Conductors are different. What is the difference between a cable and a wire and when to use them. What is a conductor

Overhead power lines are another area of ​​application for wires. Them low specific weight allows to pull products through supports standing at a considerable distance from each other.

Of course, it is possible to lay a cable through the air, but this will require weighting of the support poles to prevent them from swinging and further damaging the conductor.

Power conductors are ideal for transmitting large amounts of power in a conductive environment. The outer insulating sheath of rubber, paper, heat-resistant polymers, lead, twisted steel tape all make the risk of fire almost impossible.

So, the difference between cable and wire is as follows. The first one consists of several wires connected by one or more layers of protection. The maximum wire voltage rating is 1000 V, the cable can be operated at any voltage. Certain structural nuances make the cable a better option for laying in water or in the depths of the earth.

In conclusion, we suggest watching an interesting and informative video, what is the difference between a cable and a wire:

Various materials are used in electrical engineering. The electrical properties of substances are determined by the number of electrons in the outer valence orbit. The fewer electrons are in this orbit, the weaker they are bound to the nucleus, the easier they can go to travel.

Under the influence of temperature fluctuations, electrons break away from the atom and move in the interatomic space. Such electrons are called free, it is they who create an electric current in conductors. Is the interatomic space large, is there room for free electrons to travel inside matter?

The structure of solids and liquids seems to be continuous and dense, resembling a ball of thread in structure. But in fact, even solid bodies are more like a fishing net or a volleyball net. At the everyday level, of course, this cannot be seen, but precise scientific research has established that the distances between electrons and the nucleus of atoms far exceed their own dimensions.

If the size of the nucleus of an atom is represented as a ball the size of a soccer ball, then the electrons in such a model will be the size of a pea, and each such pea is located at a distance of several hundred and even thousands of meters from the “nucleus”. And between the nucleus and the electron is emptiness - there is simply nothing! If we imagine the distances between the atoms of matter on the same scale, the dimensions will turn out to be generally fantastic - tens and hundreds of kilometers!

Good conductors of electricity are metals. For example, gold and silver atoms have only one electron in their outer orbit, so they are the best conductors. Iron also conducts electricity, but somewhat worse.

They conduct electricity even worse. high resistance alloys. These are nichrome, manganin, constantan, fechral and others. Such a variety of high-resistance alloys is due to the fact that they are designed to solve various problems: heating elements, strain gauges, exemplary resistors for measuring instruments, and much more.

In order to evaluate the ability of a material to conduct electricity, the concept was introduced "conductivity". Reverse meaning - resistivity. In mechanics, these concepts correspond to specific gravity.

insulators, unlike conductors, do not tend to lose electrons. In them, the bond of the electron with the nucleus is very strong, and there are almost no free electrons. More precisely, there is, but very little. At the same time, in some insulators there are more of them, and the quality of their insulation, respectively, is worse. It is enough to compare, for example, ceramics and paper. Therefore, insulators can be conditionally divided into good and bad.

The appearance of free charges even in insulators is due to thermal vibrations of electrons: under the influence of high temperature, the insulating properties deteriorate, some electrons still manage to break away from the nucleus.

Similarly, the resistivity of an ideal conductor would be zero. But fortunately there is no such conductor: imagine what Ohm's law would look like ((I \u003d U / R) with zero in the denominator !!! Goodbye mathematics and electrical engineering.

And only at a temperature of absolute zero (-273.2C °) thermal fluctuations completely stop, and the worst insulator becomes good enough. In order to determine numerically "this" bad - good use the concept of resistivity. This is the resistance in ohms of a cube with a rib length of 1 cm, the unit of resistivity is obtained in ohm / cm. The resistivity of some substances is shown below. Conductivity is the reciprocal of resistivity, - Siemens unit, - 1Sm = 1 / Ohm.

