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Published byMalcolm Walton Modified over 9 years ago
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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 +−
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Moving conductors through magnetic fields
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Poll Question B I What is the direction of the Lorentz force on the charges moving along this wire? A. B. C. D. E. F.
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Current in a Field Feels a Force F B I
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Loop in a Field Has Torque B I I
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Torque Turns a Motor I I current I force F B
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– Motion Creates a Potential v B induced potential +
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Rotating Loop Creates Circuit B V V
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B V V
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Rotation Powers a Generator v v Motion V V induces a potential which generates a current that charges the battery B through the field
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Magnetic Induction Transforming energy with magnetism
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Objective Explain the electric potential created by a changing magnetic field.
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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. ☺
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Lorentz Force Reminder F = qv B Source: Griffith, The Physics of Everyday Phenomena F is directed out of the screen.
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Magnetic Flux Field through a surface = BA Effectively the number of field lines enclosed by a loop
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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 =V = tt
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Magnetic Flux Flux changes as loop turns abcde At a, c, e: Greatest flux , least change At b, d: Zero flux, fastest flux change / t
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Induced current direction Lenz’s Law When a changing magnetic flux induces a current, the induced current’s magnetic field opposes the flux change.
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Lenz’s Law Example From a to b: –flux decreases; into face of loop ← –Loop current increases –Induced field increases out of face of loop abcde Induced current
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Lenz’s Law Explains Generator B changing field opposing field needed I
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Board Work If the loop inside this magnetic field expands, in what direction will the induced current around it be? B What is the Lorentz force on each side? Which direction is the flux change? What current would oppose it?
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How a Microphone Works microphone geophone Flux inside loop becomes greater
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Wave-Powered Generator Originally appeared in Smithsonian magazine.
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How a Loudspeaker Works loudspeaker induced motion of magnet current current- generated field attracts or repels magnet
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Reading for Next Time Electromagnetic Waves
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