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electromagnetism

Electromagnetism

Stan Zurek, Electromagnetism, Encyclopedia Magnetica, E-Magnetica.pl

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Electromagnetism - a physical phenomenon and a branch of science associated with electricity and magnetism, and the interaction between them, hence the two-part name “electro-magnetism”. Electromagnetic interactions are described mathematically by Maxwell's equations.1)2)3)

Horseshoe and bar magnets are a popular symbol of magnetism magnetic_poles_repel_and_attract_magnetica.jpg

Electromagnetism includes all of the electric, electrostatic, magnetic, magnetostatic, electromagnetic field, waves and forces, optical phenomena (in visible light as well as in invisible infrared and ultraviolet), extending also to intrinsic properties of atoms and subatomic particles (electron, proton) involving quantum mechanics, as well as interactions with other branches of science (e.g. chemistry).

The meaning of a particular name depends on the context in which it is used. Some terms can be used in a seemingly interchangeable way. The appendix “static” typically refers to static systems in which there is no variation in space or in time. If such variation is not strictly static, but small enough to be neglected or averaged out then sometimes also the name “quasi-static” is used.

However, the adjective “electrostatic” is also used for dynamic systems (e.g. electrons in atoms), but in order to specifically distinguish the forces due to electric field, from other forces.4) All the dynamic effects have to be taken into account but the generic name “electric” would imply some macroscopic behaviour. So “electrostatic” is used in such situations simply for the lack of a better term.

The term magnetism is also sometimes used to differentiate magnetostatic (non-changing) fields from electromagnetic (varying ,dynamic), whereas in a wider context magnetism includes all magnetic phenomena (magnetostatic or electromagnetic).5) Thus magnetism and electromagnetism are often used interchangeably, but with the meaning typically clear from the context. Some authors use other names such as electrodynamics6) to accentuate the importance of dynamic phenomena in electromagnetism.

Electrical and magnetic forces in atoms determine the physical and chemical properties of matter. These interactions in space and time, inside atoms and in matter, can be extremely complex and often not very intuitive, so typically the subjects are introduced in education in the order of increasing complexity: from the presence of stationary electric charges, through their movement, to their acceleration.7)8)9)

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History of electromagnetism

See also the main article: History of electromagnetism

Magnetism was first discovered in magnetised iron ores because they generated mechanical forces. Earliest documents appear to indicate that the effects were known in ancient China, around 2500 BC, and also in Greece 500 BC.

Around 11th century navigational magnetic compass was in use. Magnetic poles were described in 13th century, and in the year 1600 William Gilbert published a treatise on magnetic field, magnetism of Earth, and use of a compass.

In 18th century horseshoe magnet was invented, and laws governing attraction of magnetic poles (1750, John Mitchell) and electric charges (1785, Charles-Augustine de Coulomb) were formulated. Electric battery was invented in 1800 by Alessandro Volta.

In 19th century the first link between electricity in magnetism was discovered in 1820 by Hans Christian Ørsted. This discovery inspired many scientists internationally, and forces between conductors were demonstrated (1820, André-Marie Ampère) and described mathematically (1820, Jean-Baptiste Biot and Félix Savart). In 1831 the law of electromagnetic induction was discovered by Micheal Faraday. In 1873 all known electromagnetic laws were collected by James Clerk Maxwell, later rationalised to just four equations, known now as Maxwell equations. X-rays] were discovered in 1895 by Wilhelm Röntgen.

Foundations of ferromagnetism were developed shortly after. In 1900 James Ewing proposed existence of magnetic domains, with fuller theory of ferromagnetism given by Pierre Weiss in 1906. Other important discoveries in physics were made around the begging of 20th century, culminating with the theory of general relativity published in 1915 by Albert Einstein. Superconductivity was discovered in 1911 by Kamerlingh Onnes.

In the mean time, discovery of electron (1897, Joseph John Thomson) and proton (1911, Ernest Rutherford) improved understanding of the atomic structure, which eventually lead to postulates of quantum mechanics, because certain laws of the classical physics no longer held. Some rules were postulated theoretically, and some of them hold until now, without complete understanding, but simply referred to as “intrinsic properties” (e.g. the origin of electron spin remains unexplained).

Quantum chromodynamics was further developed in 1950-1960s, with the quark model proposed in 1964 by Murray Gell-Mann and George Zweig, with many modifications and improvements by several other physicists, too numerous to mention all names. The theory predicted existence of 6 quarks, and the final one named “top” was discovered in 1995. The work on quantum theory continues, both theoretically and experimentally.

