lect13_f03 - Physics 1304: Lecture 13, Pg 1 Faraday’s Law...

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Unformatted text preview: Physics 1304: Lecture 13, Pg 1 Faraday’s Law and Lenz’s Law ~ B(t) i 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 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 ≡ • ∫ 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 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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This note was uploaded on 02/12/2012 for the course PHYSICS 104 taught by Professor Staff during the Fall '10 term at Rutgers.

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lect13_f03 - Physics 1304: Lecture 13, Pg 1 Faraday’s Law...

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