L31_a - loop given by Faraday’s Law ± t d B d r t d d 2...

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LENZ’S LAW EXAMPLE: (from Prob. 21, page 794) A conducting loop of mass M, resistance R, width w, and length l is dropped into a field B r pointing out of the page. It reaches a terminal velocity before the top edge enters the magnetic field. Calculate the terminal velocity. v w y
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LENZ’S LAW EXAMPLE: (from Prob. 22, page 794) A rectangular coil with N turns, resistance R , length l and width w moves at constant speed v into, through, and then out of a region containing a field B r pointing into the page/screen. Find the magnitude and direction of the magnetic force on the coil as it (a) enters the field, (b) moves entirely within the field, and (c) leaves the field. l w v
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INDUCED emf AND ELECTRIC FIELD Look at a conducting loop of radius r in a region of magnetic field B r pointing into the page/screen o The magnetic flux through the loop is 2 B r B BA π = = Φ o If B is changing, there is an emf around the
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Unformatted text preview: loop given by Faraday’s Law ± t d B d r t d d 2 B ε − = Φ − = ± Picture shows direction of current for decreasing B • The induced current implies a tangential electric field E r in the conductor r B in (increasing magnitude) I r B in (increasing magnitude) E E E E IMPORTANT INSIGHT: • Changing magnetic flux ALWAYS induces an electric field E r EVEN IF NO CHARGES PRESENT Connects E r to t d B d r • work to move charge q around a loop is t d d q q s d E q W B loop around Φ − = = ⋅ = ∫ ε r r • Implies: t d d s d E B loop around Φ − = ⋅ ∫ r r o integral is around closed path o B Φ is the flux through the area enclosed by the path • This is one of MAXWELL’S EQUATIONS o E r field in this equation NOT electrostatic – does not end on charges r B in (increasing magnitude) E E E E...
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L31_a - loop given by Faraday’s Law ± t d B d r t d d 2...

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