How does a electrical transformer work?

A transformer is an electrical device that is used to convert AC power at a certain
voltage level to AC power at a different voltage, without changing the frequency.

A transformer is constructed using a ferromagnetic core around which multiple
coils, or windings, of copper wire are wound. The winding which is connected to
the electrical source or the input is called the 'primary' coil, while the load or the
output winding is the 'secondary'.

Figure 1: Basic Transformer

When an alternating current is passed through a copper wire, a magnetic field is
generated around the wire. If a second wire is brought within this magnetic field,
a corresponding alternating current is created within the second wire. In technical
terms a current is "induced" in the second wire.

The alternating current in the primary coil induces an alternating magnetic flux (f)
that flows around the ferromagnetic core, changing direction during each electrical
cycle. The alternating flux in the core in turn induces an alternating current in each
of the secondary coils. The voltage induced in each of the secondary coils is
directly related to the primary voltage by the turns ratio (N), the number of turns
in the primary coil divided by the number turns in the secondary coil. The voltage
transformation ratio (primary voltage to secondary voltage) and the current
transformation ratio (primary current to secondary current) depend on the turn's
ratio. Thus, the voltage can be easily changed lower or higher by selecting the
correct number of turns. A transformer may have multiple secondary coils to feed a
number of electrical loads

Figure 2: Transformer core with magnetic flux.