LECT17_203207

# LECT17_203207 - 20.4: A closer examination of Lenz's Law A...

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20.4: A closer examination of Lenz's Law A force is exerted by a magnet on a loop to induce current. A force must be exerted by the current on the magnet to oppose the change. The induced current creates a magnetic dipole in the loop. The magnetic dipole creates a force acting on the magnet.

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Eddy Currents Drop a magnet down a cylindrical conductor, N-pole first. Ahead of (below) the magnet: B increasing S N
Eddy Currents Drop a magnet down a cylindrical conductor, N-pole first. Currents are induced in the conductor to oppose the movement of the magnet: Ahead of (below) the magnet: B increasing S N I ind S N induced magnetic dipole two N poles: repel

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Eddy Currents In the region behind (above) the magnet: B decreasing S N I ind S N induced magnetic dipole N & S pole: attract
Eddy Currents Selected videos of eddy currents: http://www.youtube.com/watch?v=nrw- i5Ku0mI http://www.youtube.com/watch?v=pcVG6c_OvYU http://www.youtube.com/watch?v=37e_OROP9dA

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20.3 Motional EMF ε ind from a conductor which is moving through a B-field Conducting rod moving at a constant velocity v Charges experience F M = qvB upward (for + charges) Negative charges accumulate at bottom. Net + charge accumulates at top. This continues until F E and F B are balanced: qE = qvB Potential difference Δ V = E l = B l v Upper end is at higher potential than lower end.
What if the moving conductor is part of a closed circuit? Conducting rod is being pulled with velocity v = Δ x/ Δ t B is constant, but the AREA of the circuit (and hence Φ B ) is increasing v Δ V = B l v = B l = B = = | ε | Δ x Δ t Δ t Δ A ΔΦ B Δ t I = = B l v | ε | R R I

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Rail gun/space catapult When switch is closed: Horizontal conducting rods create B-field into page. Current in bar is downward, and a force F m = IB l pushes the bar to the right causing it to accelerate. Payloads can be accelerated to several km/s (U.S. Navy has tested one…)
Uses mechanical work to generate electrical current. Rotary motion from falling water (hydroelectric) or steam (coal-fired plant) directed against turbine blades. A coil rotates,

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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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LECT17_203207 - 20.4: A closer examination of Lenz's Law A...

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