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Lecture 12.1 :Electromagnetic Induction
Lecture Outline:Induced Current
Motional emfMagnetic Flux
Lenz’s Law
April 2, 2013
Textbook Reading:Ch. 33.1 - 33.4
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Announcements
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•No new homework this week.
•Exam #3 is Thursday. Covers Ch. 30-32. Remember your calculator and one sheet of notes.
•Review session for Exam #3 will be offered tonight, from 7-9pm in Stolkin. Bring your clickers.
‣NOTE: If you are in the Tue. lab section (from 6:30-8:30pm), you can go to another lab section this week, or you can join the review session late. Apologies for this overlap (again).
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Last Lecture...
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�Fwire = I�l × �B
�Fon q = q�v × �B
q = I∆t = Il
v
|�Fwire| = IlB sinα Direction from right-hand rule.
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Last Lecture...
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Last Lecture...
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A current loop placed in an external magnetic field will rotate.
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Induced Current
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Faraday observed that a changing magnetic field creates an induced current.
Closing the switch creates a current in top loop, which causes a changing magnetic field in the other loop.
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Induced Current
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Faraday observed that a changing magnetic field creates an induced current.
Changing magnetic field can also be due to relative motion.
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Motional emf
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Motional emf
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An electric field builds up in the moving conductor. It reaches maximum strength when the force due to the electric field exactly balances the magnetic force.
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Clicker Question #1
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A metal bar moves through a magnetic field. The induced charges on the bar are
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Motional emf
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Imagine sliding a bar over a conducting rail located in a magnetic field. A current is induced.
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Motional emf
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Force is required to maintain a constant speed of the moving wire. We can study the energy in the circuit.
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Clicker Question #2
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An induced current flows clockwise as the metal bar is pushed to the right. The magnetic field points
A. Up.B. Down.C. Into the screen.D. Out of the screen.E. To the right.
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Motional emf
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What happens to the bar if we add a potential (battery) across the two rails, as shown?
It would appear such a scheme will accelerate the bar arbitrarily fast...making a “rail gun”! We will study next week why there are limitations to the speed.https://www.youtube.com/watch?v=-uV1SbEuzFU
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Magnetic Flux
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It is once again useful to introduce the idea of the “flux” of field (magnetic) passing through a current loop.
Units of magnetic flux:1 weber = 1 Wb = 1 Tm2
Φm = �A · �BFor a uniform magnetic field
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Clicker Question #3
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Which loop has the larger magnetic flux through it?
A. Loop A.B. Loop B.C. The fluxes are the same.D. Not enough information ! to tell.
Φm = L2B
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Lenz’s Law
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We’ve seen that a changing magnetic flux through a loop induces a current in that loop, but we don’t yet have a way of defining the direction the induced current flows.
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Lenz’s Law
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Lenz’s Law
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Lenz’s Law
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Reminders
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•Exam #3 Thursday.•Review session tonight from 7-9 pm in Stolkin.