Magnetism

Electromagnetic Waves

Electromagnetic Waves

Text Box: Introduction In physics, magnetism is one of the phenomena by which materials exert attractive or repulsive forces on other materials. Some well-known materials that exhibit easily detectable magnetic properties (called magnets) are nickel, iron, cobalt, and their alloys; however, all materials are influenced to greater or lesser degree by the presence of a magnetic field. Physics of Magnetism Every electron, on account of its spin, is a small magnet (see Electron magnetic dipole moment). In most materials, the countless electrons have randomly oriented spins, leaving no magnetic effect on average. However, in a bar magnet many of the electron spins are aligned in the same direction, so they act cooperatively, creating a net magnetic field.

In addition to the electron's intrinsic magnetic field, there is sometimes an additional magnetic field that results from the electron's orbital motion about the nucleus. This effect is analogous to how a current-carrying loop of wire generates a magnetic field (see Magnetic dipole). Again, ordinarily, the motion of the electrons is such that there is no average field from the material, but in certain conditions, the motion can line up so as to produce a measurable total field. Electromagnetic Induction Induction (electricity), in electricity, the creation of an electric current in a conductor moving across a magnetic field (hence the full name, electromagnetic induction). The effect was discovered by the British physicist Michael Faraday and led directly to the development of the rotary electric generator, which converts mechanical motion into electric energy. Magnetic fields and forces The phenomenon of magnetism is "mediated" by the magnetic field -- i.e., an electric current or magnetic dipole creates a magnetic field, and that field, in turn, imparts magnetic forces on other particles that are in the fields.
To an excellent approximation (but ignoring some quantum effects---see quantum electrodynamics), Maxwell's equations (which simplify to the Biot-Savart law in the case of steady currents) describe the origin and behavior of the fields that govern these forces. Therefore magnetism is seen whenever electrically charged particles are in motion---for example, from movement of electrons in an electric current, or in certain cases from the orbital motion of electrons around an atom's nucleus. They also arise from "intrinsic" magnetic dipoles arising from quantum effects, i.e. from quantum-mechanical spin.

The same situations which create magnetic fields (charge moving in a current or in an atom, and intrinsic magnetic dipoles) are also the situations in which a magnetic field has an effect, creating a force.

Electromagnetic Theory In the late 18th and early 19th centuries, the theories of electricity and magnetism were investigated simultaneously. In 1819 an important discovery was made by the Danish physicist Hans Christian Oersted, who found that a magnetic needle could be deflected by an electric current flowing through a wire. This discovery, which showed a connection between electricity and magnetism, was followed up by the French scientist André Marie Ampère, who studied the forces between wires carrying electric currents, and by the French physicist Dominique François Jean Arago, who magnetized a piece of iron by placing it near a current-carrying wire. In 1831 the English scientist Michael Faraday discovered that moving a magnet near a wire induces an electric current in that wire, the inverse effect to that found by Oersted: Oersted showed that an electric current creates a magnetic field, while Faraday showed that a magnetic field can be used to create an electric current. The full unification of the theories of electricity and magnetism was achieved by the English physicist James Clerk Maxwell, who predicted the existence of electromagnetic waves and identified light as an electromagnetic phenomenon. JAMES CLERK MAXWELL British mathematician and physicist James Maxwell publishes his electromagnetic theory of light and suggests that a whole family of electromagnetic radiation must exist, of which visible light is only one part.
One of the greatest scientists of the 1800s, James Clerk Maxwell developed a mathematical theory relating the properties of electric and magnetic fields. Maxwell’s work led him to predict the existence of electromagnetic waves, energy carriers that travel at the speed of light. His ideas also helped lead to some of the major innovations made in physics in the 20th century, including Einstein’s special theory of relativity and quantum theory. Subsequent studies of magnetism were increasingly concerned with an understanding of the atomic and molecular origins of the magnetic properties of matter. In 1905 the French physicist Paul Langevin produced a theory regarding the temperature dependence of the magnetic properties of paramagnets (discussed below), which was based on the atomic structure of matter. This theory is an early example of the description of large-scale properties in terms of the properties of electrons and atoms. Langevin's theory was subsequently expanded by the French physicist Pierre Ernst Weiss, who postulated the existence of an internal, “molecular” magnetic field in materials such as iron. This concept, when combined with Langevin's theory, served to explain the properties of strongly magnetic materials such as lodestone.

