Each substance is capable of conducting current to varying degrees, this value is affected by the resistance of the material. The resistivity of copper, aluminum, steel and any other element is denoted by the letter ρ of the Greek alphabet. This value does not depend on such characteristics of the conductor as size, shape and physical condition; ordinary electrical resistance takes these parameters into account. Resistivity is measured in Ohms multiplied by mm² and divided by meter.
Categories and their descriptions
Any material is capable of exhibiting two types of resistance depending on the electricity supplied to it. The current can be variable or constant, which significantly affects the technical performance of the substance. So, there are such resistances:
- Ohmic. Appears under the influence direct current. Characterizes friction, which is created by the movement of electrically charged particles in a conductor.
- Active. It is determined according to the same principle, but is created under the influence of alternating current.
In this regard, there are also two definitions of specific value. For direct current, it is equal to the resistance exerted by a unit length of conductive material of a unit fixed cross-sectional area. The potential electric field affects all conductors, as well as semiconductors and solutions capable of conducting ions. This value determines the conductive properties of the material itself. The shape of the conductor and its dimensions are not taken into account, so it can be called basic in electrical engineering and materials science.
Subject to passing alternating current specific value calculated taking into account the thickness of the conductive material. Here the influence of not only potential, but also eddy current occurs, and in addition, the frequency of electric fields is taken into account. Resistivity of this type is greater than with direct current, since here the positive value of the resistance to the vortex field is taken into account. This value also depends on the shape and size of the conductor itself. It is these parameters that determine the nature of the vortex motion of charged particles.
Alternating current causes certain electromagnetic phenomena. They are very important for the electrical characteristics of the conductive material:
- The skin effect is characterized by a weakening of the electromagnetic field, the more it penetrates into the medium of the conductor. This phenomenon is also called the surface effect.
- The proximity effect reduces current density due to the proximity of adjacent wires and their influence.
These effects are very important when calculating optimal thickness conductor, since when using a wire whose radius is greater than the depth of current penetration into the material, the rest of its mass will remain unused, and therefore this approach will be ineffective. In accordance with the calculations carried out, the effective diameter of the conductive material in some situations will be as follows:
- for a current of 50 Hz - 2.8 mm;
- 400 Hz - 1 mm;
- 40 kHz - 0.1 mm.
In view of this, the use of flat multicore cables, consisting of many thin wires, is actively used for high-frequency currents.
Characteristics of metals
Specific indicators of metal conductors are contained in special tables. Using these data, you can make the necessary further calculations. An example of such a resistivity table can be seen in the image.
The table shows that silver has the greatest conductivity - it is an ideal conductor among all existing metals and alloys. If you calculate how much wire from this material is required to obtain a resistance of 1 ohm, you will get 62.5 m. Iron wire for the same value will require as much as 7.7 m.
No matter how wonderful properties silver has, it is too expensive material for mass use in electrical networks, therefore copper has found wide application in everyday life and industry. In terms of specific indicator, it is in second place after silver, and in terms of prevalence and ease of extraction, it is much better than it. Copper has other advantages that have allowed it to become the most common conductor. These include:
For use in electrical engineering, refined copper is used, which, after smelting from sulfide ore, goes through the processes of roasting and blowing, and then necessarily undergoes electrolytic purification. After such processing, it is possible to obtain a material that is very High Quality(grades M1 and M0), which will contain from 0.1 to 0.05% impurities. An important nuance is the presence of oxygen in extremely small quantities, since it negatively affects the mechanical characteristics of copper.
Often this metal is replaced by cheaper materials - aluminum and iron, as well as various bronzes (alloys with silicon, beryllium, magnesium, tin, cadmium, chromium and phosphorus). Such compositions have higher strength compared to pure copper, although they have lower conductivity.
Advantages of aluminum
Although aluminum has greater resistance and is more fragile, its widespread use is due to the fact that it is not as scarce as copper and therefore costs less. Aluminum has a resistivity of 0.028 and its low density makes it 3.5 times lighter than copper.
