What resistivity substances? To reply simple words On this question, you need to remember the course of physics and present the physical embodiment of this definition. Through the substance passes the electric current, and it, in turn, prevents some power under the passage.

The concept of resistivity of the substance

It is this value that shows how much the substance has a strength of the current and there is a resistivity (Latin letter "RO"). In the international system of units resistance it is expressed in Omahmultiplied by meter. The formula for calculation sounds like this: "The resistance is multiplied by the cross-sectional area and is divided into the length of the conductor."

The question arises: "Why is one more resistance are used when setting resistivity?". The answer is simple, there are two different quantities - resistivity and resistance. The second shows how much the substance can interfere with the passage through it current, and the first shows almost the same thing, only we are no longer about the substance in the general sense, but about the conductor with a specific length and cross-section area, which are made of this substance.

The inverse value that characterizes the ability of a substance to pass electricity is referred to as the specific electrical conductivity and the formula for which the specific resistance is calculated directly related to the specific conductivity.

Application of media

The concept of specific resistance is widely used in calculating the conductivity of the electric current by various metals. On the basis of these calculations, decisions are made on the feasibility of using a metal for the manufacture of electrical conductors, which are used in construction, instrument making and other areas.

Metal resistance table

There are certain tables? In which the present information about transmitting and resistance of metals is reduced, as a rule, these tables are calculated for certain conditions.

In particular, widely known metal single crystal resistance table At a temperature of twenty degrees Celsius, as well as a table of resistance of metals and alloys.

These tables use to calculate various data in the so-called ideal conditions to calculate the values \u200b\u200bfor specific purposes you need to use formulas.

Copper. Her characteristics and properties

Description of the substance and properties

Copper is a metal that has been opened to humanity a long time ago and has also been used for various technical purposes. Copper is very damp and plastic metal with high electrical conductivity, it makes it very popular for making different wires and conductors.

Physical properties of copper:

• melting point - 1084 degrees Celsius;
• boiling point - 2560 degrees Celsius;
• density at 20 degrees - 8890 kilograms divided into a cubic meter;
• specific heat capacity at constant pressure and temperature 20 degrees - 385 kJ / J * kg
• specific electrical resistance - 0,01724;

Marks of media

This metal can be divided into several groups or brands, each of which has its own properties and its use in industry:

1. The M00, M0, M1 brands are excellent for the production of cables and conductors, during its swathing is eliminated by oxygen.
2. M2 and M3 brands - cheap options that are intended for small rolled and satisfy most technical and industrial tasks A small scale.
3. M1, M1F, M1P, M1P, M3P, M3P, is expensive copper brands that are manufactured for a particular consumer with specific requirements and requests.

Between Marki differ in several parameters:

The effect of impurities on the properties of copper

Impurities can affect the mechanical, technical and operational properties of products.

In conclusion, it should be emphasized that copper is a unique metal with unique properties. It is used in the automotive industry, the manufacture of elements for electric industry, electrical appliances, consumption items, hours, computers and many others. With its low resistivity, this metal is an excellent material for the manufacture of conductors and others. electrical devices. With this property, copper overtakes only silver, but due to higher cost it did not find the same use in the electric industry.

Specific resistance - applied concept in electrical engineering. It denotes how the resistance per unit length has a unit of a single section with current flowing through it - in other words, which resistance has a millimeter cross section with a length of one meter. This concept is used in various electrical calculations.

It is important to understand the differences between the specific electrical resistance of the DC and the specific electrical resistivity of the variable current. In the first case, the resistance is caused solely by the action of DC to the conductor. In the second case, alternating current (it can be any shape: a sinusoidal, rectangular, triangular or arbitrary) causes an additional vortex field in the conductor, which also creates resistance.

Physical view

In technical calculations involving the cable laying of various diameters, parameters are used to calculate the required cable length and its electrical characteristics. One of the main parameters is the resistivity. Specific electrical resistance formula:

ρ \u003d R * S / L, where:

• ρ is the specific resistance of the material;
• R - ohmic electrical resistivity of a particular conductor;
• S - cross section;
• l - Length.

The dimension ρ is measured in OM MM 2 / m, or, reducing the formula - OM M.

The value of ρ for the same substance is always the same. Consequently, this is a constant, characterizing the material of the conductor. It is usually indicated in reference books. Based on this, it is already possible to calculate technical quantities.

