Fundamental Principles of Transformer:

The processor is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism) and secondly, that a changing magnetic field inside a coil of wire induces a voltage between the ends of coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that develops. The changing magnetic flux induces a voltage in the secondary coil.

An ideal transformer is shown in the adjacent figure. Passage of current through the primary coil creates a magnetic field. The primary and secondary coils are wound around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both primary and secondary coils.