Introto Magnetism:Sourcesof Magnetic FieldsMagneticFields and also Forces
InducedCurrents:Faraday\"sLawInductionand Magnetic Recording
Summary
Despite the increasing prevalence that CD-ROMsand the use of electronic storage in RAM, many data is quiet stored magnetically.This reading assignment evaluate the basic concepts the magnetism, then introducesthe three different effects which have actually been used to check out magnetic data.

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Sourcesof Magnetic Fields

Discussion Question:What produce magnetic fields? Is there any type of difference between thefields of long-term magnets and also the areas of electromagnets? Arethe resources of the fields the very same in these 2 cases, or are they different?Our thinking about magnetic sources has adjusted considerably over thecenturies. The only kind of magnetism recognized until the 19th centurywas ferromagnetism. Details materials, as soon as \"magnetized\", would certainly attractcertain other materials. The only materials attracted through a magnetwere those that could end up being magnetized themselves. Because only certainmaterials showed magnetic properties, scientists concluded that magnetismwas an natural property of materials.Then, in the 19th century, researchers studying the relatively brand-new fieldof electrical currents found that relocating charges develop magneticeffects. A existing traveling v a loop of cable creates a magneticfield along the axis of the loop. The direction the the ar insidethe loop can be discovered by curling the finger of the appropriate hand in the directionof the current through the loop; the ignorance then points in the directionof the magnetic field. With this discovery, magnetism showed up tooccur in two different manners: ferromagnetism depending upon the material,and electromagnetsim resulted in by currents.
Classical see of the (a) orbit motionand (b) turn of an electron.As atom physics and also chemistry began to define the routine tablewith the help of the Bohr design of the atom in the early on 1900s, magneticproperties were assigned to the electron in atoms. Electron appearedto exhibit two species of movement in an atom: orbital and spin.Orbital movement referred come the activity of an electron roughly the nucleusof the atom. Due to the fact that a charged bit was moving, a magnetic fieldwas created. Yet electrons (and protons and also other particles) alsoappeared to it is in spinning approximately their centers, creating yet another magneticfield. The magnetic field as result of the orbital motion and also the magneticfield due to the spin could cancel or add, but expressions because that the exactcoupling between the two are too complicated to enter here. Sinceelectrons to be moving and also spinning in ~ atoms, ferromagnetism could nowbe defined by the movement of dues within different materials.If every one of the electron in an object line up v their spins in the samedirection, the spins will add and create an observable field.
That last sentence is contempt unrealistic.Solids contain incredably big numbers of electrons, and also they will certainly neverall fully line up. Instead, a heavy generally consists of magneticdomains.In a domain, the majority of electron which have the right to (unpaired valance electrons)will have actually spins aligned. Adjacent domains will generally not be orientedin identical directions. In magnetized materials, some domain names willcancel, but the mean domain orientation will certainly be in one direction, producinga network magnetic field. In unmagnetized materials, the domains arerandomly oriented and also cancel, for this reason no observable field is created.The figure to the appropriate illustrates this concepts.The principle of magnetism gift entirely because of the movement of chargeshas to be modified substantially in the 20th century, many thanks to quantummechanics. The Bohr model of the atom need to be modified come includeuncertainty. We have the right to never determine specifically the trajectory that anelectron or to speak for certain where it will be found. The uncertaintyprinciple requires that we rather say just where the electron is mostlikely to be found. Till we measure up the place of the electron,its wave function is spread out out over all space, with a greater probabilityof recognize the electron in the classical orbit described by Bohr.(a)Sample electron spinsin a solid. No all arealigned, yet . . .(b). . . When canceling spinsare accounting for, a netmagnetic field remains.(c)This residual field in aregion is the network magnet-ization of the region, ordomain.
(d)Solids save severalsuch domains, whichare normally notaligned completely, but(e)The fields of the individualdomains in a magnetizedsolid don\"t fully cancelbut leaving a net field(f)In one unmagnetized solidthe fields of nearbydomains completelycancel, leaving no net field
Our principle of rotate must also be adjusted to fit through the discoveriesof the 20th century. Electrons space thought to be \"point particles,\"which way they have actually no spatial extent. Which means they can\"t bephysically spinning roughly their centers. While the word \"spin\" hassurvived, it now refers to an intrinsic property of a fragment rather thanto any kind of physical rotation through space. Because electrons and also otherparticles have actually intrinsic spin, they develop magnetic fields automatically.After considering quantum mechanics, we are when again left through two typesof magnetism: intrinsic magnetism as result of the \"spins\" the electrons,and electromagnetism because of the activity of electrons.Just as an aside, the reason that molecules such as He room not magnetizedis the Pauli exclusion principle. The 2 electrons in helium atomsoccupy the same power shell, filling it (the an initial shell contains only2 states). The exemption principle claims that no two electrons canhave the same exact properties. Because that them both to accounting the sameenergy shell, their spins need to be oppositely directed and also cancel.Electrons in solids with partially-filled valence shells may, however,line up v the same spin as other electrons, thereby producing a non-zeronet magnetic field.

