mag_ckt_ex_notes - Magnetic Circuits and Examples Review of...

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Magnetic Circuits and Examples EE201 Denard Lynch Page 1 of 14 Sep 3, 2009 Review of Magnetism Originates at the molecular level: electrons orbiting a nucleus induce a magnetic “moment” each “orbit induces its own separate moment depending on particular molecule, can have a net magnetic moment (i.e. ferromagnetic) This implies and illustrates a fundamental relationship between electricity and magnetism Two important and related facts: a moving charge generates a magnetic field, and although not obvious from the above… relative motion between a magnetic field and a charge exerts a force on the charge(s) (and on that which contains them; i.e. a conductor) So what is this “magnetic field”? a force field visualized as “lines of magnetic flux” that appear to follow rules: o form closed loops from one “pole” to another o directional (by convention, from north to south pole of magnet) o repel each other * o follow the path of least resistance* o *these last two rules conflict with one another! Remember… the field consists of continuous lines, but are really a visual aid Magnetic flux can also be visualized as flowing, similar to water or electric current (symbol: Φ ) We are often interested in its density (symbol B ) Depending on the material, the magnetic moments from each set of orbiting electrons in a molecule can reinforce or cancel each other to give the molecule a net magnetic moment . Various molecular arrangements lead to three types of materials: o Paramagnetic – only very slightly receptive to a magnetic field (e.g. air, glass, wood, paper, plastics etc.) o Diamagnetic (anti-magnetic) – form opposite dipoles in response to an imposed magnetic field (bismuth, pyrolytic graphite), but only a weak response o Ferromagnetic – net magnetism at the molecular level; get together into polarized “domains”. They are normally oriented randomly, but may be aligned temporarily or permanently. (E.g. iron, cobalt etc.) We are interested in this last category because it is relatively easy * to establish magnetic flux, Φ , in these materials in response to a magnetic field. this “relative ease” is somewhat temperature dependent. Each material has a “Currie temperature”, above which they behave like paramagnetic materials.
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Magnetic Circuits and Examples EE201 Denard Lynch Page 2 of 14 Sep 3, 2009 Why is this (ferromagnetic) ability of interest to us? So we can design “magnetic circuits” to route flux in certain ways and to certain places; the same way we route electric current or hydraulic fluid. We route this to certain places for one of two reasons: 1) The force it can provide a. Speakers, sound reproduction b. Motors, generators c. Door bells, electromagnets 2) its magnetizing effect (i.e. channelizing/routing flux) a. HDDs, magnetic tapes b. Transformers, inductors How can we create a magnetic field, or a magnet?
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This note was uploaded on 09/25/2010 for the course EE 201 taught by Professor Linch during the Spring '10 term at University of Saskatchewan- Management Area.

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mag_ckt_ex_notes - Magnetic Circuits and Examples Review of...

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