How does a transformer work?

A transformer changes the size of an alternating voltage using electromagnetic induction between two coils wound on an iron core.

GCSE Physics

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In more detail

Alternating current in the primary coil produces a constantly changing magnetic field in the iron core. This changing magnetic field passes through the secondary coil and induces an alternating voltage in it. Because the field is always changing, a voltage is continuously induced — which is why transformers only work with alternating current (AC), not direct current (DC).

Key points for the exam

  • A step-up transformer has more turns on the secondary coil and increases the voltage; a step-down transformer has fewer and decreases it.
  • The turns ratio sets the voltage ratio: Vp/Vs = Np/Ns.
  • For an ideal (100% efficient) transformer, power in = power out: Vp × Ip = Vs × Is.
Exam tip: A common exam question asks WHY transformers need AC — because DC gives a steady field that does not change, so no voltage is induced.

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