Topic 6 - Electric and Magnetic Fields

• magnetism

  • like poles repel
  • opposites attract
  • magnetic materials are typically cobalt, steel, iron, and nickel
  • permanent magnets - always have magnetic and always have poles
• induced magnets

  • materials that are magnetic but dont have fixed poles - magnetism must be induced
  • can be made into temporary magnets by 'touching' them with a permanent magnet
  • this aligns all domains in the material in the same direction - creates temporary magnet - electromagnets use temporary magnetic material in their core
  • after time or a knock - domains move into random positions - magnetism lost
• magnetic fields

  • field lines point from n to s
  • field strength decreases with distance from magnet
  • direction always points to south pole away from north pole - at any point
  • plotting compasses - small compasses which show direction and shape of magnetic field at any given point
• earths core

  • magnetic - creates large magnetic field around earth
  • we know this because a freely suspended magnetic compass will align itself with the earths field lines and point n
  • compass is basically a suspended bar magnet with its own n pole lining up with earths n pole - wrong as like poles repel - earths magnetic pole in n is a magnetic s pole
• current

  • produces magnetic field around wire
  • direction dictated by 'right hand rule'
  • plotting compasses on piece of paper through which a wire is placed show this
  • current direction is perpendicular to magnetic field direction
• strength of magnetic field

  • current size - greater current, stronger magnetic field
  • distance from conductor - greater distance from wire, weaker field
• solenoids

  • magnetic field shape is similar to bar magnet
  • coiling wire causes field to align and form giant single field along centre of solanoid
  • iron core in centre - increases strength - easier for magnetic field lines to pass through than air
  • fields from individual coils cancel inside - produce weaker field outside the solanoid
• factors that affect strength of field

  • size of current
  • length
  • cross sectional area
  • number of turns (coils)
  • using soft iron core
• wire with current near a magnet

  • current produces magnetic field - interacts with magnets field
  • force experianced on conductor is equal and opposite to the force felt on magnet
  • magnetic forces are felt due to interaction between any two magnetic fields
• force

  • two magnets will interact feeling magnetic force of attraction / repulsion
  • magnet and wire will also exert a force - two magnetic fields will interact
  • magnetic field around wire is circular - the one between two magnets is straight
  • when two interact - wire will be pushed away from the field between poles (at right angles to wire direction and field direction)
• flemings left hand rule

  • use this to work out unknown factor of the three
  • thumb - thrust / motion
  • index - field
  • middle - current
• force = magnetic flux density X current X length
• factors that affect size of current / voltage

  • number of coils of wire
  • speed of rotation
  • magnetic field strength
• transformers

  • alternating current in first coil creates changing magnetic field
  • changing magnetic field cuts through secondary coil
  • this induces current in secondary coil - also ac - if primary current was dc magnetic field produced would be constant
• national grid

  • electrical energy is transferred at high volatages from power station
  • in domestic uses - electrical energy transformed to lower voltages
  • done to improve efficiency of transmission
  • larger the current - greater the heating effect occurs in wires
  • means large current - lots of energy lost
  • as P=IV and power is constant - increases voltage out of power station so reduces current
  • when close to towns voltages decreased - current increases - safer to use - makes sure less energy is lost
• step up transformer

  increase voltage and reduce current
• step down transformer

  decrease voltage and increase current