lect13_f03

# lect13_f03 - Faraday's Law and Lenz's Law B(t i ~ Physics...

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Physics 1304: Lecture 13, Pg 1 Faraday’s Law and Lenz’s Law ~ B(t) i

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Physics 1304: Lecture 13, Pg 2 Overview of Lecture Overview of Lecture Induction Effects Faraday’s Law (Lenz’ Law) Energy Conservation with induced currents? Faraday’s Law in terms of Electric Fields Text Reference: Chapter 31.1-4
Physics 1304: Lecture 13, Pg 3 Induction Effects Induction Effects v v S N v N S N S S N Bar magnet moves through coil Current induced in coil Change pole that enters Induced current changes sign Bar magnet stationary inside coil No current induced in coil Coil moves past fixed bar magnet Current induced in coil

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Physics 1304: Lecture 13, Pg 4 Induction Effects Induction Effects from Currents from Currents Switch closed (or opened) current induced in coil b Steady state current in coil a   no current induced in coil   b a b Conclusion: A current is induced in a loop when: • there is a change in magnetic field through it • loop moves through a magnetic field How can we quantify this?
Physics 1304: Lecture 13, Pg 5 Faraday's Law Faraday's Law Define the flux of the magnetic field through a surface (closed or open) from: Faraday's Law: The emf induced in a circuit is determined by the time rate of change of the magnetic flux through that circuit. The minus sign indicates direction of induced current (given by Lenz's Law). ε = - d dt B Φ dS B B Φ B B dS

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Physics 1304: Lecture 13, Pg 6 Lenz's Lenz's Law Law Lenz's Law: The induced current will appear in such a direction that it opposes the change in flux that produced it. Conservation of energy considerations: Claim: Direction of induced current must be so as to oppose the change; otherwise conservation of energy would be violated. » Why??? If current reinforced the change, then the change would get bigger and that would in turn induce a larger current which would increase the change, etc.. v B S N v B N S
Physics 1304: Lecture 13, Pg 7 Lecture 18, CQ Lecture 18, CQ A conducting rectangular loop moves with constant velocity v in the +x direction through a region of constant magnetic field B in the -z direction as shown. What is the direction of the induced current in the loop? (a) ccw (b) cw (c) no induced current A conducting rectangular loop moves with constant velocity v in the -y direction and a constant current I flows in the +x direction as shown. What is the direction of the induced current in the loop? 1A X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X v x y (a) ccw (b) cw (c) no induced current 1B v I x y

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Physics 1304: Lecture 13, Pg 8 Lecture 16, ACT 1 Lecture 16, ACT 1 A conducting rectangular loop moves with constant velocity v in the +x direction through a region of constant magnetic field B in the -z direction as shown.
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