Specific resistance of conductors. Resistivity and superconductivity

14.04.2018

Conductors made of copper, aluminum, their alloys and iron (steel) are used as conductive parts in electrical installations.

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 slightly chemically active, but easily dissolves in nitric acid, and in dilute hydrochloric and sulfuric acids it dissolves only in the presence of oxidizing agents (oxygen). In air, copper quickly becomes covered with a thin layer of dark 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.

For production it is used 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 treatment. at 20°C it is 0.0172-0.018 ohm x mm2/m.

For the manufacture of conductors, soft, semi-hard or hard copper with a specific gravity of 8.9, 8.95 and 8.96 g/cm3, respectively, is used.

It is widely used for the manufacture of live parts. copper in alloys with other metals. The following alloys are most widely used.

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 - tombak with a copper content of more than 72% (has high ductility, anti-corrosion and anti-friction properties) and special brass with addition of aluminum, tin, lead or manganese.

Brass contact

Bronze is an alloy of copper and tin with additives of various metals. Depending on the content of the main component in the alloy, bronze is called tin, aluminum, silicon, phosphorus, and cadmium. Bronze resistivity 0.021 - 0.052 ohm x mm 2 /m.

Brass and bronze have good mechanical and physical-chemical properties. They are easily processed by casting and injection, and are resistant to atmospheric corrosion.

Aluminum - according to its qualities second conductive material after copper. Melting point 659.8° C. The density of aluminum at a temperature of 20° is 2.7 g/cm 3 . Aluminum is easy to cast and easy to machine. At a temperature of 100 - 150 ° C, aluminum is malleable and ductile (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 aluminum composition, the higher its electrical conductivity and better resistance to chemical influences. Machining, rolling and annealing significantly affect the mechanical strength of aluminum. Cold working of aluminum increases its hardness, elasticity and tensile strength. Aluminum resistivity at 20° C 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 conductivity, i.e. 1.63 times.

With equal conductivity, an aluminum conductor will be 2 times lighter than a copper one.

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 they use aluminum alloys with other metals, for example: Duralumin - an alloy of aluminum with copper and manganese.

Silumin is a lightweight casting alloy made of aluminum with an admixture of silicon, magnesium, and manganese.

Aluminum alloys have good casting properties and high mechanical strength.

The following are most widely used in electrical engineering: aluminum alloys:

Aluminum deformable alloy of the AD grade, having an aluminum content of at least 98.8 and other impurities up to 1.2.

Aluminum deformable alloy of AD1 grade, having aluminum content of at least 99.3 n and other impurities up to 0.7.

Aluminum deformable alloy brand AD31, having aluminum 97.35 - 98.15 and other impurities 1.85 -2.65.

Alloys of the AD and AD1 grades are used for the manufacture of housings and dies of hardware clamps. AD31 grade alloy is used to make profiles and busbars used for electrical conductors.

As a result of heat treatment, products made of aluminum alloys acquire high strength and yield (creep) limits.

Iron - melting point 1539°C. The density of iron is 7.87. Iron dissolves in acids and is oxidized by halogens and oxygen.

Various grades of steel are used in electrical engineering, for example:

Carbon steels are malleable alloys of iron with carbon and other metallurgical impurities.

The resistivity of carbon steels is 0.103 - 0.204 ohm x mm 2 /m.

Alloy steels are alloys with additives of chromium, nickel and other elements added to carbon steel.

Steels have good properties.

The following are widely used as additives in alloys, as well as for the manufacture of solders and the production of conductive metals:

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 plating) on ​​metal surfaces. In terms of its anti-corrosion 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. Nickel resistivity 0.068 - 0.072 ohm x mm 2 /m. At ordinary 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 resistivity of tin is 0.124 - 0.116 ohm x mm 2 /m. Tin is used for soldering the protective coating (tinning) of metals in its pure form and in the form of alloys with other metals.

Lead - melting point 327.4°C. Specific resistance 0.217 - 0.227 ohm x mm 2 /m. Lead is used in alloys with other metals as an acid-resistant material. 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 resistivity 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 with a melting point of 631°C. Antimony is used as an additive in soldering alloys (solders).

Chrome is a hard, shiny metal. Melting point 1830°C. In air at ordinary temperature it does not change. The resistivity 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. Zinc resistivity 0.053 - 0.062 ohm x mm 2 /m. In humid air, zinc oxidizes, becoming covered with a layer of oxide, which is protective against subsequent chemical influences. In electrical engineering, zinc is used as additives in alloys and solders, as well as for protective coating (zinc plating) of the surfaces of metal parts.

As soon as electricity left the laboratories of scientists and began to be widely introduced into the practice of everyday life, the question arose of searching for materials that have certain, sometimes completely opposite, characteristics in relation to the flow of electric current through them.

