A transformer is used to change the voltage of an alternating current supply. Explain why a transformer does not work with a direct current supply.

Cambridge International IGCSE Physics (0625) — 4.5 Electromagnetic effects · Explain · 4 marks · View as Markdown

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

Transformers are widely used in the national grid to step up or step down voltages. They consist of two coils of wire wound around a soft iron core.

Model answer (4 marks)

A direct current produces a magnetic field that is constant and does not change.
A changing magnetic field is required to induce an e.m.f. in the secondary coil.
An alternating current continuously changes direction, so the magnetic field continuously changes.
Therefore no voltage (e.m.f.) is induced in the secondary coil with d.c., so no current flows in the secondary circuit.

Examiner tips

Use the key phrase "changing magnetic field" to link to induction.
Show the logical sequence: DC → steady field → no induction → no secondary current.
Keep answer concise and use exam terminology such as "e.m.f." and "secondary coil".

Common mistakes

Confusing DC with AC; writing that DC can induce voltage.
Omitting the step that a steady magnetic field gives zero induced e.m.f.
Using vague language like "no current" without explaining the lack of induced voltage.

Mark scheme (4 marks)

A direct current produces a magnetic field that does not change / is constant / is steady
A changing magnetic field is required to induce a voltage / e.m.f. in the secondary coil
Alternating current continuously changes direction, so the magnetic field continuously changes
Therefore no voltage / e.m.f. is induced in the secondary coil with d.c., so no current flows in the secondary circuit

Key terms in this question

alternating current · direct current