MagneticFields and Forces

DiscussionQuestion:
Think around what youhave learned during your lifetime around electricity and magnetism.How room they alike? just how are lock different? any charged objectin an electrical field experiences an electrical force. Will any chargedobject in a magnetic ar experience a magnetic force? go an objecthave to be charged to experience a magnetic force?Magnets can exert a force at a distance, similar to electric charges.So that is valuable to define the effects of magnets in terms of amagnetic field, B1, much in thesame way that the impacts of charges are defined by the electrical field.We have already invoked this ide of a magnetic field in the previoussection. Magnetic fields permeate room and are strongest near apermanent magnet or electromagnet. IThe SI unit because that B is thetesla(1 T = 1 Vs/m2). The tesla is a fairly big unit the magneticfield, for this reason we frequently list magnetic ar strengths in regards to Gauss (1 G= 10-4 T). The magnetic field of the earth is about one-halfgauss in strength.
Like an electric field, a magnetic ar may be stood for with fieldlines. This lines (and the magnetic field)point indigenous the northpole the a magnet to the southern pole that a magnet, as displayed in the figureto the left.. Unlike electrical field lines, magnetic field lines arealways closed - they never have a starting point or preventing point.Whenever you have a phibìc pole, you must have actually a south pole together well.Another method to say this is that magnetic monopoles (single poles) carry out notexist. Electric monopoles, top top the various other hand, exist in abundance.Examples space an electron, a proton, or any kind of other fee particle.
Even the magnetic field produced by a current-carrying wire have to formcomplete loops. Above, you were told that a loop the current-carryingwire produces a magnetic field along the axis that the wire. The right-handrule provides the direction the the field inside the loop of wire.The magnetic ar turns earlier the other method outside of the loop.As displayed in the figure on the right, this magnetic field from a loop ofcurrent-carrying wire looks comparable to the ar from a long-term bar magnet.
Anyone who has used a compass knows that a magnet experiences a forcein a magnetic field. Simply as for electric charges, the opposite magneticpoles repel and like poles attract. For this reason the magnetic ar pointingfrom phibìc to south points in the direction that the pressure on a north POLEof a magnet. One interesting an outcome of this is that the Earth\"s geographicnorth pole is its magnetic south pole. A compass needle\"s magneticnorth pole will allude toward the geographical north pole that the Earth.Since the north pole that a magnet is attractive to the south pole the anothermagnet, this method that the geographical north pole the the earth is reallya south magnetic pole.Permanent magnets are not the only objects which experience the magneticforce. Electrical charges deserve to experience a magnetic pressure if 2 conditionsare met:The charge have to be moving through a magnetic fieldThe velocity that the fee cannot be parallel (or antiparallel) come thedirection the the magnetic fieldThe magnitude of the force is proportional come the fee q, themagnetic ar B, the rate of the fee v, and also the sineof the angle q between the velocity and also themagnetic field:FB = qvB sin qThe direction of the pressure is perpendicular to both the velocity andthe magnetic field. The force is much more accurately to express in termsof a cross-product:FB = qv x BThe size of a cross-product depends on sin q,giving the vault expression. Because that our purposes, the first expressionis sufficient, noted you remember the the pressure is perpendicular toboth the velocity and the magnetic field.A current-carrying wire additionally experiences a force in a magnetic field,since current is nothing more than moving charges. Together for singlecharges, the present must be moving in a direction various other than the directionof the field. The magnitude of the magnetic force on a current-carryingwire is discovered fromFB = iLB sin qwhere ns is the current and also L is the size of wire inthe uniform magnetic ar of toughness B.
1To it is in exact, the price B represents magnetic flux density,also called magnetic induction, no magnetic field. The true magneticfield is denoted by H. H and also B differ only bya material-dependent constant. For most purposes, the differenceis inconsequential, so we will refer to B as the magnetic field.If you take further courses in magnetism, girlfriend will find out the distinction.

