Transformer
Definition
A device that steps AC voltage up or down using two coils and a shared magnetic field — the backbone of power grids.
Technical definition
Two (or more) coils linked by a common magnetic core. Faraday's law gives the turns ratio rule: V₂/V₁ = N₂/N₁. Works only with AC, since it needs a changing magnetic flux. Ideal transformers conserve power: V₁I₁ = V₂I₂.
Example
A phone charger transformer steps 230 V mains down to 5 V — a turns ratio of about 46:1.
Common confusion
Transformers do not work with DC — a steady current makes a steady field, and only a changing field induces voltage in the second coil. They also do not create power; stepping voltage up steps current down.