For example, when transmitting electrical energy over long distances, the wire material was required 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 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.

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 the USA contributed their efforts to the development of such a now familiar device as an incandescent lamp. Thomas Edison, having conducted more than a thousand experiments testing the properties of materials suitable for the role of filaments, created a lamp with a platinum spiral. Edison's lamps, although they had a long service life, were not practical due to the high cost of the source material.

Subsequent work by the Russian inventor Lodygin, who proposed using relatively cheap, refractory tungsten and molybdenum with a higher resistivity as filament materials, found practical application. In addition, Lodygin proposed pumping air out of incandescent lamp cylinders, 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 the General Electric company, 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, since the inquisitive human mind is so inventive that sometimes, to solve a certain technical problem, it needs materials with hitherto unprecedented properties or with incredible combinations of these properties. Nature can no longer keep 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 the casing or housing of electrical devices to a protective grounding device. Typically, grounding is carried out in the form of steel or copper strips, pipes, rods or corners 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 - housing or casing - ground - neutral wire of the alternating current 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 circuit protection devices in one way or another turn off the emergency device.

When calculating protective grounding elements, knowledge of the resistivity of soils, which can vary widely, plays a significant role.

In accordance with the data in 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 structural elements of the protective grounding device are connected by welding.

Electrical tomography

Electrical prospecting studies the near-surface geological environment and 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 prospecting is electrical tomography (Electrical Resistivity Tomography) - a method for determining the properties of rocks by their resistivity.

The essence of the method is 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 location 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 obtained data is processed and interpreted using modern computer processing methods, which make it possible to visualize information in the form of two-dimensional and three-dimensional images.

Being a very accurate search method, electrical tomography provides invaluable assistance to geologists, archaeologists and paleozoologists.

Determining the form of occurrence of mineral deposits and the boundaries of their distribution (contouring) makes it possible to identify the occurrence of vein deposits of minerals, which significantly reduces the costs 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 electrical tomography to search for the fossilized remains of ancient animals; the results of their work can be seen in natural science museums in the form of stunning reconstructions of the skeletons of prehistoric megafauna.

In addition, electrical tomography is used during the construction and subsequent operation of engineering structures: high-rise buildings, dams, dikes, embankments and others.

Definitions of resistivity in practice

Sometimes, in order to solve practical problems, we may be faced with the task of determining the composition of a substance, for example, a wire for cutting polystyrene foam. We have two coils of wire of suitable diameter from various materials unknown to us. To solve the problem, it is necessary to find their electrical resistivity and then, using the difference in the found values ​​or using a lookup table, determine the wire material.

We measure with a tape measure and cut 2 meters of wire from each sample. Let's determine the diameters of the wires d₁ and d₂ with a micrometer. Having turned 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.

Let us take into account that the cross-sectional area of ​​the wires is calculated by the formula

S = 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 = 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 = 1.2 ohm mm 2 /m

From a comparison of the values ​​of ρ₁ and ρ₂ with the reference data in Table 2 above, we conclude that the material of the first sample is steel, and the second is nichrome, from which we will make the cutter string.

They call the ability of a metal to pass a charged current through itself. In turn, resistance is one of the characteristics of a material. The greater the electrical resistance at a given voltage, the less it will be. It characterizes the force of resistance of a conductor to the movement of charged electrons directed along it. Since the property of transmitting electricity is the reciprocal of resistance, it means that it will be expressed in the form of formulas as the ratio 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 specific resistance property for copper is always equal to 0.0175 Ohm, for aluminum - 0.029, iron - 0.135, constantan - 0.48, nichrome - 1-1.1. The resistivity 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 understand this relationship 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 transfer of liquid through a 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 by Ohm.m. Conductivity depends on the average free flight length of charged particles, which is characterized by the structure of the material. Metals without impurities, which have the most correct values, have the lowest resistance values. Conversely, impurities distort the lattice, thereby increasing its performance. The resistivity of metals is located in a narrow range of values ​​at normal temperatures: from silver from 0.016 to 10 μΩm (alloys of iron and chromium with aluminum).

On the features of the movement of charged

electrons in a conductor are influenced by temperature, since as it increases, the amplitude of wave oscillations of existing ions and atoms increases. As a result, electrons have less free space to move normally in the crystal lattice. This means that the obstacle to orderly movement increases. The resistivity of any conductor, as usual, increases linearly with increasing temperature. Semiconductors, on the contrary, are characterized by a decrease with increasing degrees, since this results in the release of many charges that directly create an electric current.

The process of cooling some metal conductors to the desired temperature brings their resistivity to an abrupt state and drops to zero. This phenomenon was discovered in 1911 and called superconductivity.

