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OUTLINE 1. 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 #3 Discoveries by Faraday and Henry: Changing magnetic field produces current in a wire loop. • Induced current i: depends on { rate of change of B wire resistance • Induced emf that drives i: depends only on the rate of change of B. Magnetic flux Magnetic flux Electric flux r r r r ΦB = ∫ B ⋅ dA (T m2) Φ = ∫ E ⋅ dA (N m2/C) Surface defined by a conductive loop Closed surface (Gaussian surface) • 1 T m2 = 1 weber = 1 Wb ΦB = BAcosθ 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 ФB through a coil 1. Change B 2. Change the area A of the coil 3. Change the angle θbetween B and A Let’s examine an example of each method. Check Point for Changing B Q1 Is the induced emf and current clockwise or counterclockwise? Clockwise Q2 What does the ammeter read, when the magnet is held in place? Zero Q3 What happens while the magnet is being pulled back? Counterclockwise emf and current Q4 What happens if the S pole of the magnet is pushed toward the loop? Counterclockwise emf and current Changing A