Level 1: 220 V to 11 V Transformer: Which Coil Has the Thicker Wire?
Find the current in the 220 volt coil.
An ideal step-down transformer (220 V to 11 V) supplies a 110 W lamp. The current in its primary (220 V) coil is:
The lamp draws 110 W at 11 V, so the secondary current is 10 A. An ideal transformer takes the same power from the mains as it delivers to the lamp.
The primary carries only 0.5 A, one twentieth of the secondary current. Multiplying by the turns ratio instead of dividing gives 200 A, the classic slip of assuming that current steps up along with voltage.
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