It is important to say about the specific electrical conductivity. This value is the inverse resistivity of the material, and is used on a par with it. It is also called electrical conductivity. The higher this value, the better metal Conducts current. For example, the specific conductivity of the copper is equal to 58.14 m / (Om mm 2). Or, in units adopted in the SI system: 58 140,000 cm / m. (Siemens per meter is an electrical conductivity unit in SI).

It is possible to talk about the specific resistance only in the presence of elements conducting the current, as dielectrics have infinite or close to it by electrical resistance. Unlike them, metals are very good current conductors. You can measure the electrical resistivity of the metal conductor using the device of a millimeter, or even more accurate - micrommeter. The value is measured between their apps attached to the conductor site. They allow you to check the chains, wiring, engine windings and generators.

Metals are based on the ability to carry out the current. The resistivity of various metals is a parameter characterizing this difference. The data are shown at a temperature of 20 degrees on the Celsius scale:

The parameter ρ shows how resistance will be a meter conductor with a cross section of 1 mm 2. The more this value, the more the electrical resistance will be at the desired wire of a certain length. The smallest ρ, as can be seen from the list, in silver, the resistance of one meter from this material will be only 0.015 ohms, but it is too expensive metal to use it on an industrial scale. The next is copper, which in nature is occurring much more often (not precious, and non-ferrous metal). Therefore, the copper wiring is very common.

Copper is not only a good electrical current conductor, but also a very plastic material. Thanks to this property, the copper wiring is better stacked, it is resistant to bending and stretching.

Copper is very in demand in the market. Of this material produce many different products:

• A huge variety of conductors;
• Auto parts (for example, radiators);
• Hour mechanisms;
• Computer components;
• Details of electrical and electronic devices.

The specific electrical resistance of the copper is one of the best among the conducting current of the materials, so it is based on a variety of electric industrial goods. In addition, copper is easily soldered, so it is very common in amateur radio.

High thermal conductivity of copper allows it to be used in cooling and heating devices, and plasticity makes it possible to create the smallest details And the finest conductors.

Electric current conductors are the first and second kind. Conductors of the first kind are metals. Second-type conductors are conductive solutions of liquids. The current in the first tolerate electrons, and current carriers in the second-kind conductions of -yons, charged particles of the electrolytic fluid.

It is possible to talk about the conductivity of materials only in the context of temperature. ambient. With more high temperatures Conductors of the first kind increase their electrical resistance, and the second, on the contrary, is reduced. Accordingly, there is a temperature coefficient of resistance of materials. The specific impedance of copper OM increases with increasing heating. The temperature coefficient α also depends only on the material, this value does not have dimension and for different metals and alloys are equal to the following indicators:

• Silver - 0.0035;
• Iron - 0.0066;
• Platinum - 0.0032;
• Copper - 0.0040;
• Tungsten - 0.0045;
• Mercury - 0.0090;
• Konstanta - 0.000005;
• Nickel - 0.0003;
• Nichrome - 0.00016.

The determination of the electrical resistance of the conductor section at an elevated temperature R (T) is calculated by the formula:

R (T) \u003d R (0) ·, where:

• R (0) - resistance at the initial temperature;
• α - temperature coefficient;
• t - T (0) - temperature difference.

For example, knowing the electrical resistance of copper at 20 degrees Celsius, it can be calculated that it will be equal at 170 degrees, that is, when heated is 150 degrees. The initial resistance will increase at times, that is, 1.6 times.

With increasing temperature, the conductivity of materials, on the contrary, decreases. Since this is a quantity, inverse electrical resistance, it decreases exactly the same amount. For example, the specific electrical conductivity of copper when heating the material by 150 degrees will decrease by 1.6 times.

There are alloys that practically do not change their electrical resistance when temperatures change. Such, for example, Constanta. When the temperature changes for one hundred degrees, its resistance increases only 0.5%.

If the conductivity of the materials deteriorates with heating, it improves with a decrease in temperature. This is a phenomenon as superconductivity. If you lower the temperature of the conductor below -253 degrees Celsius, its electrical resistance will decrease sharply: almost to zero. In this regard, the transmission costs fall electrical Energy. The only problem was the cooling of the conductors to such temperatures. However, due to the recent discoveries of high-temperature superconductors based on copper oxides, the materials of the cooled are already before acceptable values.

The specific impedance of metals is their ability to counteract electric Toku.passing through them. The unit of measurement of this value is OM * M (OM-meter). As a symbol used greek letter ρ (RO). High rates The resistivity means the poor conductivity of the electrical charge with one or another material.

