Lecture 9 Electromagnetic Induction

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Lecture 9Electromagnetic Induction
Chapter 20.1 ? 20.4
Outline

• Induced Emf
• Magnetic Flux
• Faradays Law of Induction

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Electromagnetic Induction
If an electric current produces a magnetic field,
then a magnet should be able to generate an
electric current.
A current is produced in a wire when there is
relative motion between the wire and a magnetic
field.
Such a current is called an induced current
(emf). The effect is called electromagnetic
induction (discovered by Michael Faraday in the
XIX century). The strength of the current depends
on the magnetic field strength and the wires
speed.
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Magnetic Flux
The emf is induced by the magnetic flux, rather
than the magnetic field.
Magnetic flux is a measure of the total number of
magnetic lines passing through an area.
Consider a loop of wire in the presence of a
uniform magnetic field B. If the loop area is A,
then the magnetic flux ?B through it is defined
as
B? is the component of B, perpendicular to the
plane of the loop
?B B?A BA cos ?
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Faradays Law of Induction
The induced emf exists only when there is a
relative motion between the magnet and the
circuit.
Faradays law states that the instantaneous
induced emf equals the rate of change of magnetic
flux through the circuit.
If a circuit has N loops and the flux through
each loop changes by the amount of ??B during
interval of time ?t, the average emf E induced in
the circuit during this time is
E ? N ??B/?t
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Polarity of the Induced Emf
The induced emf produces an induced current. The
induced current produces its own magnetic
field. The induced magnetic field opposes the
change of the external magnetic field.
The polarity (direction) of the induced emf is
determined by Lenzs law.
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Motional Emf
Let us consider an application of Faradays law
to a conductor of length l moving at a constant
velocity v through a uniform magnetic field.
? Magnetic force on electrons in the conductor
Picture
F qvB
qE qvB
E is the electric field induced in the conductor
E is uniform ? the potential difference across
the conductor ends ?V E l
E vB
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Summary

• The phenomenon of electromagnetic induction
  refers to appearance of an electric field under
  influence of a magnetic field.
• Faradays law connects the induced emf and time
  change in the magnetic flux.
• Lenzs law defines the polarity of the induced
  emf
• Motional emf is an emf produced in a moving
  conductor under influence of a magnetic field.