Lecture 26 - EMF. Induced fields. Displacement currents

# Lecture 26 - EMF. Induced fields. Displacement currents -...

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Lecture 26 EMF. Induced field. Displacement current. ACT: Two rings Two copper rings with the same geometry move toward identical magnets with the same velocity as shown. The ring in case 2, though, has a small slit. Compare the magnitudes of: • the induced emf’s in the rings: A. ε 1 < 2 B. 1 = 2 C. 1 > 2 • the magnetic force on the rings: A. F 1 < 2 B. 1 = 2 C. 1 > 2 N S v 1 N S 2 EMF is independent of the actual ring. It would be the same for a wooden ring, or even in vacuum! No current in case 2 because ring is open. 2 = 0 DEMO: Jumping rings In-class example: AC generator A coil of copper wire consists of 120 loops of wire wound around a 10 cm × 20 cm rectangular form. If this coil is used to generate an induced emf by rotating it in a 0.3 T field at 60 Hz, what is the maximum emf that can be produced? A. 33 V B. 43 V C. 120 V D. 217 V E. 27 100 V () sin B d NBA t dt ωω Φ =− = ( ) max 2 turns 2 rad 60 120 0.3 T 0.1 0.2 m s 1 turn 271 V εω π = ⎛⎞ ⎜⎟ ⎝⎠ = cos NB A ω Φ= = G G A G G This is an AC generator! sin = ( ) cos BA

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AC generator Water turns wheel Æ rotates magnet Æ changes flux Æ induces emf Æ drives current DEMOs: Loop rotating between electromagnets Hand-driven generator with bulbs Eddy currents F Pull a sheet of metal through B-field External force required to pull sheet because induced current opposes change.
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Lecture 26 - EMF. Induced fields. Displacement currents -...

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