4)TYPES OF CURRENT

DIRECT CURRENT
Between the terminals of a battery, there is a continuos,stable flow of energy. If we use a volmeter to measure the current in a car battery, the result will always be 12 volts.This is called direct current.In the same way, if we connected a light bulb to a battery, the electrons always flow in the same direction with the same current.
ALTERNATING CURRENT
In alternating current the flow of electrons changes direction 50 times every second,as if the positive and negative poles of the socket were constantly changing places.In addition, the electric current is not always the same.
If we measured the voltage of an electrical socket,the results could be represented in a graph like the one below:

• The current begins at 0V and increases to 325V.
• The current decreases from 325V to 0V.
• The current become negative and decreases to -325V.
• The current increases to 0V.

The variation of any electrical parameter over a period of time is an electrical signal.
The tension or voltage of domestic electricity is an alternating signal because it alternates between positive and negative values.Its waveform is also sinusoidal, with a smooth, regular shape.
Alternating current is produced by generators, such as the ones in large electrical generating stations.There are stations may use conventional source of energy or non-conventional sources.

THE EFFICIENCY OF ALTERNATING CURRENT
The average power of alternating current is equal to the direct current that is needed to produce the same effect.In the case of an alternating sinusoidal current, the average power would be as follows:
                                              Vef=Vmax/Raíz cuadrada de -2
TRANSFORMERS
There is an important difference between alternating and direct current.Alternating current can be increased or decreased by a transformer.This allows us to transfer electrical energy at very high voltages and over hundrers of kilometres,without losing any of the energy as heat.
Transformers consit of two windings made of copper wires.If we apply an alternating current to one of them(V1), it will be produce a certain voltage in 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