p240_ct19_f07 - Physics 240 Fall 2007 Lecture #19 Dr. Dave...

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Physics 240 Fall 2007 Lecture #19 Dr. Dave Winn 2405 Randall Lab winn@umich.edu
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Note about a problem from MP When can you use Ampere’s law to find a magnetic field? To find B, you must be able to pull it out of the integral on left. To do this, you choose a path where either: 1. B is constant and along dl 2. B is zero, or perpendicular to dl In both cases you infer this solely from symmetry… = enclosed I l d B 0 μ r r
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Note about problem from MP What’s the field inside the loop? It points up. It gets weaker as you go up or down away from the loop It changes magnitude when you move away from the center or toward a corner Since it is no constant in magnitude or direction along any path, it is impossible to use this to determine B.
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Reminder of induction basics Define magnetic flux (like electric flux): Faraday’s Law for the induced EMF in a loop: Lenz’s law: EMF will be induced to resist the change in flux Can induce EMF by: Changing field Changing area Changing θ Anything that changes magnetic flux through a loop induces EMF around the loop = Φ A d B B r r dt d B Φ = ξ
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A circular loop of wire undergoes thermal expansion while it is in a constant magnetic field. If the current induced in it is in the clockwise direction, what way is the field pointing? 1: To the right 2: To the left 3: Up the page 4: Down the page 5: Out of the page 6: Into the page I Current produces a field to oppose the change in flux! Field produced is into page, must be counteracting an out field
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p240_ct19_f07 - Physics 240 Fall 2007 Lecture #19 Dr. Dave...

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