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As conductive parts in electrical installations, conductors made of copper, aluminum, their alloys and iron (steel) are used.
Copper is one of the best conductive materials. The density of copper at 20 ° C is 8.95 g / cm 3, the melting point is 1083 ° C. Copper is chemically slightly active, but easily dissolves in nitric acid, and dissolves in dilute hydrochloric and sulfuric acids only in the presence of oxidizing agents (oxygen). In air, copper is quickly covered with a thin layer of dark-colored oxide, but this oxidation does not penetrate deep into the metal and serves as protection against further corrosion. Copper lends itself well to forging and rolling without heating.
Used for manufacturing electrolytic copper in ingots containing 99.93% pure copper.
The electrical conductivity of copper strongly depends on the amount and type of impurities and, to a lesser extent, on mechanical and thermal processing. at 20 ° C is 0.0172-0.018 ohm x mm2 / m.
For the manufacture of conductors, soft, semi-hard or hard copper is used with a specific gravity of 8.9, 8.95 and 8.96 g / cm 3, respectively.
For the manufacture of parts of current-carrying parts is widely used copper in alloys with other metals. The most commonly used alloys are:
Brass is an alloy of copper and zinc, containing at least 50% copper in the alloy, with the addition of other metals. brass 0.031 - 0.079 ohm x mm2/m. There are brass - tompak with a copper content of more than 72% (it has high ductility, anti-corrosion and anti-friction properties) and special brasses with addition of aluminium, tin, lead or manganese.
Brass contact
Bronzes are an alloy of copper and tin with an additive of various metals. Depending on the content of the main component in the alloy, bronzes are called tin, aluminum, silicon, phosphorous, and cadmium. Resistivity of bronze 0.021 - 0.052 ohm x mm 2 /m.
Brass and bronze have good mechanical and physico-chemical properties. They are easy to process by casting and pressure, resistant to atmospheric corrosion.
Aluminum - by its qualities the second conductive material after copper. Melting point 659.8 ° C. The density of aluminum at a temperature of 20 ° - 2.7 g / cm 3. Aluminum is easy to cast and well machined. At a temperature of 100 - 150 ° C, aluminum is forged and ductile (it can be rolled into sheets up to 0.01 mm thick).
The electrical conductivity of aluminum is highly dependent on impurities and little on mechanical and heat treatment. The purer the composition of aluminum, the higher its electrical conductivity and better resistance to chemical attack. Machining, rolling and annealing significantly affect the mechanical strength of aluminum. Cold working aluminum increases its hardness, elasticity and tensile strength. Resistivity of aluminum at 20 ° С 0.026 - 0.029 ohm x mm 2 / m.
When replacing copper with aluminum, the cross section of the conductor must be increased in terms of conductivities, i.e., 1.63 times.
With equal conductivity, an aluminum conductor will be 2 times lighter than a copper conductor.
For the manufacture of conductors, aluminum is used, containing at least 98% pure aluminum, silicon not more than 0.3%, iron not more than 0.2%
For the manufacture of parts of current-carrying parts, use aluminum alloys with other metals, for example: Duralumin - an alloy of aluminum with copper and manganese.
Silumin is a light cast aluminum alloy with an admixture of silicon, magnesium, manganese.
Aluminum alloys have good casting properties and high mechanical strength.
The most widely used in electrical engineering are the following aluminum alloys:
Wrought aluminum alloy grade AD, having aluminum not less than 98.8 and other impurities up to 1.2.
Wrought aluminum alloy brand AD1, having aluminum not less than 99.3 n other impurities up to 0.7.
Wrought aluminum alloy brand AD31, having aluminum 97.35 - 98.15 and other impurities 1.85 -2.65.
Alloys of grades AD and AD1 are used for the manufacture of cases and dies of hardware clamps. Profiles and tires used for electrical conductors are made from the AD31 grade alloy.
Products made of aluminum alloys as a result of heat treatment acquire high tensile strength and yield (creep).
Iron - melting point 1539°C. The density of iron is 7.87. Iron dissolves in acids, oxidizes with halogens and oxygen.
In electrical engineering, steels of various grades are used, for example:
Carbon steels are malleable alloys of iron with carbon and other metallurgical impurities.
The specific resistance of carbon steels is 0.103 - 0.204 ohm x mm 2 /m.
Alloy steels are alloys with additions of chromium, nickel and other elements added to carbon steel.
Steels are good.
As additives in alloys, as well as for the manufacture of solders and the implementation of conductive metals, the following are widely used:
Cadmium is a malleable metal. The melting point of cadmium is 321°C. Resistivity 0.1 ohm x mm 2 /m. In electrical engineering, cadmium is used for the preparation of low-melting solders and for protective coatings (cadmium) on metal surfaces. In terms of its anticorrosion properties, cadmium is close to zinc, but cadmium coatings are less porous and are applied in a thinner layer than zinc.
Nickel - melting point 1455°C. The specific resistance of nickel is 0.068 - 0.072 ohm x mm 2 /m. At normal temperatures, it is not oxidized by atmospheric oxygen. Nickel is used in alloys and for protective coating (nickel plating) of metal surfaces.
Tin - melting point 231.9 ° C. The specific resistance of tin is 0.124 - 0.116 ohm x mm 2 /m. Tin is used for soldering a protective coating (tinning) of metals in pure form and in the form of alloys with other metals.
Lead - melting point 327.4°C. Resistivity 0.217 - 0.227 ohm x mm 2 /m. Lead is used in alloys with other metals as an acid-resistant material. It is added to soldering alloys (solders).
Silver is a very malleable, malleable metal. The melting point of silver is 960.5°C. Silver is the best conductor of heat and electric current. The specific resistance of silver is 0.015 - 0.016 ohm x mm 2 / m. Silver is used for protective coating (silvering) of metal surfaces.
Antimony is a shiny brittle metal, melting point 631°C. Antimony is used in the form of additives in soldering alloys (solders).
Chrome is a hard, shiny metal. Melting point 1830°C. It does not change in air at normal temperature. The specific resistance of chromium is 0.026 ohm x mm 2 /m. Chromium is used in alloys and for protective coating (chrome plating) of metal surfaces.
Zinc - melting point 419.4°C. Resistivity of zinc 0.053 - 0.062 ohm x mm 2 /m. In humid air, zinc oxidizes, becoming covered with an oxide layer, which is protective against subsequent chemical attack. In electrical engineering, zinc is used as an additive in alloys and solders, as well as for a protective coating (galvanizing) of the surfaces of metal parts.
