Topic 12 - Magnetism and the motor effect

• Like poles repel (North-North, South-South)
• Opposite poles attract
• Magnetic materials

  • Cobalt
  • Steel
  • Iron
  • Nickel
• Permanent Magnets

  • Always magnetic, always have poles
  • Used in speakers, compasses, and electric generators
• Induced Magnets

  • Materials that are "magnetic" but do not have fixed poles
  • Can be made into temporary magnets by 'stroking' them with a permanent magnet
  • After time or after a knock, the domains move into random positions, so magnetism will be lost
• Creating temporary magnets

  'Stroking' a material with a permanent magnet aligns all domains in the material in the same direction, creating a temporary magnet
• Magnetic Fields

  • Field Lines point from North to South
  • Field strength decreases with distance from the magnet
  • Direction always points to the south pole and away from the north pole at any point
  • Plotting Compasses show the direction and shape of a magnetic field at a given point
• The Earth's core is magnetic and creates a large magnetic field around the Earth
• A freely suspended magnetic compass will align itself with the Earth's field lines and point North
• A compass is effectively a suspended Bar Magnet, with its own north pole lining up with Earth's North pole
• Earth's magnetic pole in the north is a magnetic South Pole and the geographic south pole is close to the magnetic North Pole
• Current

  • Produces a magnetic field around a wire
  • Direction is dictated by the "right hand rule"
  • Current direction is perpendicular to the magnetic field direction
• Strength of Magnetic Field

  • Depends on current size; Greater current, stronger magnetic field
  • Varies with distance from the conductor; Greater distance from wire, weaker field
• Solenoids

  • Magnetic Field Shape is similar to a bar magnet
  • Coiling the wire causes the field to align and form a giant single, almost uniform field along the centre of the Solenoid
  • Having an iron core in the centre increases its strength as it is easier for magnetic field lines to pass through than air
  • The fields from individual coils cancel inside to produce a weaker field outside the solenoid
• Solenoid

  • Almost uniform field along the centre
• Solenoid with iron core

  • Increases strength as it is easier for magnetic field lines to pass through than air
• Solenoid

  • Fields from individual coils cancel inside to produce a weaker field outside the solenoid
• Factors affecting strength of field

  • Size of current
  • Length
  • Cross sectional area
  • Number of turns (coils)
  • Using a soft iron core
• Wire with a current near a magnet

  The current produces a magnetic field, which interacts with the magnet’s field
• Wire with a current near a magnet

  The force experienced on the conductor is equal and opposite to the force felt on the magnet
• Two magnets

  Interact, feeling a magnetic force of attraction/repulsion
• Magnet and a wire

  Exert a force as the two magnetic fields (generated by the magnet and the current in the wire) interact
• Magnetic field around a wire

  Is circular, but the magnetic field between two magnets is straight
• Wire and two magnets interact

  The wire will be pushed away from the field between the poles (at right angles to the wire direction and the field direction)
• Fleming’s Left Hand Rule: Each component is at 90⁰ to the others. Use this to work out the unknown factor out of the three (usually the direction of the force felt). Remember current is conventional current (motion of positive charge), which moves in opposite direction to electron flow.
• Formula: Force = (magnetic flux density) × (current) × (length). F = BIL. Where Magnetic Flux Density is measured in Tesla [T] - The number of flux lines per metre squared
• Motor

  • A coil of wire in between two permanent magnets
• Current flows through the wire in a motor

  The magnetic field it produces interacts with the magnets
• One side of the coil in a motor


  Gets forced down, the other side gets forced up. This causes the coil to rotate
• Use the Left Hand Rule to verify which side moves up or down