General solutions to Maxwell equations with relativistic effects (such as retarded time) were given by Oleg Jefimenko in 1966-1992.

Soft magnetic materials were developed in form of electrical steels, with the process for grain orientation patented in 1934. Soft ferrites were first developed in 1930s. Many iron, cobalt and nickel alloys were developed throughout 20th century. Amorphous and nanocrystalline materials were developed in 1970-1980s. Many of these have well-established magnetic properties, but new materials such as soft magnetic composites continue to be developed.

Performance of hard magnetic materials (permanent magnets) is linked to the coercivity value. Initially, horseshoe magnets were made of iron and carbon steel. Alnico magnets and hard ferrites were first developed in 1930s. Samarium-cobalt magnets with much higher energy density were invented in 1960-1970s and very high energy neodymium magnets in 1980s.10)

Semi-hard magnetic materials used for information storage were developed for magnetic tapes (1920-1930s), but are continued to be used for computer data storage. Magnetic hard drives were invented in 1950s, and the improvement in technology continues to make them a suitable data storage medium. Semi-hard materials are also used for temporary energy storage, for semi-permanent magnets.

Complexity of electromagnetism

Electromagnetic effects arise from the presence, movements and acceleration of electric charges:

Magnetostatic field cannot be used as an energy source, but it can store energy. For example, in a superconducting loop once the current is started it can flow indefinitely, generating static magnetic field, in a lossless way. But extraction of energy is equivalent to suppressing the current.

This is one of the reasons why the distinction is made between “static magnetism” (static magnetic fields, due to DC current or intrinsic magnetic moment) and “dynamic magnetism” (electromagnetism, inseparable combination of changing electric and magnetic fields).

Electrostatics - interactions between static electric charges: positive and a negative charges attract in an electric dipole
By convention, direction of electric current is opposite to the movement of electrons
Magnetostatics - magnetic effects of moving charges: constant electric current generates constant magnetic field right-hand_rule_compass_magnetica.jpg
Electromagnetic radiation - radiating field is produced by an accelerated electric charge11)

Further details can be also included:

  • Atomic scale magnetic effects (magnetic moments) - electrons, protons, neutrons, nuclei and atoms exhibit microscopic magnetic behaviour, due to the intrinsic spin magnetic moment, and orbital magnetic moment. These give rise to strong magnetic effects in bulk materials, as for example ferromagnetism in iron.
  • Velocity fields - charges moving at constant speed produce local electric and magnetic fields around themselves, such that they travel with the charges and do not radiate energy away.
  • Acceleration fields - electric charges are accelerated or decelerated, and this produces electromagnetic field (wave) which radiates energy away.
  • Quantum effects inside of atoms - movement of electrons is so fast that relativistic effects need to be included in order to represent calculate all the aspects of interactions between the electrons, and between the electrons and the nucleus.12)
Electromagnetic waves - radiating electromagnetic fields include radio waves and visible light heathrow_radar_magnetica.jpg
Atomic magnetic moments - electrons exhibit intrinsic orbital magnetic moment $m_o$ and spin magnetic moment $m_s$13)
Velocity field - localised magnetic field around a moving electron14)

Maxwell equations

See also the main article: Maxwell's equations

All electromagnetic phenomena are described mathematically by just four Maxwell's equations, which fully describe mathematically the interrelation between electric and magnetic fields.15)

Maxwell's equations
Description Mathematical form (differential)16)
Gauss's law for electrostatics17) relates distribution of electric charge q (expressed by the charge density ρcharge) to the electric field vector E; “div” is the divergence vector calculus operator, and ε0 is the permittivity of vacuum $$ \text{div } \mathbf{E} = \frac {\rho_{charge}}{\epsilon_0}$$
Gauss's law for magnetism18) states that there are no magnetic monopoles (the lines of magnetic field B are closed contours, not having beginnings or ends) $$ \text{div } \mathbf{B} = 0$$
Faraday's law of electromagnetic induction19) states that the electric field vector E is produced by the varying magnetic field vector B; “curl” is the curl vector calculus operator, and is the partial derivative operator $$ \text{curl } \mathbf{E} = - \frac {\partial \mathbf{B}}{\partial t}$$
Ampère's circuital law20) states that the magnetic field B is produced by the electric current I (expressed by the current density J) or the changing electric field E, μ0 is the permeability of vacuum $$ \text{curl } \mathbf{B} = \mu_0 · \epsilon_0 \frac {\partial \mathbf{E}}{\partial t} + \mu_0 · \mathbf{J} $$

The equations can be mathematically written in several ways (e.g. differential or integral form) or different units (e.g. CGS or MKS). They can also be formulated on the basis of more fundamental theory of quantum electrodynamics.