One of the most popular metals in industries is copper. It is most widely used in electrical and electronics. Most often it is used in the manufacture of windings for electric motors and transformers. The main reason for using this particular material is that copper has the lowest electrical resistivity of any material currently available. Until a new material with a lower value of this indicator appears, we can say with confidence that there will be no replacement for copper.

General characteristics of copper

Speaking about copper, it must be said that at the dawn of the electrical era it began to be used in the production of electrical equipment. It began to be used largely due to the unique properties that this alloy has. By itself, it is a material characterized by high properties in terms of ductility and good malleability.

Along with the thermal conductivity of copper, one of its most important advantages is its high electrical conductivity. It is due to this property that copper and has become widespread in power plants, in which it acts as a universal conductor. The most valuable material is electrolytic copper, which has a high degree of purity of 99.95%. Thanks to this material, it becomes possible to produce cables.

Pros of using electrolytic copper

The use of electrolytic copper allows you to achieve the following:

• Ensure high electrical conductivity;
• Achieve excellent styling ability;
• Provide a high degree of plasticity.

Areas of application

Cable products made from electrolytic copper are widely used in various industries. Most often it is used in the following areas:

• electrical industry;
• electrical appliances;
• automotive industry;
• production of computer equipment.

What is the resistivity?

To understand what copper is and its characteristics, it is necessary to understand the main parameter of this metal - resistivity. It should be known and used when performing calculations.

Resistivity is usually understood as a physical quantity, which is characterized as the ability of a metal to conduct electric current.

It is also necessary to know this value in order to correctly calculate electrical resistance conductor. When making calculations, they are also guided by its geometric dimensions. When carrying out calculations, use the following formula:

This formula is familiar to many. Using it, you can easily calculate the resistance of a copper cable, focusing only on the characteristics of the electrical network. It allows you to calculate the power that is inefficiently spent on heating the cable core. Besides, a similar formula allows you to calculate resistance any cable. It does not matter what material was used to make the cable - copper, aluminum or some other alloy.

A parameter such as electrical resistivity is measured in Ohm*mm2/m. This indicator for copper wiring laid in an apartment is 0.0175 Ohm*mm2/m. If you try to look for an alternative to copper - a material that could be used instead, then only silver can be considered the only suitable one, whose resistivity is 0.016 Ohm*mm2/m. However, when choosing a material, it is necessary to pay attention not only to resistivity, but also to reverse conductivity. This value is measured in Siemens (Cm).

Siemens = 1/ Ohm.

For copper of any weight, this composition parameter is 58,100,000 S/m. As for silver, its reverse conductivity is 62,500,000 S/m.

In our world of high technology, when every home has a large number of electrical devices and installations, the importance of a material such as copper is simply invaluable. This material used to make wiring, without which no room can do. If copper did not exist, then man would have to use wires made from other available materials, such as aluminum. However, in this case one would have to face one problem. The thing is that this material has a much lower conductivity than copper conductors.

Resistivity

The use of materials with low electrical and thermal conductivity of any weight leads to large losses of electricity. A this affects power loss on the equipment used. Most experts call copper as the main material for making insulated wires. It is the main material from which individual elements of equipment powered by electric current are made.

• Boards installed in computers are equipped with etched copper traces.
• Copper is also used to make a wide variety of components used in electronic devices.
• In transformers and electric motors it is represented by a winding, which is made of this material.

There is no doubt that the expansion of the scope of application of this material will occur with the further development of technological progress. Although there are other materials besides copper, designers still use copper when creating equipment and various installations. The main reason for the demand for this material is in good electrical and thermal conductivity this metal, which it provides at room temperature.

Temperature coefficient of resistance

All metals with any thermal conductivity have the property of decreasing conductivity with increasing temperature. As the temperature decreases, conductivity increases. Experts call the property of decreasing resistance with decreasing temperature particularly interesting. Indeed, in this case, when the temperature in the room drops to a certain value, the conductor may lose electrical resistance and it will move into the class of superconductors.

In order to determine the resistance value of a particular conductor of a certain weight at room temperature, there is a critical resistance coefficient. It is a value that shows the change in resistance of a section of a circuit when the temperature changes by one Kelvin. To calculate the electrical resistance of a copper conductor in a certain time period, use the following formula:

ΔR = α*R*ΔT, where α is the temperature coefficient of electrical resistance.

Conclusion

Copper is a material that is widely used in electronics. It is used not only in windings and circuits, but also as a metal for the manufacture of cable products. For machinery and equipment to work effectively, it is necessary correctly calculate the resistivity of the wiring, laid in the apartment. There is a certain formula for this. Knowing it, you can make a calculation that allows you to find out the optimal size of the cable cross-section. In this case, it is possible to avoid loss of equipment power and ensure its efficient use.

