A basic transformer consists of a primary coil and a
secondary coil wound on an iron core
Iron is used because it is easily magnetised (and
demagnetised), so it carries the changing magnetic field from one coil to
the otherAn alternating current in the primary coil produces a changing
magnetic field in the iron core. This changing field passes through
the secondary coil and induces an alternating potential difference
across it (the generator effect). If the secondary is part of a complete
circuit, an alternating current flows
Transformers only work with ac : a steady dc gives a constant field
and no induced potential difference
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A transformer iron core PRIMARY few turns SECONDARY more turns a.c. supply a CHANGING magnetic field in the core The alternating current in the primary makes an alternating magnetic field in the iron core. That changing field passes through the secondary coil, and induces an alternating p.d. across it. Vp ÷ Vs = np ÷ ns More turns on the secondary → a STEP-UP transformer (p.d. goes up). Fewer turns on the secondary → a STEP-DOWN transformer. If it is 100% efficient, the power in equals the power out: Vp × Ip = Vs × Is So stepping the p.d. UP steps the current DOWN — which is exactly what the National Grid needs. A transformer will NOT work on d.c.: a steady current gives a steady field, and a steady field induces nothing. A transformer: a rectangular iron core with a primary coil of a few turns on the left connected to an ac supply and a secondary coil of more turns on the right connected to a lamp; the changing magnetic field shown as lines around the core
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In the full topic
• How a transformer works• The turns ratio• Power in a transformer• Transformers and the National GridRead the free physics topics →