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Faraday’s Law - Induction Textbook Sections 23-1 -- 23-4 Physics 1161: PreLecture 14 Changing Magnetic Fields create Electric Fields

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Faraday’s Law Key to EVERYTHING in E+M –Generating electricity –Microphones, Speakers and Tape Decks –Amplifiers –Computer disks and card readers –Ground Fault Interrupters Changing B creates new E

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Motional EMF circuit Direction of Current Direction of force (F=ILB sin( )) on bar due to magnetic field I = E /R Magnitude of current Clockwise (+ charges go down thru bar, up thru bulb) To left, slows down Moving bar acts like battery E = vBL B -+-+ V What changes if B points into page? = vBL/R

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Magnetic Flux Count number of field lines through loop. Uniform magnetic field, B, passes through a plane surface of area A. A Magnetic flux = B A Magnetic flux B A cos( ) is angle between normal and B B A normal B Note: The flux can be negative (if field lines go thru loop in opposite direction)

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Physics 1161: Lecture 10, Slide 5 Faraday’s Law (EMF Magnitude) Emf = Change in magnetic Flux/Time Since = B A cos( 3 things can change 1.Area of loop 2.Magnetic field B 3.Angle between A and B

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Lenz’s Law (EMF Direction) Emf opposes change in flux If flux increases: New EMF makes new field opposite to the original field (to oppose the increase) If flux decreases: New EMF makes new field in the same direction as the original field (to oppose the decrease)

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