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# lecture10 - Physics 102 Lecture 10 Faradays Law Changing...

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Faraday’s Law Physics 102: Lecture 10 Changing Magnetic Fields create Electric Fields Exam 1 tonight Be sure to bring your ID and go to correct room All you need is a #2 pencil and calculator No cell phones No I-pods, laptops, etc.

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Last Two Lectures Magnetic fields Forces on moving charges and currents Torques on current loops Magnetic field due to Long straight wire Solenoid
Today: Faraday’s Law The principle that unifies electricity and magnetism Key to many things in E&M Generating electricity Microphones, speakers and tape decks Amplifiers Computer disks and card readers 7

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First a preliminary: Magnetic Flux “Counts” number of field lines through loop. Uniform magnetic field, B , passes through a plane surface of area A . A Magnetic flux Φ = B A Magnetic flux Φ B A cos( φ ) φ is angle between normal and B 22 B A φ n o rmal B Note: The flux can be negative (if field lines go thru loop in opposite direction)
Preflight 10.7 Compare the flux through loops a and b. 1) Φ a > Φ b 2) Φ a < Φ b a b n n B Φ A = B A cos(0) = BA Φ B = B A cos(90) = 0 73% 27% 24 “more lines pass through its surface in that position.”

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Faraday’s Law of Induction: This is new physics and not simply an application of stuff you already know “induced emf” = rate of change of magnetic flux i f i f t t t - Φ - Φ - = ∆Φ - = ε Since Φ = B A cos( φ29, 3 things can change Φ 1. Area of loop 2. Magnetic field B 3. Angle φ between A and B 26
28 Lenz’s Law (EMF Direction) Induced emf opposes change in flux EMF does NOT oppose B field, or flux!

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