CONDUCTORS, INSULATORS AND ELECTRIC CURRENT
The purpose of this lesson is to master the theoretical information regarding concepts such as: conductors, dielectrics, semiconductors, and electric current. To understand the chemical and physical nature of the generation of electric current and the basic conditions for its occurrence. In this lesson, we will touch upon such crucial concepts and definitions as direct and alternating voltage and current. At the end of this lesson, there will be a more interesting practical assignment than in the first lesson. I think it will not seem difficult to you.
Not every body provides the conditions for the flow of electric current. The fact is that the atoms and molecules of different substances have different properties. In metals, for example, electrons easily leave their shells and move randomly and chaotically between atoms. Metals have a particularly large number of free electrons. Essentially, a metal consists of positive ions arranged in a specific order, the space between which is filled with free electrons. In a metal, it is impossible to distinguish which electron belongs to which atom; they merge into a single electron cloud. The huge number of free electrons in metals creates the most favorable conditions for an electric current. We only need to organize the chaotic movement of electrons, to force them to move in one direction.
In some bodies and substances, there are almost no free electrons, as they are tightly held by the nuclei. It is difficult to take away or force extra electrons onto the molecules and atoms of such bodies. In such bodies, it is impossible to create an electric current. Bodies and substances in which an electric current can be created are called conductors. Those bodies and substances in which it cannot be created are called dielectrics or insulators. Conductors, besides metals, also include carbon, salt solutions, acids, alkalis, living organisms, and many other bodies and substances. Moreover, in salt solutions, the electric current is created not only by electrons but also by positive ions. Dielectrics include air, glass, paraffin, mica, varnishes, porcelain, rubber, plastics, various resins, oily liquids, dry wood, dry cloth, paper, and other substances. Porcelain, for example, is used to make insulators for electrical wiring, and varnishes are used to coat wires to isolate them from each other and from other objects.
But there is also a large group of substances called semiconductors. Semiconductors, in particular, include germanium and silicon. In terms of electrical conductivity, they occupy an intermediate position between conductors and insulators. Once considered unsuitable for practical purposes, they have now become the main material for the production of modern semiconductor devices, such as transistors, with which a large part of your creative work will be connected.
ELECTRIC CURRENT
How do we force the abundance of free electrons, say, in the filament of a light bulb, to move in an orderly manner, in one direction? We need to create an electric field in the conductor by connecting it, for example, to a galvanic cell or a battery of galvanic cells. The structure of a simple galvanic cell, which is a chemical current source, is shown in the figure. The cell consists of zinc and copper plates, called electrodes, which are placed in an electrolyte - a solution of salt or acid, such as sulfuric acid.
As a result of the chemical reaction occurring between the electrodes and the electrolyte, an excess of electrons is formed on the zinc electrode, and it acquires a negative electrical charge, while the copper electrode, conversely, lacks electrons and acquires a positive charge. At the same time, an electric field arises between the oppositely charged electrical charges of such a current source, and an electromotive force (EMF for short) or voltage acts. I will tell you about the difference between EMF and voltage later, during an excursion into electrical engineering.
You already know that the poles of a cell or battery are denoted by plus and minus signs. You have seen them, for example, near the tin lead plates of a battery intended to power an incandescent lamp of a pocket flashlight. Incidentally, this battery also consists of galvanic cells, only not liquid ones, like the cell shown in the figure, but dry ones. There are three of them. Several cells connected together into a single current source are called a battery.
Remember: in circuit diagrams, the negative pole of a cell or battery is conventionally denoted by a short line, and the positive one - by an elongated line.
As soon as a conductor is connected to the poles of a cell or battery, an electric field will arise in it, under the influence of which electrons, as if on a bridge thrown across a ravine, will move to where there is a shortage of them, from the negative pole through the conductor to the positive pole of the electrical energy source. This ordered movement of electrons in a conductor is the electric current. The current flows through the conductor because an electromotive force acts in the resulting circuit (positive pole of the cell, conductors, negative pole of the cell, electrolyte). Such a simple electrical circuit can be divided into two main sections: external and internal. The external section of the circuit includes everything connected to the poles of the current source, and the internal section is the part of the circuit enclosed within the current source itself.
Remember: a closed electrical circuit is a mandatory condition for the existence of current in it. Current does not flow in an open circuit.
Opposite charges can be imparted to two isolated bodies, for example, small balls suspended on silk threads. The balls will attract each other, but there will be no current between them, since they are separated by the dielectric air.
