The magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include: loops of electric current, permanent magnets, elementary particles, various molecules, many astronomical objects. More the term magnetic moment refers to a system's magnetic dipole moment, the component of the magnetic moment that can be represented by an equivalent magnetic dipole: a magnetic north and south pole separated by a small distance; the magnetic dipole component is sufficient for large enough distances. Higher order terms may be needed in addition to the dipole moment for extended objects; the magnetic dipole moment of an object is defined in terms of the torque that object experiences in a given magnetic field. The same applied magnetic field creates larger torques on objects with larger magnetic moments; the strength of this torque depends not only on the magnitude of the magnetic moment but on its orientation relative to the direction of the magnetic field.
The magnetic moment may be considered, therefore. The direction of the magnetic moment points from the south to north pole of the magnet; the magnetic field of a magnetic dipole is proportional to its magnetic dipole moment. The dipole component of an object's magnetic field is symmetric about the direction of its magnetic dipole moment, decreases as the inverse cube of the distance from the object; the magnetic moment can be defined as a vector relating the aligning torque on the object from an externally applied magnetic field to the field vector itself. The relationship is given by: τ = m × B where τ is the torque acting on the dipole, B is the external magnetic field, m is the magnetic moment; this definition is based on how one could, in principle, measure the magnetic moment of an unknown sample. For a current loop, this definition leads to the magnitude of the magnetic dipole moment equaling the product of the current times the area of the loop. Further, this definition allows the calculation of the expected magnetic moment for any known macroscopic current distribution.
An alternative definition is useful for thermodynamics calculations of the magnetic moment. In this definition, the magnetic dipole moment of a system is the negative gradient of its intrinsic energy, with respect to external magnetic field: m = − x ^ ∂ U i n t ∂ B x − y ^ ∂ U i n t ∂ B y − z ^ ∂ U i n t ∂ B z. Generically, the intrinsic energy includes the self-field energy of the system plus the energy of the internal workings of the system. For example, for a hydrogen atom in a 2p state in an external field, the self-field energy is negligible, so the internal energy is the eigenenergy of the 2p state, which includes Coulomb potential energy and the kinetic energy of the electron; the interaction-field energy between the internal dipoles and external fields is not part of this internal energy. The unit for magnetic moment in International System of Units base units is A⋅m2, where A is ampere and m is meter; this unit has equivalents in other SI derived units including: A ⋅ m 2 = N ⋅ m T = J T, where N is newton, T is tesla, J is joule.
Although torque and energy are dimensionally equivalent, torques are never expressed in units of energy. In the CGS system, there are several different sets of electromagnetism units, of which the main ones are ESU, EMU. Among these, there are two alternative units of magnetic dipole moment: 1 statA ⋅ cm 2 = 3.33564095 × 10 − 14 A ⋅ m 2 1 erg G = 10 − 3 A ⋅ m 2,where statA is statamperes, cm is centimeters, erg is ergs, G is gauss. The ratio of these two non-equivalent CGS units is equal to the speed of light in free space, expressed in cm⋅s−1. All formula
Electrostatic induction known as "electrostatic influence" or "influence" in Europe and Latin America, is a redistribution of electric charge in an object, caused by the influence of nearby charges. In the presence of a charged body, an insulated conductor develops a positive charge on one end and a negative charge on the other end. Induction was discovered by British scientist John Canton in 1753 and Swedish professor Johan Carl Wilcke in 1762. Electrostatic generators, such as the Wimshurst machine, the Van de Graaff generator and the electrophorus, use this principle. Due to induction, the electrostatic potential is constant at any point throughout a conductor. Electrostatic Induction is responsible for the attraction of light nonconductive objects, such as balloons, paper or styrofoam scraps, to static electric charges. Electrostatic induction laws apply in dynamic situations as far as the quasistatic approximation is valid. Electrostatic induction should not be confused with Electromagnetic induction.