As soon as electricity left the laboratories of scientists and began to be widely introduced into practice Everyday life, the question arose of searching for materials that have certain, sometimes completely opposite, characteristics with respect to the flow of electric current through them.
For example, when transmitting electrical energy over a long distance, requirements were imposed on the material of the wires to minimize losses due to Joule heating in combination with low weight characteristics. An example of this is the familiar high-voltage power lines made of aluminum wires with a steel core.
Or, conversely, to create compact tubular electric heaters, materials with a relatively high electrical resistance and high thermal stability were required. The simplest example of a device that uses materials with similar properties is the burner of an ordinary kitchen electric stove.
The conductors used in biology and medicine as electrodes, probes and probes require high chemical resistance and compatibility with biomaterials, combined with low contact resistance.
A whole galaxy of inventors from different countries: England, Russia, Germany, Hungary and USA. Thomas Edison, having conducted more than a thousand experiments to test the properties of materials suitable for the role of filaments, created a lamp with a platinum spiral. Edison lamps, although they had a long service life, were not practical due to the high cost of the source material.
The subsequent work of the Russian inventor Lodygin, who proposed using relatively cheap refractory tungsten and molybdenum with a higher resistivity as thread materials, found practical use. In addition, Lodygin proposed pumping air out of incandescent bulbs, replacing it with inert or noble gases, which led to the creation of modern incandescent lamps. The pioneer of mass production of affordable and durable electric lamps was General Electric, to which Lodygin assigned the rights to his patents and then successfully worked in the company's laboratories for a long time.
This list can be continued, because the inquisitive human mind is so inventive that sometimes, in order to solve a certain technical problem, it needs materials with hitherto unknown properties or with incredible combinations of these properties. Nature no longer keeps up with our appetites, and scientists from all over the world have joined the race to create materials that have no natural analogues.
It is the intentional connection of an electrical enclosure or housing to a protective earthing device. Usually, grounding is carried out in the form of steel or copper strips, pipes, rods or angles buried in the ground to a depth of more than 2.5 meters, which, in the event of an accident, ensure the flow of current along the circuit device - case or casing - earth - neutral wire of the AC source. The resistance of this circuit should be no more than 4 ohms. In this case, the voltage on the body of the emergency device is reduced to values that are safe for humans, and automatic devices for protecting the electrical circuit in one way or another turn off the emergency device.
When calculating the elements of protective grounding, knowledge of the resistivity of soils plays a significant role, which can vary over a wide range.
In accordance with the data of the reference tables, the area of the grounding device is selected, the number of grounding elements and the actual design of the entire device are calculated from it. The connection of structural elements of the protective earthing device is carried out by welding.
Electrotomography
Electrical exploration studies the near-surface geological environment, is used to search for ore and non-metallic minerals and other objects based on the study of various artificial electric and electromagnetic fields. A special case of electrical exploration is electrical resistivity tomography - a method for determining the properties of rocks by their resistivity.
The essence of the method lies in the fact that at a certain position of the electric field source, voltage measurements are taken on various probes, then the field source is moved to another place or switched to another source and the measurements are repeated. Field sources and field receiver probes are placed on the surface and in wells.
Then the received data is processed and interpreted using modern computer processing methods that allow visualizing information in the form of two-dimensional and three-dimensional images.
Being a very accurate search method, electrotomography provides invaluable assistance to geologists, archaeologists and paleozoologists.
Determining the form of occurrence of mineral deposits and the boundaries of their distribution (outlining) makes it possible to identify the occurrence of vein deposits of minerals, which significantly reduces the cost of their subsequent development.
For archaeologists, this search method provides valuable information about the location of ancient burials and the presence of artifacts in them, thereby reducing excavation costs.
Paleozoologists use electrotomography to look for fossilized remains of ancient animals; the results of their work can be seen in natural science museums in the form of amazing reconstructions of the skeletons of prehistoric megafauna.
In addition, electrical tomography is used in the construction and subsequent operation of engineering structures: high-rise buildings, dams, dams, embankments, and others.
Resistivity definitions in practice
Sometimes, to solve practical problems, we may face the task of determining the composition of a substance, for example, a wire for a polystyrene foam cutter. We have two coils of wire of a suitable diameter from various materials unknown to us. To solve the problem, it is necessary to find their electrical resistivity and then determine the material of the wire using the difference between the values found or using a reference table.
We measure with a tape measure and cut off 2 meters of wire from each sample. Let's determine the wire diameters d₁ and d₂ with a micrometer. Turning on the multimeter to the lower limit of resistance measurement, we measure the resistance of the sample R₁. We repeat the procedure for another sample and also measure its resistance R₂.
We take into account that the cross-sectional area of the wires is calculated by the formula
S \u003d π ∙ d 2 / 4
Now the formula for calculating electrical resistivity will look like this:
ρ = R ∙ π ∙ d 2 /4 ∙ L
Substituting the obtained values of L, d₁ and R₁ into the formula for calculating the resistivity given in the article above, we calculate the value of ρ₁ for the first sample.
ρ 1 \u003d 0.12 ohm mm 2 / m
Substituting the obtained values of L, d₂ and R₂ into the formula, we calculate the value of ρ₂ for the second sample.
ρ 2 \u003d 1.2 ohm mm 2 / m
From comparing the values of ρ₁ and ρ₂ with the reference data of the above Table 2, we conclude that the material of the first sample is steel, and the second sample is nichrome, from which we will make the cutter string.
The ability of a metal to pass a charged current through itself is called. In turn, resistance is one of the characteristics of the material. The greater the electrical resistance at a given voltage, the smaller it will be. It characterizes the resistance force of the conductor to the movement of charged electrons directed along it. Since the transmission property of electricity is the reciprocal of resistance, it means that it will be expressed in the form of formulas as a ratio of 1 / R.
Resistivity always depends on the quality of the material used in the manufacture of devices. It is measured based on the parameters of a conductor with a length of 1 meter and a cross-sectional area of 1 square millimeter. For example, the property of specific resistance for copper is always 0.0175 Ohm, for aluminum - 0.029, iron - 0.135, constantan - 0.48, nichrome - 1-1.1. The specific resistance of steel is equal to the number 2 * 10-7 Ohm.m
The resistance to current is directly proportional to the length of the conductor along which it moves. The longer the device, the higher the resistance. It will be easier to learn this dependence if you imagine two imaginary pairs of vessels communicating with each other. Let the connecting tube remain thinner for one pair of devices, and thicker for the other. When both pairs are filled with water, the transition of the liquid into the thick tube will be much faster, because it will have less resistance to the flow of water. By this analogy, it is easier for him to pass along a thick conductor than a thin one.