Many people have heard about Ohm's law, but not everyone knows what it is. The study begins with a school physics course. They are taught in more detail at the Faculty of Physics and Electrodynamics. This knowledge is unlikely to be useful to the average person, but it is necessary for general development, and for others, for a future profession. On the other hand, basic knowledge about electricity, its structure, and its features at home will help protect yourself from harm. It is not for nothing that Ohm’s law is called the fundamental law of electricity. A home handyman needs to have knowledge in the field of electricity to prevent overvoltage, which can lead to an increase in load and a fire.

Concept of electrical resistance

The relationship between the basic physical quantities of an electrical circuit - resistance, voltage, current strength - was discovered by the German physicist Georg Simon Ohm.

The electrical resistance of a conductor is a value that characterizes its resistance to electric current. In other words, some of the electrons under the influence of electric current on the conductor leave their place in the crystal lattice and are directed to the positive pole of the conductor. Some electrons remain in the lattice, continuing to rotate around the nuclear atom. These electrons and atoms form electrical resistance that prevents the movement of released particles.

The above process applies to all metals, but resistance occurs differently in them. This is due to the difference in size, shape, and material of which the conductor is made. Accordingly, the dimensions of the crystal lattice have different shapes for different materials, therefore, the electrical resistance to the movement of current through them is not the same.

From this concept follows the definition of the resistivity of a substance, which is an individual indicator for each metal separately. Electrical resistivity (SER) is a physical quantity, denoted by the Greek letter ρ, and characterized by the ability of a metal to prevent the passage of electricity through it.

Copper is the main material for conductors

The resistivity of a substance is calculated using the formula, where one of the important indicators is the temperature coefficient of electrical resistance. The table contains the resistivity values ​​of three known metals in the temperature range from 0 to 100°C.

If we take the resistivity of iron, as one of the available materials, equal to 0.1 Ohm, then for 1 Ohm you will need 10 meters. Silver has the lowest electrical resistance; for its value of 1 ohm it will be 66.7 meters. A significant difference, but silver is an expensive metal that is not practical to use everywhere. The next best indicator is copper, where 57.14 meters are required per 1 ohm. Due to its availability and cost compared to silver, copper is one of the popular materials for use in electrical networks. The low resistivity of copper wire or the resistance of copper wire makes it possible to use copper conductor in many branches of science, technology, as well as for industrial and domestic purposes.

Resistivity value

The resistivity value is not constant; it varies depending on the following factors:

• Size. The larger the diameter of the conductor, the more electrons it allows through itself. Therefore, the smaller its size, the greater the resistivity.
• Length. Electrons pass through atoms, so the longer the wire, the more electrons have to travel through them. When making calculations, it is necessary to take into account the length and size of the wire, because the longer or thinner the wire, the greater its resistivity and vice versa. Failure to calculate the load of the equipment used can lead to overheating of the wire and a fire.
• Temperature. It is known that temperature has a great influence on the behavior of substances in different ways. Metal, like nothing else, changes its properties at different temperatures. The resistivity of copper directly depends on the temperature coefficient of resistance of copper and increases when heated.
• Corrosion. The formation of corrosion significantly increases the load. This happens due to environmental influences, moisture, salt, dirt, etc. manifestations. It is recommended to insulate and protect all connections, terminals, twists, install protection for equipment located on the street, and promptly replace damaged wires, components, and assemblies.

Resistance calculation

Calculations are made when designing objects for various purposes and uses, because everyone’s life support is provided by electricity. Everything is taken into account, from lighting fixtures to technically complex equipment. At home, it would also be useful to make a calculation, especially if it is planned to replace the electrical wiring. For private housing construction, it is necessary to calculate the load, otherwise the “makeshift” assembly of electrical wiring can lead to a fire.

The purpose of the calculation is to determine the total resistance of the conductors of all devices used, taking into account their technical parameters. It is calculated using the formula R=p*l/S, where:

R – calculated result;

p – resistivity indicator from the table;

l – length of wire (conductor);

S – section diameter.

Units

In the International System of Units of Physical Quantities (SI), electrical resistance is measured in Ohms (Ohms). The unit of measurement of resistivity according to the SI system is equal to the resistivity of a substance at which a conductor made of one material 1 m long with a cross-section of 1 sq. m. has a resistance of 1 Ohm. The use of 1 ohm/m for different metals is clearly shown in the table.

