepsilon_0 \frac\partial \mathbfE\partial t Ampère-Maxwell law where Ï? is the charge density, which can and often does depend on time and position, ε0 is the permittivity of free space, μ0 is the permeability of free space, and \mathbf J is the current density vector, also a function of time and position. The units used above are the standard SI units. Inside a linear material, Maxwell's equations change by switching the permeability and permittivity of free space with the permeability and permittivity of the linear material in question. Inside other materials which possess more complex responses to electromagnetic fields, these terms are often represented by complex numbers, or tensors. The Lorentz force law governs the interaction of the electromagnetic field with charged matter. Properties of the field Reciprocal behaviour of electric and magnetic fields The two Maxwell equations, Faraday's Law and the Ampère-Maxwell Law, illustrate a very practical feature of the electromagnetic field. Faraday's Law may be stated roughly as 'a changing magnetic field creates an electric field'. This is the principle behind the electric generator. The Ampère-Maxwell Law roughly states that 'a changing electric field creates a magnetic field'. Thus, this law can be applied to generate a magnetic field and run an electric motor. Light as an electromagnetic disturbance Maxwell's equations take the form of an electromagnetic wave in an area that is very far away from any charges or currents free space - that is, where Ï? and \mathbf J are zero. It can be shown, that, under these conditions, the electric and magnetic fields satisfy the electromagnetic wave equation: \left \nabla^2 - 1 \over c^2 \partial^2 \over \partial t^2 \right \mathbfE \ \ = \ \ 0 \left \nabla^2 - 1 \over c^2 \partial^2 \over \partial t^2 \right \mathbfB \ \ = \ \ 0 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 Main article: Fundamental forces Please help improve this section by expanding it. Further information might be found on the talk page or at requests for expansion. June 2008 Being one of the four fundamental forces of nature, it is 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 is 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: Theory Interaction mediator Relative Magnitude Behavior Range Chromodynamics Strong interaction gluon 1038 1 10-15 m Electrodynamics Electromagnetic interaction photon 1036 1/r2 infinite Flavordynamics Weak interaction W and Z bosons 1025 1/r5 to 1/r7 10-16 m Geometrodynamics Gravitation graviton 100 1/r2 infinite Applications Please help improve this section by expanding it. Further information might be found on the talk page or at requests for expansion. June 2008 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 is not 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 US 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. 2 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 with details of possible dangers MRI and some currently unfounded fears mobile phones: 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 ray Mobile telephony: see Mobile phone radiation and health See also Afterglow plasma Antenna factor Classification of electromagnetic fields Electric field Electromagnetism Electromagnetic tensor Fundamental interaction Electromagnetic radiation Electromagnetic spectrum Gravitational field List of environment topics Magnetic field Maxwell's equations Photoelectric effect Photon Quantum electrodynamics Free space SI units References ^ Electromagnetic Fields 2nd ion, Roald K. Wangsness, Wiley, 1986. ISBN 0-471-81186-6 intermediate level textbook ^ NIOSH Fact Sheet: EMFs in the Workplace. United States National Institute for Occupational Safety and Health. Retrieved on 2007-10-28. External links On the Electrodynamics of Moving Bodies by Albert Einstein, June 30, 1905. On the Electrodynamics of Moving Bodies pdf Non-Ionizing Radiation, Part 1: Static and Extremely Low-Frequency ELF Electric and Magnetic Fields 2002 by the IARC. Report on the efficacy of electromagnetic screening for sports injuries National Institute for Occupational Safety and Health - EMF Topic Page Electric Magnetic Fields In Your Environment Retrieved from http://en..org/wiki/Electromagnetic_field Categories: Fundamental physics concepts | Electromagnetism | Equations | Occupational safety and healthHidden categories: Articles to be expanded since June 2008 | All articles to be expanded Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages العربية Bosanski БългарÑ?ки Català Česky Dansk Ελληνικά Español Français Galego í•œêµì–´ Hrvatski Bahasa Indonesia Italiano Lietuvių Magyar Malagasy Nederlands 日本語 ‪Norsk nynorsk‬ Polski Português РуÑ?Ñ?кий Shqip SlovenÄ?ina Svenska Tiếng Việt УкраїнÑ?ька ä¸æ–‡ עברית This page was last modified on 19 August 2008, at 16:32
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