Magnetism

• North and South Poles


  Opposite poles attract, same poles repel
• Permanent Magnets


  • Always magnetic, always have poles
• 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
• Magnetic Fields


  • Field Lines point from North to South
  • Strength decreases with distance from the magnet
  • Direction always points to south pole and away from north pole, at any point
• Plotting Compasses are small compasses which show the direction of the magnetic field at a certain 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
  • The compass is effectively a suspended Bar Magnet, with its own north pole lining up with Earth's 'North pole'
  • However, Earth's magnetic pole above Canada is actually a magnetic South Pole
• Current


  • Current produces a magnetic field around the wire
  • The direction is dictated by the "right hand grip rule"

  
• Greater current


  Stronger magnetic field
• Greater distance from wire


  Weaker magnetic field
• Solenoid


  • Magnetic field shape is similar to a bar magnet
  • It enhances the magnetic effect as coiling the wire causes the field to align and form a giant single field, rather than lots of them all perpendicular to the direction of the current
  • Having an iron core in the centre increases its strength as it is easier for magnetic field lines to pass through than air
  • Factors that affect the strength: size of current, length, cross sectional area, number of turns (coils), using a soft iron core
• Motor Effect


  • Two magnets will interact, feeling a magnetic force of attraction/repulsion
  • A magnet and a wire will also exert a force, as the two magnetic fields (generated by the magnet and the current in the wire) will also interact
• Fleming's Left Hand Rule


  • Each direction is 90° to each other
  • Use this to work out the unknown factor out of the three (usually the direction of the force felt)
  • Remember current is conventional current, which moves in opposite direction to the electrons

  
• Magnetic Flux Density


  Measured in Tesla, it is the number of flux lines per metre squared
• How Electric Motors work


  1. Permanent Magnets lie in fixed positions

  2. In between, a coil of current-carrying wire lies on an axis

  3. Force on one side moves that side up, force on the other side (where current is flowing in opposite direction) moves down

  4. Hence it rotates
• Electromagnetic Induction


  • When there is a relative movement between a conductor and a magnetic field, a potential difference is induced across the conductor
  • This happens if the magnetic field changes as well
  • A current flows if the conductor forms a complete circuit
  • This current will produce its own magnetic field, which oppose the change inducing it
• How Electric Generators (dynamos) work

  1. Same setup as a motor, with a coil of wire able to rotate between two permanent magnets

  2. A turbine spins turning the coil of wire

  3. The movement of the wire causes the wire to cut through the magnetic field

  4. It experiences a change in magnetic field

  5. This creates a potential difference

  6. If the coil of wire is connected to a complete circuit, an alternating current (AC) will flow

  7. Direct current (DC) current is produced if the ends are connected to a split ring commutator, which reverses the current each half-rotation so current remains positive
• AC produced by Alternator

  Alternating current
• DC produced by Dynamo

  Direct current
• How Transformers work


  1. AC in first coil creates a changing magnetic field

  2. This changing magnetic field cuts through the secondary coil

  3. This induces a current in the secondary coil

  4. More coils on secondary: Step up transformer, as voltage will be increased

  5. Fewer coils on secondary: Step down transformer, as smaller pd forms on secondary
• How Dynamic Microphones Work


  1. Fixed magnet is at the centre, and the coil of wire around the magnet is free to move

  2. Pressure variations in the sound waves cause the coil to move, and as it moves current is induced in the coil (because it cuts the magnetic field)

  3. This current is then sent to a loudspeaker

  
• How Loudspeakers Work


  1. The current flows into the coil

  2. The magnetic field from magnet and from current interact, causing the coil to move

  3. The cone therefore moves, producing pressure variations and making sound