Some Weird StuffcalendarImportant Definition – Magnetic FluxSlide 4The Essence of this TopicHow Can You Change the Magnetic Flux Going Through The Loop? Huh?The Magnetic Flux Going Through The Loop:Changing FWAIT A SECOND …….Slide 10REMEMBER when I said E Fields start and end on CHARGES???The truthWhy do you STILL think I am a liar?Slide 14The POTENTIAL between two pointsSo, consider the following:THEREFORE WHAT WILL A VOLTMETER READ FROM A to A?Slide 18Q: Which way does E point?Faraday’s ExperimentsInsert Magnet into CoilRemove Coil from Field RegionPush a magnet into a coil of two wires and a current is produced via an emf.The Strange World of Dr. LentzLenz’s LawExample of Nasty LenzSlide 27Slide 28An inductor resists CHANGES in the current going through it.Slide 30Slide 31Inductance DefinedSo …Move on to the experiment in UNIT 14SOME WEIRD STUFFNOVEMBER 5, 20101INDUCTIONCALENDARToday – QUIZDiscussion about Lenz and InductionExperiments on InductorsMonday – Finish the UnitAdditional DiscussionExam Date will be AnnouncedLabs will be collected again shortly … make sure you are ready.2INDUCTIONIMPORTANT DEFINITION – MAGNETIC FLUXINDUCTION 3AREAMagnetic FieldINDUCTION 4THE ESSENCE OF THIS TOPICINDUCTION 5Consider a conductor that is shaped in a loop but is continuous.The conductor has a magnetic field through the loop that is not necessarily uniform.There is a MAGNETIC FLUX through this loop.If the FLUX CHANGES, an “emf” will be induced around the loop.This emf can cause a current to flow around the loop.HOW CAN YOU CHANGE THE MAGNETIC FLUX GOING THROUGH THE LOOP? HUH?INDUCTION 6Divide the area of the loop into a very large number of small areas A.Find the Magnetic Field through each area as well as the angle that it makes with the normal to the area.Compute the total flux through the loop.THE MAGNETIC FLUX GOING THROUGH THE LOOP:INDUCTION 7iiiiAB )cos(Add up all of these piecesthat are INSIDE the loop.CHANGING INDUCTION8Change any or all of the•Bi••i•Change the SHAPE of the loop•Change the ANGLE that the loop makes with the magnetic field (subset of above)And the Flux will change!iiiiAB )cos(WAIT A SECOND …….INDUCTION 9You said that there is a conducting loop.You said that there is therefore a VOLTAGE or emf around the loop if the flux through the loop changes.But the beginning and end point of the loop are the same so how can there be a voltage difference around the loop?‘tis a puzzlement!INDUCTION 10REMEMBER WHEN I SAID E FIELDS START AND END ON CHARGES???INDUCTION 11DID I LIE??THE TRUTHINDUCTION 12Electric fields that are created by static charges must start on a (+) charge and end on a (–) charge as I said previously.Electric Fields created by changing magnetic fields can actually be shaped in loops.WHY DO YOU STILL THINK I AM A LIAR?INDUCTION 13Because you said that an emf is a voltage so if I put a voltmeter from one point on the loop around to the same point, I will get ZERO volts, won’t IINDUCTION 14THE POTENTIAL BETWEEN TWO POINTSINDUCTION 15Is the WORK that an external agent has to do to move a unit charge from one point to another.But we also have (neglecting the sign): sEVsSO, CONSIDER THE FOLLOWING:INDUCTION 16EConductorx x x x x x xx x x x x x xx x x x x x xx x x x x x xx x x x x x xx x x x x x xzeroREemfsEsEemf2THEREFORE WHAT WILL A VOLTMETER READ FROM A TO A?INDUCTION 17EConductorx x x x x x xx x x x x x xx x x x x x xx x x x x x xx x x x x x xx x x x x x xAA The emfB ZeroC Can’t tellINDUCTION 18temflooptheThrough Michael Faraday (1791-1867)MINUS????A: The way that you don’t want it to point! (Lenz’s Law).Lenz’s Law Explains the (-) sign!Q: WHICH WAY DOES E POINT?19 INDUCTIONFARADAY’S EXPERIMENTS??20INDUCTIONINSERT MAGNET INTO COIL21INDUCTIONREMOVE COIL FROM FIELD REGION22INDUCTIONPUSH A MAGNET INTO A COIL OF TWO WIRES AND A CURRENT IS PRODUCED VIA AN EMF.INDUCTION 23In this case, 2 coils, each have the SAME emf so we add..Ohm’s Law still works, socoilRemfitNemfTHE STRANGE WORLD OF DR. LENTZ24INDUCTIONLENZ’S LAWINDUCED MAGNETIC FIELDS ALWAYS FIGHT TO STOP WHAT YOU ARE TRYING TO DO!... MURPHY’S LAW FOR MAGNETS25INDUCTIONEXAMPLE OF NASTY LENZThe induced magnetic field opposes thefield that does the inducing!26INDUCTION27INDUCTIONLet’s do theLentz Warpagain !28INDUCTIONAN INDUCTOR RESISTS CHANGESCHANGES IN THE CURRENT GOING THROUGH IT.01/13/19 INDUCTION 29AN INDUCTOR RESISTS CHANGESCHANGES IN THE CURRENT GOING THROUGH IT.01/13/19 INDUCTION 30AN INDUCTOR RESISTS CHANGESCHANGES IN THE CURRENT GOING THROUGH IT.01/13/19 INDUCTION 31INDUCTANCE DEFINED01/13/19 INDUCTION 32iNLBIf the FLUX changes a bit during a short time t, then the current will change by a small amount i.tiLtNNLiBBThis is actually acalculus equationFaraday says this is the emf!SO …01/13/19 INDUCTION 33tiLemfE=The UNIT of “Inductance – L” of a coil is the henry.SYMBOL:There should bea (-) sign but weuse Lenz’s Lawinstead!MOVE ON TO THE EXPERIMENT IN UNIT 14INDUCTION
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