Significance of resistivity

The relationship between resistivity and conductivity can be considered as reciprocal quantities. The higher the indicator of one conductor, the lower the indicator of the other and vice versa. Therefore, when calculating electrical conductivity, the calculation 1/r is used, because the inverse of X is 1/X and vice versa. The specific indicator is denoted by the letter g.

Advantages of Electrolytic Copper

Copper is not limited to its low resistivity index (after silver) as an advantage. It has properties unique in its characteristics, namely plasticity and high malleability. Thanks to these qualities, electrolytic copper is produced to a high degree of purity for the production of cables that are used in electrical appliances, computer equipment, the electrical industry and the automotive industry.

Dependence of resistance index on temperature

The temperature coefficient is a value that is equal to the change in the voltage of a part of the circuit and the resistivity of the metal as a result of changes in temperature. Most metals tend to increase resistivity with increasing temperature due to thermal vibrations of the crystal lattice. The temperature coefficient of resistance of copper affects the resistivity of copper wire and at temperatures from 0 to 100°C is 4.1 10− 3(1/Kelvin). For silver, this indicator under the same conditions is 3.8, and for iron it is 6.0. This once again proves the effectiveness of using copper as a conductor.

Electrical resistance -a physical quantity that shows what kind of obstacle is created by the current as it passes through the conductor. The units of measurement are Ohms, in honor of Georg Ohm. In his law, he derived a formula for finding resistance, which is given below.

Let's consider the resistance of conductors using metals as an example. Metals have an internal structure in the form of a crystal lattice. This lattice has a strict order, and its nodes are positively charged ions. Charge carriers in a metal are “free” electrons, which do not belong to a specific atom, but move randomly between lattice sites. It is known from quantum physics that the movement of electrons in a metal is the propagation of an electromagnetic wave in a solid. That is, an electron in a conductor moves at the speed of light (practically), and it has been proven that it exhibits properties not only as a particle, but also as a wave. And the resistance of the metal arises as a result of the scattering of electromagnetic waves (that is, electrons) by thermal vibrations of the lattice and its defects. When electrons collide with nodes of a crystal lattice, part of the energy is transferred to the nodes, as a result of which energy is released. This energy can be calculated at constant current, thanks to the Joule-Lenz law - Q=I 2 Rt. As you can see, the greater the resistance, the more energy is released.

Resistivity

There is such an important concept as resistivity, this is the same resistance, only in a unit of length. Each metal has its own, for example, for copper it is 0.0175 Ohm*mm2/m, for aluminum it is 0.0271 Ohm*mm2/m. This means that a copper bar 1 m long and a cross-sectional area of ​​1 mm2 will have a resistance of 0.0175 Ohm, and the same bar, but made of aluminum, will have a resistance of 0.0271 Ohm. It turns out that the electrical conductivity of copper is higher than that of aluminum. Each metal has its own specific resistance, and the resistance of the entire conductor can be calculated using the formula

Where p– metal resistivity, l – conductor length, s – cross-sectional area.

Resistivity values ​​are given in metal resistivity table(20°C)

In addition to resistivity, the table contains TCR values; more on this coefficient a little later.

Dependence of resistivity on deformation

During cold forming of metals, the metal experiences plastic deformation. During plastic deformation, the crystal lattice becomes distorted and the number of defects increases. With an increase in crystal lattice defects, the resistance to the flow of electrons through the conductor increases, therefore, the resistivity of the metal increases. For example, wire is made by drawing, which means that the metal undergoes plastic deformation, as a result of which the resistivity increases. In practice, recrystallization annealing is used to reduce resistance; this is a complex technological process, after which the crystal lattice seems to “straighten out” and the number of defects decreases, and therefore the resistance of the metal too.

When stretched or compressed, the metal experiences elastic deformation. During elastic deformation caused by stretching, the amplitudes of thermal vibrations of the crystal lattice nodes increase, therefore, electrons experience great difficulty, and in connection with this, the resistivity increases. During elastic deformation caused by compression, the amplitudes of thermal vibrations of the nodes decrease, therefore, it is easier for electrons to move, and the resistivity decreases.

Effect of temperature on resistivity

As we have already found out above, the cause of resistance in metal is the nodes of the crystal lattice and their vibrations. So, as the temperature increases, the thermal vibrations of the nodes increase, which means the resistivity also increases. There is such a quantity as temperature coefficient of resistance(TKS), which shows how much the resistivity of the metal increases or decreases when heated or cooled. For example, the temperature coefficient of copper at 20 degrees Celsius is 4.1 · 10 − 3 1/degree. This means that when, for example, copper wire is heated by 1 degree Celsius, its resistivity will increase by 4.1 · 10 − 3 Ohm. The resistivity with temperature changes can be calculated using the formula

where r is the resistivity after heating, r 0 is the resistivity before heating, a is the temperature coefficient of resistance, t 2 is the temperature before heating, t 1 is the temperature after heating.