The solutions of these equations for specific interactions can involve extremely complex mathematics, especially when relativistic effects have to be taken into account.

Analytical solutions can be often found only for simple cases (e.g. involving symmetry), whereas for a generic case numerical methods are often employed, such as finite element modelling.

Frame of reference

Several additional equations are also used when describing or calculating electromagnetic phenomena. Relativistic effects (Lorentz transformation) are needed for particles moving close to the speed of light, where time dilation effects take place. Solutions of Maxwell equations which include these effects were given by Jefimenko.

From a different viewpoint, it is possible to qualitatively explain the existence of magnetic field as an artefact of relativistic effects acting on charges in motion (even for slowly moving particles).21)

By choosing an appropriate frame of reference, such that it moves at the same speed as the charges, it is possible to show that magnetic field vanishes, and the electrostatic effects produce the same forces. Thus, existence of magnetic field depends on the frame of reference.

Furthermore, careful mathematical analysis shows that magnetic fields do no work, but instead they redirect the forces such that the work is done by electric fields.22)

Theory of quantum mechanics states that behaviour of subatomic particles is probabilistic, rather than deterministic, so that precise instants of quantum interactions cannot be predicted, only their probabilities. Recently proposed theories postulate that the lack of determinism can be explained if existence of faster-than-light particles is allowed in the mathematical analysis. However, this cannot be confirmed experimentally.23)

Electrostatic and magnetic force

See also the main article: Lorentz force

The presence of electromagnetic, electric or magnetic field is detected by placing an electric test charge q and observing the forces acting on it.

Electrostatic force on a test charge (stationary or moving) acts in the direction of electric field (for positive charges in the same sense as the field, for a negative charge in the opposite sense).

Magnetic force on a moving test charge acts perpendicularly to the direction of magnetic field (for positive charges the path is affected in one direction, for a negative charge in the opposite). Magnetic force does not act on a stationary charge, or a charge moving in the direction of the magnetic field. Such force constitutes the definition of magnetic field B.24)

These two components add vectorially producing total force (called Lorentz force), expressed by:25)

$$ \vec F = q · \vec E + q · \vec v × \vec B $$
(N)
where: $q$ - electric charge (C), $\vec E$ - electric field vector (V/m), $\vec v$ - moving charge velocity vector (m/s), $\vec B$ - magnetic field vector (T)
Lorentz force acting on a positive charged particle q: a) force Fe in electric field E, b) force Fm in magnetic field B, c) total force F = Fe + Fm in both fields

Electromagnetic waves

Electron in an atom is excited to a higher energy state by absorbing a photon, and photon is released when electron drops to a lower energy level

Accelerated electric charges produce electromagnetic radiation or electromagnetic field.26) The velocity field surrounding a charge is distorted and the distortion propagates away from the charge.

Frequency of such radiated wave depends on the frequency of movement of the charges, for example related to the frequency of electric current which cases for the charges to move or oscillate.

Visible light is also a form of electromagnetic radiation, with photons representing quanta of energy.

In vacuum, in the absence of charges and currents, the equations simplify, and because of the linearity of vacuum they can be written with respect to magnetic flux density B (as shown in the table below), or magnetic field strength H. The two notations are exactly equivalent, with the latter seemingly more popular for analysing radiation from antennas.27)

Maxwell's equations in vacuum (in differential form)28)
$$ \text{div } \mathbf{E} = 0$$ $$ \text{div } \mathbf{B} = 0$$
$$ \text{curl } \mathbf{E} = - \frac {\partial \mathbf{B}}{\partial t}$$ $$ \text{curl } \mathbf{B} = \mu_0 · \epsilon_0 \frac {\partial \mathbf{E}}{\partial t} $$

Depending on the chosen mathematical approach, electromagnetic radiation can be treated as a wave or particle (wave-particle duality). Both approaches are simplifications, and quantum mechanics is required for fuller description of the electromagnetic phenomena.29)

Magnetism in materials

See also the main article: Types of magnetisms.