After Weiss's theory, magnetic properties were explored in greater and greater detail. The theory of atomic structure of Danish physicist Niels Bohr, for example, provided an understanding of the periodic table and showed why magnetism occurs in transition elements such as iron and the rare earth elements, or in compounds containing these elements. The American physicists Samuel Abraham Goudsmit and George Eugene Uhlenbeck showed in 1925 that the electron itself has spin and behaves like a small bar magnet. (At the atomic level, magnetism is measured in terms of magnetic moments—a magnetic moment is a vector quantity that depends on the strength and orientation of the magnetic field, and the configuration of the object that produces the magnetic field.) The German physicist Werner Heisenberg gave a detailed explanation for Weiss's molecular field in 1927, on the basis of the newly-developed quantum mechanics (see Quantum Theory). Other scientists then predicted many more complex atomic arrangements of magnetic moments, with diverse magnetic properties. Electromagnetic Induction
MAGNITIC FLUX Magnetic flux, represented by the Greek letter Φ (phi), is a measure of quantity of magnetism, taking into account the strength and the extent of a magnetic field. The SI unit of magnetic flux is the weber (in derived units: volt-seconds), and the unit of magnetic field is the weber per square meter, or tesla.

Description
The flux through an element of area perpendicular to the direction of magnetic field is given by the product of the magnetic field and the area element. More generally, magnetic flux is defined by a scalar product of the magnetic field and the area element vector.

The direction of the magnetic field vector B is by definition from the south to the north pole of a magnet (within the magnet). Outside of the magnet, the field lines will go from north to south.

The magnetic flux through a surface is proportional to the number of magnetic field lines that pass through the surface. This is the net number, i.e. the number passing through in one direction, minus the number passing through in the other direction.