Substituting our values, we get: r=0.0175*(1+0.0041*(154-20))=0.0271 Ohm*mm 2 /m. As you can see, our copper bar with a length of 1 m and a cross-sectional area of ​​1 mm 2, after heating to 154 degrees, would have the same resistance as the same bar, only made of aluminum and at a temperature of 20 degrees Celsius.

The property of changing resistance with temperature changes is used in resistance thermometers. These devices can measure temperature based on resistance readings. Resistance thermometers have high measurement accuracy, but small temperature ranges.

In practice, the properties of conductors to prevent the passage current are used very widely. An example is an incandescent lamp, where a tungsten filament is heated due to the high resistance of the metal, its large length and narrow cross-section. Or any heating device where the coil heats up due to high resistance. In electrical engineering, an element whose main property is resistance is called a resistor. A resistor is used in almost any electrical circuit.

Resistivity of popular conductors (metals and alloys). Steel resistivity

Resistivity of iron, aluminum and other conductors

Transmitting electricity over long distances requires taking care to minimize losses resulting from current overcoming the resistance of the conductors that make up the electrical line. Of course, this does not mean that such losses, which occur specifically in circuits and consumer devices, do not play a role.

Therefore, it is important to know the parameters of all elements and materials used. And not only electrical, but also mechanical. And have at your disposal some convenient reference materials that allow you to compare the characteristics of different materials and choose for design and operation exactly what will be optimal in a particular situation. In energy transmission lines, where the task is set to be 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 device 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 the materials selected for each structural element. , its work and operating costs. In addition, in lines transmitting electricity, there are higher requirements for ensuring the safety of both the lines themselves and everything around them where they pass. And this adds costs both for providing electricity wiring and for an additional margin of safety of all structures.

For comparison, data are usually reduced to a single, comparable form. Often the epithet “specific” is added to such characteristics, and the values ​​themselves are considered based on certain standards unified by physical parameters. For example, electrical resistivity is the resistance (ohms) of a conductor made of some metal (copper, aluminum, steel, tungsten, gold) having a unit length and a unit cross-section in the system of units of measurement used (usually 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 environmental parameters (temperature, pressure), coefficients are introduced and additional tables and dependency graphs are compiled.

Types of resistivity

Since resistance happens:

• active - or ohmic, resistive - resulting from the expenditure of electricity on heating the conductor (metal) when an electric current passes through it, and
• reactive - capacitive or inductive - which occurs from the inevitable losses due to the creation of any changes in the current passing through the conductor of electric fields, then the resistivity of the conductor comes in two varieties:

1. Specific electrical resistance to direct current (having a resistive nature) and
2. Specific electrical resistance to alternating current (having a reactive nature).

Here, type 2 resistivity is a complex value; it consists of two TC components - 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 the circuits. In DC circuits, reactance occurs only during transient processes that are associated with turning on the current (change in current from 0 to nominal) or turning off (difference from nominal to 0). And they are usually taken into account only when designing overload protection.

In alternating current circuits, the phenomena associated with reactance are much more diverse. They depend not only on the actual passage of current through a certain cross 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” the actual main movement of charges, from the depths of the entire cross-section of the conductor to its surface, the so-called “skin effect” (from skin - skin). It turns out that eddy currents seem to “steal” its cross-section from the conductor. The current flows in a certain layer close to the surface, the remaining thickness of the conductor 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, resistance is measured in such sections of conductors where its entire 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, reducing the thickness of round wires does not exhaust the effective conduction of alternating current. 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, and accordingly, the resistance will be 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 stranded wire instead of single-core; moreover, stranded wire is more flexible than single-core wire, which is often valuable. On the other hand, taking into account the skin effect in wires, it is possible to make the wires composite by making the core from a metal that has good strength characteristics, for example, steel, but low electrical characteristics. In this case, 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 surrounding conductors. Such currents are called induction currents, and they are induced both in metals that do not play the role of wiring (load-bearing structural elements), and in the wires of the entire conductive complex - playing the role of wires of other phases, neutral, grounding.

All of these phenomena occur in all electrical structures, making it even more important to have a comprehensive reference for a wide variety of materials.

Resistivity for conductors is measured with very sensitive and precise instruments, since metals with the lowest resistance are selected for wiring - on the order of ohms * 10-6 per meter of length and sq. m. mm. sections. To measure insulation resistivity, you need instruments, on the contrary, that have ranges of very large resistance values ​​- usually megohms. It is clear that conductors must conduct well, and insulators must insulate well.

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 for the strength of various structures.

