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Lecture Notes (Len's Law)

Lecture Notes (Len's Law) - Physics 122 Lenz’s Law —...

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Unformatted text preview: Physics 122 Lenz’s Law — Concepts already covered: 1 l b I E 'b . - ~ 4 5'— E2523 1. Electric Field - E 2. Magnetic Field - B Like poles repel each other. Unlike poles attract each other. Magnetic force acting on moving charges: F = qu sine? Magnetic force acting on a current carrying conductor: F = BIL sinfi 3. Magnetic fields produced by moving charges: a) 1819 - Hans Christian Oersted - the beginning of the unification theory of magnetic and electric fields. First demonstration, that an electric current in a wire deflected a nearby compass needle. (There is a magnetic field produced by an electric current - or by moving charges) b) 1827 - Andre-Marie Ampere — based solely on experimental data derived a precise mathematical formulae of electromagnetism known as Ampere’s Law EB. A]: WWW, (simplified form) Textbook: Chapter 20, p. 628 4. CAN A MAGNETIC FIELD PRODUCE AN ELECTRIC CURRENT ? a) Let us define the magnetic flux 4) q): B‘LA 7— EAL = BvA-eosa Q is proportional to the total number of lines passing through the loop. b) Experimental evidence: (b) The induced current in a conducting loop is proportional to the rate of change of magnetic flux through the loop. THE FARADAY’S LAW or MAGNETIC INDUCTION: “The instantaneous emf ( E ) induced in a circuit equals the rate of change of magnetic flux through the circuit.” [8) ._—. JV MD where N = the number of tightiy wound loops. The “ - “ Sign is included to indicate the polarity of the induced emf, which can be found using LENZ’S LAW “Induced emf acts so as to oppose the changein magnetic flux that creates it” (The induced current tends to maintain the original flux through the circuit). _ E ddj Cu. v w: mats Worksheet 1 Applications of Faraday’s Law and Lenz’s Law LMOTIONALemf Ad): I5Lé3x _‘ .— AC9 - Ax Tipler: Physics for Scientists and Engineers Transparency 107 Third Edition, Voiume 2 Figure 26—8, page 846, Figure 26—9, page 847 Demonstration of Lenz's law Copyright © 199‘} by Worth Publishers, Inc. Transparency 108 Tipler: Physics for Scientists and Engineers Figure 26-14, page 849, Figure 26-15, page 849 Third Edition, Volume 2 Motional emf ' Copyrighl © 1991 by Worth Publishers, Inc. Ax = Udt quUB EDDY CURRENTS (a) (is) (a) Eddy currents are produced in a copper disk swinging through a magnetic field. (b) Slots cut in the disk reduce the eddy currents and allow the disk to swing more freely‘ GENERATORS AND MOTORS Brushes (a) (b) 312“ cnnncczad to load Rocaling shlionan split 'ng brushes (commutator; Sliding Cuntacls) Battery (a) [m Elements ofa dc motor. Genemwr _/ Bunnng mail m 01'] ...
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