Moving electric charges (electric current) are always a source of magnetic field.

However, fundamental properties of subatomic particles (such as spin magnetic moment or orbital magnetic moment) are also sources or magnetic field. Magnetic properties of matter are dictated mostly by the intrinsic properties of electrons, or their interactions.

All materials respond to applied magnetic field in some way. This is also true for those materials which are commonly referred to as “non-magnetic”, whose response can be of much lower magnitude as compared to the “magnetic” materials. The magnetic response is also typically affected by other parameters, such as: temperature, pressure and mechanical stress, chemical composition, crystallography, and many more.30)

A specific class of a response can be categories as a type of magnetism, with the three principal ones: 31)

And from theoretical physics point of view these can be further subdivided to over twenty other types, depending on the involved atomic structure, spin ordering, etc.

In every day life the materials are often referred to as “magnetic” and “non-magnetic”. A simple test is to touch a given material with a permanent magnet (e.g. a fridge magnet) - if a mechanical force can be felt (e.g. the magnet “sticks”) then the material is “magnetic”. Otherwise it is “non-magnetic”. This layperson classification does not follow the same classes as the theoretical - for instance a magnet does not attract antiferromagnetic material, but it is a magnetically ordered structure.

Other types of magnetisms

Also, there are multiple other terms which are commonly used in relation to other branches of science. These do not refer to phenomena different from those listed above, but strongly linked with the specific scientific or technological area, and with the topic being significant enough so it gained its own name:

Scales, nature and practical importance

Path of moving electrons can be bent into a circle by applied magnetic field ASCII by M. Białek, Wikimedia Commons, CC-BY-SA-3.0

The study of magnetic phenomena extends from subatomic particles39) to cosmic scales.40) Electrons (which are responsible for ferromagnetism) have an estimated radius41) at the level of 10-22 m (the estimates vary several orders of magnitude, depending on the theoretical or experimental approach) and magnetic-like effects are observed also for structures as large as galaxies42) with dimensions 1021 m. Therefore, the magnetic phenomena extend over an extremely wide range of dimensions, and affect nature in multitude ways.

There are numerous types of magnetic behaviour, many of them being highly non-linear. For instance ferromagnetism43) continues to have a major impact on the evolution of various technologies, mainly through its involvement in energy generation and conversion. Most of the electricity generated worldwide is converted, transmitted and consumed with the use of ferromagnetic and electromagnetic phenomena.

Because of the many interrelated types of magnetic behaviours magnetism is a difficult branch of science, which was recognised by the authors of Encyclopaedia Britannica, who wrote in 1983:44)

Few subjects in science are more difficult to understand than magnetism.
The thread-like long structures in galaxy NGC 1275 are believed to be caused by magnetic field galaxy_ngc_1275_heic0817a.jpg by NASA, ESA and A. Fabian, Public domain

The quote was also used by Prof. D. Jiles in his popular book Introduction to Magnetism and Magnetic Materials.45)

On the macroscopic level, magnetic field can be analysed as being generated by electric current. However, it was shown that in some materials the magnetic field can be also attributed to a property known as “spin” of subatomic particles, a phenomenon which cannot be fully explained yet by the the current state of knowledge. Also, electromagnetic waves travel in absence of any matter (e.g. in vacuum). Hence, a question asked by a student:46)

If this space in front of my eyes contains a magnetic field what is in there sustaining it?

remains without satisfactory answer. Many theories have been proposed by theoretical physicists, but some of them (e.g. the superstring theory) remain impossible to verify with the current state of science, knowledge and technology.

Lightning is a sudden discharge of electric current which generates an impulse of magnetic field by Nico36, Public Domain

From practical point of view magnetism is widely used in electricity generation, transformation and consumption. 47) Magnetic phenomena are employed in various sensors, which indirectly influence most branches of science and technology, but there are also a lot of examples of direct use in: physics48), electrical engineering49), telecommunication50) medicine51), biology52), finances53), space exploration54), computer data storage55), security56), food production57) and many more.

The plethora of practical applications can be classified by a few basic magnetic and electromagnetic effects, as mentioned throughout this article.

In nature, an example of magnetic field generation is a lightning, which is a sudden discharge of electric current producing an impulse of magnetic field around itself, as well as electromagnetic waves throughout wide spectrum, including the visible light. Lightnings are capable of magnetising naturally occurring minerals like lodestone, which retained the magnetised state58) so that humans could discover the phenomenon of magnetism.

