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

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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 so some highly dynamic effects can be neglected then sometimes also the name “quasi-static” is used.⁴⁾

However, the adjective “electrostatic” is also used for very dynamic systems (e.g. electrons in atoms), but in order to specifically distinguish the forces due to electric field, from other forces.⁵⁾ 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).⁶⁾ Thus magnetism and electromagnetism are often used interchangeably, but with the meaning typically clear from the context. Some authors use other names such as electrodynamics⁷⁾ to accentuate the importance of dynamic phenomena in electromagnetism.

Electromagnetic forces (electrostatic and magnetic) 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. For full description of dynamic processes, especially involving large velocities, the relativistic effects must be taken into account.^8)9)10)

See also the main article: History of electromagnetism

Magnetism was first discovered in magnetised iron ores because they generated mechanical forces large enough to be felt by hand or attract small pieces of iron. 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.¹¹⁾

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.¹²⁾

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 Michael Faraday. In 1873 all electromagnetic laws (20 equations, as known then) were collected by James Clerk Maxwell, and later rationalised (by Oliver Heaviside)¹³⁾ to just four equations, now known as Maxwell's equations.

Investigation of many other phenomena was ongoing, with 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.¹⁴⁾

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 did not seem to apply at the sub-atomic level. 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).¹⁵⁾

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 research work continues on many aspects of quantum theory, sub-atomic particles and fundamental understanding of causality in physics, theoretically and experimentally.^16)17)18)

General solutions to Maxwell equations with relativistic effects (such as retarded time) were given by Oleg Jefimenko in 1992.¹⁹⁾

Soft magnetic materials were developed in a 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.²⁰⁾

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.²¹⁾

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, currently exceeding the capacity of 1 TB/in². Semi-hard materials are also used for temporary energy storage, for semi-permanent magnets.²²⁾

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

• Electrostatics - electric charges are stationary in space in time.
• Electric current (electricity) - electric charges move, but only the macroscopic electric effects are considered.
• Magnetostatics - electric charges move, but only the macroscopic magnetic field due to moving charges is considered.
• Electromagnetic field - changing electric and magnetic fields are two components of of an electromagnetic field (wave). Frequency can extend over a very wide scale, including radio waves, microwaves, all optical effects (infrared, visible light, ultraviolet, X-ray), and beyond.

Electrostatics - interactions between static electric charges: positive and a negative charges attract in an electric dipole

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By convention, direction of electric current is opposite to the movement of electrons

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Magnetostatics - magnetic effects of moving charges: constant electric current generates constant magnetic field

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Electromagnetic radiation - radiating field is produced by an accelerated electric charge²³⁾

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The topics are typically introduced starting from electrostatics, which involves mechanical forces between charged bodies. An analogy is given to gravity, which also generates attractive forces, but electrostatic forces can also repel. Names such as voltage and electric charge can be introduced. Simple analogies or often used to aid explanation of many electromagnetic phenomena.

For electricity, flow of electric current in a circuit can be likened to a flow of water in a pipe, pressure representing voltage, and volume of flowing water representing current. Laws such as Ohm's and Kirchhoff's can be used to calculate distribution of voltages and currents.

DC circuits involve only voltage, current and resistance, whereas complexity increases for alternating currents, where also capacitive and inductive effects have to be taken into account. Resonance can occur in AC circuits, and tuned circuits are made to resonate on purpose.

Inductive effects allow inductive coupling and transmission of energy to another part of a circuit, without galvanic connection. Such coupling can be improved by using magnetic materials, with magnetic properties such as: saturation, permeability and coercivity.

When physical dimensions of an electric circuit are comparable with the wavelength of the frequency then effects such as distributed impedance of a transmission line begin to play significant role. Transmission, radiation (generation) and reception of electromagnetic waves can happen over a very wide range of frequencies, depending on the dimensions of the circuit. Analogue tuned circuits improve radiation/reception and are a basis for all wireless telecommunication devices.

Optical phenomena (reflection, interference, refraction, etc.) can be introduced independently from electromagnetics, but they are electromagnetic in nature, with all the consequences of electromagnetic interactions between the electromagnetic waves and atoms in matter.

