One of the most sought-after metals in the industries is copper. She received the most widespread in electrics and electronics. It is most often 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 material currently existing electrical resistance. Until a new material appears with a lower value of this indicator, it is safe to say that the replacement of copper will not be.
General characteristic of media
Speaking about copper, it must be said that even at the dawn of an electric era, it began to be used in the manufacture of electrical engineering. It began to use it in many respects due to the unique properties that this alloy possesses. By itself, it represents a material that is distinguished by high properties in terms of plasticity and possessing good patience.
Along with the thermal conductivity of copper, one of its most important advantages is high electrical conductivity. It is due to this property copper and gained widespread in power plantsin which it acts as a universal conductor. The most valuable material is electrolytic copper, which has a high degree of purity -99.95%. Thanks to this, the material appears the possibility for the production of cables.
Pluses of using electrolytic copper
The use of electrolytic copper allows you to achieve the following:
- Provide high electrical conductivity;
- Achieve excellent causing ability;
- Provide a high degree of plasticity.
Scope of application
Cable products manufactured from electrolytic copper gained widespread in various industries. Most often it is used in the following areas:
- electric industry;
- electrical appliances;
- automotive;
- production of computer equipment.
What is the specific resistance?
To understand what copper and its characteristics represent, it is necessary to deal with the main parameter of this metal - resistivity. It should be known and used when performing calculations.
Under the resistivity, it is customary to understand the physical quantity, which is characterized as the ability of the metal to carry out an electric current.
Know this magnitude is also necessary in order to calculate electrical resistance Explorer. Calculations also focus on its geometric dimensions. When calculating, the following formula is used:
This formula is familiar to many well. Using it, you can easily calculate the resistance of the copper cable, focusing only on the characteristics of the electrical network. It allows you to calculate the power that is inefficiently spent on the heating of the cable core. Besides, similar formula allows you to perform resistance calculations Any cable. It does not matter which material was used for the manufacture of cable - copper, aluminum or some other alloy.
Such a parameter as a specific electrical resistance is measured in OM * mm2 / m. This indicator for copper wiring laid in the apartment is 0.0175 Ohm * mm2 / m. If you try to search for an alternative to copper - the material that could be used instead of it, then only suitable can only be considered silverwhich has a specific resistance of 0.016 Ohm * mm2 / m. However, it is necessary to pay attention to the selection of material not only on the resistivity, but also for reverse conductivity. This value is measured in Siemens (cm).
Siemens \u003d 1 / Ohm.
In copper of any weight, this parameter composition is 58,100,000 cm / m. As for silver, the amount of reverse conductivity is equal to 62,500,000 cm / m.
In our world of high technologies, when there is a large number of electrical devices and installations in each house, the value of such a material as copper is simply invaluable. This material use for wiring, without which no room costs. If the cop did not exist, then the person had to use the wires from other available materials, for example, from aluminum. However, in this case, I would have to face one problem. The thing is that this material has a specific conductivity much less than the copper conductors.
Resistivity
The use of materials with low electrical and thermal conductivity of any weight leads to large loss of electricity. BUT it affects the loss of power Used equipment. Most experts as the main material for the manufacture of wires with insulation are copper. It is the main material from which the individual elements of electric current equipment are manufactured.
- Boards installed in computers are equipped with treated copper tracks.
- Copper is also used to make various elements used in electronic devices.
- In transformers and electric motors, it is represented by winding, which is made of this material.
You can not doubt that the expansion of the scope of this material will occur with the further development of technical progress. Although, except for copper, there are other materials, but still the designer when creating equipment and various installations use copper. The main reason for the demand of this material is in good electrical and thermal conductivity This metal, which it provides in room temperature.
Temperature resistance coefficient
All metals with any thermal conductivity possess the property to reduce conductivity with an increase in temperature. When decreasing temperature, the conductivity increases. Especially interesting specialists call the property reduction decrease in temperature. Indeed, in this case, when the temperature is reduced to a certain value in the room, the conductor may disappear the electrical resistance And he will go to the class of superconductors.
