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Board Work What are the directions of the forces on the opposite charges moving with velocity v through magnetic field B? B − + v Moving conductors through magnetic fields Poll Question What is the direction of the Lorentz force on the charges moving along this wire? A. D. B. E. C. F. B I Current in a Field Feels a Force B F I Loop in a Field Has Torque B I I Torque Turns a Motor current I I force F I B Motion Creates a Potential – B v + induced potential Rotating Loop Creates Circuit B V V Rotating Loop Creates Circuit B V V Rotation Powers a Generator Motion through the field induces a potential which generates a current that charges the battery v V V B v Magnetic Induction Transforming energy with magnetism Objective • Explain the electric potential created by a changing magnetic field. What’s the Point? • This is how electric current is harnessed to do mechanical work. • It’s another way to think about the Lorentz force. ☺ Lorentz Force Reminder F = qv B Source: Griffith, The Physics of Everyday Phenomena F is directed out of the screen. Magnetic Flux • Field through a surface F = BA • Effectively the number of field lines enclosed by a loop Flux Change Creates Potential Faraday’s Law The rate of magnetic flux change through a loop equals the electric potential induced around the loop. V= DF Dt Magnetic Flux a b c d e • Flux changes as loop turns • • At a, c, e: Greatest flux F, least change At b, d: Zero flux, fastest flux change DF/Dt Induced current direction Lenz’s Law When a changing magnetic flux induces a current, the induced current’s magnetic field opposes the flux change. Lenz’s Law Example Induced current a b c d e • From a to b: – flux decreases; DF into face of loop ← – Loop current increases – Induced field increases out of face of loop Lenz’s Law Explains Generator changing field e e opposing field B needed I Board Work What is the Lorentz force on each side? B Which direction is the flux change? What current would oppose it? If the loop inside this magnetic field expands, in what direction will the induced current around it be? How a Microphone Works microphone geophone Flux inside loop becomes greater Wave-Powered Generator Originally appeared in Smithsonian magazine. How a Loudspeaker Works loudspeaker currentgenerated field attracts or repels magnet induced motion of magnet current Reading for Next Time • Electromagnetic Waves