Energy Transferred by Working
Electricity and Magnetism

Explaining how a transformer works

Teaching Guidance for 14-16 Supporting Physics Teaching

An explanation of several interlinked steps – so take particular care

Wrong Track: The electric current flows around the primary coil… it then goes through the iron core and into the secondary coil.

Right Lines: There is no direct electrical link between the primary and secondary coil. The transformer core does not carry an electric current; it carries a changing magnetic field.

Building up an explanation, step by step

Thinking about the teaching

The explanation for the working of a transformer is a multi–step story that most students will find pretty demanding.

What are the key steps in explaining how a simple transformer works? Let's think about a simple transformer set-up with input and output coils and a changing (alternating) potential difference across the primary.

  1. The changing potential difference drives a changing electric current round the primary.
  2. The changing electric current in the input produces a changing magnetic field: when the current is zero, the field is zero; when the current is at a maximum, the magnetic field is at maximum strength.
  3. The changing magnetic field is carried by the transformer core and linked to the output coil.
  4. The changing magnetic field linking the output induces a changing potential difference across the secondary coil.
  5. The changing potential difference across the secondary coil drives a changing current through that coil.

Two points to bear in mind with this explanation are:

  • There is no electrical connection between input and output coils: the linkage is through the changing magnetic field.
  • Even though the explanation involves going through a sequence of five steps, the steps occur simultaneously in real time.
Energy Transferred by Working
appears in the relation dU=dQ+dW
is used in analyses relating to Working Engines Thermionic Emission
is a special case of Work
has the special case Potential Energy Kinetic Energy
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