For electrical work use purified aluminum grade A1, containing no more than 0.5% impurities. The higher grade AB00 is used for the manufacture of electrolytic capacitors, electrodes and aluminum foil. The impurity content in this aluminum is no more than 0.03%. There is also pure metal AB0000, including no more than 0.004% additives. The impurities themselves also matter: nickel, silicon and zinc have a slight effect on the conductivity of aluminum, and the content of copper, silver and magnesium in this metal has a noticeable effect. Thallium and manganese reduce conductivity the most.
Aluminum has good anti-corrosion properties. Upon contact with air, it becomes covered with a thin film of oxide, which protects it from further destruction. For improvement mechanical characteristics the metal is alloyed with other elements.
Indicators of steel and iron
The resistivity of iron compared to copper and aluminum is very high, however, due to its availability, strength and resistance to deformation, the material is widely used in electrical production.
Although iron and steel, whose resistivity is even higher, have significant disadvantages, manufacturers of conductor materials have found methods to compensate for them. In particular, low corrosion resistance is overcome by coating the steel wire with zinc or copper.
Properties of sodium
Sodium metal is also very promising in conductor production. In terms of resistance, it significantly exceeds copper, but has a density 9 times less than that. This allows the material to be used in the manufacture of ultra-light wires.
Sodium metal is very soft and completely unstable to any kind of deformation, which makes its use problematic - a wire made of this metal must be covered with a very strong sheath with extremely little flexibility. The shell must be sealed, since sodium exhibits strong chemical activity under the most neutral conditions. It instantly oxidizes in air and exhibits a violent reaction with water, including water contained in the air.
Another benefit of using sodium is its availability. It can be obtained through the electrolysis of molten sodium chloride, of which there is an unlimited amount in the world. Other metals are clearly inferior in this regard.
To calculate the performance of a specific conductor, it is necessary to divide the product of the specific number and length of the wire by its cross-sectional area. The result will be the resistance value in Ohms. For example, to determine the resistance of 200 m of iron wire with a nominal cross-section of 5 mm², you need to multiply 0.13 by 200 and divide the result by 5. The answer is 5.2 Ohms.
Rules and features of calculation
Microohmmeters are used to measure the resistance of metallic media. Today they are produced in a digital version, so the measurements taken with their help are accurate. It can be explained by the fact that metals have high level conductivity and have extremely low resistance. For example, the lower threshold of measuring instruments has a value of 10 -7 Ohms.
Using microohmmeters, you can quickly determine how good the contact is and what resistance is exhibited by the windings of generators, electric motors and transformers, as well as electrical buses. It is possible to calculate the presence of inclusions of another metal in the ingot. For example, a piece of tungsten plated with gold exhibits half the conductivity of all gold. In the same way you can determine internal defects and cavities in the conductor.
The resistivity formula is as follows: ρ = Ohm mm 2 /m. In words it can be described as the resistance of 1 meter of conductor, having a cross-sectional area of 1 mm². The temperature is assumed to be standard - 20 °C.
Effect of temperature on measurement
Heating or cooling of some conductors has a significant effect on the performance of measuring instruments. An example is the following experiment: it is necessary to connect a spirally wound wire to the battery and connect an ammeter to the circuit.
The more the conductor heats up, the lower the readings on the device become. The current has the opposite proportional dependence from resistance. Therefore, we can conclude that as a result of heating, the conductivity of the metal decreases. To a greater or lesser extent, all metals behave this way, but in some alloys there is practically no change in conductivity.
It is noteworthy that liquid conductors and some solid nonmetals tend to decrease their resistance as temperature increases. But scientists have also turned this ability of metals to their advantage. Knowing the temperature coefficient of resistance (α) when heating some materials, it is possible to determine the external temperature. For example, a platinum wire placed on a mica frame is placed in an oven and the resistance is measured. Depending on how much it has changed, a conclusion is drawn about the temperature in the oven. This design is called a resistance thermometer.
If at temperature t 0 conductor resistance is r 0, and at temperature t equals rt, then the temperature coefficient of resistance is equal to 
Calculation using this formula can only be done in a certain temperature range (up to approximately 200 °C).
Specific electrical resistance is a physical quantity that indicates the extent to which a material can resist passage through it electric current. Some people may get confused 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.