In order to determine the resistance indicator of a specific conductor of a certain weight in room temperature, there is a critical resistance coefficient. It is a value that shows a change in the resistance of the chain section when the temperature is changed per Kelvin. To perform the calculation of the electrical resistance of the copper conductor, the following formula is used in a certain time period:
ΔR \u003d 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 the winding and schemes, but also as a metal for the manufacture of cable products. To machinery and equipment work efficiently, it is necessary correctly calculate the specific wiring resistancedeployed in the apartment. For this there is a certain formula. Knowing it, you can make a calculation that allows you to find out the optimal value of the cable cross section. In this case, you can avoid the loss of equipment power and ensure the effectiveness of its use.
We know that the cause of the electrical resistance of the conductor is the interaction of electrons with the ions of the crystal metal lattice (§ 43). Therefore, it can be assumed that the resistance of the conductor depends on its length and the cross-sectional area, as well as from the substance from which it is manufactured.
Figure 74 shows the installation for such experience. The current source circuit in turns includes various conductors, for example:
- nickeline wire of the same thickness, but of different lengths;
- nickeline wires of the same length, but of different thickness (different cross-sectional area);
- nickelines and nichrome wires of the same length and thickness.
The current strength in the chain is measured by an ammeter, voltage - voltmeter.
Knowing the voltage at the ends of the conductor and the current strength in it, according to the law of Ohm, you can define the resistance of each of the conductors.
Fig. 74. Dependence of the resistance of the conductor from its size and kind of substance
After performing the experiments, we install that:
- of the two nickelovic wires of the same thickness, longer wire has greater resistance;
- of the two nickeline wires of the same length, greater resistance has a wire, the cross section is less;
- nickelin and nichrome wires of the same sizes have different resistance.
The dependence of the resistance of the conductor from its size and the substance from which the conductor was made, for the first time on the experiments, studied OM. It found that the resistance is directly proportional to the length of the conductor, inversely proportional to the area of \u200b\u200bits cross-section and depends on the substance of the conductor.
How to take into account the dependence of resistance from the substance from which the conductor is manufactured? For this calculate the so-called specific resistivity.
The resistivity is a physical value that determines the resistance of the conductor from this substance with a length of 1 m, cross-sectional area of \u200b\u200b1 m 2.
We introduce alphabetic notation: ρ is the resistivity of the conductor, I is the length of the conductor, S is the area of \u200b\u200bits cross-section. Then the resistance of the conductor R will express the formula
From her we get that:
From the last formula, you can define a unit of specific resistance. Since the resistance unit is 1 ohm, the unit of the cross-sectional area is 1 m2, and the unit of length is 1 m, then the unit of the resistivity will be:
It is more convenient to express the cross-sectional area of \u200b\u200bthe conductor in square millimetepax, as it is most often small. Then the unit of resistivity will be:
Table 8 shows the specific resistivity values \u200b\u200bof some substances at 20 ° C. Specific resistance with temperature change varies. The experimental way was found that metals, for example, the resistivity with increasing temperature increases.
Table 8. Specific electrical resistance of some substances (at T \u003d 20 ° C)
Of all metals, silver and copper have the smallest resistivity. Consequently, silver and copper are the best conductors of electricity.
When wiring electrical circuits, aluminum, copper and iron wires are used.
In many cases there are devices that have great resistance. They are made of specially created alloys - substances with high resistivity. For example, as can be seen from Table 8, the nichoy nichrome has a specific resistance of almost 40 times greater than aluminum.
Porcelain and ebonite have such a high resistivity that the electric current is almost at all, they are used as insulators.
Questions
- How depends the resistance of the conductor from its length and from the cross-sectional area?
- How to show the dependence of the resistance of the conductor from its length, cross-sectional area and the substance from which it is made?
