LECT16_201202

LECT16_201202 - Ch. 20: Induced Voltages & Inductance...

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Ch. 20: Induced Voltages & Inductance We’ve seen that electrical current can produce magnetic fields. Can magnetic fields be used to produce electrical current? The answer, as discovered by Michael Faraday, is YES: Applications include electrical generators, ground-fault interrupters, microphones
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Faraday’s Experiments
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Faraday’s Experiments Close switch: immediately after switch closed, ammeter measures current in secondary circuit +1
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Faraday’s Experiments After the switch has been closed for a while, ammeter has returned to zero.
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Faraday’s Experiments Open switch: immediately after opening switch, ammeter registers a current in the OPPOSITE direction –1
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Faraday’s Experiments After the switch has been opened for a while, ammeter has returned to zero.
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Conclusions from Faraday’s experiment: B-field going from OFF to ON-- induces a current in secondary circuit B-field going from ON to OFF -- induces a current (opposite sign) in secondary circuit. B-field level constant (zero or non-zero) -- no current induced! Conclusion: The B-field itself does not induce any current -- only a CHANGE in B-field.
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Electromagnetic Induction While the magnet is held stationary, there is no current While the magnet is moving away from the loop, the ammeter shows a current in the opposite direction While a magnet is moving toward a loop of wire, the ammeter shows the presence of a current
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Electromagnetic Induction If the magnet is held stationary and the LOOP is moving, you get the same effect. A current is induced whenever there exists RELATIVE motion between the magnet & loop. Direction of current depends on direction of motion.
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Magnetic Flux Assume B-field is uniform. Area of loop = A. Magn. Flux Φ B through an area A: Φ B = B A = BAcos θ SI unit: Weber (Wb) = T × m 2 Φ B is proportional to the total number of lines passing through the loop
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Magnetic Flux Edge view of loop in uniform B-field: θ =90º: Φ B is zero Change B-field direction so θ =180º: Φ B = –BA θ =0º: Φ B is maximized: Φ B = BA
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Φ B & ΔΦ B Example: A hexagon-shaped loop with area 0.5 m
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This note was uploaded on 09/30/2011 for the course PHYS 1B taught by Professor Briankeating during the Summer '07 term at UCSD.

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LECT16_201202 - Ch. 20: Induced Voltages & Inductance...

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