In electrical engineering, iron is used as a conductor in the form of flexible steel wires where physical strength and flexibility are needed, and the required resistance can be achieved through the appropriate cross-section.

Having a table of resistivities of various metals and alloys, you can 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 wire. Let's take aluminum wire with a cross-section of 2.5 mm as the initial one.

We need that over a length of 1 m the resistance of the wire made of all these metals is equal to the resistance of the original one. The resistance of aluminum per 1 m length and 2.5 mm section will be equal to

Substituting the original values, we get the resistance of a meter-long piece of aluminum wire in ohms.

After this, let us solve the formula for S

Since the resistivity in the table is measured on a wire 1 m long, in microohms per 1 mm2 section, then we got it in microohms. To get it in ohms, you need to multiply the value by 10-6. But we don’t necessarily need to get the number ohm with 6 zeros after the decimal point, since we still find the final result in mm2.

As you can see, the resistance of the iron is quite high, the wire is thick.

But there are materials for which it is even greater, for example, nickel or constantan.

Similar articles:

domelectrik.ru

Table of electrical resistivity of metals and alloys in electrical engineering

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Specific resistance of metals.

Specific resistance of alloys.

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Electrical resistivity | Welding world

Electrical resistivity of materials

Electrical resistivity (resistivity) is the ability of a substance to prevent the passage of electric current.

Unit of measurement (SI) - Ohm m; also measured in Ohm cm and Ohm mm2/m.

LITERATURE

• Alpha and Omega. Quick reference book / Tallinn: Printest, 1991 – 448 p.
• Handbook of elementary physics / N.N. Koshkin, M.G. Shirkevich. M., Science. 1976. 256 p.
• Handbook on welding of non-ferrous metals / S.M. Gurevich. Kyiv: Naukova Dumka. 1990. 512 p.

weldworld.ru

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 strongly depends on impurities; the table shows the values ​​of p for chemically pure metals, and for insulators they are given approximately. Metals and insulators are arranged in the table in order of increasing p values.

Metal resistivity table

Table of resistivity of insulators

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).

Resistance ratio Rt/Rq of pure metals at temperatures T ° K and 273 ° K.

The reference table gives the ratio Rt/Rq of the resistances of pure metals at temperatures T ° K and 273 ° K.

Specific resistance of electrolytes

The table gives the values ​​of the resistivity of electrolytes in ohm cm at a temperature of 18 ° C. The concentration of solutions is given in percentages, which determine the number of grams of anhydrous salt or acid in 100 g of solution.

Source of information: BRIEF PHYSICAL AND TECHNICAL GUIDE / Volume 1, - M.: 1960.

infotables.ru

Electrical resistivity - steel

Page 1

The electrical resistivity of steel increases with increasing temperature, with the greatest changes observed when heated to the Curie point temperature. After the Curie point, the electrical resistivity changes slightly and at temperatures above 1000 C remains virtually constant.

Due to the high electrical resistivity of steel, these iuKii create a very large slowdown in the flow decline. In 100 A contactors, the drop-off time is 0 07 sec, and in 600 A contactors - 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 switch drives, the electromagnetic mechanism of these contactors allows adjustment of the actuation voltage and release voltage by adjusting the force of the return spring and a special break-off spring. Contactors of the KMV type must operate with a deep voltage drop. Therefore, the minimum operating voltage for these contactors can drop to 65% UH. Such a low operating voltage results in current flowing through the winding at rated voltage, resulting in increased heating of the coil.

The silicon additive increases the electrical resistivity of steel almost proportionally to the silicon content and thereby helps reduce losses due to eddy currents that occur in steel when it operates in an alternating magnetic field.

The silicon additive increases the electrical resistivity of steel, which helps reduce eddy current losses, but at the same time silicon worsens the mechanical properties of steel and makes 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 soft magnetic steel was put on a massive rotor made of the optimal SM-19 alloy. The electrical resistivity of steel differs little from the resistivity of the alloy, and the CG of steel is approximately an order of magnitude higher. The screen thickness is selected according to the penetration depth of first-order tooth harmonics and is equal to 0 8 mm. For comparison, the additional losses, W, are given for 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 alloy electrical steel containing from 2 to 5% silicon. The silicon additive increases the electrical resistivity of steel, as a result of which eddy current losses are reduced, the 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 magnetic core is made in the form of a package assembled from sheets of stamped steel.

Electrical steel is low carbon steel. To improve the magnetic characteristics, silicon is introduced into it, which causes an increase in the electrical resistivity of the steel. This leads to a reduction in eddy current losses.