- What is called the specific resistance of the conductor?
- What formula can you calculate the resistance of the conductors?
- what units is the specific resistance of the conductor?
- What substances make conductors used in practice?
The term "resistivity" means a parameter that has copper or any other metal, and is often found in special literature. It is worth dealing with what is understood by this.
One of the varieties of copper cable
General information about electrical resistance
To begin with, consider the concept of electrical resistance. As is known, under the action of electrical current on the conductor (and copper is one of the best conductors metal) part of the electrons in it leave its place in the crystal lattice and rushed towards the positive pole of the conductor. However, not all electrons leave the crystal lattice, some of them remain in it and continue to perform a rotational movement around the atomic core. These electrons, as well as atoms located in the nodes of the crystal lattice, create an electrical resistance that prevents the promotion of the released particles.
This process, which we briefly described, is typical for any metal, for copper, including. Naturally, various metals, each of which has a special form and dimensions of the crystal lattice, resist the progress of electric current in different ways. Just these differences and characterizes the resistivity - an indicator, individual for each metal.
Application of copper in electrical and electronic systems
In order to understand the reason for the popularity of copper as a material for the manufacture of elements of electrical and electronic systems, it is enough to look at the table value of its resistivity. In copper, this parameter is 0.0175 Ohm * mm2 / meter. In this regard, copper is inferior only to silver.
It is the low resistivity measured at a temperature of 20 degrees Celsius, is the main reason that without copper today does not cost virtually no electronic and electrical device. Copper is the main material for the production of wires and cables, printed circuit boards, electric motors and details of power transformers.
Low resistivity, which is characterized by copper, allows you to use it for the manufacture of electrical devices that differ in high energy-saving properties. In addition, the temperature of the copper conductors increases very slightly when the electric current passes through them.
What affects the validity of the resistance?
It is important to know that there is a dependence of the specific resistance from the chemical purity of the metal. In the content of even a minor amount of aluminum (0.02%), the value of its parameter can increase significantly (up to 10%).
Affects this coefficient and temperature of the conductor. This is explained by the fact that when the temperature increases, the oscillations of metal atoms in the nodes of its crystalline lattice are enhanced, which leads to the fact that the specific resistance coefficient increases.
That is why in all reference tables, the value of this parameter is given taking into account the temperature of 20 degrees.
How to calculate the overall conductor resistance?
To know what is equal to the resistance, it is important in order to conduct preliminary calculations of the parameters of electrical equipment during its design. In such cases, the overall resistance of the conductors of the designed device with certain sizes and shape is determined. Looking at the value of the resistivity of the conductor on the reference table by defining its size and cross-sectional area, it is possible to calculate the value of its overall resistance by the formula:
This formula uses the following notation:
- R is the overall resistance of the conductor, which must be determined;
- p is the resistivity of the metal from which the conductor is made (defined by the table);
- l - Explorer length;
- S is the area of \u200b\u200bits cross section.
The electric current occurs as a result of the circuit with the difference in potentials on the clips. Field forces affect free electrons and they move through the conductor. In the process of this trip, electrons are found with atoms and transmit part of their accumulated energy. As a result, their speed is reduced. But, due to the effects of the electric field, it is gaining momentum again. Thus, electrons constantly experiencing resistance, that is why the electric current is heated.
The property of a substance, to convert electricity to heat during current exposure, and is electrical resistance and is indicated, as R, its measuring unit is OM. The magnitude of the resistance depends mainly on the ability of various materials to carry out the current.
For the first time, the resilience said the German explorer G. Om.
In order to find out the dependence of the strength of the current from the resistance, the well-known physicist conducted many experiments. For experiments, he used various conductors and received various indicators.
The first thing that I defined the city is that the specific resistance depends on the length of the conductor. That is, if the length of the conductor increased, the resistance also increased. As a result, this connection was defined as directly proportional.
