Topic 12 ~ Magnetism and the Motor Effect

• Like poles


  Repel (North-North, South-South)
• 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, ie. Magnetism must be induced
• Making temporary magnets


  1. Stroking with a permanent magnet

  2. Aligns all domains in the material in the same direction, creating a temporary magnet

  3. Electromagnets use temporary magnetic material in their core
• After time, or after a knock

  Magnetism will be lost as the domains move into random positions
• Magnetic Fields


  • Field Lines point from North to South
  • Field strength decreases with distance from the magnet
  • Direction always points to south pole and away from north pole, at any point
• Plotting Compasses


  Small compasses which show the direction and shape of a magnetic field at a given point
• Earth's Core


  • The 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
  • 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
• Magnetic field strength


  • 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
• Factors affecting strength of solenoid 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
  • The force experienced on the conductor is equal and opposite to the force felt on the magnet
• Magnetic forces


  Felt due to interaction between any two magnetic fields
• Visualising magnetic force on a wire

  1. Fixed permanent magnets have field lines along the x axis

  2. Wire is along the y axis, where current is moving up

  3. The Force felt on the wire is at right angles to both the direction of the current and magnetic field lines along the z axis
• Fleming's Left Hand Rule


  • Each component (force, field, current) 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
• Magnetic Flux Density


  • Measured in Tesla [T]
  • The number of flux lines per metre squared
• Motors


  • A coil of wire in between two permanent magnets
  • Current flows through the wire, and the magnetic field it produces interacts with the magnets
  • One side of the coil 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