THE TRANSFORMER
3 marks · NormalExplain why transformers cannot be used with a direct current (d.c.) supply.
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Also accept: Transformers only work with changing (alternating) magnetic fields. Direct current provides a constant magnetic field, so no voltage is induced in the secondary coil.
Common mistakes: Saying that transformers 'work better' with ac rather than exclusively
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Marking points
B1 · 1 mark · States that transformers require a changing or alternating magnetic field.
B2 · 1 mark · Explains that direct current produces a constant (unchanging) magnetic field.
B3 · 1 mark · States that no voltage (or current) is induced in the secondary coil with d.c.
Worked solution
Transformers only work with alternating current because they require a changing magnetic field to induce a voltage in the secondary coil. With direct current, the magnetic field around the primary coil does not change; it stays constant. Therefore, no voltage is induced in the secondary coil, so a transformer cannot work with d.c.