A normal uncharged piece of matter has equal numbers of positive and negative electric charges in each part of it, located close together, so no part of it has a net electric charge. The positive charges are the atoms' nuclei which are bound into the structure of matter and are not free to move; the negative charges are the atoms' electrons. In electrically conductive objects such as metals, some of the electrons are able to move about in the object; when a charged object is brought near an uncharged, electrically conducting object, such as a piece of metal, the force of the nearby charge due to Coulomb's law causes a separation of these internal charges. For example, if a positive charge is brought near the object, the electrons in the metal will be attracted toward it and move to the side of the object facing it; when the electrons move out of an area, they leave an unbalanced positive charge due to the nuclei. This results in a region of negative charge on the object nearest to the external charge, a region of positive charge on the part away from it.
These are called induced charges. If the external charge is negative, the polarity of the charged regions will be reversed. Since this process is just a redistribution of the charges that were in the object, it doesn't change the total charge on the object; this induction effect is reversible. However, the induction effect can be used to put a net charge on an object. If, while it is close to the positive charge, the above object is momentarily connected through a conductive path to electrical ground, a large reservoir of both positive and negative charges, some of the negative charges in the ground will flow into the object, under the attraction of the nearby positive charge; when the contact with ground is broken, the object is left with a net negative charge. This method can be demonstrated using a gold-leaf electroscope, an instrument for detecting electric charge; the electroscope is first discharged, a charged object is brought close to the instrument's top terminal. Induction causes a separation of the charges inside the electroscope's metal rod, so that the top terminal gains a net charge of opposite polarity to that of the object, while the gold leaves gain a charge of the same polarity.
Since both leaves have the same charge, they spread apart. The electroscope has not acquired a net charge: the charge within it has been redistributed, so if the charged object were to be moved away from the electroscope the leaves will come together again, but if an electrical contact is now made between the electroscope terminal and ground, for example by touching the terminal with a finger, this causes charge to flow from ground to the terminal, attracted by the charge on the object close to the terminal. This charge neutralizes the charge in the gold leaves, so the leaves come together again; the electroscope now contains a net charge opposite in polarity to that of the charged object. When the electrical contact to earth is broken, e.g. by lifting the finger, the extra charge that has just flowed into the electroscope cannot escape, the instrument retains a net charge. The charge is held in the top of the electroscope terminal by the attraction of the inducing charge, but when the inducing charge is moved away, the charge is released and spreads throughout the electroscope terminal to the leaves, so the gold leaves move apart again.
The sign of the charge left on the electroscope after grounding is always opposite in sign to the external inducing charge. The two rules of induction are: If the object is not grounded, the nearby charge will induce equal and opposite charges in the object. If any part of the object is momentarily grounded while the inducing charge is near, a charge opposite in polarity to the inducing charge will be attracted from ground into the object, it will be left with a charge opposite to the inducing charge. A remaining question is; the movement of charges is caused by the force exerted on them by the electric field of the external charged object, by Coulomb's law. As the charges in the metal object continue to separate, the resulting positive and negative regions create their own electric field, which opposes the field of the external charge; this process continues until quickly an equilibrium is reached in which the induced charges are the right size to cancel the external electric field throughout the interior of the metal object.
The remaining mobile c
Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship: I = V R, where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, R is the resistance of the conductor in units of ohms. More Ohm's law states that the R in this relation is constant, independent of the current. Ohm's law is an empirical relation which describes the conductivity of the vast majority of electrically conductive materials over many orders of magnitude of current; however some materials do not obey Ohm's law, these are called non-ohmic. The law was named after the German physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage and current through simple electrical circuits containing various lengths of wire.
