magnetic fields

magnetic fields

1. electromagnets

   • magnetic fields exist around magnet
   • magnetic elements (eg iron) will attract to both poles
   • when current flows through an insulated wire, it becomes an electromagnet
     • around wire = electromagnetic field
     • eg. wrap insulated wire round iron (core). iron becomes magnetised
   • creating stronger field stregth
     • increase current
     • add iron core
     • increase number of coils
     • increase surface area
   • uses
     • scrapyard cranes
     • circuit breakers
     • electric bells

2. the motor effect

   • force = maximum when wire at 90degrees to magnetic field = zero when parallel to magnetic field
   • Fleming's left hand rule
     • thuMb = Movement
     • seCond finger = Current
     • First finger = Field
   • size of force increased by:
     • increase magnetic field strength
     • increase size of current
   • current flows through wire - magnetic field produced
     • electromagnetic field placed within permanent magnetic field
       • two fields interact + conductor experiences force (moves)
   • direction of force reversed if direction of either current or magnetic field is reversed
     • split-ring commutator reverses direction of current every half turn
       • because sides swap over each half turn, coil always pushed in same direction

3. electromagnetic induction

   • if magnet moved into coil of wire, pd is induced across ends of coil
     • electrical conductor (coil) 'cuts through' magnetic field, inducing pd
     • pd only induced during movement
     • if direction of movement, coil or polarity of magnet is reversed, direction of pd also reversed
   • size of induced pd increased by increasing:
     • speed of movement
     • strength of magnetic field
     • number of turns on coil

4. transformers

   • iron core
   • ac flows into primary coil + alternating electromagnetic field in core produced
     • passed to secondary coil + coil cuts through magnetic field
       • pd is induced + ac exits
   • wire = insulated so current doesn't short across either core of other turns
   • national grid
     • step up transformers - more turns on secondary coil
     • step down transformers - less turns on secondary coil
     • the higher the pd, the smaller the energy wasted
   • switch mode transformers
     • operate at higher frequency
     • lighter and smaller
     • use little power when no device connected to output

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