A transformer is a device that changes the magnitude of alternating voltage and current.
It has two windings with different numbers of turns: primary and secondary winding. The windings are connected via an iron core in such a way that the magnetic fluxes 
on the primary and secondary sides are the same. The powers on the primary and secondary sides are also approximately the same.
When we connect the primary voltage 
to the primary winding, we get the secondary voltage 
on the secondary side.
Transformer
Alternating current causes a change in the magnetic flux on the primary side. According to the Faraday's law of induction therefore, the voltage 
of the primary side is:
The magnetic field is located practically only in the iron core, which surrounds the primary and secondary sides of the transformer. Let us assume that there is no magnetic field shedding. The change in magnetic flux on the primary side is transmitted to the secondary side.
Due to the change in magnetic flux, a voltage is induced on the secondary side:
We have stated that the two magnetic fluxes are the same. Let's now divide equation 1 by 2 and obtain:
The voltage is transformed in proportion to the number of turns.
The efficiency of the transmission of electricity or power is quite high on the devices, somewhere between 92% for low-power transformers and 98% for high-power energy transformers.
Efficiency is affected by:
Heating the iron core. A voltage can also be induced in the core, which drives a short-circuit current in the form of concentric circles (eddy currents) in a plane perpendicular to the lines of the magnetic field. We reduce them by using a core made of thin layers, which are electrically insulated from each other, instead of a compact iron core.
Ohmic resistance of transformer windings. This is the resistance of the electrical conductor, e.g. copper wire wound on the primary and secondary side of the transformer.
Magnetic field shedding - the magnetic flux on the secondary side is not exactly the same as the flux on the primary side.
In many cases, we will assume that the efficiency is 100%, i.e. the entire power is transferred from the input to the output of the transformer.
Let's now consider the equality of powers on the primary and secondary sides:
The current is transformed in inverse proportion to the number of turns.
