PY208N-Lecture-20 - Physics 208N-004 Lecture#20 2 March...

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1 Physics 208N-004 Lecture #20 – 2 March 2011 Chapter 29: 1-2 – Induction and Faraday’s Law Induced EMF Faraday’s Law of Induction; Lenz’s Law EMF Induced in a Moving Conductor Electric Generators Back EMF and Counter Torque; Eddy Currents Transformers and Transmission of Power A Changing Magnetic Flux Produces an Electric Field Almost 200 years ago, Faraday looked for evidence that a magnetic field would induce an electric current with this apparatus: 29-1 Induced EMF Field observed when switch closed or opened.
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2 He found no evidence when the current was steady, but did see a current induced when the switch was turned on or off. 29-1 Induced EMF Therefore, a changing magnetic field induces an emf . Faraday’s experiment used a magnetic field that was changing because the current producing it was changing; graphic shows a magnetic field that is changing because the magnet is moving. The induced emf in a wire loop is proportional to the rate of change of magnetic flux through the loop. Magnetic flux: Unit of magnetic flux: weber, Wb:1 Wb = 1 T·m 2 . 29-2 Faraday’s Law of Induction; Lenz’s Law This drawing shows the variables in the flux equation:
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