There are many other mechanisms in which magnetic field can be generated, for example with the core of a planet, on a global level (see: Earth's magnetic field).

Chemical

All chemical reactions are mediated by electromagnetic interactions of atoms.59) Atoms have affinity of completing the outer shell which causes chemical bonding, broadly classified as ionic or covalent. Atoms with fully occupied outer shell are chemically inert (noble gas).

Atoms can form stable isotopes, but also unstable ones, which can decay through radioactivity into other isotopes or other types of chemical elements. The process can produce significant amount of heat which is utilised for example in nuclear power generation.

Biological

Green leaves perform photosynthesis in plants leaf_1_web.jpg by Jon Sullivan, CC-0

Most of the life forms on Earth are supported by the energy delivered from the Sun in a form of light or electromagnetic radiation. Plants convert light into chemical energy (such as sugars) in photosynthesis.

Plants are consumed by animals like herbivores, which in turn are consumed by carnivores. Most food chains utilise electromagnetic energy converted initially by green plants.

Moreover, even the current state of human technology was originally achieved and is still supported mostly by the same source, which in the past was stored as fossil fuels (like coal, crude oil and natural gas).60)

Life on the Earth would not be possible to the same extent without the electromagnetic energy. However, there are some primitive organisms which can use other sources of energy (e.g. heat at the ocean floor).

Magnetic field can also be utilised for purposes such as navigation by birds or other animals.61)

Mechanical forces

See also the main article: Magnetic force

Magnetic effects can generate mechanical force, often referred to as magnetic force. Devices such as electric motors use electromagnetic effects to convert electric energy into mechanical force. On the other hand, electric generators convert mechanical energy into electricity.

Electric motor converts electricity to mechanical energy x-default

Permanent magnets (common name: “magnets”) are used widely for generation or conversion of mechanical forces. This is also true for electromagnets, actuators and sensors. The mechanical force is then used for working with or against other forces.

Magnets could be used for very high power applications e.g. a generator in a power plant or electric motor in propulsion of electric cars, as well as atomic and sub-atomic particles, whose trajectories are affected by the mechanical forces of particle accelerators.

A few examples can be given as:

Permanent magnets are used in motors, generators, actuators, loudspeakers, toys, etc. hard_ferrites_1_magnetica.jpg

Deflection coils in a CRT affect trajectory of charged particles deflection_coils_1_magnetica.jpg

Electromagnetic energy conversion

Power transformer is a crucial part of electric grid The Taza power plant, photographed Nov. 2, 2008, in Kirkuk, Iraq, is the largest
and newest power plant in Kirkuk province.  The V94 turbine generator generates
electricity for the Northern regions of Iraq. (U.S. Army photo by Sgt. 1st Class... by Marvin L. Daniels, U.S. Army, Public domain

Electromagnetism is used for electromagnetic coupling of energy between the source and the load. Although some mechanical effect can be generated during the operation (e.g. magnetostriction) the energy is converted primarily through non-moving parts, due to the laws of electromagnetic induction. This is therefore a different application from motors and generators. Examples:

  • transformer - converting one level of variable current to a different level
  • wireless charger - delivering energy in a contactless way

There are also other physical phenomena, which can transfer electromagnetic energy into different type of energy (e.g. heat) but the electromagnetic-electromagnetic conversion is a special case, and it is currently used as the pivotal component of global grid supplying electricity. This is possible because the transformers can increase the voltage to very high level for more efficient transmission of electricity. At the same time the transformers are very efficient devices, with figures up 99% for high-power devices.62)

High-voltage high-power transformers can be very big devices, with rating 500 MVA, 400 kV, weighing 380 tons.63) They are used for long-distance power transmission grid.

Another inherent feature of electromagnetic conversion is that it allows galvanic separation between the circuits, which is a very important factor from the viewpoint of safety of electric circuits. 64) For example, mains-powered chargers for portable appliances (such as mobile phones, tablets, laptops) are not required to have connection to ground/earth only if they have full galvanic isolation between the mains input and the low-voltage output.65)

Electromagnetic waves

See also the main article: Electromagnetic waves.
Mobile phones use electromagnetic waves to transmit and receive signals by Anders K. Larsen, Public domain

Each variation of magnetic field or electric field in time produces electromagnetic waves. Such electromagnetic radiation is referred to as electromagnetism and for instance can be analysed as the so-called near field or far field phenomena. In electric and electronic circuits there can be transmission line effects, which are caused by the link between the wave length and physical circuit dimensions.

