L8-1 - L8-1Chapter 27-28 The Magnetic Field Goals:1)...

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Unformatted text preview: L8-1Chapter 27-28 The Magnetic Field Goals:1) Introduction to the magnetic field3) Forces due to magnetic fields-some applications3) Sources of magnetic field- Biot- Savart law- Amperes LawMeta-comments:- previously static charges electric fields- charges in motion (currents) make magnetic fields- and magnetic fields influence charges in motionWATCH OUT:- a lot of cross products here- right hand rules- in addition to usual minus sign issuesMagnetic fields are different:-no magnetic charges- magnet poles cannot be isolated- cut a magnet in two - two magnets-current and moving charges are important- also currents inside permanent magnetsIntroduction and demos:features of magnetic fieldsBasic facts:Two poles on magnet:-opposites attract, likes repelCompass is a magnet- north pole is defines as that which points northCurrents in wires also produce magnetic fieldsMagnetic forces act on moving charges- neither attractive nor repulsive- perpendicular forceMagnetic Poles and Field LinesOpposite magnetic poles attractand like magnetic poles repelNorthmagnetic pole: field lines emerge .Southmagnetic pole: field lines go in.By convention, field linesrun from north to southpoles of magnetsSymbol:magnetic field= BHistorical origin of the convention:- compass needlesNorth poles are those that point northConsequence:Earths magnet in northern hemisphere is really a SOUTH poleDefined so that the north magnetic pole of a compass needle is the end that points north Magnetic field lines neither start nor endThere are no magnetic charges,Looking inside the permanent magnet, we see that the field lines do not start or endCompare: E for DipoleCurrents generate magnetic fields:Useful notation:Now on to a mathematical description of magnetic fields:- recall 2 ways to calculate electric field- here there are also two ways- dB =I ds- Bdl =.2encEdQdEkrrQ==rintegrating over chargesintegral ruleintegrating over currentsintegral ruleYou cant derive these laws proven by experimentThe vector cross productACBACrrr=forming a vector from two other vectorsThe magnitude of C is ABsin-maximum when perpendicularThe direction of C is perpendicular to bothA and B-right hand rulegives directionUnits for magnetic fieldsOlder (cgs) unit, still commonly used:1 gauss = 10 teslaThe SI unit of Bis1 newton/coulomb-(meter/sec) = 1 tesla1 N/C-(m/s) = 1 TExamples:Field of Earth at the surfaceB~ 5e-5 T = 0.5 GStrong (rare-earth) permanent magnet B~ 1 TNow on to Magnetic ForcesRecall demos:- neither attractive or repulsive- force perpendicular to fieldThe Magnetic Force on a Moving Charge...
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This note was uploaded on 04/07/2008 for the course PHY 211 taught by Professor Souder during the Spring '08 term at Syracuse.

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L8-1 - L8-1Chapter 27-28 The Magnetic Field Goals:1)...

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