Specifications Steel

Before detailing the resistivity of the steel, it should be familiar with its main physico-mechanical properties. Thanks to its qualities, this material was widespread in the production sector and other areas of life and activities of people.

Steel is an alloy of iron and carbon contained in an amount not exceeding 1.7%. In addition to carbon, steel contains a certain amount of impurities - silicon, manganese, sulfur and phosphorus. On its qualities it is significantly better cast iron, it is easy to harden, forging, renting and other types of processing. All types of steels are distinguished by high strength and plasticity.

In its purpose, steel is divided into structural, instrumental, as well as special physical properties. Each of these contains a different amount of carbon, thanks to which the material acquires certain specific qualities, for example, heat resistance, heat resistance, resistance to rust and corrosion.

Special place is occupied by electrical steel produced in sheet format and used in the manufacture of electrical products. To obtain this material, silicon doping is made capable of improving its magnetic and electrical properties.

In order to have electrical steel acquired necessary characteristicsIt is necessary to comply with certain requirements and conditions. The material should be easily magnifying and reclicing, that is, to have a high magnetic permeability. Such steels are good, and their reclamation is carried out with minimal losses.

On compliance with these requirements, dimensions depend on magnetic cores and windings, as well as the coefficient useful action transformers and the magnitude of their operating temperature. Many factors affect the fulfillment of the conditions, including steel resistivity.

Specific resistance and other indicators

The value of the specific electrical resistance is the ratio of the electric field strength in the metal and the density of the current flowing in it. For practical calculations, the formula is used: in which ρ is a specific metal resistance (OM * M), E. - electric field strength (in / m), and J. - Electrotock density in metal (A / m 2). With a very large electric field strength and low current density, the resistivity of the metal will be high.

There is another value called the specific electrical conductivity, inverse the resistivity, indicating the degree of electrical conductivity in one way or another. It is determined by the formula and expressed in units of CM / M - Siemens per meter.

Specific resistance is closely related to electrical resistance. However, they have differences among themselves. In the first case, this is the property of the material, including steel, and in the second case, the property of the entire object is determined. The quality of the resistor affects the combination of several factors, first of all, forms and resistivity of the material from which it is made. For example, if a thin and long wire was used to manufacture a wire resistor, then its resistance will be greater than that of the resistor made of the thick and short wire of the same metal.

As another example, you can bring resistors from wire with the same diameter and length. However, if in one of them the material has a high resistivity, and in another low, then, respectively, in the first resistor, the electrical resistance will be higher than in the second.

Knowing the basic properties of the material, the resistivity of the steel can be used to determine the resistance value of the steel conductor. For calculations, in addition to the electrical resistance, the diameter and the length of the wire itself will be required. Calculations are performed according to the following formula: in which R. is (ohms) ρ - Steel resistivity (Om * m), L. - corresponds to the length of the wire, BUT - Square of its cross section.

There is a dependence of the resistivity of steel and other metals from temperature. Most calculations are used room temperature - 20 0 C. All changes under the influence of this factor are taken into account using the temperature coefficient.

Content:

In the electrical engineering one of the main elements of the electrical circuits are wires. Their task is to skip electrical current with minimal loss. An experimental way has long been determined that to minimize the loss of electricity the wire is best made from silver. It is this metal that provides the properties of the conductor with minimal resistance in Omah. But since this noble metal of roads, its use is very limited in industry.

And the main metals for wires were aluminum and copper. Unfortunately, the resistance of iron as an electricity conductor is too large so that it turned out a good wire from it. Despite more low costIt is applied only as the carrier of the wire of power lines.

Such different resistance

Resistance is measured in ohms. But for the wires, this value is very small. If you try to measure the tester in the resistance measurement mode, it will be difficult to get the correct result. Moreover, whatever the wire we take, the result on the scoreboard will differ little. But this does not mean that in fact the electrical resistance of these wires will equally affect the loss of electricity. To make sure that it is necessary to analyze the formula for which the resistance calculation is made:

This formula uses such values \u200b\u200bas:

It turns out that the resistance determines the resistance. There is a resistance calculated by the formula using other resistance. This is a specific electrical resistance of ρ (Greek letter RO) just causes the advantage of a metal as an electric conductor:

Therefore, if applying copper, iron, silver or any other material for the manufacture of identical wires or conductors of a special design, the main role in its electrical properties is the material.