Resistivity, as an SI unit, is measured in ohm.m. Conductivity depends on the mean free path of charged particles, which is characterized by the structure of the material. Metals without impurities, in which the most correct one, have the lowest counteraction values. Conversely, impurities distort the lattice, thereby increasing its performance. The resistivity of metals is located in a narrow range of values at normal temperature: from silver from 0.016 to 10 μOhm.m (iron and chromium alloys with aluminum).
On the features of the movement of charged
electrons in a conductor is affected by temperature, since as it increases, the amplitude of wave oscillations of existing ions and atoms increases. As a result, the electrons have less free space for normal movement in the crystal lattice. And this means that the obstacle to orderly movement is increasing. The resistivity of any conductor, as usual, increases linearly with increasing temperature. And for semiconductors, on the contrary, a decrease with increasing degrees is characteristic, since because of this, many charges are released that create a direct electric current.
The process of cooling some metal conductors to the desired temperature, brings their resistivity to a jump-like state and drops to zero. This phenomenon was discovered in 1911 and called superconductivity.
Most of the laws of physics are based on experiments. The names of the experimenters are immortalized in the titles of these laws. One of them was Georg Ohm.
Georg Ohm's experiments
He established in the course of experiments on the interaction of electricity with various substances, including metals, the fundamental relationship between density, electric field strength and the property of a substance, which was called "conductivity". The formula corresponding to this pattern, called "Ohm's Law" is as follows:
j= λE , wherein
- j- electric current density;
- λ — specific conductivity, also referred to as "electrical conductivity";
- E- electric field strength.
In some cases, another letter of the Greek alphabet is used to denote conductivity - σ . The specific conductivity depends on some parameters of the substance. Its value is influenced by temperature, substances, pressure, if it is a gas, and most importantly, the structure of this substance. Ohm's law is observed only for homogeneous substances.
For more convenient calculations, the reciprocal of the conductivity is used. It was called "resistivity", which is also associated with the properties of the substance in which the electric current flows, denoted by the Greek letter ρ and has the dimension of Ohm*m. But since for various physical phenomena different theoretical justifications apply, alternative formulas can be used for resistivity. They are a reflection of the classical electronic theory of metals, as well as quantum theory.
Formulas
In these tedious, for ordinary readers, formulas such factors as Boltzmann's constant, Avogadro's constant and Planck's constant appear. These constants are used for calculations that take into account the free path of electrons in a conductor, their speed during thermal motion, the degree of ionization, the concentration and density of the substance. In a word, everything is quite difficult for a non-specialist. In order not to be unfounded, further you can get acquainted with how everything looks in reality:
Features of metals
Since the movement of electrons depends on the homogeneity of the substance, the current in a metal conductor flows according to its structure, which affects the distribution of electrons in the conductor, taking into account its inhomogeneity. It is determined not only by the presence of impurity inclusions, but also by physical defects - cracks, voids, etc. The inhomogeneity of the conductor increases its resistivity, which is determined by the Matthiesen rule.
This simple-to-understand rule, in fact, says that several separate resistivities can be distinguished in a current-carrying conductor. And the resulting value will be their sum. The terms will be the resistivity of the crystal lattice of the metal, impurities and conductor defects. Since this parameter depends on the nature of the substance, the corresponding regularities are determined for its calculation, including for mixed substances.
Despite the fact that alloys are also metals, they are considered as solutions with a chaotic structure, and for calculating the resistivity it matters which metals are included in the composition of the alloy. Basically, most of the two-component alloys that do not belong to the transition and rare earth metals fall under the description of Nodheim's law.
As a separate topic, the resistivity of metallic thin films is considered. The fact that its value should be greater than that of a bulk conductor made of the same metal is quite logical to assume. But at the same time, a special Fuchs empirical formula is introduced for the film, which describes the interdependence of the resistivity and film thickness. It turns out that in films, metals exhibit the properties of semiconductors.
And the process of charge transfer is influenced by electrons that move in the direction of the film thickness and interfere with the movement of "longitudinal" charges. At the same time, they are reflected from the surface of the film conductor, and thus one electron oscillates for a sufficiently long time between its two surfaces. Another significant factor in increasing resistivity is the temperature of the conductor. The higher the temperature, the greater the resistance. Conversely, the lower the temperature, the lower the resistance.
Metals are substances with the lowest resistivity at the so-called "room" temperature. The only non-metal that justifies its use as a conductor is carbon. Graphite, which is one of its varieties, is widely used to make sliding contacts. He has very good combination properties such as resistivity and coefficient of sliding friction. Therefore, graphite is an indispensable material for motor brushes and other sliding contacts. The resistivity values of the main substances used for industrial purposes are shown in the table below.
Superconductivity
At temperatures corresponding to the liquefaction of gases, that is, up to the temperature of liquid helium, which is equal to - 273 degrees Celsius, the resistivity decreases almost to complete disappearance. And not only good metal conductors such as silver, copper and aluminum. Almost all metals. Under such conditions, which are called superconductivity, the metal structure has no inhibitory effect on the movement of charges under the action of an electric field. Therefore, mercury and most metals become superconductors.
But, as it turned out, relatively recently in the 80s of the 20th century, some varieties of ceramics are also capable of superconductivity. And for this you do not need to use liquid helium. Such materials are called high-temperature superconductors. However, several decades have already passed, and the range of high-temperature conductors has expanded significantly. But the mass use of such high-temperature superconducting elements is not observed. In some countries, single installations have been made with the replacement of conventional copper conductors with high-temperature superconductors. To maintain the normal mode of high-temperature superconductivity, liquid nitrogen is necessary. And this turns out to be too expensive a technical solution.
Therefore, the low value of resistivity, bestowed by Nature on copper and aluminum, still makes them indispensable materials for the manufacture of various conductors of electric current.
Experience has shown that the resistance R metal conductor is directly proportional to its length L and inversely proportional to its cross-sectional area BUT:
R = ρ L/ BUT (26.4)
where coefficient ρ is called resistivity and serves as a characteristic of the substance from which the conductor is made. This is in line with common sense: the resistance of a thick wire should be less than that of a thin wire, since electrons can move over a larger area in a thick wire. And we can expect an increase in resistance with an increase in the length of the conductor, since the number of obstacles in the path of the electron flow increases.