The second dependence is the cross-sectional area. It could be determined by cross cutting conductor. The area of \u200b\u200bthe figure that was formed on the cut and there is a cross-sectional area. Here the connection turned back proportional. That is, the larger the cross-sectional area, the less the resistance of the conductor was becoming.
And the third, an important value from which resistance depends is the material. As a result of the fact that OM used various materials in experiments, he discovered various properties of resistance. All these experiments and indicators were reduced to the table from which it can be seen, the different meaning of the resistivity in various substances.
It is known that the best conductors are metals. And which of the metals the best conductors? The table shows that copper and silver have the smallest resistance. Copper is used more often due to less cost, and silver is used in the most important and responsible devices.
The substances with high resistivity in the table are poorly conducted by electric current, and therefore can be excellent insulating materials. Substances possessing this property to the greatest extent, it is porcelain and ebonite.
In general, the specific electrical resistance is a very important factor, because, by defining its indicator, we can learn from which substance a conductor is made. To do this, it is necessary to measure the area of \u200b\u200bthe cross section, find out the current strength with a voltmeter and ammeter, and also measure the voltage. Thus, we learn the value of the resistivity and, with the help of the table, easily come out on the substance. It turns out that the resistivity is in the genus fingerprint substance. In addition, the specific resistance is important when planning long electrical chains: we need to know this indicator to comply with the balance between the long and area.
There is a formula that determines that the resistance is 1 ohms, if at a voltage of 1B, its current strength is 1a. That is, the resistance of a single area and a single length made from a certain substance and has a specific resistance.
It should also be noted that the specific resistance indicator is directly dependent on the frequency of the substance. That is, whether he has impurities. That, adding only one percent of manganese increases the resistance of the conductive substance itself - copper, three times.
This table demonstrates the value of the specific electrical resistance of some substances.
High conduction materials
Copper
As we have already spoken copper, most often used as a conductor. This is explained not only to its low resistance. Copper has such advantages as high strength, resistance to corrosion, ease of use and good workability. Good brands of copper is considered m0 and m1. In them, the number of impurities does not exceed 0.1%.
The high cost of metal and its prevailing recent deficiency prompts manufacturers to apply aluminum as conductor. Also, copper alloys with different metals are used.
Aluminum
This metal is much easier than copper, but aluminum has large values \u200b\u200bof heat capacity and melting temperature. In this regard, in order to bring it to the molten state, more energy is required than copper. Nevertheless, it is necessary to take into account the fact of copper deficiency.
The manufacture of electrical products is used, as a rule, aluminum brand A1. It contains no more than 0.5% impurities. And the highest frequency metal is aluminum brand AB0000.
Iron
The cheapness and availability of iron is overshadowed by its high resistivity. In addition, it quickly undergoes corrosion. For this reason, steel conductors are often covered with zinc. The so-called bimetal is widely used - it is steel covered with copper protection.
Sodium
Sodium, also affordable and promising material, but its resistance is almost three times more copper. In addition, the metallic sodium has a high chemical activity, which obliges to cover such a conductor with hermetic protection. It should protect the conductor from mechanical damage, as sodium is very soft and sufficiently continuing material.
Superconductivity
The table below indicates the resistivity of the substances at a temperature of 20 degrees. The temperature indication is no coincidence, because the resistivity directly depends on this indicator. This is explained by the fact that when heated, the speed of atoms increases, which means the probability of meeting them with electrons will also increase.
I wonder what happens with the resistance in cooling conditions. For the first time, the behavior of atoms at very low temperatures was noticed by G. Chalning-Onnes in 1911. It cooled the mercury wire to 4k and discovered the drop in its resistance to zero. Change the indicator of the specific resistance in some alloys and metals under conditions of low temperature, the physicist called superconductivity.
Superconductors go to the state of superconductivity during cooling, and, with 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 usual state, as well as from the properties of other metals, which, with a decrease in temperature, cannot move into this state.
