Chapter 34. Electromagnetic Induction

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Chapter 34. Electromagnetic Induction
Electromagnetic induction is the scientific
principle that underlies many modern
technologies, from the generation of electricity
to communications and data storage. Chapter Goal
To understand and apply electromagnetic
induction.
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Chapter 34. Electromagnetic Induction

• Topics
• Induced Currents
• Motional emf
• Magnetic Flux
• Lenzs Law
• Faradays Law
• Induced Fields
• Induced Currents Three Applications
• Inductors
• LC Circuits
• LR Circuits

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Stop to think 34.1 page 1044 Stop to think
34.2 page 1045 Stop to think 34.3 page
1048 Stop to think 34.4 page 1054 Stop to
think 34.5 page 1058 Stop to think
34.6 page 1067 Stop to think 34.7 page 1073
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Faradays Discovery
Faraday found that there is a current in a coil
of wire if and only if the magnetic field
passing through the coil is changing. This is an
informal statement of Faradays law.
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Magnetic data storage encodes information in a
pattern of alternating magnetic fields. When
these fields move past a small pick-up coil, the
changing magnetic field creates an induced
current in the coil. This current is amplified
into a sequence of voltage pulses that represent
the 0s and 1s of digital data.
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Motional emf
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Potential difference along a rotating bar
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Induced Current in a Circuit
P 1046
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Magnetic flux can bedefined in terms of an area
vector
The magnetic flux measures the amount of magnetic
field passing through a loop of area A if the
loop is tilted at an angle ? from the field, B.
As a dot-product, the equation becomes AB cos?
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Example 34.5 Magnetic flux from the current in a
long straight wire (P1051)
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From the Chapter 33. we know the magnetic field
at distance x from a long straight wire
The infinitesimal flux through a little area is
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The Induced current for six different situation
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Tactics Using Lenzs Law
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Inductors

• Capacitors as devices that produce a uniform
  electric field.
• Capacitance C Q/?V
• Capacitor stores potential energy UC 1/2 C(?V)2
  
• Inductors A coil of wire in the form of a
  solenoid is a device that produces a uniform
  magnetic field. An idea inductor is one for which
  the wire forming a coil has no electric
  resistance .
• The magnetic filed inside a solenoid having N
  turn and length l
• We define the inductance L as
• The inductance of the solenoid
• L (solenoid)

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From Faradays Law
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Inductance of a solenoid
The inductance of a solenoid having N turns,
length l and cross-section area A is
Here we also used the magnetic field B (solenoid)

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Energy density
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The potential difference across an inductor
The potential difference across an inductor with
an inductance of L and carrying a current I
measured along the direction of the current is
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C
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P 1070
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LR Circuits
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A current-carrying wire is pulled away from a
conducting loop in the direction shown. As the
wire is moving, is there a cw current around the
loop, a ccw current or no current?

1. There is no current around the loop.
2. There is a clockwise current around the loop.
3. There is a counterclockwise current around the
   loop.