Introto Magnetism: | Sourcesof Magnetic Fields | MagneticFields and Forces |
InducedCurrents: | Faraday"sLaw | Inductionand Magnetic Recording |
Summary |
Sourcesof Magnetic Fields
Discussion Question:![]() | As atomic physics and chemistry began to explain the periodic tablewith the help of the Bohr model of the atom in the early 1900s, magneticproperties were assigned to the electrons in atoms. Electrons appearedto exhibit two types of motion in an atom: orbital and spin.Orbital motion referred to the motion of an electron around the nucleusof the atom. Since a charged particle was moving, a magnetic fieldwas created. But electrons (and protons and other particles) alsoappeared to be spinning around their centers, creating yet another magneticfield. The magnetic field due to the orbital motion and the magneticfield due to the spin could cancel or add, but expressions for the exactcoupling between the two are too complicated to go into here. Sinceelectrons were moving and spinning within atoms, ferromagnetism could nowbe explained by the motion of charges within different materials.If all of the electrons in an object line up with their spins in the samedirection, the spins will add and create an observable field. |
That last sentence is slightly unrealistic.Solids contain incredably large numbers of electrons, and they will neverall completely line up. Instead, a solid generally consists of magneticdomains.In a domain, the majority of electrons which can (unpaired valance electrons)will have spins aligned. Adjacent domains will generally not be orientedin identical directions. In magnetized materials, some domains willcancel, but the average domain orientation will be in one direction, producinga net magnetic field. In unmagnetized materials, the domains arerandomly oriented and cancel, so no observable field is created.The figure to the right illustrates these concepts.The concept of magnetism being entirely due to the motion of chargeshas been modified significantly in the 20th century, thanks to quantummechanics. The Bohr model of the atom must be modified to includeuncertainty. We can never determine exactly the trajectory of anelectron or say for certain where it will be found. The uncertaintyprinciple requires that we instead say only where the electron is mostlikely to be found. Until we measure the position of the electron,its wave function is spread out over all space, with a higher probabilityof finding the electron in the classical orbit described by Bohr. | ![]() | ![]() | ![]() |
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MagneticFields and Forces
DiscussionQuestion:Magnets can exert a force at a distance, just like electric charges.So it is advantageous to describe the effects of magnets in terms of amagnetic field, B1, much in thesame way that the effects of charges are described by the electric field.We have already invoked this concept of a magnetic field in the previoussection. Magnetic fields permeate space and are strongest near apermanent magnet or electromagnet. IThe SI unit for B is thetesla(1 T = 1 Vs/m2). The tesla is a fairly large unit of magneticfield, so we often list magnetic field strengths in terms of Gauss (1 G= 10-4 T). The magnetic field of the earth is about one-halfgauss in strength.![]() | Like an electric field, a magnetic field may be represented with fieldlines. These lines (and the magnetic field)point from the northpole of a magnet to the south pole of a magnet, as shown in the figureto the left. Unlike electric field lines, magnetic field lines arealways closed - they never have a starting point or stopping point.Whenever you have a north pole, you must have a south pole as well.Another way to say this is that magnetic monopoles (single poles) do notexist. Electric monopoles, on the other hand, exist in abundance.Examples are an electron, a proton, or any other charged particle. |
Even the magnetic field produced by a current-carrying wire must formcomplete loops. Above, you were told that a loop of current-carryingwire produces a magnetic field along the axis of the wire. The right-handrule gives the direction of the field inside the loop of wire.The magnetic field turns back the other way outside of the loop.As shown in the figure on the right, this magnetic field from a loop ofcurrent-carrying wire looks similar to the field from a permanent bar magnet. | ![]() |
1 | To be exact, the symbol B represents magnetic flux density,also called magnetic induction, not magnetic field. The true magneticfield is denoted by H. H and 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, you will learn the distinction. |
InducedCurrents, Induced EMF, and Faraday"s Law
