chapter31

# chapter31 - Chapter 31 Faraday's Law Electricity generator,...

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Chapter 31 Faraday’s Law Electricity generator, or from B to E. 1. Battery b Chemical emf 2. Motional emf 3. Faraday’s Law of Induction 4. Lentz Law about the emf direction

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A dry-cell battery Chemical reactions in the battery cells transport charge carriers (electrons) from one terminal to the other to create the needed electric potential (emf) to drive the current through the outside load, a light bulb here.
Motional emf, the concept c With We can group charges by moving them in a magnetic field b motional emf. c A motional emf is the emf induced in a conductor moving through a magnetic field c The electrons in the conductor experience a force, that is directed along c Charges are accumulated at the ends of the conductor to create an electric field inside the conductor to stop further charge transportation. q = × F v B a a a q = × F v B a a a = = = B E F qvB F qE When equilibrium

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Motional emf, the calculation c Start from the equilibrium condition One has Or the emf, potential difference: c As long as the bar is kept being moved with a velocity v, the motional emf is maintained to be vB . = = = B E F qvB F qE = E vB = Δ = = emf V E vB l l
Motional emf, put in use to power a resistor PLAY ACTIVE FIGURE Bar moved by a app F I Two issues need attention: 1. The moving bar carrying current I , inside the magnetic field, experiences a force from the field is F B =I B 2. The magnetic flux in the enclosed area (bar, rails and resistor) is Φ B =x B, and it is changing with time as Equivalent circuit diagram Condition: A bar moving on two rails. The bar and the rails have negligible resistance. A resistor of R is connected to the end of the two rails. Result: The emf = vB , so the current I = vB /R ( ) Φ = = = = b d d dx x B B vB dt dt dt emf l l l

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Example, what is the terminal velocity? A bar of mass m sides on two vertical rails. A resistor is connected to the end of the rails. When the bar is released at t = t 0 , (a) calculate the velocity of the bar at time t , (b) what is the terminal velocity? Assuming that the rails and the magnetic field is long/large enough. I
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## This note was uploaded on 05/06/2008 for the course PHYS 1304 taught by Professor Ye during the Spring '08 term at SMU.

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chapter31 - Chapter 31 Faraday's Law Electricity generator,...

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