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 properties such as 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.²⁴⁾

Electromagnetic waves include radio waves and optical range

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Atomic magnetic moments - electrons exhibit intrinsic orbital magnetic moment $m_o$ and spin magnetic moment $m_s$²⁵⁾

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Velocity field - localised magnetic field around a moving electron²⁶⁾

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See also the main article: Maxwell's equations

Just four Maxwell's equations fully describe mathematically the interrelation between electric and magnetic fields.²⁷⁾ However, there are several implicit assumptions which need to be fulfilled, such as conservation of energy and conservation of charge.²⁸⁾

The equations can be mathematically written in several ways (e.g. differential or integral form) or different units (e.g. CGS or MKS, but also others).

The equations themselves are deceptively simple, but their solutions for a specific problem can take extremely complex mathematical formulae, especially when relativistic effects are taken into account.³⁴⁾

Analytical solutions can be often found only for simple cases (e.g. involving symmetry), whereas for a generic case numerical methods are often employed in practice, such as finite element modelling for design of electromagnetic devices.³⁵⁾

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.

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).³⁶⁾

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.³⁷⁾

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.³⁸⁾

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.

Opposite electric charges attract, like charges repel, as schematically shown in the image. Such intuitive interactions are described for instance by the Coulomb's law, but more generically by the Lorentz force equation.

Opposite charges attract, like charges repel, there is no force between neutral bodies

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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.³⁹⁾

These two components add vectorially producing total force (called Lorentz force), expressed by:⁴⁰⁾

Lorentz force acting on a positive charged particle q: a) force F_e in electric field E, b) force F_m in magnetic field B, c) total force F = F_e + F_m in both fields; green arrow shows velocity and direction of movement

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Electron in an atom can absorb a photon, or release it when dropping to a lower energy level

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Accelerated electric charges produce electromagnetic radiation or electromagnetic field.⁴¹⁾ The velocity field surrounding a charge is distorted for the duration of acceleration 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. The energy is quantised in the units of Planck constant, but can increase proportionally to the frequency of a photon:⁴²⁾ $E = h·f$ (J).

Only sufficiently energetic photons can produce the photoelectric effect, which is the reason why infrared photons do not contribute significantly to energy production in photovoltaic panels.

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.

In vacuum the two notations, with B or H are exactly equivalent, with the latter quite popular for analysing radiation from antennas.⁴⁴⁾ For example, using the Poynting vector which represents power, as a product of electric field E in V/m and magnetic field H in A/m, the result is V·A/m² or W/m² (power density).

Electromagnetic wave with wavelength λ, propagating in direction x, comprising the component of electric field E in phase with the magnetic field B; all three directions (E, B and x are orthogonal to each other⁴⁵⁾

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In vacuum, far from the radiating dipole (plane wave), the component of electric field E is in phase with the magnetic field B, whereas for standing waves (e.g. in waveguides, or near reflections) they can be shifted by 90° in space and time.⁴⁶⁾⁴⁷⁾

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.⁴⁸⁾

See also the main article: Types of magnetisms.

Hysteresis loop (B-H loop) of a ferromagnetic material

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

Magnetic field penetrates space, be it vacuum or some matter. Magnetic field can be defined as flux density B or magnetic field strength H, and in vacuum they are linked by the magnetic constant (permeability) such that: $B = \mu_0 · H$.

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 the interactions between themselves or with the external field.

All materials respond to the 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.⁴⁹⁾

A specific class of a response can be categorised as a type of magnetism, with the three principal ones: ⁵⁰⁾

• paramagnetism
• diamagnetism
• ferrromagnetism (and other ordered structures)

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.