A whole important sub-class of magnetic phenomena is transmission of signals through electromagnetic waves. For efficient transmission tuned circuits are used, and are ubiquitously employed in terrestrial and outer space telecommunication.

Examples:

  • tuned circuit - a basis for all signal transmission based on electromagnetic waves of various length (from radio waves, through GPS and mobile phone telecommunication, to radar, and beyond)
  • radar - detection of signals reflected from objects
  • X-ray - inner structure of materials or bodies can be detected due to differences in absorption of electromagnetic waves
X-ray image of human chest taken for medical purposes x-ray_of_child_chest_fda_public_domain_.jpg FDA, Public Domain

Transmission of signals is actually also transmission of energy, but with smaller power. The same principles can be used for transmission of energy, for instance in some types of wireless charging.

At much higher frequencies the electromagnetic waves constitute visible spectrum, so that all optical devices in effect employ electromagnetic waves in the form of invisible (infra-red, ultraviolet) and visible light (see next section).

The same applies to lasers.

Optical

Fire flames are a display of electromagnetic waves fire_flames.jpg by J. Sullivan, Public domain

Other optical effects are related to electromagnetic waves, but with specific range of wavelengths.

Visible light can be generated in a number of ways: from thermal heating (burning flame, incandescent light bulb), through electroluminescence (light-emitting diode), ionised gasses (compact fluorescent light bulb), chemical reactions, bioluminescence, etc.

A rainbow is an example of spectrum of visible light double_rainbow.jpg by A. McMillan, Public domain

Visible and near-visible spectrum is suitable for a whole range of applications: energy transfer (photovoltaic cells), heat generation (infrared halogen heaters), signal and information transmission (traffic lights, fibre optic computer networks), sensing (all optical sensors), lasers, and many many more.

Optics itself its a very wide scientific and technological field and is a separate branch of physics, but because of its diversity in its own right it overlaps with almost every aspect of science and technology.

Interestingly, there are also direct phenomena occurring between light (electromagnetic waves) and magnetic or electromagnetic fields. For instance in the Faraday effect magnetic field can twist a polarised beam of light, and there are scientific indications that the vision of pigeons is affected by Earth's magnetic field.66)

Sensors and transducers

Compass detects direction of Earth's magnetic field compass_magnetica.jpg

A multiplicity of other physical quantities can be measured by employing phenomena related to magnetics. In sensors and transducers the amount of processed energy is usually small, and focus is given to such aspects as accuracy and linearity of signal transformation, rather than efficiency of energy conversion.

Examples:

Information storage

Hard drive uses magnetic technology to store digital information hard_drive_with_ide_magnetica.jpg

Magnetism is still widely used as a major technology for information storage. A layer of ferromagnetic substance can be magnetised, and the direction of local magnetisation can store information in an analogue or digital way.

Magnetism and electromagnetism are widely used for such applications, because they offer inexpensive way of manufacturing such products. Importantly, it is possible to have completely contactless interaction, for instance in anti-theft protection systems.

Examples:

Thermal effects

Induction heating of a metal bar induction_heating_of_bar_commons.jpg by Vector1 nz, CC-BY-SA-3.0

There are several applications in which magnetism is used for creating thermal effects. Only few of these exhibit a direct link between magnetic field and thermal phenomena, rather than having an intermediate electromagnetic-electromagnetic coupling.

Cooling can be achieved by adiabatic demagnetisation through the magnetocaloric effect. In theory it should be possible to build efficient magnetic refrigerators, without any moving parts. Research is carried out to find appropriate materials and configurations which could facilitate commercially viable devices.67)

Examples:

Other magneto-thermal effects rely on some intermediate physical phenomena to generate heat. For instance, electric current is induced in any conducting medium which is exposed to a varying magnetic field. These so-called eddy currents are capable of heating up the medium in which they flow, and it is a basis for all induction heating devices. However, it is the eddy currents which are ultimately the source of heat - so electromagnetism is used only to transfer the energy and induce the currents.

Examples:

References


60) Photosynthesis, Encyclopædia Britannica Ultimate Reference Suite, Chicago: Encyclopædia Britannica, 2013
electromagnetism.txt · Last modified: 2021/04/22 18:52 by stan_zurek

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