Quantitatively, the magnetic flux through a surface S is defined as the integral of the magnetic field over the area of the surface. Magnetic flux through a closed surface Gauss's law for magnetism, which is one of the four Maxwell's equations, states that the total magnetic flux through a closed surface is zero. (A "closed surface" is a surface without boundaries, such as the surface of a sphere or a cube, but not like the surface of a disk.) This law is a consequence of the empirical observation that magnetic monopoles do not exist or are not measurable. Apparatus to demonstrate electromagnetic induction Faraday's law of induction Faraday's law of induction describes a basic law of electromagnetism, which is involved in the working of transformers, inductors, and many forms of electrical generators.
The law states
The induced electromotive force or EMF in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit. Magnetic flux through an open surface While the magnetic flux through a closed surface is always zero, the magnetic flux through an open surface is an important quantity in electromagnetism. For example, a change in the magnetic flux passing through a loop of conductive wire will cause an electromotive force, and therefore an electric current, in the loop. The relationship is given by Faraday's law. Application of Magnetism ELERIC BELL LOUD SPEAKER MICROPHONES Electric bell An electric bell is a mechanical bell that functions by means of an electromagnet.
DC BELLS
In DC electric bells, when power is applied, current flows through the coil. The coil becomes an electromagnet, attracting the metal strip. This moves the clanger to hit the bell, but also breaks the circuit. The coil is no longer a magnet, so the clanger moves back. The circuit is thus restored. The process repeats continuously until the power is removed. WORKING OF ELECTRIC BELL
AC BELLS
AC electric bells do not have interrupting contacts and their coils are powered directly by the source. Their hammers vibrate at same frequency as the frequency of voltage they are powered by. Lack of contacts makes them more reliable than DC bells.
Some electric bells have two cups which generate different tones. When the hammer goes in one direction, it hits one cup, when it moves back, it hits another cup. The sound of such two-tone electric bells is more pleasant. ELECTRIC BELL Applications Of Electric bell Two early applications of the electric bell were the telephone and doorbell. Early telephones used electric bells to indicate that there was an incoming call. Doorbells were used by visitors to indicate their presence at the external door of a dwelling or business. Though still in use, the electric bell mechanisms in both telephones and doorbells now compete with non-mechanical noisemaking technologies including electronic oscillators and digitally recorded sounds played back through a speaker.
A common style of doorbell uses an AC solenoid coil and a plunger. When the doorbell button is depressed, the plunger is drawn into the solenoid and strikes a gong; a shading coil on the solenoid prevents the plunger from vibrating at the same frequency as the power supply. When the button is released, a spring retracts the plunger which then strikes a second gong, giving a two-tone sound. A variant has a second solenoid which is wired to the back door and only strikes one gong, allowing front or rear door callers to be identified . Dynamo The Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic principles to convert mechanical rotation into a pulsing direct electric current through the use of a commutator. The first dynamo was built by Hippolyte Pixii in 1832.
A dynamo machine consists of a stationary structure, which provides a constant magnetic field, and a set of rotating windings which turn within that field. On small machines the constant magnetic field may be provided by one or more permanent magnets; larger machines have the constant magnetic field provided by one or more electromagnets, which are usually called field coils.
Large power generation dynamos are now rarely seen due to the now nearly universal use of alternating current for power distribution and solid state electronic AC to DC power conversion. But before the principles of AC were discovered, very large direct-current dynamos were the only means of power generation and distribution. Now power generation dynamos are mostly a curiosity. A DYNAMO Loudspeaker system design Used in multi-driver speaker systems, the crossover is a device that separates the input signal into different frequency ranges suited to each driver. Each driver, therefore, receives the frequency range it was designed for, so the distortion in each driver, and interference between the drivers, is reduced.
Crossovers can be passive or active. A passive crossover is an electronic circuit using a combination of one or more resistors, inductors and non-polar capacitors. These parts are formed into carefully designed networks, and placed between the amplifier and the loudspeaker drivers to divide the amplifier's signal into the necessary frequency bands before being delivered to the individual drivers. Passive crossover circuits need no external power beyond the audio signal itself. An active crossover is an electronic filter circuit which divides the complete signal into individual frequency bands before amplification, thus requiring one amplifier for each bandpass. The active crossover requires an external power supply.