Reciprocal of this material is the specific 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 mm 2 /m. To transfer from this system to an international one you will not need to use complex formulas, since 1 Ohm mm 2 /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 in certain conditions. For example, outdoors, where temperature values depend on the time of year, necessary materials with less susceptibility 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. Carriers electric charges 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 generally not applicable for electrical engineering if they have high value this parameter. Other properties may interfere with this. For example, the electrical conductivity of water will not have of great importance for this area. Here are the values of some substances with high indicators.
| High resistivity materials | ρ (Ohm m) |
| Bakelite | 10 16 |
| Benzene | 10 15 ...10 16 |
| Paper | 10 15 |
| Distilled water | 10 4 |
| Sea water | 0.3 |
| Dry wood | 10 12 |
| The ground is wet | 10 2 |
| Quartz glass | 10 16 |
| Kerosene | 10 1 1 |
| Marble | 10 8 |
| Paraffin | 10 1 5 |
| Paraffin oil | 10 14 |
| Plexiglass | 10 13 |
| Polystyrene | 10 16 |
| Polyvinyl chloride | 10 13 |
| Polyethylene | 10 12 |
| Silicone oil | 10 13 |
| Mica | 10 14 |
| Glass | 10 11 |
| Transformer oil | 10 10 |
| Porcelain | 10 14 |
| Slate | 10 14 |
| Ebonite | 10 16 |
| Amber | 10 18 |
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.
| Low resistivity materials | ρ (Ohm m) |
| Aluminum | 2.7·10 -8 |
| Tungsten | 5.5·10 -8 |
| Graphite | 8.0·10 -6 |
| Iron | 1.0·10 -7 |
| Gold | 2.2·10 -8 |
| Iridium | 4.74·10 -8 |
| Constantan | 5.0·10 -7 |
| Cast steel | 1.3·10 -7 |
| Magnesium | 4.4·10 -8 |
| Manganin | 4.3·10 -7 |
| Copper | 1.72·10 -8 |
| Molybdenum | 5.4·10 -8 |
| Nickel silver | 3.3·10 -7 |
| Nickel | 8.7·10 -8 |
| Nichrome | 1.12·10 -6 |
| Tin | 1.2·10 -7 |
| Platinum | 1.07·10 -7 |
| Mercury | 9.6·10 -7 |
| Lead | 2.08·10 -7 |
| Silver | 1.6·10 -8 |
| Gray cast iron | 1.0·10 -6 |
| Carbon brushes | 4.0·10 -5 |
| Zinc | 5.9·10 -8 |
| Nikelin | 0.4·10 -6 |
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 the parameter when different temperatures widely used in electrical engineering. Most simple example is an incandescent lamp where it is used nichrome thread. 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.
Widespread use has also affected noble metals, which have suitable characteristics for electrical engineering. For critical circuits that require high speed, silver contacts are selected. They have high cost, but given the relatively small amount of materials, their use is quite justified. Copper is inferior to silver in conductivity, but has 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 they are not always appropriate for outdoor use, 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.
It has been experimentally established that resistance R metal conductor is directly proportional to its length L and inversely proportional to its area cross section A:
R = ρ L/ A (26.4)
where is the coefficient ρ is called resistivity and serves as a characteristic of the substance from which the conductor is made. This is common sense: a thick wire should have less resistance than a thin wire because electrons can move over a larger area in a thick wire. And we can expect an increase in resistance with increasing length of the conductor, as the number of obstacles to the flow of electrons increases.
Typical values ρ For different materials are given in the first column of the table. 26.2. (Actual values depend on the purity of the substance, heat treatment, temperature and other factors.)
|
Table 26.2. Specific resistance and temperature coefficient of resistance (TCR) (at 20 °C) |
||
| Substance | ρ ,Ohm m | TKS α ,°C -1 |
| Conductors | ||
| Silver | 1.59·10 -8 | 0,0061 |
| Copper | 1.68·10 -8 | 0,0068 |
| Aluminum | 2.65·10 -8 | 0,00429 |
| Tungsten | 5.6·10 -8 | 0,0045 |
| Iron | 9.71·10 -8 | 0,00651 |
| Platinum | 10.6·10 -8 | 0,003927 |
| Mercury | 98·10 -8 | 0,0009 |
| Nichrome (alloy of Ni, Fe, Cr) | 100·10 -8 | 0,0004 |
| Semiconductors 1) | ||
| Carbon (graphite) | (3-60)·10 -5 | -0,0005 |
| Germanium | (1-500)·10 -5 | -0,05 |
| Silicon | 0,1 - 60 | -0,07 |
| Dielectrics | ||
| Glass | 10 9 - 10 12 | |
| Hard rubber | 10 13 - 10 15 | |
| 1) Real values strongly depend on the presence of even small amounts of impurities. | ||
Silver has the lowest resistivity, which thus turns out to be the best conductor; however it is expensive. Copper is slightly inferior to silver; It is clear why wires are most often made of copper.