Good conductivity or low resistivity have: silver 1.5 * 10 ^ (-6), read as (one and a half ten to the power of minus six), copper 1.78 * 10 ^ (-6), aluminum 2.8 * 10^(-6). The conductivity is much worse for alloys with high resistance: constantan 0.5 * 10 ^ (-4), nichrome 1.1 * 10 ^ (-4). These alloys can be called poor conductors. After all these complex numbers, ohm / cm should be substituted.

Further, semiconductors can be distinguished into a separate group: germanium 60 ohm / cm, silicon 5000 ohm / cm, selenium 100,000 ohm / cm. The resistivity of this group is greater than that of poor conductors, but less than that of poor insulators, not to mention good ones. Probably, with the same success, semiconductors could be called semi-insulators.

After such a short acquaintance with the structure and properties of the atom, one should consider how atoms interact with each other, how atoms interact with each other, how molecules are obtained from them, which make up various substances. To do this, we again have to recall the electrons in the outer orbit of the atom. After all, they are involved in the connection of atoms into molecules and determine the physical and chemical properties of matter.

How molecules are made from atoms

Any atom is in a stable state if there are 8 electrons in its outer orbit. He does not seek to take electrons from neighboring atoms, but does not give up his own. To verify the validity of this, it is enough to look at the inert gases in the periodic table: neon, argon, krypton, xenon. Each of them has 8 electrons in the outer orbit, which explains the reluctance of these gases to enter into any relations (chemical reactions) with other atoms, to build chemical molecules.

The situation is completely different for those atoms that do not have the cherished 8 electrons in the outer orbit. Such atoms prefer to unite with others in order to supplement their outer orbit with up to 8 electrons due to them and acquire a calm stable state.

Take, for example, the well-known water molecule H2O. It consists of two hydrogen atoms and one oxygen atom, as shown in Figure 1.

Picture 1

At the top of the figure, two hydrogen atoms and one oxygen atom are shown separately. There are 6 electrons in the outer orbit of oxygen and two electrons in the vicinity of two hydrogen atoms. Oxygen, up to the cherished number 8, lacks just two electrons in the outer orbit, which it will receive by attaching two hydrogen atoms to itself.

Each hydrogen atom lacks 7 electrons in its outer orbit to be completely happy. The first hydrogen atom receives 6 electrons from oxygen and one more electron from its twin, the second hydrogen atom, into its outer orbit. There are now 8 electrons in its outer orbit along with its electron. The second hydrogen atom also completes its outer orbit to the coveted number 8. This process is shown at the bottom of Figure 1.

Figure 2 shows the process of combining sodium and chlorine atoms. As a result, sodium chloride is obtained, which is sold in stores under the name table salt.

Figure 2. The process of combining sodium and chlorine atoms

Here, too, each of the participants receives the missing number of electrons from the other: chlorine adds a single sodium electron to its own seven electrons, while giving its own to the sodium atom. Both atoms have 8 electrons in the outer orbit, thus achieving complete agreement and well-being.

Valence of atoms

Atoms that have 6 or 7 electrons in their outer orbit tend to add 1 or 2 electrons to themselves. Such atoms are said to be monovalent or divalent. But if there are 1, 2 or 3 electrons in the outer orbit of an atom, then such an atom tends to give them away. In this case, the atom is considered one, two or three valent.

If the outer orbit of an atom contains 4 electrons, then such an atom prefers to unite with the same one, which also has 4 electrons. This is how the atoms of germanium and silicon are combined, which are used in the manufacture of transistors. In this case, the atoms are called tetravalent. (Atoms of germanium or silicon can combine with other elements, such as oxygen or hydrogen, but these compounds are not interesting for our story.)

Figure 3 shows a germanium or silicon atom that wants to combine with the same atom. The small black circles are the atom's own electrons, and the light circles indicate the places where the electrons of the four neighboring atoms will fall.

Figure 3. Atom of germanium (silicon).

Crystal structure of semiconductors

The germanium and silicon atoms in the periodic table are in the same group as carbon (the chemical formula of diamond is C, which is just large crystals of carbon obtained under certain conditions), and therefore, when combined, they form a diamond-like crystal structure. The formation of such a structure is shown, in a simplified form, of course, in Figure 4.