Passive crossovers are generally installed inside speaker boxes and are by far the most common type of crossover for home and low power use. In car audio systems, passive crossovers may be in a small separate box, necessary to accommodate the size of the components used. Passive crossovers may be simple, or quite elaborate, although steep slopes such as 24dB per octave require components of unusually close tolerances. Passive crossovers, like the driver units that they feed, have power handling limits, and have a modest amount of insertion loss as they convert a small portion of the amplifier power into heat. So, when the highest output levels are required, active crossovers may be preferable. Active crossovers may be simple circuits which emulate the response of a passive network, or may be more complex allowing audio adjustments. A LOUDSPEAKER INTERIOR OF A LOUD SPEAKER Ribbon and planar magnetic loudspeakers A ribbon speaker consists of a thin metal-film ribbon suspended in a magnetic field. The electrical signal is applied to the ribbon which moves with it, thus creating the sound. The advantage of a ribbon driver is that the ribbon has very little mass; thus, it can accelerate very quickly, yielding very good high-frequency response. Ribbon loudspeakers are often very fragile -- some can be torn by a strong gust of air. Most ribbon tweeters emit sound in a dipole pattern; a very few have backings which limit the dipole radiation pattern. Above and below the ends of the more or less rectangular ribbon, there is less audible output due to phase cancellation, but the precise amount of directivity depends on ribbon length. Ribbon designs generally require exceptionally powerful magnets which make them costly to manufacture. Ribbons have a very low resistance that most amplifiers cannot drive directly. As a result, a step down transformer is typically used to increase the current through the ribbon. The amplifier "sees" a load that is the ribbon's resistance times the transformer turns ratio squared. The transformer must be carefully designed so that its frequency response and parasitic losses do not degrade the sound, further increasing cost and complication relative to conventional designs.
Planar magnetic speakers (having printed or embedded conductors on a flat diaphragm) are sometimes described as "ribbons", but are not truly ribbon speakers. The term planar is generally reserved for speakers which have roughly rectangular shaped flat surfaces that radiate in a bipolar (i.e., front and back) manner. Planar magnetic speakers consist of a flexible membrane with a voice coil printed or mounted on it. The current flowing through the coil interacts with the magnetic field of carefully placed magnets on either side of the diaphragm, causing the membrane to vibrate more or less uniformly and without much bending or wrinkling. Ribbon and planar magnetic loudspeakers Loudspeaker system design A loudspeaker, speaker, or speaker system is an electroacoustical transducer that converts an electrical signal to sound. The term loudspeaker can refer to individual transducers (known as drivers), or to complete systems consisting of a enclosure incorporating one or more drivers and electrical filter components. Loudspeakers, just as with other electroacoustic transducers, are the most variable elements in an audio system and are responsible for the greatest degree of audible differences between sound systems.
To adequately reproduce a wide range of frequencies, most loudspeaker systems require more than one driver, particularly for high sound pressure level or high accuracy. Individual drivers are used to reproduce different frequency ranges. The drivers are named subwoofers (very low frequencies), woofers (low frequencies), mid-range speakers (middle frequencies), tweeters (high frequencies) and sometimes supertweeters optimized for the highest audible frequencies.
The terms for different speaker drivers differ depending on the application. In 2-way loudspeakers, there is no "mid-range" driver, so the task of reproducing the midrange sounds falls upon the woofer and tweeter. Home stereos use the designation "tweeter" for high frequencies whereas professional audio systems for concerts may designate high frequency drivers as "HF" or "highs" or "horns".
When multiple drivers are used in a system, a "filter network", called a crossover, separates the incoming signal into different frequency ranges, and routes them to the appropriate driver. A loudspeaker system with n separate frequency bands is described as "n-way speakers": a 2-way system will have woofer and tweeter speakers; a 3-way system is either a combination of woofer, mid-range and tweeter or subwoofer, woofer and tweeter. Active crossovers called Digital Loudspeaker management systems may include facilities for precise alignment of phase and time between frequency bands, equalization, and dynamics (compression and/or limiting) control.
Some hi-fi and professional loudspeaker systems now include an active crossover circuit as part of an onboard amplifier system. These designs are identifiable by their need for AC power in addition to a signal cable. This 'active' topology may also include driver protection circuits, and other features of a digital loudspeaker management system. Powered speaker systems are common in computer sound (for a single listener) and, at the other end of the size spectrum, in concert sound systems. Powered speaker systems for concert sound, by virtue of no external adjustments, have the potential to provide predictabile, if not necessarily good, sound quality by removing control of crossover, delay and limiter settings from the concert sound engineer. MODERN SPEAKER Other rotating electromagnetic generators Without a commutator, the dynamo is an example of an alternator, which is a synchronous singly-fed generator. With an electromechanical commutator, the dynamo is a classical direct current (DC) generator. The alternator must always operate at a constant speed that is precisely synchronized to the electrical frequency of the power grid for non-destructive operation. The DC generator can operate at any speed within mechanical limits but always outputs a direct current waveform.
Other types of generators, such as the asynchronous or induction singly-fed generator, the doubly-fed generator, or the brushless wound-rotor doubly-fed generator, do not incorporate permanent magnets or field windings (i.e, electromagnets) that establish a constant magnetic field, and as a result, are seeing success in variable speed constant frequency applications, such as wind turbines or other renewable energy technologies.
The full output performance of any generator can be optimized with electronic control but only the doubly-fed generators or the brushless wound-rotor doubly-fed generator incorporate electronic control with power ratings that are substantially less than the power output of the generator under control, which by itself offer cost, reliability and efficiency benefits. Hard disk drive A hard disk drive (HDD), commonly referred to as a hard drive, hard disk, or fixed disk drive,[1] is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.