Topic 13 - Electromagneticism

• Generating a current


  1. Magnet moves relative to a conductor

  2. Coil of wire moves or spins in a magnetic field
• Direction of current


  Depends on the commutator
• Generating a current in a coil


  Magnetic field lines cut through the coil of wire
• Faster magnet movement


  Greater current induced
• Fewer turns of wire

  Smaller current induced
• Alternating current generation


  1. Magnetic poles constantly reversed

  2. Electrons driven backwards and forwards
• Faster pole reversal


  Greater frequency of AC
• Simple generator


  • Water wheel
• Faster wheel turns


  Greater current generated
• Induced potential difference


  • Can be increased by: moving magnet faster, more turns of wire, stronger magnet
• Induced current


  Generates its own opposing magnetic field
• Electromagnetic braking


  • Aluminium plate swinging in magnetic field
  • Solid aluminium sheet
• Electromagnetic braking


  Induced current produces magnetic field which slows down the motion
• Magnetic levitation


  • Metal rings jumping in magnetic field
  • Continuous ring
  • Ring with single slit
• Future high-speed trains could use magnetic levitation to reduce global warming effects
• Alternator


  Produces alternating current
• Dynamo


  Generates direct current
• Induced current is used in microphones and loudspeakers
• Transformer


  Only works with alternating current
• Transformer operation

  1. Alternating potential difference drives alternating current in primary circuit

  2. Alternating magnetic field induces alternating current in secondary circuit
• Transformer efficiency


  Not 100% efficient, but calculations assume primary power equals secondary power
• Transformers are essential for the national grid to distribute electricity efficiently
• Higher voltage distribution

  Lower current, less power loss
• Power loss is proportional to the square of the current
• Electricity distribution in the national grid


  1. Step-up transformers increase voltage

  2. Step-down transformers reduce voltage to safe levels