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From this page you can on the references get on pages describing bases of an electro-radio engineering.


Imagine the large tank, in which there is under pressure a water ready an any minute to escape outside. From the tank the pipe with the crane departs. Have opened the crane, and water has directed through a pipe, for example, in pool. If the diameter of a pipe is small, speed of a flow small. Have increased a diameter of a pipe - the speed of a flow has increased also. Occurs so because with increase of a diameter the pipe renders smaller resistance to a pressure of water, and water follows with the greater speed.
We admit, that the tank with water is a source of the electrical energy having a certain pressure (pressure of water), and the pipe - loading, resistance (diameter of a pipe) with which can change. Then the water flow can be accepted for an electrical current proceeding through loading.
While resistance of loading it is not enough (diameter of a pipe large), through it the significant current (large speed of a flow) flows. When the resistance grows (the diameter of a pipe), electrical current (speed of a flow), on the contrary decreases, falls. With the help of this analogy you, probably, can independently define, how the current will change at increase of a pressure (increase of pressure of water in the tank).
And now we shall proceed to units of measurements of a pressure, current and resistance. A pressure measure in volt, designating this unit by the letter In. If you look at a label of the flat battery from a pocket lantern, will notice on it an inscription " 4,5 V ". It means, that a pressure of the battery of 4,5 Volts. On a label of the round battery (is more correct than her to name as an element) already other figure - 1,5 V, that is pressure her 1,5 V.
And for a label there are marks "+" and "-". It is polarity of conclusions. She specifies, in what direction the current will flow, if to battery to connect loading, we shall tell a bulb from a pocket lantern. All of you, certainly, saw such bulb and know, that inside a glass cylinder in it is suspended thin metal wolfram a string. One end it is soldered to a groove  of a part of a bulb, and another - to contact below.  Groove a part and contact are conclusions of a bulb. As soon as they appear connected to conclusions of the battery, through a string of a bulb begins a leak an electrical current. The direction it will be certain - from positive of a conclusion of the battery to negative. As the current flows constantly in one direction, it name constant, pressure too constant.
At galvanic both storage batteries and elements is  still one parameter is an ELECTRICAL CAPACITY. Electrical capacity shows, during what the source can give time the certain current. Electrical capacity measure in AMPERE-HOURS. 
FOR an EXAMPLE: galvanic the element such as 316, or "AA" has (depending on the manufacturer and technology of manufacturing) electrical capacity about 0,6 A/Hour . It means, that the given element can give back a current in 60 mill ampere during 10 hours continuously (600 MA/Hour  to divide on 60 MA). If a working current of an element to reduce up to 10 mill ampere, the time of continuous work of an element will increase till 60 hours. Now, after the expiration 60 hours the pressure on an element under loading will fall up to 0,8 volts (at an initial voltage in 1,5 V).
In a storage time  galvanic of elements there is their self-category (is caused by a number of the reasons), that lowers capacity of elements and batteries. The decrease of capacity occurs, basically, at the expense of irreversible chemical processes occurring in a storage time. After the expiration any of time, the battery can in general lose capacity. For  measurement of parameters of such elements it is possible to use the measuring device - VOLTMETER, but thus it is necessary necessarily to connect to an element the resistor! Business that the voltmeter, having high internal resistance, practically does not load an element and the voltage on conclusions of an element will differ a little from specified on a label. 
" And why do not specify polarity on jacks of the network socket? " - you ask. Business that mains voltage variable. That in one jack of the socket plus of a voltage, in the friend - minus, on the contrary. Such change of polarity occurs 100 times per one second. At inclusion in the socket, for example, desktop lamp, through its string the current will begin to flow, which direction will vary as much of time per one second, how many and polarity of a voltage.
Electrical current measure in ampere, designating this unit by the letter And. But in practice of the radio fan with such currents meet seldom, therefore use fineer unit of measurements - milli ampere - thousand share ,ampere designated by the letters MA, or  micro ampere- one thousand share milli ampere, designated by the letters Mca.
Resistance measure in ohm (conditional designation Om). Except for this unit, larger are used: kiloohm (1 whom = 1000 Ohm) and MegaOhm (1 Mohm=1000 KOhm=1 000 000 Ohm).
In practice of designing of electronic devices to you repeatedly will come to carry out various accounts of electrical circuits. 
For the majority of accounts use the law Ohm. It is the basic law of all radio engineering: " The FORCE of a CURRENT In an ELECTRICAL CIRCUIT IS DIRECTLY PROPORTIONAL to the ENCLOSED VOLTAGE And BACK IS PROPORTIONAL to RESISTANCE to THIS CIRCUIT ". How it to decipher? The current in a circuit the is more, than more voltage of a circuit. At increase of resistance in a circuit (at a constant voltage) the current decreases. And on the contrary.
For practical purposes it is enough to remember this triangle:

The force of a current is designated by the latin letter I, voltage - U and resistance - R.

The second law of an electrical circuit refers to as as the Law Kirhgoff, which says:
For example, if to take with what or unit (connection of several wires or elements in one point), the force of a current which is included in this unit will be equal to force of a current leaving this unit.

If you are interested by a theme of chemical sources of a current - you can in more detail learn about it under this reference. 





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