Typical values ρ for different materials are given in the first column of the table. 26.2. (Actual values may vary depending on purity, heat treatment, temperature, and other factors.)
|
Table 26.2. Resistivity and temperature coefficient of resistance (TCR) (at 20 °C) |
||
| Substance | ρ ,Ohm m | tks α ,°C -1 |
| conductors | ||
| Silver | 1.59 10 -8 | 0,0061 |
| Copper | 1.68 10 -8 | 0,0068 |
| Aluminum | 2.65 10 -8 | 0,00429 |
| Tungsten | 5.6 10 -8 | 0,0045 |
| Iron | 9.71 10 -8 | 0,00651 |
| Platinum | 10.6 10 -8 | 0,003927 |
| Mercury | 98 10 -8 | 0,0009 |
| Nichrome (Ni, Fe, Cr alloy) | 100 10 -8 | 0,0004 |
| Semiconductors 1) | ||
| Carbon (graphite) | (3-60) 10 -5 | -0,0005 |
| Germanium | (1-500) 10 -5 | -0,05 |
| Silicon | 0,1 - 60 | -0,07 |
| Dielectrics | ||
| Glass | 10 9 - 10 12 | |
| Rubber hard | 10 13 - 10 15 | |
| 1) The actual values strongly depend on the presence of even a small amount of impurities. | ||
Silver has the lowest resistivity and is thus the best conductor; however, it is expensive. Copper is slightly inferior to silver; it is clear why wires are most often made of copper.
The specific resistance of aluminum is higher than that of copper, but it has a much lower density, and in some cases it is preferred (for example, in power lines), since the resistance of aluminum wires of the same mass is less than that of copper. The reciprocal of resistivity is often used:
σ = 1/ρ (26.5)
σ called specific conductivity. Conductivity is measured in units of (Ohm m) -1 .
The resistivity of a substance depends on temperature. Generally, the resistance of metals increases with temperature. This should not be surprising: as the temperature rises, the atoms move faster, their arrangement becomes less ordered, and they can be expected to interfere more with the flow of electrons. In narrow temperature ranges, the resistivity of the metal increases almost linearly with temperature:
where ρT- resistivity at temperature T, ρ 0 - resistivity at standard temperature T 0 , and α - temperature coefficient of resistance (TCR). The values of a are given in Table. 26.2. Note that for semiconductors, TCR can be negative. This is obvious, since with increasing temperature the number of free electrons increases and they improve the conductive properties of the substance. Thus, the resistance of a semiconductor can decrease with increasing temperature (although not always).
The values of a depend on the temperature, so you should pay attention to the temperature range within which the given value(for example, according to the reference book of physical quantities). If the range of temperature change is wide, then the linearity will be violated, and instead of (26.6), an expression containing terms that depend on the second and third degrees of temperature should be used:
ρT = ρ 0 (1+αT+ + βT 2 + γT 3),
where coefficients β and γ usually very small (we put T 0 = 0°C), but at high T the contribution of these members becomes significant.
At very low temperatures, the resistivity of some metals, as well as alloys and compounds, drops to zero within the accuracy of modern measurements. This property is called superconductivity; it was first observed by the Dutch physicist Geike Kamerling-Onnes (1853-1926) in 1911 when mercury was cooled below 4.2 K. At this temperature, the electrical resistance of mercury suddenly dropped to zero.
Superconductors go into the superconducting state below the transition temperature, which is usually a few degrees Kelvin (slightly above absolute zero). An electric current was observed in the superconducting ring, which practically did not weaken in the absence of voltage for several years.
In recent years, superconductivity has been intensively studied in order to elucidate its mechanism and find materials that have superconductivity at more high temperatures to reduce the cost and inconvenience of having to cool to very low temperatures. The first successful theory of superconductivity was created by Bardeen, Cooper and Schrieffer in 1957. Superconductors are already being used in large magnets, where the magnetic field is generated by electric current (see Chap. 28), which significantly reduces power consumption. Of course, energy is also expended to maintain a superconductor at a low temperature.
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Content:The appearance of an electric current occurs when the circuit is closed, when a potential difference occurs at the terminals. The movement of free electrons in a conductor is carried out under the action of an electric field. In the process of movement, electrons collide with atoms and partially transfer their accumulated energy to them. This leads to a decrease in their speed of movement. Later, under the influence of the electric field, the speed of the electrons increases again. The result of such resistance is the heating of the conductor through which the current flows. Exist various ways calculations of this quantity, including the resistivity formula used for materials with individual physical properties.
Electrical resistivity
The essence of electrical resistance lies in the ability of a substance to convert electrical energy into thermal energy during the action of a current. This value is denoted by the symbol R, and Ohm is used as the unit of measurement. The value of resistance in each case is related to the ability of one or another.
In the process of research, a dependence on resistance was established. One of the main qualities of the material is its resistivity, which varies depending on the length of the conductor. That is, with an increase in the length of the wire, the value of the resistance also increases. This dependence is defined as directly proportional.
Another property of a material is its cross-sectional area. It represents the dimensions of the cross section of the conductor, regardless of its configuration. In this case, an inversely proportional relationship is obtained, when decreases with an increase in the cross-sectional area.
Another factor that affects resistance is the material itself. During the research, different resistance was found in different materials. Thus, the values of specific electrical resistances for each substance were obtained.
It turned out that the best conductors are metals. Among them, silver has the lowest resistance and high conductivity. They are used in the most critical places of electronic circuits, besides, copper has a relatively low cost.
Substances with a very high resistivity are considered poor conductors of electric current. Therefore, they are used as insulating materials. The dielectric properties are most characteristic of porcelain and ebonite.
Thus, the resistivity of the conductor is of great importance, since it can be used to determine the material from which the conductor was made. To do this, the cross-sectional area is measured, the current strength and voltage are determined. This allows you to set the value of electrical resistivity, after which, using a special table, you can easily determine the substance. Therefore, resistivity is one of the most characteristic features of a material. This indicator allows you to determine the most optimal length of the electrical circuit so that balance is maintained.
Formula
Based on the data obtained, it can be concluded that the resistivity will be considered the resistance of any material with a unit area and a unit length. That is, a resistance equal to 1 ohm occurs at a voltage of 1 volt and a current of 1 ampere. This indicator is influenced by the degree of purity of the material. For example, if only 1% manganese is added to copper, then its resistance will increase by 3 times.
Resistivity and conductivity of materials
Conductivity and resistivity are considered as a rule at a temperature of 20 0 C. These properties will differ for different metals:
- Copper. Most often used for the manufacture of wires and cables. It has high strength, corrosion resistance, easy and simple processing. In good copper, the proportion of impurities is no more than 0.1%. If necessary, copper can be used in alloys with other metals.