Figure 4.

There is a germanium atom in the center of the cube, and 4 more atoms are located at the corners. The atom depicted in the center of the cube is connected with its nearest neighbors by its valence electrons. In turn, the corner atoms donate their valence electrons to the atom located in the center of the cube and to its neighbors - atoms not shown in the figure. Thus, the outer orbits are completed to eight electrons. Of course, there is no cube in the crystal lattice, it is simply shown in the figure so that the mutual, volumetric arrangement of atoms is clear.

But in order to simplify the story of semiconductors as much as possible, the crystal lattice can be depicted as a flat schematic drawing, despite the fact that the interatomic bonds are still located in space. Such a scheme is shown in Figure 5.

Figure 5. The crystal lattice of germanium in a flat form.

In such a crystal, all electrons are firmly attached to atoms by their valence bonds, so there are apparently simply no free electrons here. It turns out that we have an insulator in the figure, since there are no free electrons in it. But actually it is not.

Own conductivity

The fact is that under the influence of temperature, some electrons still manage to break away from their atoms, and for some time free themselves from bonding with the nucleus. Therefore, a small number of free electrons in the germanium crystal exists, due to which it is possible to conduct an electric current. How many free electrons exist in a germanium crystal under normal conditions?

There are no more than two such free electrons per 10 ^ 10 (ten billion) atoms, so germanium is a poor conductor, or, as they say, a semiconductor. It should be noted that only one gram of germanium contains 10 ^ 22 (ten thousand billion billion) atoms, which allows you to "get" about two thousand billion free electrons. It seems to be enough to pass a large electric current. To deal with this issue, it is enough to remember what a current of 1 A is.

A current of 1 A corresponds to the passage through the conductor in one second of an electric charge of 1 Coulomb, or 6 * 10 ^ 18 (six billion billion) electrons per second. Against this background, two thousand billion free electrons, moreover, scattered over a huge crystal, can hardly ensure the passage of large currents. Although, due to thermal motion, a small conductivity of germanium exists. This is the so-called intrinsic conductivity.

Electronic and hole conductivity

As the temperature rises, additional energy is imparted to the electrons, their thermal vibrations become more energetic, as a result of which some electrons manage to break away from their atoms. These electrons become free and, in the absence of an external electric field, make chaotic movements, move in free space.

Atoms that have lost electrons cannot make random movements, but only slightly oscillate relative to their normal position in the crystal lattice. Such atoms that have lost electrons are called positive ions. We can assume that in place of electrons torn out of their atoms, free places are obtained, which are commonly called holes.

In general, the number of electrons and holes is the same, so a hole can capture an electron that is nearby. As a result, the atom from a positive ion again becomes neutral. The process of combining electrons with holes is called recombination.

The detachment of electrons from atoms occurs with the same frequency, therefore, on average, the number of electrons and holes for a particular semiconductor is equal, is a constant value and depends on external conditions, primarily temperature.

If a voltage is applied to a semiconductor crystal, then the movement of electrons will become ordered, a current will flow through the crystal, due to its electronic and hole conductivity. This conductivity is called intrinsic, it has already been mentioned a little higher.

But semiconductors in their pure form, which have electronic and hole conductivity, are unsuitable for the manufacture of diodes, transistors and other parts, since the basis of these devices is the p-n (read “pe-en”) junction.

To obtain such a transition, two types of semiconductors are needed, two types of conductivity (p - positive - positive, hole) and (n - negative - negative, electronic). These types of semiconductors are obtained by doping, adding impurities to pure germanium or silicon crystals.

Although the amount of impurities is very small, their presence to a large extent changes the properties of the semiconductor, making it possible to obtain semiconductors of different conductivity. This will be discussed in the next part of the article.

Boris Aladyshkin,

conductor resistance. Conductivity. Dielectrics. The use of conductors and insulators. Semiconductors.

