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The University of Vermont
Phys 012 - Elementary Physics
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Lecture 16 Outline of Last Lecture II. Problem: Find the force on the rectangular coil as it enters the magnetic field B at constant velocity v. a. Entering field: flux is increasing  induced electromagnetic force (εmf)  induced current i. ΦB = ΣBΔAcosθ = Bly ii. εind = -N (ΔΦB/Δt) = -NBL(Δy/Δt) = -NBLv iii. Iind = εind/R = -NBLv/R 1. Causes forces on sides of coil inside B; forces up and down cancel; net force to the lef iv. Fnet = NILBsinθ = N[(NBLv)/R]LB = (N2B2L2v)/R (to the lef) b. Inside field: no change in flux  no εmf or current induced c. Leaving field: flux decreasing  induced εmf  induced current in opposite direction as entering i. Net force also to the lef with same magnitude III. Lenz’s Law: the polarity of an induced εmf is such that it produces a current whose magnetic field opposes the change in magnetic flux through the loop a. Ex) Conductive bar moving in a circuit in a magnetic field b. Ex) Circuit next to a wire carrying a rapidly decreasing current Phys 012 1st Edition

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