Electromagnetic field. The field can be viewed as the combination of an electric field and a magnetic field. The electric field is produced by stationary charges, and the magnetic field by moving charges (currents); these two are often described as the sources of the field. The way in which charges and currents interact with the electromagnetic field is described by Maxwell's equations and the Lorentz force law. From a classical perspective in the history of electromagnetism, the electromagnetic field can be regarded as a smooth, continuous field, propagated in a wavelike manner; whereas from the perspective of quantum field theory, the field is seen as quantized, being composed of individual particles. [citation needed] Structure of the electromagnetic field[edit] The electromagnetic field may be viewed in two distinct ways: a continuous structure or a discrete structure. Continuous structure[edit] Classically, electric and magnetic fields are thought of as being produced by smooth motions of charged objects.
Magnetostriction. Animation of magnetostriction Magnetostriction (cf. electrostriction) is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials's magnetization due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value, λ. The effect was first identified in 1842 by James Joule when observing a sample of iron.[1] This effect causes losses due to frictional heating in susceptible ferromagnetic cores.
Explanation[edit] Internally, ferromagnetic materials have a structure that is divided into domains, each of which is a region of uniform magnetic polarization. The reciprocal effect, the change of the susceptibility (response to an applied field) of a material when subjected to a mechanical stress, is called the Villari effect. On magnetization, a magnetic material undergoes changes in volume which are small: of the order 10−6. Magnetostrictive materials[edit] Inverse magnetostrictive effect. The inverse magnetostrictive effect (also known as Villari effect) is the name given to the change of the magnetic susceptibility of a material when subjected to a mechanical stress. Explanation[edit] Whereas magnetostriction characterizes the shape change of a ferromagnetic material during magnetization, the inverse magnetostrictive effect characterizes the change of domain magnetization when a stress is applied to a material. This magnetostriction can be positive (magnetization increased by tension) like in pure iron, or negative (magnetization decreased by tension) like in nickel.
In the case of a single stress applied on a single magnetic domain, the magnetic strain energy density can be expressed as:[1] where is the magnetostrictive expansion at saturation, and the angle between the saturation magnetization and the stressed direction. And are both positive (like in iron under tension), the energy is minimum for = 0, i.e. when tension is aligned with the saturation magnetization. Magnetic susceptibility. In electromagnetism, the magnetic susceptibility Definition of volume susceptibility[edit] The volume magnetic susceptibility, represented by the symbol (often simply , sometimes – magnetic, to distinguish from the electric susceptibility), is defined in the International System of Units — in other systems there may be additional constants — by the following relationship, it is same as residual magnet. where M is the magnetization of the material (the magnetic dipole moment per unit volume), measured in amperes per meter, and H is the magnetic field strength, also measured in amperes per meter.
The magnetic induction B is related to H by the relationship where μ0 is the magnetic constant (see table of physical constants), and and the magnetic permeability are related by the following formula: This allows an alternative description of all magnetization phenomena in terms of the quantities I and B, as opposed to the commonly used M and H. Conversion between SI and CGS units[edit] Examples[edit] Deformation in Liquid. Ferromagnetic resonance. History[edit] Description[edit] FMR arises from the precessional motion of the (usually quite large) magnetization of a ferromagnetic material in an external magnetic field .
The basic setup for an FMR experiment is a microwave resonant cavity with an electromagnet. Furthermore, the resonant absorption of microwave energy causes local heating of the ferromagnet. The resonant frequency of a film with parallel applied external field is given by the Kittel formula:[1] where is the magnetization of the ferromagnet and is the gyromagnetic ratio.[2] See also[edit] References[edit] Vonsovskii, S. External links[edit] References[edit] Absorption (electromagnetic radiation) An overview of electromagnetic radiation absorption. This example discusses the general principle using visible light as specific example. A white light source — emitting light of multiple wavelengths — is focused on a sample (the pairs of complementary colors are indicated by the yellow dotted lines). Upon striking the sample, photons that match the energy gap of the molecules present (green light in this example) are absorbed, exciting the molecules.
Other photons are transmitted unaffected and, if the radiation is in the visible region (400–700 nm), the transmitted light appears as the complementary color (here red). By recording the attenuation of light for various wavelengths, an absorption spectrum can be obtained. In physics, absorption of electromagnetic radiation is the way in which the energy of a photon is taken up by matter, typically the electrons of an atom. All these quantities measure, at least to some extent, how well a medium absorbs radiation. Electromagnetic absorption by water. The absorption of electromagnetic radiation by water in the gas phase occurs in three regions of the spectrum.
Rotational transitions are responsible for absorption in the microwave and far-infrared, vibrational transitions in the mid-infrared and near-infrared. Vibrational bands have rotational fine structure. Electronic transitions occur in the vacuum ultraviolet regions. Liquid water has no rotational spectrum but does absorb in the microwave region. Ice has a similar spectrum to liquid water. Overview[edit] The water molecule, in the gaseous state, has three types of transition that can give rise to absorption of electromagnetic radiation Rotational transitions, in which the molecule gains a quantum of rotational energy.
In reality, vibrations of molecules in the gaseous state are accompanied by rotational transitions, giving rise to a vibration-rotation spectrum. Units[edit] wavenumber (cm-1) = 104 / wavelength (μm) Wavenumber per centimeter is the reciprocal of the wavelength in cm. Electromagnetic Fields (genotoxic injury) Piezoelectricity. A piezoelectric system (without contact tabs) Piezoelectricity is found in useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation, microbalances, and ultrafine focusing of optical assemblies. It is also the basis of a number of scientific instrumental techniques with atomic resolution, the scanning probe microscopies such as STM, AFM, MTA, SNOM, etc., and everyday uses such as acting as the ignition source for cigarette lighters and push-start propane barbecues.
History[edit] Discovery and early research[edit] The pyroelectric effect, by which a material generates an electric potential in response to a temperature change, was studied by Carl Linnaeus and Franz Aepinus in the mid-18th century. A piezoelectric disk generates a voltage when deformed (change in shape is greatly exaggerated) The Curies, however, did not predict the converse piezoelectric effect. World War I and post-war[edit] World War II and post-war[edit]