Magnetism and electromagnetism

• Magnets:

  • North and south poles
  • Same Poles repel
  • Opposite Poles attract
• Permanent magnets:

  • Always magnetic, always have poles
• Induced magnets:

  • Materials that are "magnetic" but do not have fixed poles
• Induced magnets can be made into temporary magnets by stroking them with a permanent magnet

  • These align the domains in the material all in the same direction, creating a temporary magnet
  • Iron, Nickel, Cobalt
• 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
  • Use plotting compasses
• Plotting compasses = small compasses which show the direction of the magnetic field at a certain point
• Earths core is magnetic, and creates a large magnetic field around the Earth
• We know the Earth's core is magnetic because a freely suspended magnetic compass will align itself with the Earth's field lines and point north
• A magnetic compass doesn't point to the geographic North pole - it is over North Canada
• The compass is effectively a suspended Bar Magnet, with its own north pole lining up with Earth's 'North pole'

  • However this cannot be right, as like poles repel
  • So in fact, Earth's magnetic pole above Canada is a magnetic South Pole (and the geographic south pole is close to the Magnetic North Pole)
• • Current produces a magnetic field around the wire
  • The direction is dictated by the "right hand grip rule"
  • Plotting compasses on a piece of paper through which a wire is pierced shows this
• • Greater current, stronger magnetic field
  • Greater distance from wire, weaker 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
• • two magnets Will interact, feeling a magnetic force of attraction/repulsion
  • So 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
• • The magnetic field around a wire is circular, but the magnetic field between two magnets is straight
  • When the two interact, the wire is 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 direction is 90 degrees to each other
  • Use this to work out the unknown factor out of three (usually the direction of the force felt)
  • Remember current is conventional current, which moves in opposite direction to the electrons
• force = magnetic flux density x current x length
• magnetic flux density is measured in Tesla

  • and it is the number of lux lines per metre squared
• How electric motors work:

  • knowledge of structure Is not expected
  • Permanent magnets lie in fixed positions
  • in between, a coil of current-carrying wire lies up on an axis ( force on one side moves that side up. Force on the other side, where current is flowing in opposite direction, moves down. This can be verified issuing Fleming’s left hand rule.)
  • Hence it rotates