TYPES OF CURRENT
Some electrical devices use batteries and some must be connected to the electric mains. Both provide electricity, but in different ways.

Direct current
Between the terminals of a battery, there is a continuous, stable flow of energy. If we use a volmmeter to measure the direct current in  a car battery, the result will always be 12 volts. This is called direct current. The value of direct current doesn't change over time.

Alternating current
The tension or voltage of domestic electricity is an alternating signal because it alternates between positive and negative values.
If we measure the voltage of an electrical socket, the results could be represented in graph like the one below:

-The current begins at 0V and increases to 325V
-The current decreases from 325V to 0V
-The current becomes negative and decreases to -325V
-The current increases to 0V

The value of alternating current changes over time. The flow of energy also changes direction in a cyclical way.


The efficiency of alternating current
The average power of alternating current is the power that a direct current should have in order to produce the same electrical effect. The average power would be as follows:
                                  Vef= Vmax/√2


Transformers
Alternating current can be increased or decreased by a transformer. They consist of two windings made of copper wire. If we apply an alternating current to one of them (V1) it'll produce a certain voltage in the other (V2). The value will depend on the number of times that the copper wire has been wrapped around each winding, represented as n1 and n2:
                                    V1/V2=n1/n2