Aluminum has a higher resistivity than copper, but it has a much lower density and is preferred in some applications (for example, in power lines) because the resistance of aluminum wires of the same mass is less than that of copper. The reciprocal of resistivity is often used:
σ = 1/ρ (26.5)
σ called specific conductivity. Specific conductivity is measured in units (Ohm m) -1.
The resistivity of a substance depends on temperature. As a rule, the resistance of metals increases with temperature. This should not be surprising: as temperature increases, atoms move faster, their arrangement becomes less ordered, and we can expect them to interfere more with the flow of electrons. In narrow temperature ranges, the resistivity of the metal increases almost linearly with temperature:
Where ρ T- resistivity at temperature T, ρ 0 - resistivity at standard temperature T 0 , a α - temperature coefficient of resistance (TCR). The values of a are given in table. 26.2. Note that for semiconductors the TCR can be negative. This is obvious, since with increasing temperature the number of free electrons increases and they improve the conductive properties of the substance. Thus, the resistance of a semiconductor may decrease with increasing temperature (although not always).
The values of a depend on temperature, so you should pay attention to the temperature range within which given value(for example, according to a reference book of physical quantities). If the range of temperature changes turns out to be wide, then linearity will be violated, and instead of (26.6) it is necessary to use an expression containing terms that depend on the second and third powers of temperature:
ρ T = ρ 0 (1+αT+ + βT 2 + γT 3),
where are the coefficients β And γ usually very small (we put T 0 = 0°С), but at large T the contributions of these members become significant.
At very low temperatures ah, the resistivity of some metals, as well as alloys and compounds, drops to zero within the accuracy of modern measurements. This property is called superconductivity; it was first observed by the Dutch physicist Geike Kamerling Onnes (1853-1926) in 1911 when mercury was cooled below 4.2 K. At this temperature, the electrical resistance of mercury suddenly dropped to zero.
Superconductors enter a superconducting state below the transition temperature, which is typically a few degrees Kelvin (just above absolute zero). An electric current was observed in a superconducting ring, which practically did not weaken in the absence of voltage for several years.
IN last years Superconductivity is being intensively researched to understand its mechanism and to find materials that superconduct at higher temperatures to reduce the cost and inconvenience of having to cool to very low temperatures. The first successful theory of superconductivity was created by Bardeen, Cooper and Schrieffer in 1957. Superconductors are already used in large magnets, where the magnetic field is created by an electric current (see Chapter 28), which significantly reduces energy consumption. Of course, maintaining a superconductor at a low temperature also requires energy.
Comments and suggestions are accepted and welcome!
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 differently affect the passage of current in it. These properties are assessed by electrical resistance R.
Georg Ohm empirically determined the factors influencing the electrical resistance of a substance and derived it 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 resistance value measured at a temperature of 0 ° C for a homogeneous mercury column 106.3 cm long with a cross-sectional area of 1 mm 2.
Definition
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.
It is taken into account that the resistance of the conductor increases with increasing its length and decreasing cross-section. The SI system uses a volume of a homogeneous conductor with a length of 1 meter and a cross-section of 1 m 2. In technical calculations, an outdated but convenient non-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 was introduced - specific conductivity b. It is inversely proportional to the resistivity value and 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 working for outdoors in heat and cold.
The material and resistivity of the wire are selected taking into account the operating conditions.
The increase in the resistance of conductors to the passage of current when heated is explained by the fact that as the temperature of the metal increases, the intensity of movement of atoms and electric charge carriers in it increases in all directions, which creates unnecessary obstacles to the movement of charged particles in one direction and reduces the amount of their flow.