InducedCurrents, Induced EMF, and Faraday\"s Law

DiscussionQuestion:
deserve to you develop a currentthrough a cable without connecting the wire straight to a voltage sourcelike a battery? Do every one of your appliances have direct connections?What around your auto engine?If a coil of cable is put in a transforming magnetic field, a currentwill it is in induced in the wire. This current flows because somethingis producing an electric field that forces the charges around the wire.(It cannot be the magnetic force since the charges space not initially moving).This \"something\" is referred to as an electromotive force, or emf,even though it is no a force. Instead, emf is favor the voltage providedby a battery. A changing magnetic ar through a coil of cable thereforemust induce one emf in the coil which in turn causes current come flow.The law describing induced emf is named after the brothers scientistMichael Faraday, yet Faraday\"s law should yes, really be dubbed Henry\"s Law.Joseph Henry, one American indigenous the Albany area, discovered that changingmagnetic areas induced current prior to Faraday did. Unfortunately,he stayed in the age prior to instantaneous electronic communication betweenEurope and also America. Faraday obtained published and also got famous prior to Henrycould report his findings. Interestingly enough, Henry had actually to explainthe results to Faraday as soon as the two met a few years later.Briefly stated, Faraday\"s legislation says that a transforming magnetic ar producesan electrical field. If fees are cost-free to move, the electrical fieldwill cause an emf and a current. Because that example, if a loop of wire isplaced in a magnetic ar so the the ar passes through the loop, achange in the magnetic field will induce a present in the loop of wire.A present is additionally induced if the area that the loop changes, or if the areaenclosing magnetic ar changes. So it is the change in magneticflux, identified asthat identify the induced current. A is the area vector;its size is the area of the loop, and its direction is perpendicularto the area the the loop, and also q is theangle between A and the magnetic field B. The lastequality (removing the integral) is valid only if the ar is uniformover the whole loop.Faraday\"s legislation says the the emf induced (and thus the current induced)in the loop is proportional come the price of change in magnetic flux:e is the emf, which is the occupational done movingcharges roughly the loop, separated by the charge. The is comparable inconcept to voltage, except that no fee separation is necessary.The magnetic flux FBequals the magnetic ar B time the area A the the loopwith magnetic field through the if (a) the magnetic ar is perpendicularto the plane of the loop, and (b) the magnetic ar is uniform throughoutthe loop. For our purposes, we will assume these two problems aremet; in handy applications, however, magnetic field will vary througha loop, and also the field will not constantly be perpendicular to the loop.Since every applications the Faraday\"s legislation to magnetic warehouse involvea coil of cable of solved area, we will likewise assume that (c) the area doesnot adjust in time. Us then have a less complicated expression for the currentinduced in the coil:The induced current depends on both the area of the coil and the changein magnetic field. In a coil the wires, every loop contributes an areaAto the right-hand next of the equation, for this reason the induced emf will be proportionalto the number of loops in a coil. However doubling the number of loopsdoubles the length of wire used and also so doubles the resistance, for this reason the inducedcurrent will not rise when loops are added.