Ohm explained his experimental results by a more complex equation than the modern form above. In physics, the term Ohm's law is used to refer to various generalizations of the law; this reformulation of Ohm's law is due to Gustav Kirchhoff. In January 1781, before Georg Ohm's work, Henry Cavendish experimented with Leyden jars and glass tubes of varying diameter and length filled with salt solution, he measured the current by noting how strong a shock he felt as he completed the circuit with his body. Cavendish wrote that the "velocity" varied directly as the "degree of electrification", he did not communicate his results to other scientists at the time, his results were unknown until Maxwell published them in 1879. Francis Ronalds delineated “intensity” and “quantity” for the dry pile – a high voltage source – in 1814 using a gold-leaf electrometer, he found for a dry pile that the relationship between the two parameters was not proportional under certain meteorological conditions. Ohm did his work on resistance in the years 1825 and 1826, published his results in 1827 as the book Die galvanische Kette, mathematisch bearbeitet.
He drew considerable inspiration from Fourier's work on heat conduction in the theoretical explanation of his work. For experiments, he used voltaic piles, but used a thermocouple as this provided a more stable voltage source in terms of internal resistance and constant voltage, he used a galvanometer to measure current, knew that the voltage between the thermocouple terminals was proportional to the junction temperature. He added test wires of varying length and material to complete the circuit, he found that his data could be modeled through the equation x = a b + l, where x was the reading from the galvanometer, l was the length of the test conductor, a depended on the thermocouple junction temperature, b was a constant of the entire setup. From this, Ohm published his results. In modern notation we would write, I = E r + R, where E is the open-circuit emf of the thermocouple, r is the internal resistance of the thermocouple and R is the resistance of the test wire. In terms of the length of the wire this becomes, I = E r + R l, where R is the resistance of the test wire per unit length.
Thus, Ohm's coefficients are, a = E R, b = r R. Ohm's law was the most important of the early quantitative descriptions of the physics of electricity. We consider it obvious today; when Ohm first published his work, this was not the case. They called his work a "web of naked fancies" and the German Minister of Education proclaimed that "a professor who preached such heresies was unworthy to teach science." The prevailing scientific philosophy in Germany at the time asserted that experiments need not be performed to develop an understanding of nature because nature is so well ordered, that scientific truths may be deduced through reasoning alone. Ohm's brother Martin, a mathematician, was battling the German educational system; these factors hindered the acceptance of Ohm's work, his work did not become accepted until the 1840s. However, Ohm received recognition for his contributions to science. In the 1850s, Ohm's law was known as such and was considered proved, alternatives, such as "Barlow's law", were discredited, in terms of real ap
In electromagnetics and communications engineering, the term waveguide may refer to any linear structure that conveys electromagnetic waves between its endpoints. However, the original and most common meaning is a hollow metal pipe used to carry radio waves; this type of waveguide is used as a transmission line at microwave frequencies, for such purposes as connecting microwave transmitters and receivers to their antennas, in equipment such as microwave ovens, radar sets, satellite communications, microwave radio links. A dielectric waveguide employs a solid dielectric rod rather than a hollow pipe. An optical fibre is a dielectric guide designed to work at optical frequencies. Transmission lines such as microstrip, coplanar waveguide, stripline or coaxial cable may be considered to be waveguides; the electromagnetic waves in a waveguide may be imagined as travelling down the guide in a zig-zag path, being reflected between opposite walls of the guide. For the particular case of rectangular waveguide, it is possible to base an exact analysis on this view.
Propagation in a dielectric waveguide may be viewed in the same way, with the waves confined to the dielectric by total internal reflection at its surface. Some structures, such as non-radiative dielectric waveguides and the Goubau line, use both metal walls and dielectric surfaces to confine the wave. Depending on the frequency, waveguides can be constructed from either conductive or dielectric materials; the lower the frequency to be passed the larger the waveguide is. For example, the natural waveguide the earth forms given by the dimensions between the conductive ionosphere and the ground as well as the circumference at the median altitude of the Earth is resonant at 7.83 Hz. This is known as Schumann resonance. On the other hand, waveguides used in high frequency communications can be less than a millimeter in width. During the 1890s theorists did the first analyses of electromagnetic waves in ducts. Around 1893 J. J. Thomson derived the electromagnetic modes inside a cylindrical metal cavity.
