Electric current is a directed flow of electrons from one pole of the current source to another.If this direction is constant and does not change over time, talking about DC.One terminal of the power supply at the same time is considered to be + EV, the second - minus.It is considered that the current flows from plus to minus.
classic example of the constant current source is normal penlight batteries.Such batteries are widely used as a power source in a small-sized electronic equipment - for example, remote controls, in cameras, radios, etc.etc.
alternating current, in turn, is characterized in that periodically reverses direction.For example, in Russian standard adopted, according to which the mains voltage is 220 V and the curre
nt frequency of 50 Hz.It is the second parameter characterizes the frequency with which changes the direction of the electric current.If the current frequency is 50 Hz then it changes direction 50 times per second.
Does this mean that in a conventional electrical outlet having two contacts, change periodically, plus-minus?That is, the first contact on one plus the other negative, then vice versa, etc.etc.?In fact, things are a little different.Electrical outlets in power are two conclusions: the phase and ground.They are usually referred to as "phase" and "earth".Grounding terminal safe voltage across it is not.In the same phase output 50 Hz per second are changing the plus and minus.If you touch the "earth", nothing will happen.Wire phase is better not to touch because it is always under the voltage of 220 V.
Some devices are powered by direct current from the other variable.Why do we require such a separation?In fact, most electronic devices use just DC voltage, even if included in the AC line.In this case, the alternating current is converted to DC rectifier, in the simplest case consists of a diode, a half-wave shear, and a capacitor for smoothing the ripples.
Alternating current is used because it is very convenient to transmit over long distances, the losses in this case is minimized.In addition, it is easy to transform - ie voltage change.DC can not transform.The higher the voltage, the lower the transmission loss of alternating current, so the main line voltage reaches several tens or even hundreds of thousands of volts.To submit to the settlements on the high voltage substations reduced, resulting in homes supplied already low voltage of 220 V.
Different countries have different standards of supply voltage.Thus, in the European countries is 220, then in the United States - 110 V. Another interesting fact is that the famous inventor Thomas Edison was unable at the time to evaluate all the advantages of AC and advocated the need for the use in electric networks is DC.Only later he was forced to admit he was wrong.