Lecture 30b

# Lecture 30b - Magnetic Induction (contd.) Magnetic...

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Magnetic Induction (contd.) Magnetic Induction can be used to power a Bike light

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Lenz’s Law: Opposition to motion I must always do positive work to move the magnet. It takes energy to make a current flow.

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4 Sliding metal bar on metal rails L x=vt v X X X X X X B Bar is pulled by an external agent to the right at constant velocity. There is a uniform B- field surrounding the rails and loop system.
5 L x=vt v X X X X X X B This creates a wire loop whose area is growing with time. Lvt Lx A Thus, the Magnetic Flux is increasing with time BLv Lvt dt d B BA dt d dt d B ) ( ) ( | |

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6 L x=vt v X X X X X X B What is the direction of the induced EMF and thus current in the wire loop? A) Clockwise B) Counter Clockwise BLv | | Induced emf: What is the magnitude of the current in the loop? ° = ± ²
There is another way to see why the current in the bar is up. Charges in the bar are moving to the right with the bar itself. v X B There is a B-field in this region. Electrons feel a force ) ( down evB B v q F Thus, the direction of conventional current is up! i

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## This note was uploaded on 02/05/2011 for the course PHYS 220B taught by Professor Preminger during the Fall '10 term at CSU Northridge.

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Lecture 30b - Magnetic Induction (contd.) Magnetic...

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