Geomagnetism is a study of Earth's magnetic field

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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:

• electromagnetism - branch of physics concerned with analysing magnetic and electric field as a single electromagnetic phenomenon ⁵¹⁾
• biomagnetism - magnetic phenomena in living organisms ⁵²⁾
• geomagnetism - study of Earth's magnetic field ⁵³⁾
• paleomagnetism - magnetic properties of geological structures ⁵⁴⁾
• cryomagnetism - magnetic phenomena at very low temperatures ⁵⁵⁾
• micromagnetism - magnetic phenomena in small physical structures (e.g. at atomic level) ⁵⁶⁾
• quantum magnetism - magnetic phenomena in quantum physics⁵⁷⁾
• and many more.

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

The study of magnetic phenomena extends from subatomic particles⁵⁸⁾ to cosmic scales.⁵⁹⁾ Electrons (which are responsible for ferromagnetism) have an estimated radius⁶⁰⁾ 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 galaxies⁶¹⁾ with dimensions 10⁺²¹ m. Therefore, the magnetic phenomena extend over an extremely wide range of dimensions, and affect nature in a multitude ways.

There are numerous types of magnetic behaviour, many of them being highly non-linear. For instance ferromagnetism⁶²⁾ 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 1983:⁶³⁾

The thread-like long structures in galaxy NGC 1275 are believed to be caused by magnetic field ^{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.⁶⁴⁾

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:⁶⁵⁾

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. ⁶⁶⁾ 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: physics⁶⁷⁾, electrical engineering⁶⁸⁾, telecommunication⁶⁹⁾ medicine⁷⁰⁾, biology⁷¹⁾, finances⁷²⁾, space exploration⁷³⁾, computer data storage⁷⁴⁾, security⁷⁵⁾, food production⁷⁶⁾ 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 charges, through an impulse 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 state⁷⁷⁾ 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).

All chemical reactions are mediated by electromagnetic interactions of atoms.⁷⁸⁾ 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.

Green leaves perform photosynthesis in plants ^{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).⁷⁹⁾

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.⁸⁰⁾

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

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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:

• fridge magnet - working with friction against gravity
• loudspeaker - balancing spring force acting on the membrane
• generator - generate electricity from mechanical force
• electric motor - generate mechanical force from electricity
• compass - aligning the needle against friction
• particle accelerator - a charged particle path is deflected in magnetic field (this includes applications like CRT or Aurora Borealis in which charged particles are guided by the geomagnetic field)
• ferrofluid - mechanical forces act on the particles suspended in a fluid and change its behaviour (e.g. against gravity)
• magnetic levitation
• magnetic bearing
• microwave heating - mechanical movement of water particles generates heat through mechanical friction

Relays are electromagnetic actuators

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

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Deflection coils in a CRT affect trajectory of charged particles

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Power transformer is a crucial part of electric grid ^{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. High-power transformers are very efficient, with figures even up 99%.⁸¹⁾

High-voltage high-power transformers can be very big devices, with rating 500 MVA, 400 kV, weighing 380 tons.⁸²⁾ 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. ⁸³⁾ 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.⁸⁴⁾

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 ^{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.

Fire flames are a display of electromagnetic waves ^{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 ^{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.⁸⁵⁾

Compass detects direction of Earth's magnetic field

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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.

Fluxgate magnetometer is a sensor of magnetic field

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Examples:

• Hall effect - output voltage is proportional to the input magnetic field (or electric current which produces it)
• Faraday effect - light twisting angle is proportional to the input magnetic field (or electric current)
• fluxgate magnetometer - asymmetry of magnetic saturation can be used for measurement of magnetic field
• Kerr effect - magnetic domain wall movements can be used as a detector of magnetic field (or electric current)
• compass - sensing direction of Earth's magnetic field
• magnetic resonance imaging - electromagnetic field generated by protons can be used for construction of 3D images of inside of living organisms in a non-invasive way

Hard drive uses magnetic technology to store digital information

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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:

• magnetic tape (music cassette and video tape)
• hard disk
• anti-theft protection

Induction heating of a metal bar ^{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.⁸⁶⁾

Examples:

• magnetocaloric effect - cooling through demagnetisation
• Nernst effect - generation of temperature gradient due to magnetic field

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:

• induction heating - heating by inducing eddy currents
• microwave heating - heating by friction due to mechanical movements of water particles