Magnetic Fields and Induced EMFs

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Magnetic Fields and Induced EMFs
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Electromagnetic Induction

• Just as a magnetic field can be formed by a
  current in a circuit, a current can be produced
  using an external magnetic field.
• The process of inducing a current in a circuit
  with a magnetic field is called electromagnetic
  induction

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But How???

• Remember, a moving charge in a magnetic field
  experiences a force.
• Well a charge that is still will also experience
  a force if the magnetic field is moving around
  it.
• The moving magnetic field causes the force on
  charges which will cause them to go around the
  circuit with a current.

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Electromagnetic Induction

• A current can only be produced if the magnetic
  field is changing, or what physicist call, a
  magnetic flux
• There are three ways that a magnetic flux can be
  produced

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Induced Current method 1

• The circuit is moved into or out of a magnetic
  field.
• This is relative, so the magnetic field could be
  moving, the circuit could be moving, or both
  could be moving.

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Induced Current method 2

• The circuit is rotated in the magnetic field.
• The strength of a magnetic field is dependent on
  the angle to the circuit.
• By rotating the circuit, the strength of the
  magnetic field is changing and therefore a
  current can be induced.

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Induced current 3

• The intensity of the magnetic field is varied.
• This change in the strength of the magnetic field
  will cause a current to be induced.

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Faradays Law

• The emf produced is proportional to the change in
  the magnetic flux per unit of time and the number
  of loops in a circuit.
• Or

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Magnetic Flux

• F is the magnetic flux, or how much magnetic
  field is going through an area.
• Picture a hula-hoop in the rain

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Hula-hooping in the rain

• The harder the rain, the more water that will go
  through the hoop.
• The bigger the hula-hoop, the more water that
  will go through the hoop.
• Magnetic flux is the same as this. It is a
  factor of how much magnetic field there is and
  how much room does it have to flow through.

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What if you turn the hoop?

• Lets say you are hula-hooping around your arm.
  Can any rain get through?
• The angle also determines the amount of magnetic
  field that can get through.

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Magnetic Flux

• Therefore
• Where, A is the area of the circuit loop, and B
  is the magnetic field strength

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Faradays Law again

• In total, Faradays Law is

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Summary

• The things that effect electromagnetic induction
  according to Fardays law are
• 1) The angle between the magnetic field and the
  circuit
• 2) The strength of the magnetic field
• 3) The area of the loop of the circuit
• 4) The number of loops in the circuit
• 5) How quickly the magnetic field changes

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• simulation

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Lenzs Law

• What direction is the induced current?
• Lenzs Law states
• The magnetic field of the induced current
  opposes the change in the applied magnetic field.

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Right-hand rule 1 Again

• Remember, a current can induce a magnetic field
  of its own.
• The current that it induces will oppose the
  direction of the magnetic field flux.

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Example

• A circuit of wire is pulled through a magnetic
  field pointed out of the smart board. What is
  the direction of the induced current?

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Solution

• The current induced will be such that it will
  create a magnetic field to go back into the smart
  board.
• Therefore, the direction of the current will be
  clockwise

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Door Bells

• Ever wonder why there is a small light behind the
  button of a door bell??

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Door bell continued

• Pushing the door bell disrupts the current in the
  left circuit.
• This disruption causes there to be a decrease in
  the magnetic field in the left circuit.
• According to Fardays law, this decrease in the
  left circuit, causes a current to be induced in
  the right circuit.

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Door bell continued

• The induced current causes a magnetic field in
  the right circuit directed through the middle to
  the coil.
• The iron bar is pushed outwards and hits the
  chime.

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Door Bell