The use of superconductors is mainly carried out in the preparation of a superal magnetic field, the strength of which reaches 107 a / m. Superconducting power lines are also developed.
Similar materials.
When the electrical circuit is closed, on the clips of which there is a potential difference, an electric current occurs. Free electrons under the influence of the electric forces of the field move along the conductor. In its movement, electrons are pushed to the atoms of the conductor and give them a margin of their kinetic energy. The speed of the electron movement continuously changes: when electron collisions with atoms, molecules and other electrons it decreases, then under the action of the electric field increases and again decreases with a new collision. As a result, the conductor establishes a uniform movement of the flow of electrons at a speed of several sorts of centimeter per second. Consequently, the electrons passing through the conductor always meet with its resistance to their movement. When the electric current is passed through the conductor, the latter is heated.
Electrical resistance
Electrical resistance of the conductor, which is indicated by the Latin letter r., It is called the body property or the medium to convert electrical energy into thermal when the electric current is passed.
In the diagrams, the electrical resistance is indicated as shown in Figure 1, but.
AC electrical resistance that serves to change the current in the circuit is called reostat. In the schemes, the records are indicated as shown in Figure 1, b.. In general, the rosostat is made of wire of one or another resistance wound on an insulating base. The slider or the lever of the rheostat is set to a certain position, as a result of which the necessary resistance is introduced into the circuit.
Long conductor of a small cross section creates a large resistance current. Short conductors of a large cross section have a short resistance.
If you take two conductors from a different material, but the same length and sections, then the conductors will carry out the current in different ways. This shows that the conductor resistance depends on the material of the conductor itself.
The temperature of the conductor also affects its resistance. With increasing temperature, the resistance of metals increases, and the resistance of liquids and coal decreases. Only some special metal alloys (manganine, confatetan, nickeline and others) with an increase in the temperature of their resistance almost do not change.
So, we see that the electrical resistance of the conductor depends on: 1) the length of the conductor, 2) of the cross section of the conductor, 3) of the material of the conductor, 4) the temperature of the conductor.
Per unit of resistance adopted one Ohm. Ohms is often indicated by the Greek capital letter Ω (omega). Therefore, instead of writing "Explorer resistance is 15 ohms," you can write simple: r. \u003d 15 Ω.
1 000 ohms called 1 kiloma (1kom, or 1kΩ),
1 000 000 ohms called 1 megaom (1MG, or 1MΩ).
When comparing the resistance of the conductors from various materials, it is necessary to take a certain length for each sample and cross section. Then we will be able to judge what material is better or worse conducts electric current.
Video 1. Conductor resistance
Specific electrical resistance
Resistance in Ohms conductor with a length of 1 m, cross section 1 mm² is called specific resistance and is indicated by the Greek letter ρ (RO).
Table 1 shows the specific resistances of some conductors.
Table 1
Specific resistances of various conductors
It can be seen from the table that the iron wire with a length of 1 m and a cross section of 1 mm² has a resistance of 0.13 ohms. To get 1 ohm resistance, you need to take 7.7 m such a wire. Silver has the smallest resistivity. 1 Ohm resistance can be obtained if you take 62.5 m silver wire with a cross section of 1 mm². Silver is the best conductor, but the cost of silver eliminates the possibility of its mass application. After silver in the table goes copper: 1 m copper wire with a cross section of 1 mm² has a resistance of 0.0175 ohms. To obtain resistance in 1 ohms, you need to take 57 m such a wire.
Chemically pure, obtained by refining, copper found universal use in electrical engineering for the manufacture of wires, cables, windings of electrical machines and devices. Also used as aluminum and iron conductors.
The conductor resistance can be determined by the formula:
where r. - resistance of the conductor in Omah; ρ - resistivity of the conductor; l. - the length of the conductor in m; S. - Conductor cross section in mm².
Example 1. Determine the resistance of 200 m iron wire with a cross section of 5 mm².
