OUTLINE1. Faraday’s law of induction (fundamental law #3)2. Lenz’s law (just a qualitative translation of Faraday’s law into words)Induction: fundamental law #3Discoveries by Faraday and Henry:Changing magnetic field produces current in a wire loop. • Induced current i: depends on rate of change of Bwire resistance• Induced emf that drives i: depends only on the rate ofchange of B.{Magnetic fluxElectric flux∫⋅= AdBΦrrB∫⋅= AdEΦrrMagnetic fluxSurface defined by a conductive loopClosed surface (Gaussian surface)(N m2/C)(T m2)•1 T m2 = 1 weber = 1 WbθBAΦ cosB=if B is constant and surface is flat.Lenz’s law (just a qualitative translation of F’s law)The direction of the induced emf is such that the resulting current opposes the change in ФB.Three ways of changing ФBthrough a coil1. Change B2. Change the area A of the coil3. Change the angle θbetween B and ALet’s examine an example of each method.Q1 Is the induced emfand current clockwise or counterclockwise?Check Point for Changing BQ2 What does the ammeter read, when the magnet is held in place?ClockwiseZeroQ3 What happens while the magnet is being pulled back?Counterclockwise emf and currentQ4 What happens if the S pole of the magnet is pushed toward the loop?Counterclockwise emf and currentChanging
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