Dear reader, today I would like to introduce you to the basic, theoretical aspects of electricity. This knowledge will be the basis for your future DIY work. A good grounding in theory will later enable you to understand more and more complex electrical and electronic schemes and hopefully also to build simple electronic devices and circuits together. We will start with quite basic concepts, conductivity, electric current and electric voltage. As is well known, the electron is the smallest negative electric charge. A large number of electrons flowing through a metal in one direction creates a stream of electrons. This stream of electrons flowing through the metal in one direction creates an electric current. The more electric charges flow through the metal, and therefore the denser the electron stream, the greater the electric current will be. The free electrons in electrically conductive matter can be compared to water droplets. If the individual drops are at rest, they do not form a water current. A large number of them, flowing in one direction, form a stream or even a large river. In such a stream or river the drops flow - as we say - "downstream". This current always has a certain so-called "intensity". However, not in every matter an electric current can flow; it depends on the presence of free electrons and the possibility of their movement. Free electrons can flow most easily in metals (preferably in silver, gold, copper), they can also flow in carbon such as is used for making electrodes for arc lamps, "brushes" for electric machines and others, and in various liquids and gases. In the latter, the flow depends on such parameters as density, pressure and temperature. We can therefore say that the above-mentioned solids, liquids and gases have a greater or lesser electrical conductivity for the flow of free electrons, and therefore offer greater or lesser resistance to the flowing current. All bodies in nature that conduct electricity are called conductors. As mentioned, electricity cannot always flow in liquids. Electricity does not flow in distilled water. In various solids, electricity may also not flow. Such bodies include glass, porcelain, mica, ebonite, many plastics, tars, oils. In these bodies the electrons are so tightly bound in the atoms that there are practically no electrons in the interatomic space; there are no free electrons, so no electric current can flow. Such bodies are called insulators. There is also a group of bodies in which electrons are released from the nucleus in the atom as a result of, for example, irradiation or temperature changes. To such bodies belong conductors: metals - selenium, tellurium, silicon or oxides of some metals. There are also materials which, when joined together, create a transition on the contact surface which conducts current only in one direction, while in the other direction it creates very high resistance to current flow. These materials are called semiconductors. These materials are widely used in electronic devices, contributing to the enormous strength of equipment, its longevity and miniaturisation. For an easier understanding of the phenomena caused by electric current, let us compare it to water current. 
[001]
The properties of water are well known. In small streams small amounts of water flow, whereas in rivers great masses of water flow. Let us assume that the intensity of a small water current is - 1. Correspondingly, the intensity of the water current of a very large river is - 100. A weak water current can move only one mill. The intensity of the water current, needed to move one mill, let us then assume - 1. A four times stronger water current will therefore be able to move four such mills. The intensity of the water current is - 4. Unlike the water current, which we can observe, the electric current flows invisibly along the wires. In the figure [002] the electric current moves only one motor. Let us then assume that the current intensity is - 1. When the electric current powers four such motors, the current flowing in the main wire is then 4. 
[002] The unit used to determine the quantity of water is the litre. We can say that the amount of water flowing is a specific value of litres in one second. A practical unit for determining the electric current flowing is the ampere. The name comes from the famous scientist who formulated this value. The intensity of electric current is measured in units of amperes, which are denoted by the letter A, and in formulas by the letter I. In practice, however, we often deal with current intensities many times smaller than one ampere. In various electric circuits, for example radio receivers, the currents are only thousandths or even millionths of an ampere. For this reason, smaller units - milliampere (mA) and microampere (µA) - have been adopted for ease of use in practice. Measuring instruments are used to determine whether an electric current is flowing in a circuit. Instruments called ammeters are used to accurately measure the intensity of electric current. When it comes to representing the amount of flowing current in the form of electrons, it can be assumed that the intensity of 1 ampere corresponds to the amount of 6*1018 electrons. It is worth knowing that it is not indifferent how large a cross-section of a conductor, e.g. a wire, is crossed by an electric current of this or that intensity. The flow of the current causes an increase in the temperature of the conductor and heat. This is more intense the higher the number of electrons flowing in the stream. There is a close relationship between the permissible current density for a given conductor (for example, copper wire), its cross-section (in mm2) and the permissible current intensity. The current density j is expressed in the unit A/mm2, while the cross-section of the conductor is expressed in mm2. 
[003]
The current density is usually chosen depending on the type of conductive material and the device in which the current flows (for example for copper wire in coils a current density of 2-3 A/cm2 is assumed). Knowing also the current which must flow through such or other devices, we can calculate the required cross-section in mm2, and from this the required diameter of the wire. Choosing the right current density in amperes per mm2 protects the wires in some devices from getting too hot, and in others, heaters (cookers, cookers, or classic light bulbs) from melting. One source of electric current can be any type of battery or electric accumulator. The size of batteries and accumulators determines the amount of current that can be drawn from them. Different amounts of current flow through different electrical devices. However, not all light bulbs, motors or electron tubes have the same electrical current. The glow current of electron lamps, for example, is specific to the type and size of lamp. The second electrical quantity that is closely related to current is voltage. In order to understand more easily what voltage is, it can be compared to the difference in level when water falls. It can be assumed that for a small difference in level of flowing water the drop is 1. For a large difference in level of water the drop is correspondingly large and amounts to, say, 100. Water falling from a small height can run only one mill. Let's take the water drop, which is the difference in level, to be 1. Five times the water drop will be able to run five such mills with the same amount of water and the same intensity of the water current that turns one mill. Drop - 5. Similar phenomena occur with electrical voltage. Another example - a light bulb glowing, at a voltage of 2. If two identical bulbs connected in series are glowing, the voltage is then 4. If five identical bulbs are glowing, the voltage is 10. In the examples discussed, with the bulbs connected in series, the electric current flowing through the bulbs is the same, only the overall voltages are different. 
[004] In electrical engineering, the equivalent of different water levels are electrical potentials. The difference in electrical potentials is called voltage. Voltage is measured in volts and is denoted by the symbol V, and in formulae by the letter U. The name of the voltage unit - Volt - also comes from a famous scientist who contributed to the knowledge of the science of electricity. In various electronic circuits of radio receivers there are voltages much lower than one volt. On the other hand, classical TV sets and various industrial and scientific equipment have voltages of many thousands of volts. To facilitate the use of these values, additional units have been adopted: 1µV, 1mV, 1kV. Special meters, called voltmeters, are used to measure electrical voltage. 
[005]
Commonly known sources of constant electric voltage are batteries and rechargeable batteries. In all voltage sources, i.e. whether in electric batteries, electric generators, batteries or in various electric circuits in which currents are induced, in the unloaded state, i.e. when nothing is connected to them, there is a greater potential difference at their terminals than when they are connected to some circuit drawing current from them. This voltage, which is generated by an electric current source in an unloaded state, is called the electromotive force and is abbreviated EMF, and in the formulae it is denoted by the letter E, as opposed to the voltage U, which prevails at the terminals of the source when a current collector is connected to it. These loads, when drawing a current of a certain amperage, cause a voltage drop across the resistance of the current source (each current source has its own resistance), which is subtracted from the EMF of the source, thus establishing a voltage U slightly lower than EMF during operation. • • •
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