In 1897 Lord Rayleigh did a definitive analysis of waveguides. He showed that the waves could travel without attenuation only in specific normal modes with either the electric field or magnetic field, or both, perpendicular to the direction of propagation, he showed each mode had a cutoff frequency below which waves would not propagate. Since the cutoff wavelength for a given tube was of the same order as its width, it was clear that a hollow conducting tube could not carry radio wavelengths much larger than its diameter. In 1902 R. H. Weber observed that electromagnetic waves travel at a slower speed in tubes than in free space, deduced the reason. Prior to the 1920s, practical work on radio waves concentrated on the low frequency end of the radio spectrum, as these frequencies were better for long-range communication; these were far below the frequencies that could propagate in large waveguides, so there was little experimental work on waveguides during this period, although a few experiments were done.
In a June 1, 1894 lecture, "The work of Hertz", before the Royal Society, Oliver Lodge demonstrated the transmission of 3 inch radio waves from a spark gap through a short cylindrical copper duct. In his pioneering 1894-1900 research on microwaves, Jagadish Chandra Bose used short lengths of pipe to conduct the waves, so some sources credit him with inventing the waveguide. However, after this, the concept of radio waves being carried by a tube or duct passed out of engineering knowledge. During the 1920s the first continuous sources of high frequency radio waves were developed: the Barkhausen-Kurz tube, the first oscillator which could produce power at UHF frequencies; these made possible the first systematic research on microwaves in the 1930s. It was discovered that transmission lines used to carry lower frequency radio waves, parallel line and coaxial cable, had excessive power losses at microwave frequencies, creating a need for a new transmission method; the waveguide was developed independently between 1932 and 1936 by George C.
Southworth at Bell Telephone Laboratories and Wilmer L. Barrow at the Massachusetts Institute of Technology, who worked without knowledge of one another. Southworth's interest was sparked during his 1920s doctoral work in which he measured the dielectric constant of water with a radio frequency Lecher line in a long tank of water, he found that if he removed the Lecher line, the tank of water still showed resonance peaks, indicating it was acting as a dielectric waveguide. At Bell Labs in 1931 he resumed work in dielectric waveguides. By March 1932 he observed waves in water-filled copper pipes. Rayleigh's previous work had been forgotten, Sergei A. Schelkunoff, a Bell Labs mathematician, did theoretical analyses of waveguides and rediscovered waveguide modes. In December 1933 it was realized that with a metal sheath the dielectric is superfluous and attention shifted to metal waveguides. Barrow had become interested in high frequencies in 1930 studying under Arnold Sommerfeld in Germany. At MIT beginning in 1932 he worked on high frequency antennas to generate narrow beams of radio waves to locate aircraft in fog.
He invented a horn antenna and hit on the idea of using a hollow pipe as a feedline to feed radio waves to the antenna. By March 1936 he had derived the propagation modes and cutoff frequency in a rectangular waveguide; the source he was using had a large wavelength of
Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electrically charged particles. The electromagnetic force exhibits electromagnetic fields such as electric fields, magnetic fields, light, is one of the four fundamental interactions in nature; the other three fundamental interactions are the strong interaction, the weak interaction, gravitation. At high energy the weak force and electromagnetic force are unified as a single electroweak force. Electromagnetic phenomena are defined in terms of the electromagnetic force, sometimes called the Lorentz force, which includes both electricity and magnetism as different manifestations of the same phenomenon; the electromagnetic force plays a major role in determining the internal properties of most objects encountered in daily life. Ordinary matter takes its form as a result of intermolecular forces between individual atoms and molecules in matter, is a manifestation of the electromagnetic force.
