It has the lowest resistivity. Resistivity and other properties of copper

Electric current I in any substance is created by the movement of charged particles in a certain direction due to the application of external energy (potential difference U). Each substance has individual properties that affect the passage of current in it in different ways. These properties are evaluated by the electrical resistance R.

Georg Ohm empirically determined the factors influencing the magnitude of the electrical resistance of a substance, deduced from voltage and current, which is named after him. The unit of measurement of resistance in the international SI system is named after him. 1 Ohm is the value of resistance measured at a temperature of 0 ° C at a homogeneous mercury column 106.3 cm long with a cross-sectional area of ​​\u200b\u200b1 mm 2.

Definition

In order to evaluate and put into practice materials for the manufacture of electrical devices, the term "conductor resistivity". The added adjective "specific" indicates the factor of using the reference volume value adopted for the substance in question. This makes it possible to evaluate the electrical parameters of different materials.

At the same time, it is taken into account that the resistance of the conductor increases with an increase in its length and a decrease in its cross section. The SI system uses the volume of a homogeneous conductor with a length of 1 meter and a cross section of 1m 2. In technical calculations, an outdated but convenient off-system unit of volume is used, consisting of a length of 1 meter and an area of ​​1 mm 2. The formula for resistivity ρ is shown in the figure.

To determine the electrical properties of substances, another characteristic is introduced - specific conductivity b. It is inversely proportional to the value of resistivity, determines the ability of the material to conduct electric current: b = 1/ρ.

How does resistivity depend on temperature?

The conductivity of a material is affected by its temperature. Different groups of substances behave differently when heated or cooled. This property is taken into account in electrical wires operating outdoors in heat and cold.

The material and resistivity of the wire are selected taking into account the conditions of its operation.

The increase in the resistance of conductors to the passage of current during heating is explained by the fact that with an increase in the temperature of the metal in it, the intensity of the movement of atoms and carriers of electric charges in all directions increases, which creates unnecessary obstacles for the movement of charged particles in one direction, reduces the magnitude of their flow.

If the temperature of the metal is reduced, then the conditions for the passage of current improve. When cooled to a critical temperature, the phenomenon of superconductivity appears in many metals, when their electrical resistance is practically zero. This property is widely used in powerful electromagnets.

The influence of temperature on the conductivity of a metal is used by the electrical industry in the manufacture of ordinary incandescent lamps. During the passage of current, they heat up to such a state that it emits a luminous flux. Under normal conditions, the specific resistance of nichrome is about 1.05 ÷ 1.4 (ohm ∙ mm 2) / m.

When the light bulb is turned on, a large current passes through the filament, which heats up the metal very quickly. At the same time, the resistance of the electrical circuit increases, limiting the initial current to the nominal value necessary to obtain lighting. In this way, a simple regulation of the current strength through a nichrome spiral is carried out, there is no need to use complex ballasts used in LED and luminescent sources.

How the resistivity of materials is used in engineering

Non-ferrous noble metals have better electrical conductivity properties. Therefore, critical contacts in electrical devices are made of silver. But this increases the final cost of the entire product. The most acceptable option is to use cheaper metals. For example, the resistivity of copper, equal to 0.0175 (ohm ∙ mm 2) / m, is quite suitable for such purposes.

noble metals- gold, silver, platinum, palladium, iridium, rhodium, ruthenium and osmium, named mainly for their high chemical resistance and beautiful appearance in jewelry. In addition, gold, silver and platinum have high ductility, while platinum group metals have high melting point and, like gold, chemical inertness. These advantages of noble metals are combined.

Copper alloys with good conductivity are used to make shunts that limit the flow of high currents through the measuring head of high-power ammeters.

The specific resistance of aluminum 0.026 ÷ 0.029 (ohm ∙ mm 2) / m is slightly higher than that of copper, but the production and cost of this metal is lower. Plus, it's easier. This explains its widespread use in the energy sector for the manufacture of outdoor wires and cable cores.