- Aluminum. Its specific gravity is less than that of copper, but it has a higher heat capacity and melting point. It takes much more energy to melt aluminum than copper. Impurities in high-quality aluminum do not exceed 0.5%.
- Iron. Along with the availability and low cost, this material has a high resistivity. In addition, it has low corrosion resistance. Therefore, the coating of steel conductors with copper or zinc is practiced.
The specific resistance formula at low temperatures is considered separately. In these cases, the properties of the same materials will be completely different. For some of them, resistance may drop to zero. This phenomenon is called superconductivity, in which the optical and structural characteristics of the material remain unchanged.
- active - or ohmic, resistive - resulting from the cost of electricity for heating the conductor (metal) when an electric current passes through it, and
- reactive - capacitive or inductive - which comes from the inevitable losses to create any changes in the current passing through the conductor of electric fields, then the resistivity of the conductor can be of two varieties:
Resistivity of popular conductors (metals and alloys). Steel resistivity
Resistivity of iron, aluminum and other conductors
The transmission of electricity over long distances requires taking care to minimize the losses resulting from overcoming the resistance of the conductors that make up the electric line. Of course, this does not mean that such losses, which already occur specifically in the circuits and consumption devices, do not play a role.
Therefore, it is important to know the parameters of all the elements and materials used. And not only electrical, but also mechanical. And to have at your disposal some convenient reference materials that allow you to compare the characteristics of different materials and choose exactly what will be optimal in a particular situation for design and operation. In power transmission lines, where the task is most productive, that is, with high efficiency, to bring energy to the consumer, both the economics of losses and the mechanics of the lines themselves are taken into account. The final economic efficiency of the line depends on the mechanics - that is, the arrangement and arrangement of conductors, insulators, supports, step-up / step-down transformers, the weight and strength of all structures, including wires stretched over long distances, as well as on the materials chosen for each structural element. , its work and operating costs. In addition, in the lines that transmit electricity, the requirements for ensuring the safety of both the lines themselves and the environment where they pass are higher. And this adds costs both to ensure the wiring of electricity, and to an additional margin of safety for all structures.
For comparison, the data is usually reduced to a single, comparable form. Often, the epithet “specific” is added to such characteristics, and the values themselves are considered on some standards unified in terms of physical parameters. For example, electrical resistivity is the resistance (ohm) of a conductor made of some metal (copper, aluminum, steel, tungsten, gold) having a unit length and unit section in the system of units used (usually in SI). In addition, the temperature is specified, since when heated, the resistance of the conductors can behave differently. Normal average operating conditions are taken as a basis - at 20 degrees Celsius. And where properties are important when changing the parameters of the medium (temperature, pressure), coefficients are introduced and additional tables and graphs of dependencies are compiled.
Types of resistivity
Because resistance is:
- Specific electrical resistance to direct current (having a resistive character) and
- Specific electrical resistance to alternating current (having a reactive character).
Here, type 2 resistivity is a complex value, it consists of two components of the TP - active and reactive, since resistive resistance always exists when current passes, regardless of its nature, and reactive resistance occurs only with any change in current in circuits. In DC circuits, reactance occurs only during transients that are associated with current on (change in current from 0 to nominal) or off (difference from nominal to 0). And they are usually taken into account only when designing overload protection.
In AC circuits, the phenomena associated with reactances are much more diverse. They depend not only on the actual passage of current through a certain section, but also on the shape of the conductor, and the dependence is not linear.
The fact is that alternating current induces an electric field both around the conductor through which it flows, and in the conductor itself. And from this field, eddy currents arise, which give the effect of “pushing out” the actual main movement of charges, from the depth of the entire section of the conductor to its surface, the so-called “skin effect” (from skin - skin). It turns out that eddy currents, as it were, “steal” its cross section from the conductor. The current flows in a certain layer close to the surface, the rest of the conductor thickness remains unused, it does not reduce its resistance, and there is simply no point in increasing the thickness of the conductors. Especially at high frequencies. Therefore, for alternating current, resistances are measured in such cross sections of conductors, where its entire cross section can be considered near-surface. Such a wire is called thin, its thickness is equal to twice the depth of this surface layer, where eddy currents displace the useful main current flowing in the conductor.
Of course, the effective conduction of alternating current is not limited to a decrease in the thickness of wires that are round in cross section. The conductor can be thinned, but at the same time made flat in the form of a tape, then the cross section will be higher than that of a round wire, respectively, and the resistance is lower. In addition, simply increasing the surface area will have the effect of increasing the effective cross section. The same can be achieved by using a stranded wire instead of a single strand, in addition, a stranded wire is superior in flexibility to a single strand, which is often also valuable. On the other hand, taking into account the skin effect in the wires, it is possible to make the wires composite by making the core from a metal that has good strength characteristics, such as steel, but low electrical characteristics. At the same time, an aluminum braid is made over the steel, which has a lower resistivity.
In addition to the skin effect, the flow of alternating current in conductors is affected by the excitation of eddy currents in the surrounding conductors. Such currents are called pickup currents, and they are induced both in metals that do not play the role of wiring (bearing structural elements), and in the wires of the entire conductive complex - playing the role of wires of other phases, zero, grounding.
All of these phenomena are found in all designs related to electricity, this further reinforces the importance of having at your disposal summary reference information for a wide variety of materials.
Resistivity for conductors is measured with very sensitive and accurate instruments, since metals are selected for wiring and have the lowest resistance - of the order of ohm * 10-6 per meter of length and square. mm. sections. To measure the resistivity of the insulation, instruments are needed, on the contrary, having ranges of very large resistance values - usually megohms. It is clear that conductors must conduct well, and insulators must be well insulated.
Table
Iron as a conductor in electrical engineering
Iron is the most common metal in nature and technology (after hydrogen, which is also a metal). It is the cheapest and has excellent strength characteristics, therefore it is used everywhere as the basis of strength. various designs.
In electrical engineering, iron is used as a conductor in the form of steel flexible wires where physical strength and flexibility are needed, and the desired resistance can be achieved due to the appropriate section.
Having a table of specific resistances of various metals and alloys, it is possible to calculate the cross sections of wires made from different conductors.
As an example, let's try to find the electrically equivalent cross section of conductors made of different materials: copper, tungsten, nickel and iron wires. For the initial take aluminum wire with a cross section of 2.5 mm.
