Electromagnetism

• magnetic materials

  iron, steel, nickel, cobalt
• magnetic flux density is a measure of the strength of a magnetic field and measured in tesla (T)
• A magnetic compass contains a small bar magnet and can be used to plot the field around a bar magnet
• right hand grip method (not solenoid)

  thumb- current

  fingers- magnetic field lines
• strength of magnetic field from current in a wire depends on

  size of current

  distance from wire
• A solenoid has a strong uniform field inside it, and a strong magnetic field outside and a field like a bar magnet outside
• Increase strength of solenoid

  add an iron core (electromagnet)

  add more loops

  increase current
• right hand grip method (solenoid)

  fingers- direction of current

  thumb- north
• Electric bell

  1. when the switch is pushed, the electromagnet is magnetised
  2. the electromagnet attracts the armature
  3. the hammer strikes the gong and breaks the circuit
  4. the armature springs back, completing the circuit again and remagnetising the electromagnet
  5. the cycle repeats for as long as the button remains pushed
• The motor effect is caused by the field created by the current interacting with the magnetic field

  the force can be increased by increasing the

  • size of current
  • length of conductor in magnetic field
  • the flux density
• Fleming's left hand rule

  thumb- force

  first finger- magnetic field (n to s)

  second finger- direction of current
• Electric motors

  the commutator ensures the current direction in the coil is always in the same direction
• Loudspeakers use the variations in an alternating current to produce sound waves
• dynamos

  use a split-ring commutator
• in generators, the pd is greatest when the magnetic field and coil are parallel
• Vs x Is = Vp x Ip
• Vp / Vs = np / ns