Electromagnetic Induction

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Electromagnetic Induction

• The Discoveries of Michael Faraday and Joseph
  Henry Showed That a Current Can Be Induced by a
  Changing Magnetic Field.

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Faradays Law

• An emf is set up in a closed electric circuit
  located in a magnetic field whenever the total
  magnetic flux linking the circuit is changing.
• This is called electromagnetic induction.
• Induced emf
• Induced current

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A Galvanometer and a B-Field

• Induced current is related to the motion of the
  conductor in the magnetic field
• The direction of the induced current is directly
  related to the direction of motion of the
  conductor in the magnetic field
• Direction is dependent on the motion of the
  B-field or the conductor
• The motion of the conductor is perpendicular to
  the B-field

4
More Thoughts

• If the conductor is moved parallel to B-field,
  the galvanometer will show no deflection, thus no
  induced emf or current.
• If the conductor is looped several times, the
  deflection on the galvanometer will increase.
• Thus, the rate of motion, the number of loops and
  the magnitude of the B-field vary directly with
  the magnitude of the induced current and emf.

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Factors Affecting Emf

• Emf induced in each coil of the conductor is
  proportional to rate of change of magnetic flux
• Emf - DF/Dt
• Emf -N(DF/Dt)
• Note that negative sign indicates that induced
  emf is of such polarity as to oppose the change
  that induced it

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Induced EMF

• Induced current exists only if wire in a field is
  part of a circuit.
• Thus, if F BA then F Bxl
• So therefore, Emf - DBxl/Dt
• But x/t v (velocity)
• Therefore, Emf Blv

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Faradays Law

• The induced emf in a coil of N loops with a
  changing B field within in equal to the rate of
  change of magnetic flux times the number of loops.

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Lenzs Law
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• The net B-field thru a loop from a changing flux
  comes from two sources.

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• The original B field.
• The B field caused by the original currentthe
  induced B field.

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Induced B Field

• This occurs because of the induced current.
• This can be used to determine the polarity of the
  induced emf.

13
Lenzs Law

• 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 B field opposes the original flux
  change.