Topic 13 - Electromagnetic Induction

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

  Wire that forms a potential difference as electrons move to one side when the field changes
• Conductor connected in a circuit

  1. A current will flow

  2. This current will produce its own magnetic field

  3. The direction of this new field is in the opposite direction to the first field (opposes the original change)
• Small-scale production

  1. Spinning a coil of wire in between two permanent magnets will cause a current to flow, shown by a sensitive ammeter

  2. Passing a wire through a field will show a deflection in an ammeter
• Large-scale production

  1. In a thermal power station, water heats up and evaporates to form steam

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

  3. This causes the turbine to rotate, 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 the size of current/voltage produced

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

  1. Every half turn, the current switches direction

  2. Use the Left Hand Rule to show the direction of the current induced switches as it moves up or down

  3. This produces AC
• Dynamo

  1. At the end of the coil, there is a 'commutator' that reverses the sign of the current that flows from the coil

  2. Every half-turn the commutator switches the sign of the current, ensuring it remains positive

  3. This produces DC
• Microphones and Loudspeakers

  1. Produce a current proportional to the sound signal

  2. Pressure variations in the sound waves cause the coil to move, inducing current in the coil

  3. This current is then sent to a loudspeaker
• Producing current proportional to the sound signal

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

  2. Pressure variations in the sound waves cause the coil to move, inducing current in the coil

  3. The current is then sent to a loudspeaker
• Loudspeaker setup

  1. Current flows into the coil

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

  3. The cone moves, producing pressure variations and sound
• Transformers

  1. AC in the first coil creates a changing magnetic field

  2. This changing magnetic field cuts through the secondary coil, inducing a current in the secondary coil (which is also AC)<|More coils on the secondary mean a step-up transformer, increasing voltage<|Fewer coils on the secondary mean a step-down transformer, decreasing voltage
• National Grid

  • Electrical energy is transferred at high voltages from power stations
  • In domestic uses, electrical energy is transformed to lower voltages<|This is done to improve the efficiency of the transmission<|Increasing the voltage out of the power station reduces the current<|Decreasing voltage near towns makes it safer to use and reduces energy loss during transmission