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Electromagnetic Induction
Chapter 22
22.1 induced EMF and Induced
Current
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If a coil of conducting material and a magnet
are moving (relative to one another) an
electric current can be induced.
The changing magnetic field forces the
electrons to move one direction or another.
Since a source of emf is needed to produce a
current, the coil itself behaves as if it were a
source of emf; in this case an induced emf.
How to induce emf
Move a bar magnet near a coil
 Move a magnet away from a coil
 Move a coil toward a magnet
 Change the area of a coil in a constant
magnetic field
 MUST have either magnet or coil moving
 MUST have a closed circuit or coil
For I to exist
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22.2 Motional EMF
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Moving a conducting rod through a constant
magnetic field can separate the positive and
negative charges existing within the rod.
Once all possible charge movement has
taken place, the separated charges cause an
induced or motional emf.
Emf exists as long as the rod moves
Electric and magnetic forces balance at
equilibrium
Motional emf when v, B, and L are
perpendicular
  BL
• ε = 0 when v = 0m/s
• ε is expressed in volts
Motional EMF and Electrical Energy
Magnetic force acts on charges in a
conductor → motional emf → emf causes
a current → second magnetic force is
created
 Since emf cause a current that is
perpendicular to B, a force that would
slow down the rod is generated.
 A counterbalancing force must be applied
to the rod by an external agent.
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Energy from Motional EMF
The external force sill do work in such a
system.
 Word done is equal to the electrical
energy of the bulb.
 Remember: F = ILBsinθ can be used as a
force in both F=ma and
W=
Fxcosθ.
 This concept follows the law of
conservation of energy.
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22.3 Magnetic Flux: Motional EMF
and Magnetic Flux
Magnetic Flux: product of magnetic field
and area (BA)
 Symbolized by Greek letter phi
 Φ = BA
 magnitude of the induced emf is the
change in flux divided by the time interval
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𝜀=
∆Φ
∆𝑡
(-) appears in this equation b/c
the direction of induced I is such
that magnetic F acts on the rod to
oppose its motion, therefore
slowing down the rod
A General Expression for φ
Φ = 𝐵𝐴 𝑐𝑜𝑠φ
 If B or φ are not constant, the average
value of the product Bcosφ is used to
compute the flux.
 Magnetic flux is proportional to the
number of field lines per unit area that
pass through a surface perpendicular to
the lines
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22.4 Faraday’s Law
All hail to Michael Faraday and Joseph
Henry for discovering electromagnetic
induction!!!!
 When there is a change in flux
through a loop of wire, and emf is
induced in the loop.
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∆Φ
𝜀 = −𝑁
∆𝑡
22.5 Lenz’s Law
A method for determining the polarity of
the induced emf is needed so that + and –
terminals can be determined.
 Remember, two factors contribute to the
net magnetic field penetrating a coil of
wire
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◦ Original magnetic field producing changing
flux
◦ Induced current creating its own magnetic
field (induced magnetic field)
Lenz’s Law
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The induced emf resulting from a
changing magnetic flux has a polarity that
leads to an induced current whose
direction is such that the induced
magnetic field opposes the original flux
change.
Reasoning Strategy
Determine whether the magnetic flux that
penetrates a coil is increasing or decreasing.
 Find what the direction of the induced magnetic
field must be so that it can oppose the change in
flux by adding to or subtracting from the original
field
 Once direction of induced magnetic field is found,
use RHR-2 to determine the direction of the
induced current. Then the polarity of the induced
emf can be assigned because conventional current
is directed out of the positive terminal, through
the external circuit, and into the negative
terminal.
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Last minute hints
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Electromagnetic induction is explained well
here.
The induced field is not always opposite to
the external field, because Lenz’s law
requires only that is must oppose the change
in the flux that generates the emf.
To oppose an increase in flux, the direction
of the induced magnetic field must be
opposite to the field of a bar magnet.