Transformers

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3.4 Transformers

  • A transformer consists of two coils of insulated wire, called the primary coil and the secondary coil.
  • These coils are wound on to the same iron core.
  • When an alternating current passes through the primary coil, it produces an alternating magnetic field in the core. This field continually expands and collapses
  • The alternating magnetic field line passes through the secondary coil and induce an alternating potential difference across its ends.
  • If the secondary coil is part of a complete circuit, an alternating current is produced.
  • The coils of wire are insulated so that the current does not short across either the iron core or adjacent turns of wire, but flows around the whole coil. The core is made of iron so it is easily magnetised.
  • Transformers are used in the National Grid
  • A step-up transformer makes the pd across the secondary coil greater than the pd across the primary coil. Its secondary coil has more turns than its primary coil.
  • A step-down transformer makes the pd across the secondary coil less than the pd across the primary coil. Its secondary coil has fewer turns than its primary coil.
  • A switch mode transformer has a ferrite core. A switch mode transformer operates at a much higher frequency, is lighter and smaller, and uses very little power when there is no device connected across its output terminals.
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3.5 Transformers in action

  • The National Grid uses transformers to step-up the pd from power stations.
  • This is because the higher the pd at which electrical energy is transmitted across the Grid, the smaller the energy wasted in cables.
  • Step-down transformers are used to reduce the pd so that it is safe to be used by consumers.
  • A step-down transformer has less turns on the secondary coil than on the primary coil.
  • A step-up transformer has more turns on the secondary coil than on the primary coil.
  • Transformers are almost 100% efficient.

 

 

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