After mechanical treatment, the magnetic circuit is annealed. Since eddy currents in steel participate in the creation of deceleration, one should focus on the value of the electrical resistivity of steel on the order of Pc (Iu-15) 10 - 6 ohm cm. In the attracted position of the armature, the magnetic system is quite highly saturated, therefore the initial induction in different magnetic systems fluctuates within very small limits and for steel grade E Vn1 6 - 1 7 ch. The indicated induction value maintains the field strength in the steel on the order of Yang.

For the manufacture of magnetic systems (magnetic cores) of transformers, special thin-sheet electrical steels with a high (up to 5%) silicon content are used. Silicon promotes the decarburization of steel, which leads to an increase in magnetic permeability, reduces hysteresis losses and increases its electrical resistivity. Increasing the electrical resistivity of steel makes it possible to reduce losses in it from eddy currents. In addition, silicon weakens the aging of steel (increasing losses in steel over time), reduces its magnetostriction (changes 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 increases its brittleness and complicates its machining.

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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 accepted as a scalar quantity.

Designation - ρ

$ \vec E = \rho \vec j, $

$ \vec E $ - electric field strength, $ \vec j $ - current density.

The SI unit of measurement is the ohm meter (ohm m, Ω m).

The resistivity resistance of a cylinder or prism (between the ends) of a material with length l and section S is determined as follows:

$ R = \frac(\rho l)(S). $

In technology, the definition of resistivity is used as the resistance of a conductor of a unit cross-section and unit length.

Resistivity of some materials used in electrical engineering Edit

en.electronics.wikia.com

Electrical resistivity | formula, volumetric, table

Electrical resistivity is a physical quantity that indicates the extent to which a material can resist the passage of electric current through it. Some people may confuse this characteristic with ordinary electrical resistance. Despite the similarity of concepts, the difference between them is that specific refers to substances, and the second term refers exclusively to conductors and depends on the material of their manufacture.

The reciprocal value of this material is the electrical conductivity. The higher this parameter, the better the current flows through the substance. Accordingly, the higher the resistance, the more losses are expected at the output.

Calculation formula and measurement value

Considering how specific electrical resistance is measured, it is also possible to trace the connection with non-specific, since units of Ohm m are used to denote the parameter. The quantity 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 size. This unit of measurement corresponds to the SI system, but other variations may occur. 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 formula for electrical resistivity is as follows:

R= (ρ l)/S, where:

• R – conductor resistance;
• Ρ – resistivity of the material;
• l – conductor length;
• S – conductor cross-section.

Temperature dependence

Electrical resistivity depends on temperature. But all groups of substances manifest themselves differently when it changes. This must be taken into account when calculating wires that will operate under certain conditions. For example, on the street, where temperature values ​​depend on the time of year, the necessary materials are less susceptible to changes in the range from -30 to +30 degrees Celsius. If you plan to use it in equipment that will operate under the same conditions, then you also need to optimize the wiring for specific parameters. The material is always selected taking into account the use.

In the nominal table, electrical resistivity is taken at a temperature of 0 degrees Celsius. The increase in the indicators of this parameter when the material is heated is due to the fact that the intensity of the movement of atoms in the substance begins to increase. Electric charge carriers scatter randomly in all directions, which leads to the creation of obstacles to the movement of particles. The amount of electrical flow decreases.

As the temperature decreases, the conditions for current flow become better. Upon reaching a certain temperature, which will be different for each metal, superconductivity appears, at which the characteristic in question almost reaches zero.

The 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 unintentional human contact. Some substances are not applicable at all 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 much importance for a given area. Here are the values ​​of some substances with high indicators.

Substances with low performance are used more actively in electrical engineering. These are often metals that serve as conductors. There are also many differences between them. To find out the electrical resistivity of copper or other materials, it is worth looking at the reference table.

Specific volumetric electrical resistivity

This parameter characterizes the ability to pass current through the volume of a substance. To measure, it is necessary to apply a voltage potential from different sides of the material from which the product will be included in the electrical circuit. It is supplied with current with rated parameters. After passing, the output data is measured.

Use in electrical engineering

Changing a parameter at different temperatures is widely used in electrical engineering. The simplest example is an incandescent lamp, which uses a nichrome filament. When heated, it begins to glow. When current passes through it, it begins to heat up. As heating increases, resistance also increases. Accordingly, the initial current that was needed to obtain lighting is limited. A nichrome spiral, using the same principle, can become a regulator on various devices.

Precious metals, which have suitable characteristics for electrical engineering, are also widely used. For critical circuits that require high speed, silver contacts are selected. They are expensive, 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, which is why it is more often used to create wires.

In conditions where extremely low temperatures can be used, 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 for such conditions aluminum, copper and silver remain the leaders.

In practice, many parameters are taken into account and this is one of the most important. All calculations are carried out at the design stage, for which reference materials are used.