Topic 13: EM Induction

• Electromagnet Induction - a current is induced in a wire if it is moved to cut across a magnetic field

  • if current moves along field lines, current is neutralised as nothing pushes electrons (no current without movement)

  Current can be increased by using a stronger magnetic field and moving the wire faster
• Faraday's Law - induced voltage (positive current) can be increased by using a stronger magnet, moving the magnet faster and increasing the number of turns on the coil
• Lenz's Law - the direction of the induced current is such that it opposes the change producing it

  • when N-pole is moving in, the current flows to produce a N-pole which repels the magnet
  • when N-pole is moving out, the current flows to produce a S-pole which attracts the magnet
• Bicycle Dynamo - a magnet rotates near a coil of wire so that the lines of flux are cut by the wire
• Loudspeaker:

  • moveable coil is attached to a a large cone
  • cone fits loosely over the centre of a cylindrical permanent magnet so that the coil is in a strong magnetic field
  • current flows so coil moves inwards according to Fleming's Left Hand rule
  • current causes the coil and cone to vibrate rapidly (electric converted into sound energy via vibrations)

  A microphone does the opposite of this process, turning sound into electric energy
• Transformers are used to increase or decrease the voltage of a power supply:

  • two coils are connected by a magnetic field, meaning there is an induced alternating current in the secondary coil
  • primary coil - potential difference, current and magnetic field are alternating
  • this induces an alternating magnetic field in the iron core which induces a potential difference in the secondary coil
• Step Up Transformers - more turns in the secondary coil than the primary one (larger voltage)

  Step Down Transformers - more turns in the primary coil than the secondary one (smaller voltage)
• National Grid:

  • to transfer energy from power stations to houses, either low voltage and high current OR high voltage and low current is used
  • former is wasteful as high currents heats the power lines
  • High voltage and low current is used with transformers at each end of the grid to change voltage
  • alternating current (AC) is used because direct current (DC) doesn't work with transformers