Electrons are bound by the electromagnetic force to atomic nuclei, their orbital shapes and their influence on nearby atoms with their electrons is described by quantum mechanics. The electromagnetic force governs all chemical processes, which arise from interactions between the electrons of neighboring atoms. There are numerous mathematical descriptions of the electromagnetic field. In classical electrodynamics, electric fields are described as electric potential and electric current. In Faraday's law, magnetic fields are associated with electromagnetic induction and magnetism, Maxwell's equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents; the theoretical implications of electromagnetism the establishment of the speed of light based on properties of the "medium" of propagation, led to the development of special relativity by Albert Einstein in 1905. Electricity and magnetism were considered to be two separate forces; this view changed, with the publication of James Clerk Maxwell's 1873 A Treatise on Electricity and Magnetism in which the interactions of positive and negative charges were shown to be mediated by one force.
There are four main effects resulting from these interactions, all of which have been demonstrated by experiments: Electric charges attract or repel one another with a force inversely proportional to the square of the distance between them: unlike charges attract, like ones repel. Magnetic poles attract or repel one another in a manner similar to positive and negative charges and always exist as pairs: every north pole is yoked to a south pole. An electric current inside a wire creates a corresponding circumferential magnetic field outside the wire, its direction depends on the direction of the current in the wire. A current is induced in a loop of wire when it is moved toward or away from a magnetic field, or a magnet is moved towards or away from it. While preparing for an evening lecture on 21 April 1820, Hans Christian Ørsted made a surprising observation; as he was setting up his materials, he noticed a compass needle deflected away from magnetic north when the electric current from the battery he was using was switched on and off.
This deflection convinced him that magnetic fields radiate from all sides of a wire carrying an electric current, just as light and heat do, that it confirmed a direct relationship between electricity and magnetism. At the time of discovery, Ørsted did not suggest any satisfactory explanation of the phenomenon, nor did he try to represent the phenomenon in a mathematical framework. However, three months he began more intensive investigations. Soon thereafter he published his findings, proving that an electric current produces a magnetic field as it flows through a wire; the CGS unit of magnetic induction is named in honor of his contributions to the field of electromagnetism. His findings resulted in intensive research throughout the scientific community in electrodynamics, they influenced French physicist André-Marie Ampère's developments of a single mathematical form to represent the magnetic forces between current-carrying conductors. Ørsted's discovery represented a major step toward a unified concept of energy.
This unification, observed by Michael Faraday, extended by James Clerk Maxwell, reformulated by Oliver Heaviside and Heinrich Hertz, is one of the key accomplishments of 19th century mathematical physics. It has had far-reaching consequences, one of, the understanding of the nature of light. Unlike what was proposed by the electromagnetic theory of that time and other electromagnetic waves are at present seen as taking the form of quantized, self-propagating oscillatory electromagnetic field disturbances called photons. Different frequencies of oscillation give rise to the different forms of electromagnetic radiation, from radio waves at the lowest frequencies, to visible light at intermediate frequencies, to gamma rays at the highest frequencies. Ørsted was not the only person to examine the relationship between magnetism. In 1802, Gian Domenico Romagnosi, an Italian legal scholar, deflected a magnetic needle using a Voltaic pile; the factual setup of the experiment is not clear, so if current flew across the needle or not.
An account of the discovery was published in 1802 in an Italian newspaper, but it was overlooked by the contemporary scientific community, because Romagnosi did not belong to this community. An earlier, neglected, connec
An electromagnetic pulse sometimes called a transient electromagnetic disturbance, is a short burst of electromagnetic energy. Such a pulse's origination may be a natural occurrence or man-made and can occur as a radiated, electric, or magnetic field or a conducted electric current, depending on the source. EMP interference is disruptive or damaging to electronic equipment, at higher energy levels a powerful EMP event such as a lightning strike can damage physical objects such as buildings and aircraft structures; the management of EMP effects is an important branch of electromagnetic compatibility engineering. Weapons have been developed to deliver the damaging effects of high-energy EMP. An electromagnetic pulse is a short burst of electromagnetic energy, its short duration means. Pulses are characterized by: The type of energy; the range or spectrum of frequencies present. Pulse waveform: shape and amplitude; the last two of these, the frequency spectrum and the pulse waveform, are interrelated via the Fourier transform and may be seen as two different ways of describing the same pulse.
