Topic 13 ~ Electromagnetic Induction

• Current induction


  Current is induced if a wire is moved in a magnetic field
• Current induction


  1. Conductor (wire) forms a potential difference (electrons move to one side of the conductor as the field changes)

  2. If the conductor is connected in a circuit, a current will flow

  3. This current will produce its own magnetic field

  4. The direction of this new field is in the opposite direction to the first field (i.e. it opposes the original change)
• Small-scale current production


  • Spinning a coil of wire in between two permanent magnets will cause a current to flow in the wire, which can be shown by a sensitive ammeter (only milliamps will be generated)
  • Passing a wire through a field will also show a deflection in an ammeter (a reading)
• Large-scale current production


  1. Water heats up and evaporates to form steam (combustion of fossil fuels / nuclear fission may cause this)

  2. The steam is put under pressure and forced into a turbine

  3. This causes the turbine to rotate, which is connected to a massive coil of wire in a strong magnetic field (the generator)

  4. Current is generated in the coil by the spinning motion of the coil through the field
• Factors affecting size of current/voltage produced

  • Number of coils of wire
  • Speed of rotation
  • Magnetic field strength
• Alternator


  • Every half turn, the current switches direction, as the wire will be in the opposite orientation compared to its starting position
  • This produces AC
• Dynamo


  • At end of the coil, there is a 'commutator' (a metal ring that reverses the sign of the current that flows from the coil – ensuring current output remains positive)
  • Every half-turn the commutator switches the sign of the current, so it remains positive
  • This produces DC
• Microphones and Loudspeakers


  1. Produce a current which is proportional to the sound signal

  2. Fixed magnet is at the centre, and the coil of wire around the magnet is free to move

  3. Pressure variations in the sound waves cause the coil to move, and as it moves current is induced in the coil (because it passes through the magnetic field)

  4. This current is then sent to a loudspeaker

  5. Current flows into the coil

  6. The magnetic field from magnet and from current interact, causing the coil to move

  7. The cone therefore moves, producing pressure variations, making sound
• Transformers


  1. AC in first coil creates a changing magnetic field

  2. This changing magnetic field cuts through the secondary coil

  3. This induces a current in the secondary coil (which is also AC)

  4. If primary current was DC, magnetic field produced will be constant, not inducing anything in the secondary coil

  5. More coils on secondary? Step up transformer, as voltage will be increased, as changing field will cut through more of the secondary wire inducing a larger pd

  6. Fewer coils on secondary? Step down transformer, as smaller pd forms on secondary
• National Grid


  • Electrical energy is transferred at high voltages from power stations
  • In domestic uses, electrical energy is transformed to lower voltages (to improve the efficiency of the transmission - the larger the current, the greater the heating effect occurs in wires, so increasing the voltage out of the power station reduces the current)
  • High voltages are very dangerous, so when it is close to towns (etc.) the voltage is decreased (and so current increases) to make it safer to use