Lecture 12.1 :
Electromagnetic Induction
Lecture Outline:
Induced Current
Motional emf
Magnetic Flux
Lenz’s Law
Textbook Reading:
Ch. 33.1 - 33.4
April 2, 2013
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Announcements
•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...
F�on
q
�
= q�v × B
q = I∆t = I
l
v
�
�
�
Fwire = I l × B
|F�wire | = IlB sin α
3
Direction from
right-hand rule.
Last Lecture...
4
Last Lecture...
A current loop placed in an external magnetic field will rotate.
5
Induced Current
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.
6
Induced Current
Faraday observed that a changing magnetic field
creates an induced current.
Changing magnetic field can also be due to relative motion.
7
Motional emf
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Motional emf
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
A metal bar moves through a magnetic field. The
induced charges on the bar are
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Motional emf
Imagine sliding a bar over a conducting rail located in a
magnetic field. A current is induced.
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Motional emf
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
An induced current flows
clockwise as the metal bar
is pushed to the right. The
magnetic field points
A.
B.
C.
D.
E.
Up.
Down.
Into the screen.
Out of the screen.
To the right.
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Motional emf
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
It is once again useful to introduce the idea of the “flux” of
field (magnetic) passing through a current loop.
For a uniform
magnetic field
Φm
�
�
=A·B
Units of magnetic flux:
1 weber = 1 Wb = 1 Tm2
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Clicker Question #3
Which loop has the larger
magnetic flux through it?
Φm = L2B
Loop A.
B. Loop B.
C. The fluxes are the same.
D. Not enough information
! to tell.
A.
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Lenz’s Law
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
•Exam #3 Thursday.
•Review session tonight from 7-9 pm in Stolkin.
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