Physics 212 Lecture 17, Slide * Main Point 3 Finally, we

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Physics 212 Lecture 17
Faradays Law
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Main Point 1

• First, we introduced the concept of the magnetic
  flux and found that the motional emfs produced in
  the three examples from the last prelecture could
  all be written simply as the time rate of change
  of the magnetic flux through the circuit.

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Main Point 2

• Second, we introduced Faradays Law, which states
  that whenever magnetic flux changes in time, not
  just in the case of a moving conductor, an emf
  will be produced. In particular, this induced
  emf will just be equal to minus the time rate of
  change of the magnetic flux.

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Main Point 3

• Finally, we observed that this induced emf is
  determined by integrating the electric field
  around the loop, so that Faradays law can be
  written more generally only in terms of the
  electric and magnetic fields. A changing
  magnetic flux creates an electric field.
  Faradays law represents the first important step
  in establishing the deep connections between
  electric and magnetic fields which ultimately
  will explain the existence of electromagnetic
  waves and the identification of light as an
  electromagnetic phenomenon.

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Faradays Law
Looks scary but its not its amazing and
beautiful !
A changing magnetic flux produces an electric
field.
Electricity and magnetism are on intimate terms
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Faradays Law
where
In Practical Words 1) When the flux FB through
a loop changes, an emf is induced in the loop.
B
There are many ways to change this
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Faradays Law
In Words 1) When the flux FB through a loop
changes, an emf is induced in the loop. 2) The
emf will make a current flow if it can (like a
battery). 3) The current that flows induces a new
magnetic field. 4) The new magnetic field opposes
the change in the original magnetic field.
B
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The Ways Flux Can Change
Change Area
Change magnetic field
Change orientation
ALL THESE CHANGES CAN BE UNDERSTOOD FROM MOTIONAL
EMF
WHATS NEW WITH FARADAY?
Flux can change WITHOUT moving any conductor !!
e.g., change current that produces magnetic field
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Faradays Law
Executive Summary
emf?current?field a) induced only when flux is
changing b) opposes the
change
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Checkpoint 1a
A copper loop is placed in a uniform magnetic
field as shown. You are looking from the right.
Suppose the loop is moving to the right. The
current induced in the loop is A. zero B.
clockwise C. counterclockwise
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Checkpoint 1b
A copper loop is placed in a uniform magnetic
field as shown. You are looking from the right.
Checkpoint 1b
Now suppose the that loop is stationary and that
the magnetic field is decreasing in time. The
current induced in the loop is A. zero B.
clockwise C. counterclockwise
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Checkpoint 1c
Now suppose that the loop is spun around a
vertical axis as shown, and that it makes
one complete revolution every second.
The current induced in the loop A. Is zero B.
Changes direction once per secondC. Changes
direction twice per second
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Checkpoint 2
A horizontal copper ring is dropped from rest
directly above the north pole of a permanent
magnet
(copper is notferromagnetic)
Will the acceleration a of the falling ring in
the presence of the magnet be any different than
it would have been under the influence of just
gravity (i.e. g)? A. a gt g B. a g C. a lt g
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Calculation
y
A rectangular loop (height a, length b,
resistance R, mass m) coasts with a constant
velocity v0 in x direction as shown. At t 0,
the loop enters a region of constant magnetic
field B directed in the z direction. What is
the direction and the magnitude of the force on
the loop when half of it is in the field?
B
x x x x x x x x x x x x x x x x x
x x x x x x x x x x x
b
a
v0
x
Conceptual Analysis
Strategic Analysis
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