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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)
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 macroscopic behaviour. So “electrostatic” is used in such situations simply for the lack of better term.
The term magnetism is also sometimes used to differentiate magnetostatic (non-changing) fields from electromagnetic (varying), whereas in a wider context magnetism includes all magnetic phenomena (magnetostatic or electromagnetic).5)
Electrical and magnetic forces in atoms determine the physical and chemical properties of matter. These interactions in space, time, matter and atoms can be extremely complex, so typically the subjects are introduced in education in the order of increasing complexity.6)7)
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Electromagnetic effects are linked with the presence and movements of electric charges, and the following order of complexity can be distinguished:
All electromagnetic phenomena are described mathematically by just four Maxwell equations, but the solutions of these equations for specific interactions can involve extremely complex mathematics. Analytical solution can be often found only for simple cases (e.g. involving symmetry), whereas for generic case numerical solution is often applied, for instance through methods such as finite-element modelling.
Additional equations are also used when describing electromagnetic phenomena. For instance, the force exerted on an electric charge q due to all electromagnetic effects is expressed in the Lorentz force law, which constitutes the definition of magnetic field B:11)
$$ \vec F = q · \vec E + q · \vec v × \vec B $$
|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)|
|See also the main article: Types of magnetisms.|
Moving electric charges or electric current is always a source of magnetic field. Also, fundamental properties of subatomic particles (such as spin magnetic moment or orbital magnetic moment) are sources or magnetic field.
All materials respond to the 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 “magnetic”. The magnetic response is also typically affected by other parameters, such as: temperature, pressure and mechanical strain, chemical composition, crystallography, and many more.12)
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.
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:
The study of magnetic phenomena extends from subatomic particles21) to cosmic scales.22) Electrons (which are responsible for ferromagnetism) have an estimated radius23) 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 galaxies24) 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 ferromagnetism25) 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 1983:26)
|Few subjects in science are more difficult to understand than magnetism.|
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:28)
|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.
From practical point of view magnetism is widely used in electricity generation, transformation and consumption. 29) 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: physics30), electrical engineering31), telecommunication32) medicine33), biology34), finances35), space exploration36), computer data storage37), security38), food production39) 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 state40) 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).
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).41)
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).
|See separate article on: Magnetic force|
Magnetic effects can generate mechanical force, often referred to as magnetic force.
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:
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:
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. 43)
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. 44) 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.45)
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.46)
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.
|See also the main article: Electromagnetic waves.|
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.
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).
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.
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.47)
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.
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.
|See separate article on: History of electromagnetism|