If you reduce the temperature of the metal, the conditions for the passage of current improve. When cooled to a critical temperature, many metals exhibit the phenomenon of superconductivity, when their electrical resistance is practically zero. This property is widely used in powerful electromagnets.
The effect of temperature on the conductivity of metal is used by the electrical industry in the manufacture of ordinary incandescent lamps. When a current passes through them, it heats up to such a state that it emits a luminous flux. Under normal conditions, the resistivity 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 very quickly heats up the metal. At the same time, the resistance of the electrical circuit increases, limiting the initial current to the nominal value required to obtain lighting. In this way, the current strength is easily regulated through a nichrome spiral, eliminating the need to use complex ballasts used in LED and fluorescent sources.
How is the resistivity of materials used in technology?
Non-ferrous precious metals have best properties electrical conductivity. 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 due to their high chemical resistance and beautiful appearance in jewelry. In addition, gold, silver and platinum have high ductility, and platinum group metals have refractoriness and, like gold, chemical inertness. These advantages of noble metals are combined.
Copper alloys, which have good conductivity, are used to make shunts that limit the flow of large currents through the measuring head of high-power ammeters.
The resistivity 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 lighter. This explains its widespread use in the energy sector for the manufacture of outdoor wires and cable cores.
The resistivity of iron 0.13 (ohm ∙mm 2)/m also allows its use for transmitting electric current, but this results in greater power losses. Steel alloys have increased strength. Therefore, aluminum overhead wires high voltage lines Electric transmissions are woven with steel threads that are designed to withstand tensile loads.
This is especially true when ice forms on wires or strong gusts of wind.
Some alloys, for example, constantine and nickel, have thermally stable resistive characteristics in a certain range. Nickel's electrical resistivity remains virtually unchanged from 0 to 100 degrees Celsius. Therefore, spirals for rheostats are made of nickel.
IN measuring instruments The property of strictly changing the resistivity values of platinum depending on its temperature is widely used. If electric current from a stabilized voltage source is passed through a platinum conductor and the resistance value is calculated, it will indicate the temperature of the platinum. This allows the scale to be graduated in degrees corresponding to Ohm values. This method allows you to measure temperature with an accuracy of fractions of degrees.
Sometimes to solve practical problems you need to know cable impedance or specific resistance. To do this, in the reference books on cable products The values of inductive and active resistance of one core are given for each cross-sectional value. They are used to calculate permissible loads, heat generated, permissible operating conditions are determined and effective protection is selected.
The conductivity of metals is influenced by the method of their processing. Using pressure for plastic deformation disrupts the crystal lattice structure, increases the number of defects and increases resistance. To reduce it, recrystallization annealing is used.
Stretching or compressing metals causes elastic deformation in them, from which the amplitudes of thermal vibrations of electrons decrease and the resistance decreases somewhat.
When designing grounding systems, it is necessary to take into account. It differs in definition from the above method and is measured in SI units - Ohm∙meter. It is used to evaluate the quality of the flow of electric current inside the earth.
The conductivity of soil is influenced by many factors, including soil moisture, density, particle size, temperature, and the concentration of salts, acids and alkalis.
Resistivity is an applied concept in electrical engineering. It denotes how much resistance per unit length a material of a unit cross-section has to the current flowing through it - in other words, what resistance a wire of a millimeter cross-section one meter long has. This concept is used in various electrical calculations.
It is important to understand the differences between DC electrical resistivity and AC electrical resistivity. In the first case, the resistance is caused solely by the action of direct current on the conductor. In the second case alternating current(it can be of any shape: sinusoidal, rectangular, triangular or arbitrary) causes an additional vortex field in the conductor, which also creates resistance.
Physical representation
In technical calculations involving the laying of cables of various diameters, parameters are used to calculate the required cable length and its electrical characteristics. One of the main parameters is resistivity. Electrical resistivity formula:
ρ = R * S / l, where:
- ρ is the resistivity of the material;
- R is the ohmic electrical resistance of a particular conductor;
- S - cross section;
- l - length.
The dimension ρ is measured in Ohm mm 2 /m, or, to abbreviate the formula - Ohm m.
The value of ρ for the same substance is always the same. Therefore, this is a constant characterizing the material of the conductor. It is usually indicated in directories. Based on this, it is already possible to calculate technical quantities.