But in fact, the situation with resistance is more difficult than just calculations according to the formulas above. These formulas do not take into account the temperature and shape of the transcriptor. And with increasing temperature, the specific resistance of copper, as well as any other metal, becomes greater. Quite visual example This may be incandescent light. You can measure the resistance to its helix with a tester. Then, measuring the current strength in the circuit with this lamp, according to the law of Ohm, calculate its resistance in the state of the glow. The result will be much larger than when measuring the resistance to the tester.

Also, copper will not give the expected efficiency at the current great powerIf neglect the cross section of the conductor. The skin effect, which manifests itself directly proportional to the increase in current force, makes ineffective conductors with a round cross section, even if silver or copper is used. For this reason, the resistance of the round copper wire With a strength current, it may be higher than that of a flat wire from aluminum.

Moreover, even if their sparkms are the same. With alternating current, the skin effect is also manifested, increasing as the frequency increases. The skin effect means the striving of current to flow closer to the surface of the conductor. For this reason, in some cases it is more profitable to use silver wire coating. Even a slight decrease in the resistivity of the silver-earth surface copper conductor Significantly reduces signal loss.

Generalization of the performance of the specific resistance

As in any other case, which is associated with the display of dimensions, the resistivity is expressed in different systems of units. In SI (international system of units), OM M is used, but permissible to use alone * sq. MM / M (this is an incidental unit of measuring resistance). But in a real conductor, the specific resistance value is inconsistent. Since all materials are characterized by a certain purity, which can be changed from point to point, it was necessary to create an appropriate idea of \u200b\u200bresistance in real material. Such a manifestation was the law of Oma in differential form:

This law is likely to be applied to payments to everyday life. But during the design of various electronic components, for example, resistors, crystalline elements, it is certainly used. Because it allows you to perform calculations based on this point for which there is a current density and electric field strength. And corresponding specific resistance. The formula is used for inhomogeneous isotropic, as well as anisotropic substances (crystals, discharge in gas, etc.).

How to get clean copper

In order to maximize the losses in the wires and the veins of cables from copper, it should be particularly clean. This is achieved special technological processes:

• based on electron beam, as well as zone smelting;
• multiple electrolysis cleaning.

Or electrical circuit electric current.

Electrical resistance is defined as a proportionality coefficient R. Between voltage U. and DC power I. In the law of Ohm for the chain section.

The unit of resistance is called omom (OM) in honor of the German scientist. Ohm, who introduced this concept into physics. One Ohm (1 Ohm) is the resistance of such a conductor in which at a voltage 1 IN The current of the current is equal 1 BUT.

Resistivity.

Resistance to a homogeneous conductor of a permanent section depends on the material of the conductor, its length l. and cross-section S. and can be determined by the formula:

where ρ - The resistivity of the substance from which the conductor is made.

Specific resistivity - This is a physical value showing how the resistance has the conductor of a single length and a single cross-sectional area.

From the formula it follows that

The quantity, inverse ρ , called specific conductivity σ :

Since the system of resistance is 1 ohm. unit of 1 m 2, and a single length of 1 m, then the unit of resistivity in C will be 1 ohm · m 2 / m, or 1 ohms · m. Unit of specific conductivity in SI - OM -1 M -1.

In practice, the cross-sectional area of \u200b\u200bthin wires is often expressed in square millimeters (mm 2). In this case, a more convenient unit of resistivity is OM · mm 2 / m. Since 1 mm 2 \u003d 0.000001 m 2, then 1 ohm mm 2 / m \u003d 10 -6 ohm · m. Metals have a very low resistivity - order (1 · 10 -2) ohm mm 2 / m, dielectrics - at 10 15 -10 20 large.

Dependence of temperature resistance.

With increasing temperature, the resistance of metals increases. However, there are alloys, the resistance of which is almost not changed by increasing the temperature (for example, Konstanta, Manganin, etc.). The resistance of electrolytes with an increase in temperature decreases.

Temperature resistance coefficient The conductor is called the ratio of the value of the resistance of the conductor when heated by 1 ° C to the value of its resistance at 0 ºС:

.

The dependence of the resistivity of conductors from temperature is expressed by the formula:

.

In general α Depends on temperature, but if the temperature range is small, then the temperature coefficient can be considered constant. For pure metals α \u003d (1/273) to -1. For electrolyte solutions α < 0 . For example, for a 10% solution of the sodium salt α \u003d -0.02 to -1. For Constantane (copper alloy with nickel) α \u003d 10 -5 to -1.

The dependence of the resistance of the conductor on temperature is used in resistance thermometers.