EMP energy may be transferred in any of four forms: Electric field Magnetic field Electromagnetic radiation Electrical conductionDue to Maxwell's equations, a pulse of any one form of electromagnetic energy will always be accompanied by the other forms, however in a typical pulse one form will dominate. In general, only radiation acts with the others acting over short distances. There are a few exceptions, such as a solar magnetic flare. A pulse of electromagnetic energy comprises many frequencies from DC to some upper limit depending on the source; the range defined as EMP, sometimes referred to as "DC to daylight", excludes the highest frequencies comprising the optical and ionizing ranges. Some types of EMP events can leave an optical trail, such as lightning and sparks, but these are side effects of the current flow through the air and are not part of the EMP itself; the waveform of a pulse describes. Real pulses tend to be quite complicated, so simplified models are used; such a model is described either in a diagram or as a mathematical equation.
Most electromagnetic pulses have a sharp leading edge, building up to their maximum level. The classic model is a double-exponential curve which climbs steeply reaches a peak and decays more slowly. However, pulses from a controlled switching circuit approximate the form of a rectangular or "square" pulse. EMP events induce a corresponding signal in the surrounding environment or material. Coupling occurs most over a narrow frequency band, leading to a characteristic damped sine wave. Visually it is shown as a high frequency sine wave growing and decaying within the longer-lived envelope of the double-exponential curve. A damped sinewave has much lower energy and a narrower frequency spread than the original pulse, due to the transfer characteristic of the coupling mode. In practice, EMP test equipment injects these damped sinewaves directly rather than attempting to recreate the high-energy threat pulses. In a pulse train, such as from a digital clock circuit, the waveform is repeated at regular intervals.
A single complete pulse cycle is sufficient to characterise such a repetitive train. An EMP arises; the energy is broadband by nature, although it excites a narrow-band damped sine wave response in the surrounding environment. Some types are generated as regular pulse trains. Different types of EMP arise from natural, man-made, weapons effects. Types of natural EMP event includes: Lightning electromagnetic pulse; the discharge is an initial huge current flow, at least mega-amps, followed by a train of pulses of decreasing energy. Electrostatic discharge, as a result of two charged objects coming into close proximity or contact. Meteoric EMP; the discharge of electromagnetic energy resulting from either the impact of a meteoroid with a spacecraft or the explosive breakup of a meteoroid passing through the Earth's atmosphere. Coronal mass ejection. A burst of plasma and accompanying magnetic field, ejected from the solar corona and released into the solar wind. Sometimes referred to as a Solar EMP. Types of man-made EMP event include: Switching action of electrical circuitry, whether isolated or repetitive.
Electric motors can create a train of pulses as the internal electrical contacts make and break connections as the armature rotates. Gasoline engine ignition systems can create a train of pulses as the spark plugs are energized or fired. Continual switching actions of digital electronic circuitry. Power line surges; these can be up to several kilovolts, enough to damage electronic equipment, insufficiently protected. Types of military EMP include: Nuclear electromagnetic pulse, as a result of a nuclear explosion. A variant of this is the high altitude nuclear EMP, which produces a secondary pulse due to particle interactions with the Earth's atmosphere and magnetic field. Non-nuclear electromagnetic pulse weapons. Lightning is unusual in that it has a preliminary "leader" discharge of low energy building up to the main pulse, which in turn may be followed at intervals by several smaller bursts. ESD events are characterised by high voltages of many kV but small currents and sometimes cause visible sparks.
ESD is treated as a small, locali