Example 2.Calculate resistance 2 km aluminum wire with a cross section of 2.5 mm².
From the impedance formula, it is easy to determine the length, a resistivity and cross-section of the conductor.
Example 3. For the radio receiver, it is necessary to wind the resistance of 30 ohms from a nickeline wire by section 0.21 mm². Determine the required wire length.
Example 4. Determine the cross section of 20 m nichrome wire if it resistance is equal to 25 ohms.
Example 5. Wire with a cross section of 0.5 mm² and a length of 40 m has a resistance of 16 ohms. Determine the wire material.
The conductor material characterizes its resistivity.
At the specific resistivity table, we find that lead has such resistance.
It indicated above that the resistance of the conductors depends on the temperature. We will do the next experience. We wrap in the form of a spiral of several meters of thin metal wire and turn on this helix into the battery chain. To measure the current in the chain, turn on the ammeter. When heated spirals in the flame burner, it can be noted that the ammeter readings will decrease. This shows that with heating, the resistance of the metal wire increases.
In some metals, when heated by 100 ° resistance increases by 40 - 50%. There are alloys that slightly change their resistance with heating. Some special alloys practically do not change resistance when the temperature changes. The resistance of metal conductors when increasing the temperature increases, the resistance of electrolytes (liquid conductors), coal and some solids, on the contrary, decreases.
Metal ability to change its temperature change resistance is used for resistance thermometers. Such a thermometer is a platinum wire wound on a mica frame. Placing the thermometer, for example, in the oven and measuring the resistance of platinum wire before and after heating, you can determine the temperature in the furnace.
Changing the resistance of the conductor when he heated, per 1 Ω of the initial resistance and 1 ° temperature, is called temperature resistance coefficient And denotes the letter α.
If at temperature t. 0 conductor resistance equals r. 0, and at temperatures t. equally r T., then the temperature coefficient of resistance
Note. The calculation of this formula can be carried out only at a certain temperature range (approximately 200 ° C).
We give the value of the temperature coefficient of resistance α for some metals (Table 2).
table 2
The temperature coefficient values \u200b\u200bfor some metals
From the formula of the temperature coefficient of resistance, we define r T.:
r T. = r. 0 .
Example 6. Determine the resistance of the iron wire heated to 200 ° C if its resistance at 0 ° C was 100 ohms.
r T. = r. 0 \u003d 100 (1 + 0.0066 × 200) \u003d 232 ohms.
Example 7. The resistance thermometer made from platinum wire, indoors with a temperature of 15 ° C had a resistance of 20 ohms. The thermometer was placed in the oven and after some time its resistance was measured. It turned out to be equal to 29.6 ohms. Determine the temperature in the furnace.
Electrical conductivity
So far, we have considered the resistance of the conductor as an obstacle that has an electric current conductor. But still the current on the conductor passes. Consequently, in addition to resistance (obstacles), the conductor also has the ability to carry out an electric current, that is, conductivity.
The greater resistance has a conductor, the smaller it has a conductivity, the worse it conducts electric current, and, on the contrary, the smaller the resistance of the conductor, the greater conductivity it has, the easier the current to go through the conductor. Therefore, the resistance and conductivity of the conductor are the values \u200b\u200bof the inverse.
It is known from mathematics that the number, inverse 5, there is 1/5 and, on the contrary, the number, inverse 1/7, is 7. Therefore, if the resistance of the conductor is indicated by the letter r., the conductivity is defined as 1 / r.. Typically, the conductivity is indicated by the letter G.
Electrical conductivity is measured in (1 / Ohm) or in Siemens.
Example 8. The conductor resistance is equal to 20 ohms. Determine its conductivity.
If a r. \u003d 20 ohms, then
Example 9. The conductivity of the conductor is 0.1 (1 / ohms). Determine its resistance
If G \u003d 0.1 (1 / Ohms), then r. \u003d 1 / 0.1 \u003d 10 (OM)
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