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Unformatted text preview: Platt, David Homework 21 Due: Oct 28 2005, 4:00 am Inst: Ken Shih 1 This print-out should have 8 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. The due time is Central time. 001 (part 1 of 2) 10 points Given: B = bt , where b = 0 . 9 T / s (lower figure). Consider a changing magnetic field (from B to B , upper figure) within a circle of radius 4 m. The magnetic field is pointing into the page and its magnitude is increasing. 4 m B B B 4 m B B B ~ t ~ B B B Find the direction of the induced emf E ind . 1. clockwise 2. counter-clockwise correct 3. out of the page 4. into the page Explanation: Let : a = 4 m and b = 0 . 9 T / s . Faradays Law of Induction: E ind = I ~ E ind d~s =- d B dt Lenzs Law Induced emfs always oppose magnetic flux changes. Use a bracket, [ ], to denote the direc- tions. The direction content of Faradays Law then reads, [ E ind ] =- d dt =- = fl . The right hand rule then implies that the direction of E ind is counter-clockwise. This is consistent with Lenzs Law: the magnetic field is increasing into the page, so the induced emf E ind creates a magnetic field out of the page to counter this change....
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