The specific resistance of iron 0.13 (ohm ∙ mm 2) / m also allows its use for the transmission of electric current, but in this case there are large power losses. Steel alloys have increased strength. Therefore, steel strands are woven into the aluminum overhead wires of high-voltage power lines, which are designed to withstand tensile stresses.

This is especially true when there is ice on the wires or strong gusts of wind.

Some alloys, for example, constantine and nickeline, have thermally stable resistive characteristics in a certain range. In nickeline, the electrical resistivity practically does not change from 0 to 100 degrees Celsius. Therefore, spirals for rheostats are made of nickeline.

In measuring instruments, the property of a strict change in the values ​​of the resistivity of platinum from its temperature is widely used. If an electric current is passed through a platinum conductor from a stabilized voltage source and the resistance value is calculated, then it will indicate the temperature of the platinum. This allows you to calibrate the scale in degrees, corresponding to Ohm values. This method allows you to measure the temperature with an accuracy of fractions of a degree.

Sometimes, in order to solve practical problems, you need to know cable impedance or resistivity. To do this, in the reference books for cable products, the values ​​\u200b\u200bof the inductive and active resistance of one core for each value of the cross section are given. With their help, the permissible loads, the generated heat are calculated, the permissible operating conditions are determined and effective protections are selected.

The specific conductivity of metals is influenced by the way they are processed. The use of pressure for plastic deformation breaks the structure of the crystal lattice, increases the number of defects and increases the resistance. To reduce it, recrystallization annealing is used.

Stretching or compression of metals causes elastic deformation in them, from which the amplitudes of thermal oscillations of electrons decrease, and the resistance decreases somewhat.

When designing grounding systems, it is necessary to take into account. It has differences in definition from the above method and is measured in units of the SI system - Ohm∙meter. With its help, the quality of the spreading of electric current inside the earth is evaluated.

Soil conductivity is affected by many factors, including soil moisture, soil density, particle size, temperature, salt, acid, and alkali concentrations.

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 size. 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 mm 2 / m. To convert from this system to the international one, you will not need to use complex formulas, since 1 ohm mm 2 /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. The increase in 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. 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.

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.

Specific volume electrical resistance

This parameter characterizes the ability to pass current through the volume of the substance. To measure, it is necessary to apply a voltage potential from different sides of the 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 at different temperatures is widely used in electrical engineering. The simplest example is an incandescent lamp, where a nichrome filament is used. When heated, it starts 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 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 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.

Despite the fact that this topic may seem quite banal, in it I will answer one very important question regarding the calculation of voltage loss and the calculation of short circuit currents. I think for many of you this will be as much of a revelation as it was for me.

Recently I studied one very interesting GOST:

GOST R 50571.5.52-2011 Low-voltage electrical installations. Part 5-52. Selection and installation of electrical equipment. Wiring.

This document provides a formula for calculating voltage loss and states:

p is the resistivity of conductors under normal conditions, taken equal to the resistivity at temperature under normal conditions, that is, 1.25 resistivity at 20 ° C, or 0.0225 Ohm mm 2 / m for copper and 0.036 Ohm mm 2 / m for aluminum;

I didn't understand anything =) Apparently, when calculating voltage losses and when calculating short-circuit currents, we must take into account the resistance of the conductors, as under normal conditions.

It is worth noting that all tabular values ​​\u200b\u200bare given at a temperature of 20 degrees.

What are the normal conditions? I thought 30 degrees Celsius.

Let's remember physics and calculate at what temperature the resistance of copper (aluminum) will increase by 1.25 times.

R1=R0

R0 - resistance at 20 degrees Celsius;

R1 - resistance at T1 degrees Celsius;

T0 - 20 degrees Celsius;

α \u003d 0.004 per degree Celsius (copper and aluminum are almost the same);

1.25=1+α (T1-T0)

Т1=(1.25-1)/α+Т0=(1.25-1)/0.004+20=82.5 degrees Celsius.