Physical substances are diverse in their electrical properties. The most extensive classes of matter are conductors and dielectrics.

conductors

The main feature of conductors- the presence of free charge carriers that participate in thermal motion and can move throughout the volume of matter.
As a rule, such substances include salt solutions, melts, water (except distilled water), moist soil, the human body and, of course, metals.

Metals considered to be the best conductors of electric charge.
There are also very good conductors that are not metals.
Among such conductors, carbon is the best example.
All conductors have properties such as resistance and conductivity . Due to the fact that electric charges, colliding with atoms or ions of a substance, overcome some resistance to their movement in an electric field, it is customary to say that conductors have electrical resistance ( R).
The reciprocal of resistance is called conductivity ( G).

G = 1/R

That is, the conductivity – is the property or ability of a conductor to conduct an electric current.
You need to understand that good conductors represent a very small resistance to the flow of electric charges and, accordingly, have high conductivity. The better the conductor, the greater its conductivity. For example, a copper conductor has b about greater conductivity than an aluminum conductor, and the conductivity of a silver conductor is higher than that of a copper conductor.

Dielectrics

The dielectrics are, in the first place, gases that conduct electrical charges very poorly. As well as glass, porcelain, ceramics, rubber, cardboard, dry wood, various plastics and resins.

Items made of dielectrics are called insulators. It should be noted that the dielectric properties of insulators largely depend on the state of the environment. So, in conditions of high humidity (water is a good conductor), some dielectrics may partially lose their dielectric properties.

On the use of conductors and insulators

For example, all electrical wires in the house are made of metal (most often copper or aluminum). And the sheath of these wires or the plug that is plugged into the outlet must be made of various polymers, which are good insulators and do not allow electrical charges to pass through.

It should be noted that the terms "conductor" or "insulator" do not reflect qualitative characteristics: the characteristics of these materials in fact are in a wide range - from very good to very bad.
Silver, gold, platinum are very good conductors, but these are expensive metals, so they are used only where the price is less important compared to the function of the product (space, defense).
Copper and aluminum are also good conductors and at the same time inexpensive, which predetermined their widespread use.
Tungsten and molybdenum, on the contrary, are poor conductors and for this reason cannot be used in electrical circuits (they will disrupt the operation of the circuit), but the high resistance of these metals, combined with infusibility, predetermined their use in incandescent lamps and high-temperature heating elements.

insulators there are also very good ones, just good ones and bad ones. This is due to the fact that in real dielectrics there are also free electrons, although there are very few of them. The appearance of free charges even in insulators is due to thermal vibrations of electrons: under the influence of high temperature, some electrons still manage to break away from the nucleus and the insulating properties of the dielectric deteriorate. In some dielectrics, there are more free electrons and the quality of their insulation is, accordingly, worse. It is enough to compare, for example, ceramics and cardboard.

The best insulator is an ideal vacuum, but it is practically unattainable on Earth. Absolutely pure water would also be a great insulator, but has anyone seen it in real life? And water with the presence of any impurities is already a fairly good conductor.
The quality criterion of an insulator is its compliance with the functions that it must perform in a given circuit. If the dielectric properties of a material are such that any leakage through it is negligible (does not affect the operation of the circuit), then such a material is considered a good insulator.

Semiconductors

There are substances, which in their conductivity occupy an intermediate position between conductors and dielectrics.
Such substances are called semiconductors. They differ from conductors in the strong dependence of the conductivity of electric charges on temperature, as well as on the concentration of impurities, and can have the properties of both conductors and dielectrics.

Unlike metallic conductors, in which the conductivity decreases with increasing temperature, in semiconductors, the conductivity increases with increasing temperature, and the resistance, as the reciprocal of conductivity, decreases.

At low temperatures semiconductor resistance, as seen from rice. one, tends to infinity.
This means that at a temperature of absolute zero, a semiconductor has no free carriers in the conduction band and, unlike conductors, behaves like a dielectric.
With an increase in temperature, as well as with the addition of impurities (doping), the conductivity of the semiconductor increases and it acquires the properties of a conductor.

Rice. one. The dependence of the resistance of conductors and semiconductors on temperature