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Chapter 20
Electromagnetic Induction
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Induced Electromotive Force
Note the motion of the magnet in each image:
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(PhET Animations)
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Magnetic Flux
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Induced Electromotive Force
Faraday’s experiment: closing the switch in the
primary circuit induces a current in the secondary
circuit, but only while the current in the primary
circuit is changing.
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Faraday’s Law of Induction
An emf is induced only when the magnetic flux
through a loop changes with time.
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Faraday’s Law of Induction
There are many devices that operate on the basis of
Faraday’s law.
An electric guitar
pickup:
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Faraday’s Law of Induction
Microphone:
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AC vs. DC
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Transformers
A transformer is used to change voltage in an
alternating current from one value to another.
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Transformers
By applying Faraday’s law of induction to both coils,
we find:
Here, p stands for the primary coil and
s for the secondary.
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Transformers
The power in both circuits must be the same;
therefore, if the voltage is lower, the current
must be higher.
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Lenz’s Law
An induced current always flows in a direction
that opposes the change that caused it.
Therefore, if the magnetic field is increasing, the
magnetic field created by the induced current will
be in the opposite direction; if decreasing, it will
be in the same direction.
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Lenz’s Law
This conducting rod completes the circuit. As it falls,
the magnetic flux decreases, and a current is
induced.
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Lenz’s Law
The force due to the induced current is upward,
slowing the fall.
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Lenz’s Law
Currents can also flow in bulk conductors. These
induced currents, called eddy currents, can be
powerful brakes.
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Mechanical Work and Electrical Energy
generator demo
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Generators and Motors
An electric motor converts electrical energy into
mechanical energy:
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Generators and Motors
An electric
generator
converts
mechanical
energy into
electric energy:
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Homework
Continue work on fourth quarter project.
First weekly report is due tomorrow.
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