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How do transformers use magnetic fields to change voltage levels?

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Diagram showing a circular loop of wire with an electric current, surrounded by curved blue lines representing magnetic field lines that loop around the wire and pass through its center. — magnetism Some lines of the magnetic field B (blue) for an electric current I (gold) in a loop.

Transformers change voltage levels through the principle of electromagnetic induction, as described by Faraday’s law of induction.12

A transformer consists of two coils of wire, the primary and the secondary, typically wound around a shared iron core. When an alternating current flows through the primary coil, it creates a changing magnetic field. Because the iron core confines this magnetic flux, the changing field passes through the secondary coil, inducing an electromotive force (voltage) within it.13

The specific change in voltage is determined by the turns ratio—the ratio of the number of turns in the secondary coil ( $N_{2}$ ) to the number of turns in the primary coil ( $N_{1}$ ).13
* Step-up transformers increase voltage by having more turns in the secondary coil than the primary.13
* Step-down transformers decrease voltage by having fewer turns in the secondary coil.13

To maintain high efficiency, the iron core is laminated to minimize eddy currents, which are circular currents that would otherwise cause energy loss through heat.34