It is important to say about specific electrical conductivity. This value is the inverse of the resistivity of the material, and is used equally with it. It is also called electrical conductivity. The higher this value, the metal is better conducts current. For example, the conductivity of copper is 58.14 m/(Ohm mm2). Or, in SI units: 58,140,000 S/m. (Siemens per meter is the SI unit of electrical conductivity).
We can talk about resistivity only in the presence of elements that conduct current, since dielectrics have infinite or close to infinite electrical resistance. In contrast, metals are very good conductors of current. You can measure the electrical resistance of a metal conductor using a milliohmmeter, or an even more accurate microohmmeter. The value is measured between their probes applied to the conductor section. They allow you to check circuits, wiring, windings of motors and generators.
Metals vary in their ability to conduct current. Resistivity various metals- a parameter characterizing this difference. The data is given at a material temperature of 20 degrees Celsius:
The parameter ρ shows what resistance a meter conductor with a cross section of 1 mm 2 will have. The higher this value, the greater the electrical resistance will be. the right wire a certain length. The smallest ρ, as can be seen from the list, is silver; the resistance of one meter of this material will be equal to only 0.015 Ohms, but this is too expensive a metal to use on an industrial scale. Next comes copper, which is much more common in nature (not a precious metal, but a non-ferrous metal). Therefore, copper wiring is very common.
Copper is not only good guide electric current, but also a very plastic material. Thanks to this property, copper wiring fits better and is resistant to bending and stretching.
Copper is in great demand on the market. Many different products are made from this material:
- A huge variety of conductors;
- Auto parts (eg radiators);
- Clock mechanisms;
- Computer components;
- Parts of electrical and electronic devices.
The electrical resistivity of copper is one of the best among current-conducting materials, so many electrical industry products are created based on it. In addition, copper is easy to solder, so it is very common in amateur radio.
The high thermal conductivity of copper allows it to be used in cooling and heating devices, and its plasticity makes it possible to create the smallest parts and the thinnest conductors.
Conductors of electric current are of the first and second kind. Conductors of the first kind are metals. Conductors of the second type are conductive solutions of liquids. The current in the first type is carried by electrons, and the current carriers in conductors of the second type are ions, charged particles of the electrolytic liquid.
We can talk about the conductivity of materials only in the context of temperature environment. At higher temperatures, conductors of the first type increase their electrical resistance, and the second, on the contrary, decrease. Accordingly, there is a temperature coefficient of resistance of materials. The resistivity of copper Ohm m increases with increasing heating. Temperature coefficientα also depends only on the material, this value has no dimension and for different metals and alloys is equal to the following indicators:
- Silver - 0.0035;
- Iron - 0.0066;
- Platinum - 0.0032;
- Copper - 0.0040;
- Tungsten - 0.0045;
- Mercury - 0.0090;
- Constantan - 0.000005;
- Nickelin - 0.0003;
- Nichrome - 0.00016.
Determination of the electrical resistance value of a conductor section at elevated temperature R (t) is calculated using the formula:
R (t) = R (0) · , where:
- R (0) - resistance at initial temperature;
- α - temperature coefficient;
- t - t (0) - temperature difference.
For example, knowing the electrical resistance of copper at 20 degrees Celsius, you can calculate what it will be equal to at 170 degrees, that is, when heated by 150 degrees. The initial resistance will increase by a factor of 1.6.
As the temperature increases, the conductivity of materials, on the contrary, decreases. Since this is the reciprocal of electrical resistance, it decreases by exactly the same amount. For example, the electrical conductivity of copper when the material is heated by 150 degrees will decrease by 1.6 times.
There are alloys that practically do not change their electrical resistance when temperature changes. This is, for example, constantan. When the temperature changes by one hundred degrees, its resistance increases by only 0.5%.
While the conductivity of materials deteriorates with heat, it improves with decreasing temperature. This is related to the phenomenon of superconductivity. If you lower the temperature of the conductor below -253 degrees Celsius, its electrical resistance will sharply decrease: almost to zero. As a result, transmission costs fall electrical energy. The only problem was cooling the conductors to such temperatures. However, due to the recent discoveries of high-temperature superconductors based on copper oxides, materials have to be cooled to acceptable values.