We need that over a length of 1 m, the resistance of the wire from all these metals is equal to the resistance of the original one. The resistance of aluminum per 1 m of length and 2.5 mm of cross section will be equal to
, where R is the resistance, ρ is the resistivity of the metal from the table, S is the cross-sectional area, L is the length.Substituting the initial values, we get the resistance of a meter-long piece of aluminum wire in ohms.
After that, we solve the formula for S
, we will substitute the values from the table and get the cross-sectional areas for different metals.
Since the resistivity in the table is measured on a wire 1 m long, in microohms per 1 mm2 of cross section, we got it in microohms. To get it in ohms, you need to multiply the value by 10-6. But the number of ohms with 6 zeros after the decimal point is not necessary for us to get, since we still find the final result in mm2.
As you can see, the resistance of iron is quite large, the wire is thick.
But there are materials that have even more, such as nickeline or constantan.
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Table of electrical resistivity of metals and alloys in electrical engineering
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Specific resistance of metals.
Specific resistance of alloys.
The values are given at t = 20° C. The resistances of the alloys depend on their exact composition. comments powered by HyperCommentstab.wikimassa.org
Specific electrical resistance | world of welding
Electrical resistivity of materials
Electrical resistivity (resistivity) - the ability of a substance to prevent the passage of electric current.
Unit of measure (SI) - Ohm m; also measured in ohm cm and ohm mm2/m.
| Metals | ||
| Aluminum | 20 | 0.028 10-6 |
| Beryllium | 20 | 0.036 10-6 |
| Phosphor bronze | 20 | 0.08 10-6 |
| Vanadium | 20 | 0.196 10-6 |
| Tungsten | 20 | 0.055 10-6 |
| Hafnium | 20 | 0.322 10-6 |
| Duralumin | 20 | 0.034 10-6 |
| Iron | 20 | 0.097 10-6 |
| Gold | 20 | 0.024 10-6 |
| Iridium | 20 | 0.063 10-6 |
| Cadmium | 20 | 0.076 10-6 |
| Potassium | 20 | 0.066 10-6 |
| Calcium | 20 | 0.046 10-6 |
| Cobalt | 20 | 0.097 10-6 |
| Silicon | 27 | 0.58 10-4 |
| Brass | 20 | 0.075 10-6 |
| Magnesium | 20 | 0.045 10-6 |
| Manganese | 20 | 0.050 10-6 |
| Copper | 20 | 0.017 10-6 |
| Magnesium | 20 | 0.054 10-6 |
| Molybdenum | 20 | 0.057 10-6 |
| Sodium | 20 | 0.047 10-6 |
| Nickel | 20 | 0.073 10-6 |
| Niobium | 20 | 0.152 10-6 |
| Tin | 20 | 0.113 10-6 |
| Palladium | 20 | 0.107 10-6 |
| Platinum | 20 | 0.110 10-6 |
| Rhodium | 20 | 0.047 10-6 |
| Mercury | 20 | 0.958 10-6 |
| Lead | 20 | 0.221 10-6 |
| Silver | 20 | 0.016 10-6 |
| Steel | 20 | 0.12 10-6 |
| Tantalum | 20 | 0.146 10-6 |
| Titanium | 20 | 0.54 10-6 |
| Chromium | 20 | 0.131 10-6 |
| Zinc | 20 | 0.061 10-6 |
| Zirconium | 20 | 0.45 10-6 |
| Cast iron | 20 | 0.65 10-6 |
| plastics | ||
| Getinax | 20 | 109–1012 |
| Kapron | 20 | 1010–1011 |
| Lavsan | 20 | 1014–1016 |
| Organic glass | 20 | 1011–1013 |
| Styrofoam | 20 | 1011 |
| PVC | 20 | 1010–1012 |
| Polystyrene | 20 | 1013–1015 |
| Polyethylene | 20 | 1015 |
| Fiberglass | 20 | 1011–1012 |
| Textolite | 20 | 107–1010 |
| Celluloid | 20 | 109 |
| Ebonite | 20 | 1012–1014 |
| rubber | ||
| Rubber | 20 | 1011–1012 |
| Liquids | ||
| Transformer oil | 20 | 1010–1013 |
| gases | ||
| Air | 0 | 1015–1018 |
| Wood | ||
| Dry wood | 20 | 109–1010 |
| Minerals | ||
| Quartz | 230 | 109 |
| Mica | 20 | 1011–1015 |
| Various materials | ||
| Glass | 20 | 109–1013 |
LITERATURE
- Alpha and Omega. Brief reference / Tallinn: Printest, 1991 - 448 p.
- Handbook of elementary physics / N.N. Koshkin, M.G. Shirkevich. M., Science. 1976. 256 p.
- Reference book on welding of non-ferrous metals / S.M. Gurevich. Kyiv: Naukova Dumka. 1990. 512 p.
weldworld.com
Resistivity of metals, electrolytes and substances (Table)
Resistivity of metals and insulators
The reference table gives the resistivity p values of some metals and insulators at a temperature of 18-20 ° C, expressed in ohm cm. The value of p for metals is highly dependent on impurities, the table gives p values for chemically pure metals, for insulators they are given approximately. Metals and insulators are arranged in the table in order of increasing p values.
Table resistivity of metals
| pure metals | 104 ρ (ohm cm) | pure metals | 104 ρ (ohm cm) |
| Aluminum | |||
| Duralumin | |||
| Platinite 2) | |||
| Argentan | |||
| Manganese | |||
| Manganin | |||
| Tungsten | Constantan | ||
| Molybdenum | Wood alloy 3) | ||
| Alloy Rose 4) | |||
| Palladium | Fekhral 6) | ||
Table of resistivity of insulators
| insulators | insulators | ||
| wood dry | |||
| Celluloid | |||
| Rosin | |||
| Getinax | Quartz _|_ axis | ||
| Soda glass | Polystyrene | ||
| pyrex glass | |||
| Quartz || axes | |||
| Fused quartz |
Resistivity of pure metals at low temperatures
The table gives the resistivity values (in ohm cm) of some pure metals at low temperatures (0°C).
The ratio of resistance Rt / Rq of pure metals at a temperature of T ° K and 273 ° K.
The reference table gives the ratio Rt / Rq of the resistances of pure metals at a temperature of T ° K and 273 ° K.
| pure metals | ||
| Aluminum | ||
| Tungsten | ||
| Molybdenum | ||
Resistivity of electrolytes
The table gives the values of the specific resistance of electrolytes in ohm cm at a temperature of 18 ° C. The concentration of solutions c is given as a percentage, which determines the number of grams of anhydrous salt or acid in 100 g of solution.
