Magnetism

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Magnetism
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Like Electricity, Magnetism has Force effects

• North and South Poles
• North and North - Repel
• South and South - Repel
• North and South - Attract
• Magnetic Fields Show lines of Force
• Force depends on strength of Field - B

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Magnetic Attraction

• A north pole allowed to freely rotate will point
  North(North pole is a North Seeking pole)
• Magnetic Fields go from N to S
• A compass is just a magnet whose north pole
  points north
• Magnetic field is B measured in Teslas (T)

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Magnetic Domains and Permanent magnets

• Certain materials allow their electrons to line
  up in special ways
• Ferro magnetism is the strong magnetism of
  certain metals
• Iron, Cobalt and Nickel all exhibit
  ferromagnetism and can be picked up by magnets
• If all domains are aligned an magnet is formed
• A moving electron is current, so currents must
  produce magnetism

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Current and Magnetic Fields

• If current flows through a wire then a magnetic
  field is formed around the wire
• The magnetic field is at 90 degrees
  (perpendicular) to the flow of current
• The direction of the field, N to S, depends on
  the direction of the current- use the right hand
  rule

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Force on a wire in a magnetic field

• F BIL
• Right Hand Rule to find direction

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Force on a charged Particle

• F Bqv
• Right hand rule to find the direction

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Representing a perpendicular magnetic field

• B is a vector and vectors use arrows for
  direction
• are arrow points coming out of the page
• xxxxxxxxx are the back of the arrows going into
  the page

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Making an electromagnet

• wire loops create a strong B field in the loop
• Use the second right hand rule to find the
  direction of the field

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Lenzs Law

• When a magnet or magnetic field changes, current
  will flow to create a magnet that opposes the
  change
• Use the right hand rules to find the current
  direction

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Motors

• If you keep switching the polarity of an
  electromagnet, another magnet can be made to
  chase the changing field
• A spilt ring commutator allows the electromagnet
  to switch polarity every 1/2 turn
• A DC motor turns electrical energy into
  mechanical energy
• If the motor is turned it produces a current -
  the opposite of a motor is a generator
• As a motor spins faster it generates a back EMF
  that regulates the speed

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Generators

• Moving a wire in a magnetic field generates
  current
• EMF Blv
• As current flows in the wire a magnetic field is
  formed that opposes motionThe force pushing pack
  is proportional to the moving force resulting in
  a constant velocity
• If the wire makes a complete circle then it moves
  up and down in a magnetic field - this will
  produce an AC voltage and current
• If a Split ring is used, the connection is
  switched every 1/2 turn to produce a constant
  direction or DC voltage and current.
• Motors change electrical energy into mechanical
  energy - Generators change mechanical energy into
  electrical energy

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Transformers

• As Current flowing through a wire increases the
  magnetic field increases
• Increasing and decreasing current (AC voltage
  applied) make a moving magnetic field
• A wire in a moving magnetic field generates a
  current.
• Placing the wires near each other by using a
  special core allows for AC in and AC out
• Maximum power transfer is 100 - Pin P out or
  VpIp VsIs
• Vp/Vs number turns on primary/number of turns
  on secondary
• Voltages step up or step down

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Distribution of Power

• Wires lose energy as current flows through them
  in the form of heat
• Power companies use transformers to step up
  voltage and use thin wires with little current
• At the consumer end, a transformer steps down the
  voltage to 240 (120 each leg) and delivers a
  large current
• AC voltages and transformers make long distance
  power distribution possible