As you can see, it's not 30 degrees at all. Apparently, all calculations must be performed at the maximum allowable cable temperatures. The maximum operating temperature of the cable is 70-90 degrees, depending on the type of insulation.

To be honest, I do not agree with this, because. this temperature corresponds to almost the emergency mode of the electrical installation.

In my programs, I laid down the specific resistance of copper - 0.0175 Ohm mm 2 / m, and for aluminum - 0.028 Ohm mm 2 / m.

If you remember, I wrote that in my program for calculating short-circuit currents, the result is about 30% less than the tabular values. There, the resistance of the phase-zero loop is calculated automatically. I tried to find the error but couldn't. Apparently, the inaccuracy of the calculation lies in the resistivity, which is used in the program. And everyone can ask the resistivity, so there should be no questions for the program if you specify the resistivity from the above document.

But I most likely will have to make changes to the programs for calculating voltage losses. This will increase the calculation results by 25%. Although in the ELECTRIC program, the voltage losses are almost the same as mine.

If this is your first time on this blog, then you can get acquainted with all my programs on the page

What do you think, at what temperature should voltage losses be considered: at 30 or 70-90 degrees? Are there any regulations that will answer this question?

One of the physical quantities used in electrical engineering is electrical resistivity. Considering the specific resistance of aluminum, it should be remembered that this value characterizes the ability of a substance to prevent the passage of electric current through it.

Concepts Related to Resistivity

The value opposite to resistivity is called conductivity or electrical conductivity. The usual electrical resistance is characteristic only of a conductor, and the specific electrical resistance is characteristic only of a particular substance.

As a rule, this value is calculated for a conductor having a uniform structure. To determine electrical homogeneous conductors, the formula is used:

The physical meaning of this quantity lies in a certain resistance of a homogeneous conductor with a certain unit length and cross-sectional area. The unit of measurement is the SI unit Ohm.m or the off-system unit Ohm.mm2/m. The last unit means that a conductor of a homogeneous substance, 1 m long, having a cross-sectional area of ​​​​1 mm2, will have a resistance of 1 ohm. Thus, the resistivity of any substance can be calculated using a section of an electrical circuit 1 m long, the cross section of which will be 1 mm2.

Resistivity of different metals

Each metal has its own individual characteristics. If we compare the resistivity of aluminum, for example, with copper, it can be noted that for copper this value is 0.0175 Ohm.mm2 / m, and for aluminum - 0.0271 Ohm.mm2 / m. Thus, the resistivity of aluminum is much higher than that of copper. It follows from this that the electrical conductivity is much higher than that of aluminum.

Certain factors influence the value of the resistivity of metals. For example, during deformations, the structure of the crystal lattice is disturbed. Due to the resulting defects, the resistance to the passage of electrons inside the conductor increases. Therefore, there is an increase in the resistivity of the metal.

Temperature also has an effect. When heated, the nodes of the crystal lattice begin to oscillate more strongly, thereby increasing the resistivity. Currently, due to the high resistivity, aluminum wires are being replaced everywhere with copper wires, which have a higher conductivity.

Electric current arises as a result of closing the circuit with a potential difference at the terminals. The field forces act on free electrons and they move along the conductor. During this journey, electrons meet atoms and transfer to them part of their accumulated energy. As a result, their speed decreases. But, due to the influence of the electric field, it is gaining momentum again. Thus, the electrons are constantly experiencing resistance, which is why the electric current heats up.

The property of a substance to convert electricity into heat during the action of a current is electrical resistance and is denoted as R, its measurement unit is Ohm. The amount of resistance depends mainly on the ability of various materials to conduct current.
For the first time, the German researcher G. Ohm announced resistance.

In order to find out the dependence of current strength on resistance, a famous physicist conducted many experiments. For experiments, he used various conductors and obtained various indicators.
The first thing G. Ohm determined was that the resistivity depends on the length of the conductor. That is, if the length of the conductor increased, the resistance also increased. As a result, this relationship was determined to be directly proportional.