It is established that electrons in a conductor move from the negative pole (where there is an excess of them) to the positive pole (where there is a shortage of them), however, even today, as in the last century, it is generally accepted that the current flows from plus to minus, i.e., in the direction opposite to the movement of electrons. You might ask: why not break this tradition now? The fact is that this would require rewriting all textbooks and all technical literature directly or indirectly related to electrical and radio engineering. Furthermore, the conventional direction of current is used by scientists as the basis for a number of rules related to determining many electrical phenomena. At the same time, such a convention does not create any special inconveniences if you firmly remember that the direction of current in conductors is opposite to the direction of electron movement. In those cases when the current is created by positive electrical charges, for example, in the electrolytes of chemical direct current sources, or the flow of holes in semiconductors (we will talk about this in the following lessons), such contradictions do not exist at all, because the direction of movement of positive charges coincides with the direction of the current.
As long as the cell or battery is active, the current in the external section of the electrical circuit flows in the same direction. Such a current is called direct current (DC) and is denoted by the Latin letter (I).
In the figure above, through the connecting conductors and the incandescent lamp filament, electrons move from left to right from minus to plus. But if the poles of the cell are swapped, then the electrons in the same external section of the circuit will flow from right to left, since now the minus will be at the right end of the circuit section, and the plus at the left. Only the direction of electron movement will change, but the current will remain constant in this case as well.
And what if the poles of the current source are swapped very quickly and rhythmically? In this case, the electrons in the external section of the circuit will also alternately change the direction of their movement. First, they will flow in one direction, then, when the poles are swapped, in the other, opposite to the previous one, then again in the forward direction, again in reverse, etc. An alternating current, rather than a direct current, will flow in the external circuit.
Remember: an alternating current (AC) flows in the wires of the electrical lighting network, not a direct current, as in the circuit of an electric pocket flashlight. It is generated by machines called alternating current generators.
The signs of the electrical charges at the poles of the generator change continuously, but not abruptly, as in our example, but smoothly. The charge of the generator pole, which at a certain point in time was positive, begins to decrease and after a fraction of a second becomes negative; the negative charge first increases, then begins to decrease until it is positive again, etc. The sign of the charge of the other pole changes simultaneously. At the same time, the voltage and the value of the current in the electrical circuit also change periodically.
Graphically, alternating current is depicted by a wavy line - a sinusoid, shown in the figure. Here, the vertical axis with the arrow pointing up corresponds to one direction of current, and down - to another direction of current, opposite to the first.
What can such a graph tell us? The current in the circuit appears at the point in time indicated on the graph by point 'a'. It smoothly increases and flows in one direction, reaching its maximum value (point 'b'), and also smoothly decreases to zero (point 'v'). Having disappeared for a moment, the current reappears, smoothly increases, and flows in the circuit, but in the opposite direction. Having reached its maximum value (point 'g'), it decreases to zero again (point 'd'). And further, the current, continuously increasing and decreasing in this sequence, constantly changes its direction and value.
With alternating current, the electrons in the conductor seem to oscillate from side to side. Therefore, alternating current is also called electrical oscillations. One full, or completed, current oscillation is considered to be the ordered movement of electrons in a conductor, corresponding to the section of the graph from 'a' to 'd' or from 'v' to 'zh'. The time during which one full oscillation occurs is called the period, the time of half an oscillation - the half-period, and the maximum value of the current during each half-period is the amplitude.
Alternating current compares favorably with direct current in that it is easily transformed. For example, using a special device - a transformer - you can increase the voltage of an alternating current or, conversely, decrease it. Alternating current can also be rectified - converted into direct current. You will widely use these properties of alternating current in your amateur radio practice.
Everything I have just told you is known to every high school student and, of course, every radio amateur. You use the benefits of electricity, sometimes even wastefully, without thinking about the fact that scientists only just found ways to practically use this generous gift of nature a mere 100 years ago.
In this lesson, you became acquainted with such crucial concepts as: conductors, dielectrics, and semiconductors. What direct and alternating electric current is. And the last thing that needs to be clearly remembered and understood are the main characteristics of alternating current on the presented graph (sinusoid): these are the period, half-period, frequency, and amplitude.
PRACTICAL WORK
Your next practical work will consist of assembling a simple flashlight, which consists of a miniature incandescent lamp L (the operating voltage of the incandescent lamp should be in the range of 1.5 - 2.5 V), a galvanic cell G (a regular 1.5 V AA battery), connecting wires, and a button (you can use any button with one normally open contact; an alternative to a button could also be two wires, one of which is connected to the positive terminal of the battery, and the other to the light bulb). When the contacts of the switch or wires are closed, the light bulb should light up.
The goal of this practical work is to observe the effect of an electric current on the filament of a light bulb and to find out what happens as a result of this effect.
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