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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

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).

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.

Last Lecture...

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Last Lecture...

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A current loop placed in an external magnetic field will rotate.

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.

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.

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.

Clicker Question #1

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A metal bar moves through a magnetic field. The induced charges on the bar are

Motional emf

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Imagine sliding a bar over a conducting rail located in a magnetic field. A current is induced.

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.

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.

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

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

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

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.

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.