The second dependence is the cross-sectional area. It could be determined by a cross section of the conductor. The area of ​​the figure that formed on the cut is the cross-sectional area. Here the relationship is inversely proportional. That is, the larger the cross-sectional area, the lower the resistance of the conductor.

And the third, important quantity, on which the resistance depends, is the material. As a result of the fact that Ohm used different materials in the experiments, he found different properties of resistance. All these experiments and indicators were summarized in a table from which one can see the different values ​​of the specific resistance of various substances.

It is known that the best conductors are metals. Which metals are the best conductors? The table shows that copper and silver have the least resistance. Copper is used more often because of its lower cost, while silver is used in the most important and critical devices.

Substances with high resistivity in the table do not conduct electricity well, which means they can be excellent insulating materials. Substances with this property to the greatest extent are porcelain and ebonite.

In general, electrical resistivity is a very important factor, because by determining its indicator, we can find out what substance the conductor is made of. To do this, it is necessary to measure the cross-sectional area, find out the current strength using a voltmeter and ammeter, and also measure the voltage. Thus, we will find out the value of resistivity and, using the table, we can easily reach the substance. It turns out that resistivity is like the fingerprints of a substance. In addition, resistivity is important when planning long electrical circuits: we need to know this figure in order to strike a balance between length and area.

There is a formula that determines that the resistance is 1 ohm, if at a voltage of 1V, its current strength is 1A. That is, the resistance of unit area and unit length, made of a certain substance, is the resistivity.

It should also be noted that the resistivity index directly depends on the frequency of the substance. That is, whether it has impurities. That, the addition of only one percent of manganese increases the resistance of the most conductive substance - copper, three times.

This table shows the electrical resistivity of some substances.

Highly Conductive Materials

Copper
As we have said, copper is most often used as a conductor. This is due not only to its low resistance. Copper has the advantages of high strength, corrosion resistance, ease of use and good machinability. Good grades of copper are M0 and M1. In them, the amount of impurities does not exceed 0.1%.

The high cost of the metal and its recent scarcity encourages manufacturers to use aluminum as a conductor. Also, copper alloys with various metals are used.
Aluminum
This metal is much lighter than copper, but aluminum has a high heat capacity and melting point. In this regard, in order to bring it to a molten state, more energy is required than copper. Nevertheless, the fact of copper deficiency must be taken into account.
In the production of electrical products, as a rule, aluminum grade A1 is used. It contains no more than 0.5% impurities. And the metal of the highest frequency is AB0000 grade aluminum.
Iron
The cheapness and availability of iron is overshadowed by its high specific resistance. In addition, it quickly corrodes. For this reason, steel conductors are often coated with zinc. The so-called bimetal is widely used - this is steel coated with copper for protection.
Sodium
Sodium is also an accessible and promising material, but its resistance is almost three times that of copper. In addition, metallic sodium has a high chemical activity, which makes it necessary to cover such a conductor with hermetic protection. It should also protect the conductor from mechanical damage, since sodium is a very soft and rather fragile material.

Superconductivity
The table below shows the resistivity of substances at a temperature of 20 degrees. The indication of temperature is not accidental, because the resistivity directly depends on this indicator. This is explained by the fact that when heated, the speed of atoms also increases, which means that the probability of their meeting with electrons will also increase.

It is interesting what happens to the resistance under cooling conditions. For the first time, the behavior of atoms at very low temperatures was noticed by G. Kamerling-Onnes in 1911. He cooled the mercury wire to 4K and found its resistance to drop to zero. The physicist called the change in the specific resistance index of some alloys and metals under low temperature conditions superconductivity.

Superconductors pass into the state of superconductivity when cooled, and their optical and structural characteristics do not change. The main discovery is that the electrical and magnetic properties of metals in the superconducting state are very different from their own properties in the ordinary state, as well as from the properties of other metals, which cannot go into this state when the temperature is lowered.
The use of superconductors is carried out mainly in obtaining a superstrong magnetic field, the strength of which reaches 107 A/m. Systems of superconducting power lines are also being developed.

Similar materials.