DiscussionQuestion: Can you create a currentthrough a wire without connecting the wire directly to a voltage sourcelike a battery? Do all of your appliances have direct connections?What about your car engine?If a coil of wire is placed in a changing magnetic field, a currentwill be 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 are not initially moving).This "something" is called an electromotive force, or emf,even though it is not a force. Instead, emf is like the voltage providedby a battery. A changing magnetic field through a coil of wire thereforemust induce an emf in the coil which in turn causes current to flow.The law describing induced emf is named after the British scientistMichael Faraday, but Faraday"s Law should really be called Henry"s Law.Joseph Henry, an American from the Albany area, discovered that changingmagnetic fields induced current before Faraday did. Unfortunately,he lived in the age before instantaneous electronic communication betweenEurope and America. Faraday got published and got famous before Henrycould report his findings. Interestingly enough, Henry had to explainthe results to Faraday when the two met a few years later.Briefly stated, Faraday"s law says that a changing magnetic field producesan electric field. If charges are free to move, the electric fieldwill cause an emf and a current. For example, if a loop of wire isplaced in a magnetic field so that the field passes through the loop, achange in the magnetic field will induce a current in the loop of wire.A current is also induced if the area of the loop changes, or if the areaenclosing magnetic field changes. So it is the change in magneticflux, defined as


Inductionand Magnetic Recording
A traditional recording head for magnetic data consists of a coil of wiresattached to some current-sensitive device. A ferromagnetic materialpasses under the coil. Such an arrangement can both write magneticdata to the ferromagnetic material and read magnetic data off of the material.To write magnetic data, current is sent through the coil in proportionto the desired signal. This current produces a magnetic field proportionalto the current. The magnetic field aligns the spins in the ferromagneticmaterial. As the material moves away from the coil, the magneticfield decreases, and the spins remain aligned until they enter anothermagnetic field (when they are erased).Unlike electric storage, magnetic storage can be either analog or digital.The amount of spin alignment depends on the strength of magnetic field,so analog data can be recorded with a continually varying current producinga continually varying magnetic field. Digital data can be recordedby alternating the direction of the current. To minimize data lossor errors, binary data is not determined solely by the direction of magnitizationin a domain. Instead, it is represented by the change in magneticorientation between two domains. If one bit of magnetic field hasthe same direction as the one before it, that represents a 0 (no change).If one bit of magnetic field has the opposite direction as the one beforeit, that represents a 1 (change). So a 1 is written by changing thedirection of current between the two domains comprising a bit, and a 0is written by keeping the direction the same. Each bit starts witha change of orientation. This convention for recording data identifieserrors, since one would never have three domains of the same orientationin a row. In addition, the orientation should change with every otherdomain. If the computer thinks a bit is complete but the orientationdoes not change, it knows that some error has occurred. Some examplesof domains, bits, and strings are shown below.
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 force from Driving Force by Livingstonsummarize most of the information contained in this and the next reading.Of particular interest to the workings of computers are steps 4, 6, and8. 9 and 10 are also important concepts to remember. This readingassignment has only touched on the applications of magnets in informationsystems and other commonly-used technologies. If you are interestedin learning more, the book by Livingston is an excellent place to start.1. | If free to rotate, permanent magnets point approximately north-south. |
2. | Like poles repel, unlike poles attract. |
3. | Permanent magnets attract some things (like iron and steel) but notothers (like wood or glass). |
4. | Magnetic forces act at a distance, and they can act through nonmagneticbarriers (if not too thick). |
5. | Things attracted to a permanent magnet become temporary magnets themselves. |
6. | A coil of wire with an electric current flowing through it becomesa magnet. |
7. | Putting iron inside a current-carrying coil greatly increases the strengthof the electromagnet. |
8. | Changing magnetic fields induce electric currents in copper and otherconductors. |
9. | A charged particle experiences no magnetic force when moving parallelto a magnetic field, but when it is moving perpendicular to the field itexperiences a force perpendicular to both the field and the direction ofmotion. |
10. | A current-carrying wire in a perpendicular magnetic field experiencesa force in a direction perpendicular to both the wire and the field. |