Source of information: BRIEF PHYSICAL AND TECHNICAL HANDBOOK / Volume 1, - M .: 1960.
infotables.ru
Electrical resistivity - steel
Page 1
The electrical resistivity of steel increases with increasing temperature, and the greatest changes are observed when heated to the Curie point temperature. After the Curie point, the value of electrical resistivity changes insignificantly and at temperatures above 1000 C practically remains constant.
Due to the high electrical resistivity of the steel, these iuKii create a large slowdown in the decay of the flux. In contactors for 100 a, the drop-off time is 0 07 sec, and in contactors 600 a-0 23 sec. Due to the special requirements for contactors of the KMV series, which are designed to turn on and off the electromagnets of oil circuit breaker drives, the electromagnetic mechanism of these contactors allows adjustment of the operation voltage and release voltage by adjusting the force of the return spring and a special tear-off spring. Contactors of the KMV type must operate with a deep voltage drop. Therefore, the minimum operating voltage for these contactors can drop down to 65% UH. This low pickup voltage causes a current to flow through the winding at rated voltage, resulting in increased heating of the coil.
The silicon additive increases the electrical resistivity of the steel almost in proportion to the silicon content and thereby helps to reduce the eddy current losses that occur in the steel when it is operated in an alternating magnetic field.
Silicon additive increases the electrical resistivity of steel, which helps to reduce eddy current losses, but at the same time, silicon worsens the mechanical properties of steel, making it brittle.
Ohm - mm2 / m - electrical resistivity of steel.
To reduce eddy currents, cores are used, made of steel grades with increased electrical resistivity of steel, containing 0 5 - 4 8% silicon.
To do this, a thin screen made of magnetically soft steel was put on a massive rotor made of the optimal CM-19 alloy. The specific electrical resistance of steel differs little from the specific resistance of the alloy, and the cg of steel is approximately an order of magnitude higher. The thickness of the screen is chosen according to the penetration depth of the first-order tooth harmonics and is equal to d 0 8 mm. For comparison, additional losses are given, W, with a basic squirrel-cage rotor and a two-layer rotor with a massive cylinder made of SM-19 alloy and with copper end rings.
The main magnetically conductive material is sheet alloyed electrical steel containing from 2 to 5% silicon. Silicon additive increases the electrical resistivity of steel, resulting in reduced eddy current losses, steel becomes resistant to oxidation and aging, but becomes more brittle. In recent years, cold-rolled grain-oriented steel with higher magnetic properties in the rolling direction has been widely used. To reduce losses from eddy currents, the core of the magnetic circuit is made in the form of a package assembled from sheets of stamped steel.
Electrical steel is a low carbon steel. For improvement magnetic characteristics silicon is introduced into it, which causes an increase in the electrical resistivity of steel. This leads to a reduction in eddy current losses.
After machining, the magnetic circuit is annealed. Since eddy currents in steel are involved in creating the deceleration, one should focus on the electrical resistivity of steel on the order of Pc (Yu-15) 10 - 6 ohm cm. In the attracted position of the armature, the magnetic system is quite strongly saturated, so the initial induction in various magnetic systems fluctuates within very small limits and is for steel grade E Vn1 6 - 1 7 Ch. The specified value of induction maintains the field strength in the steel of the order of Yang.
For the manufacture of magnetic systems (magnetic cores) of transformers, special thin-sheet electrical steels are used, which have an increased (up to 5%) silicon content. Silicon contributes to the decarburization of steel, which leads to an increase in magnetic permeability, reduces hysteresis losses and increases its electrical resistivity. An increase in the specific electrical resistance of steel makes it possible to reduce losses in it from eddy currents. In addition, silicon weakens the aging of steel (an increase in losses in steel over time), reduces its magnetostriction (change in the shape and size of a body during magnetization) and, consequently, the noise of transformers. At the same time, the presence of silicon in steel leads to an increase in its brittleness and makes it difficult to machining.
Pages: 1 2
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Resistivity | Wikitronics Wiki
Resistivity is a characteristic of a material that determines its ability to conduct electric current. Defined as the ratio of the electric field to the current density. In the general case, it is a tensor, but for most materials that do not exhibit anisotropic properties, it is taken as a scalar value.
Designation - ρ
$ \vec E = \rho \vec j, $
$ \vec E $ - electric field strength, $ \vec j $ - current density.
The SI unit is an ohmmeter (ohm m, Ω m).
The resistance of a cylinder or prism (between the ends) of a material of length l and cross section S in terms of resistivity is determined as follows:
$ R = \frac(\rho l)(S). $
In technology, the definition of resistivity is used, as the resistance of a conductor of unit cross section and unit length.
Resistivity of some materials used in electrical engineering Edit
| silver | 1.59 10⁻⁸ | 4.10 10⁻³ |
| copper | 1.67 10⁻⁸ | 4.33 10⁻³ |
| gold | 2.35 10⁻⁸ | 3.98 10⁻³ |
| aluminum | 2.65 10⁻⁸ | 4.29 10⁻³ |
| tungsten | 5.65 10⁻⁸ | 4.83 10⁻³ |
| brass | 6.5 10⁻⁸ | 1.5 10⁻³ |
| nickel | 6.84 10⁻⁸ | 6.75 10⁻³ |
| iron(α) | 9.7 10⁻⁸ | 6.57 10⁻³ |
| tin gray | 1.01 10⁻⁷ | 4.63 10⁻³ |
| platinum | 1.06 10⁻⁷ | 6.75 10⁻³ |
| tin white | 1.1 10⁻⁷ | 4.63 10⁻³ |
| steel | 1.6 10⁻⁷ | 3.3 10⁻³ |
| lead | 2.06 10⁻⁷ | 4.22 10⁻³ |
| duralumin | 4.0 10⁻⁷ | 2.8 10⁻³ |
| manganin | 4.3 10⁻⁷ | ±2 10⁻⁵ |
| constantan | 5.0 10⁻⁷ | ±3 10⁻⁵ |
| mercury | 9.84 10⁻⁷ | 9.9 10⁻⁴ |
| nichrome 80/20 | 1.05 10⁻⁶ | 1.8 10⁻⁴ |
| kantal A1 | 1.45 10⁻⁶ | 3 10⁻⁵ |
| carbon (diamond, graphite) | 1.3 10⁻⁵ | |
| germanium | 4.6 10⁻¹ | |
| silicon | 6.4 10² | |
| ethanol | 3 10³ | |
| water, distilled | 5 10³ | |
| ebonite | 10⁸ | |
| hard paper | 10¹⁰ | |
| transformer oil | 10¹¹ | |
| ordinary glass | 5 10¹¹ | |
| polyvinyl | 10¹² | |
| porcelain | 10¹² | |
| wood | 10¹² | |
| PTFE (teflon) | >10¹³ | |
| rubber | 5 10¹³ | |
| quartz glass | 10¹⁴ | |
| waxed paper | 10¹⁴ | |
| polystyrene | >10¹⁴ | |
| mica | 5 10¹⁴ | |
| paraffin | 10¹⁵ | |
| polyethylene | 3 10¹⁵ | |
| acrylic resin | 10¹⁹ |
en.electronics.wikia.com
Specific electrical resistance | formula, volumetric, table
Electrical resistivity is a physical quantity that indicates the extent to which a material can resist the passage of an electric current through it. Some people may confuse this characteristic with ordinary electrical resistance. Despite the similarity of the concepts, the difference between them lies in the fact that the specific refers to substances, and the second term refers exclusively to conductors and depends on the material of their manufacture.