Inductionand Magnetic Recording

A timeless recording head for magnetic data consists of a coil the wiresattached to part current-sensitive device. A ferromagnetic materialpasses under the coil. Together an plan can both compose magneticdata to the ferromagnetic material and read magnetic data off of the material.To create magnetic data, current is sent out through the coil in proportionto the desired signal. This existing produces a magnetic field proportionalto the current. The magnetic field aligns the spins in the ferromagneticmaterial. Together the product moves far from the coil, the magneticfield decreases, and also the spins continue to be aligned until they go into anothermagnetic field (when they are erased).Unlike electric storage, magnetic storage have the right to be one of two people analog or digital.The amount of turn alignment relies on the stamin of magnetic field,so analog data have the right to be recorded with a continually varying existing producinga continually differing magnetic field. Digital data deserve to be recordedby alternative the direction the the current. To minimization data lossor errors, binary data is not figured out solely by the direction that magnitizationin a domain. Instead, the is stood for by the adjust in magneticorientation in between two domains. If one little bit of magnetic field hasthe exact same direction together the one prior to it, that represents a 0 (no change).If one little bit of magnetic field has opposing direction together the one beforeit, the represents a 1 (change). Therefore a 1 is written by changing thedirection of current between the 2 domains making up a bit, and also a 0is written by keeping the direction the same. Each little starts witha change of orientation. This convention for recording data identifieserrors, because one would certainly never have actually three domains of the very same orientationin a row. In addition, the orientation should change with every otherdomain. If the computer thinks a bit is complete yet the orientationdoes no change, it knows that part error has occurred. Some examplesof domains, bits, and also strings are presented below.To read magnetic data, the ferromagnetic product is relocated past thecoil the wire. The transforming magnetic field caused by the material\"smotion root cause a current in the coil of cable proportional come the changein field. If a 0 is represented, the magnetic ar does no changebetween the two domains of a bit, so no present is induced together the magneticmaterial overcome the coil. For a 1, the magnetic field alters fromone direction come the other; this change induces a present in the coil.Inductive analysis of magnetic data is subject to many limitations.Since the readjust in magnetic field will be higher if the ferromagneticmaterial is relocated faster, the induced existing is dependent on the speedof the material. Thus the sensitivity that inductive review heads islimited by the precision of the product speed. The other limitingfactor ~ above inductive top is the stamin of the magnetic field.As efforts to rise storage density continue, the size of a data elementshrinks. Since fewer electrons room now included in the region ofone bit, the linked magnetic ar is smaller. This smaller sized magneticfield produce less readjust and hence less induced current, request moreloops to create a measurable current. As mentioned above, more loopsmeans an ext resistance which means more heat. Since of these limitations,new magnetic storage tools use the phenomenon the magnetoresistance toread magnetic data.

Summary

FactsAbout the Force
(From driving Force: The natural Magic of Magnets, byJames D. Livingston, (Havard university Press: Cambridge), 1996)These 10 facts about the pressure from Driving force by Livingstonsummarize most of the information contained in this and the next reading.Of details interest come the operations of computer systems are steps 4, 6, and8. 9 and 10 are also important concepts to remember. This readingassignment has actually only touch on the applications of magnets in informationsystems and other commonly-used technologies. If you room interestedin finding out more, the publication by Livingston is terrific place to start.
1.If complimentary to rotate, permanent magnets point approximately north-south.
2.Like poles repel, unlike poles attract.
3.Permanent magnets lure some things (like iron and steel) however notothers (like timber or glass).
4.Magnetic forces act in ~ a distance, and also they deserve to act through nonmagneticbarriers (if not also thick).
5.Things attracted to a permanent magnet become temporary magnets themselves.
6.A coil of wire v an electric current flowing through it becomesa magnet.
7.Putting iron inside a current-carrying coil significantly increases the strengthof the electromagnet.
8.Changing magnetic fields induce electrical currents in copper and otherconductors.
9.A charged fragment experiences no magnetic force when relocating parallelto a magnetic field, but when it is moving perpendicular come the field itexperiences a force perpendicular to both the field and also the direction ofmotion.
10.

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A current-carrying cable in a perpendicular magnetic ar experiencesa pressure in a direction perpendicular to both the wire and the field.

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