USING TRANSFORMERS TO CHANGE VOLTAGES
3 marks · NormalExplain why transformers cannot be used with direct current (d.c.).
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Transformers need a changing magnetic field to induce voltage in the secondary coil, which is produced by an alternating current (a.c.). Direct current (d.c.) does not create a changing field, so no voltage is induced in the secondary coil.
Must show: transformers require changing field, d.c. is constant current/field, no induced voltage in secondary with d.c.
Common mistakes: Assuming d.c. will work but at lower efficiency
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Marking points
B1 · 1 mark · Mentions transformers need a changing magnetic field.
B2 · 1 mark · States d.c. produces a steady (unchanging) current and magnetic field.
B3 · 1 mark · Explains no voltage is induced in the secondary coil with a constant field.
Worked solution
Transformers work by electromagnetic induction, which requires a changing magnetic field to induce a voltage in the secondary coil. Alternating current (a.c.) produces a changing magnetic field, but direct current (d.c.) does not. With d.c., the magnetic field is steady, so no voltage is induced in the secondary coil, and the transformer cannot work.