The reciprocal of this material is electrical conductivity. The higher this parameter, the better the current passes through the substance. Accordingly, the higher the resistance, the more losses are expected at the output.
Calculation formula and measurement value
Considering what the electrical resistivity is measured in, it is also possible to trace the connection with the non-specific, since units of ohm m are used to designate the parameter. The value itself is denoted as ρ. With this value, it is possible to determine the resistance of a substance in a particular case, based on its dimensions. This unit of measure corresponds to the SI system, but there may be other options. In technology, you can periodically see the outdated designation Ohm mm2 / m. To convert from this system to the international one, you will not need to use complex formulas, since 1 ohm mm2 / m equals 10-6 ohm m.
The electrical resistivity formula is as follows:
R= (ρ l)/S, where:
- R is the resistance of the conductor;
- Ρ is the resistivity of the material;
- l is the length of the conductor;
- S is the cross section of the conductor.
Temperature dependence
The specific electrical resistance depends on the temperature. But all groups of substances manifest themselves differently when it changes. This must be taken into account when calculating the wires that will work in certain conditions. For example, in the street, where the temperature values depend on the season, the necessary materials are less susceptible to changes in the range from -30 to +30 degrees Celsius. If it is planned to use it in a technique that will work under the same conditions, then here it is also necessary to optimize the wiring for specific parameters. The material is always selected taking into account the operation.
In the nominal table, electrical resistivity is taken at a temperature of 0 degrees Celsius. Increasing performance given parameter when the material is heated, it is due to the fact that the intensity of the movement of atoms in the substance begins to increase. Carriers of electric charges chaotically scatter in all directions, which leads to the creation of obstacles in the movement of particles. The magnitude of the electrical flow is reduced.
As the temperature decreases, the current flow conditions become better. When a certain temperature is reached, which will be different for each metal, superconductivity appears, at which the characteristic in question almost reaches zero.
Differences in parameters sometimes reach very large values. Those materials that have high performance can be used as insulators. They help protect wiring from short circuits and inadvertent human contact. Some substances are generally not applicable for electrical engineering if they have a high value of this parameter. Other properties may interfere with this. For example, the electrical conductivity of water will not be of great importance for this sphere. Here are the values of some substances with high rates.
| Materials with high resistivity | ρ (ohm m) |
| Bakelite | 1016 |
| Benzene | 1015...1016 |
| Paper | 1015 |
| Distilled water | 104 |
| sea water | 0.3 |
| wood dry | 1012 |
| The ground is wet | 102 |
| quartz glass | 1016 |
| Kerosene | 1011 |
| Marble | 108 |
| Paraffin | 1015 |
| Paraffin oil | 1014 |
| Plexiglass | 1013 |
| Polystyrene | 1016 |
| PVC | 1013 |
| Polyethylene | 1012 |
| silicone oil | 1013 |
| Mica | 1014 |
| Glass | 1011 |
| transformer oil | 1010 |
| Porcelain | 1014 |
| Slate | 1014 |
| Ebonite | 1016 |
| Amber | 1018 |
Substances with low rates are used more actively in electrical engineering. Often these are metals that serve as conductors. They also show many differences. To find out the electrical resistivity of copper or other materials, it is worth looking at the reference table.
| Materials with low resistivity | ρ (ohm m) |
| Aluminum | 2.7 10-8 |
| Tungsten | 5.5 10-8 |
| Graphite | 8.0 10-6 |
| Iron | 1.0 10-7 |
| Gold | 2.2 10-8 |
| Iridium | 4.74 10-8 |
| Constantan | 5.0 10-7 |
| cast steel | 1.3 10-7 |
| Magnesium | 4.4 10-8 |
| Manganin | 4.3 10-7 |
| Copper | 1.72 10-8 |
| Molybdenum | 5.4 10-8 |
| Nickel silver | 3.3 10-7 |
| Nickel | 8.7 10-8 |
| Nichrome | 1.12 10-6 |
| Tin | 1.2 10-7 |
| Platinum | 1.07 10-7 |
| Mercury | 9.6 10-7 |
| Lead | 2.08 10-7 |
| Silver | 1.6 10-8 |
| Gray cast iron | 1.0 10-6 |
| carbon brushes | 4.0 10-5 |
| Zinc | 5.9 10-8 |
| Nickelin | 0.4 10-6 |
Specific volume electrical resistance
This parameter characterizes the ability to pass current through the volume of the substance. For measurement, it is necessary to apply a voltage potential with different parties material, the product from which will be included in the electrical circuit. It is supplied with current with nominal parameters. After passing, the output data is measured.
Use in electrical engineering
Changing the parameter when different temperatures widely used in electrical engineering. Most simple example is an incandescent lamp that uses a nichrome filament. When heated, it begins to glow. When current passes through it, it begins to heat up. As the heat increases, so does the resistance. Accordingly, the initial current that was needed to obtain illumination is limited. A nichrome coil, using the same principle, can become a regulator on various devices.
Precious metals, which have suitable characteristics for electrical engineering, have also been widely used. For critical circuits that require speed, silver contacts are selected. They have a high cost, but given the relatively small amount of materials, their use is quite justified. Copper is inferior to silver in conductivity, but has a more affordable price, due to which it is more often used to create wires.
In conditions where it is possible to use the maximum low temperatures superconductors are used. For room temperature and outdoor use, they are not always appropriate, since as the temperature rises, their conductivity will begin to fall, so aluminum, copper and silver remain leaders for such conditions.
In practice, many parameters are taken into account, and this one is one of the most important. All calculations are carried out at the design stage, for which reference materials are used.
