Magnetism



Magnetism - a physical phenomenon associated with electric current, motion of electric charges, properties of elementary particles (e.g. electron spin), or combination of all of these factors. The term electromagnetism refers to mutual relationship between magnetic field and electric field, which can be mathematically and physically described with Maxwell's equations.

Types of magnetism


Moving electric charges or electric current is always a source of magnetic field. Also movement or properties (like spin) of subatomic particles are sources or magnetic field.

Depending on chemical composition, physical state and ambient conditions 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" (the response can be of much lower magnitude).

A specific class of a response is a type of "magnetism", with the three principal ones being:


 * 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.



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
 * biomagnetism - magnetic phenomena in living organisms
 * geomagnetism - study of Earth's magnetic field
 * paleomagnetism - magnetic properties of geological structures
 * cryomagnetism - magnetic phenomena at very low temperatures
 * micromagnetism - magnetic phenomena in small physical structures (e.g. at atomic level)
 * and many more.

Practical importance


The study of magnetic phenomena extends from subatomic particles to cosmic scales.

There are numerous types of magnetic behaviour, many of them being highly non-linear. For instance ferromagnetism 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:

"Few subjects in science are more difficult to understand than magnetism."

- Encyclopaedia Britannica, 1983

The quote was also used by David Jiles in his popular book Introduction to Magnetism and Magnetic Materials.

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:

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

- Student's question, 1986

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. 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: physics , electrical engineering , telecommunication medicine , biology , finances , space exploration , computer data storage security , food production and many more.

However, the plethora of applications can be classified into a few basic magnetic and electromagnetic effects.

Mechanical forces
Permanent magnets are used commonly used for generation or conversion of mechanical forces. This is also true for electromagnets and electromagnetic actuators. The mechanical force is then used for working with or against other forces.



This could be a very high power applications (e.g. a generator in a power plant), 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 - spring force of 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

Electromagnetic energy conversion


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

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 a backbone component of global grid supplying electricity. This is possible because the transformers can increase the voltage to very high level for more efficient transmission. At the same time the transformers are very efficient devices, with figures up 99% for high-power devices.

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.



Thermal effects
There are several applications in which magnetism is used for inducing thermal effects. Only few of these exhibit a direct link between magnetic field and thermal phenomena.

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.

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 - 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 in mechanical movements of water particles



Electromagnetic waves
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

Transmission of signals is actually also transmission of energy, but on a smaller scale. 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 (infrared, ultraviolet) and visible light (see next section).

Optical
All other physical phenomena related to optical effects by definition employ 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.

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, fiber optic computer networks), sensing (all optical sensors), lasers, and many many more.

Optics itself its a very wide scientific and technological field. From classical point of view 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 wave) 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.

Sensors and transducers


A multiplicity of physical phenomena can be measured by employing 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.



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 - magnetic saturation of one element can be used as a basis for measurement of magnetic field from a different source (e.g. electric current)
 * 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 live organisms

Information storage


Magnetism is 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 contact-less interaction, for instance in anti-theft protection systems.

Examples:
 * magnetic tape
 * music cassette
 * video tape
 * hard disk
 * anti-theft protection