What materials have the lowest resistivity. Calculation of wire resistances

The term "resistivity" refers to the parameter that copper or any other metal has, and is quite common in the literature. It is worth understanding what is meant by this.

One of the types of copper cable

General information about electrical resistance

First, consider the concept of electrical resistance. As you know, under the action of an electric current on a conductor (and copper is one of the best conductor metals), some of the electrons in it leave their place in the crystal lattice and rush towards the positive pole of the conductor. However, not all electrons leave the crystal lattice, some of them remain in it and continue to rotate around the atomic nucleus. It is these electrons, as well as atoms located at the nodes of the crystal lattice, that create electrical resistance that prevents the movement of released particles.

This process, which we briefly outlined, is typical for any metal, including copper. Naturally, different metals, each of which has a special shape and size of the crystal lattice, resist the movement of electric current through them in different ways. It is these differences that characterize the specific resistance - an indicator that is individual for each metal.

The use 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 value of its resistivity in the table. For copper, this parameter is 0.0175 Ohm * mm2 / meter. In this regard, copper is second only to silver.

It is the low resistivity, measured at a temperature of 20 degrees Celsius, that is the main reason that almost no electronic and electrical device can do without copper today. Copper is the main material for the production of wires and cables, printed circuit boards, electric motors and power transformer parts.

The low resistivity that characterizes copper makes it possible to use it for the manufacture of electrical devices with high energy-saving properties. In addition, the temperature of copper conductors rises very little when an electric current passes through them.

What affects the value of resistivity?

It is important to know that there is a dependence of the resistivity value on the chemical purity of the metal. When copper contains even a small amount of aluminum (0.02%), the value of this parameter can increase significantly (up to 10%).

This coefficient is also affected by the temperature of the conductor. This is explained by the fact that with an increase in temperature, vibrations of metal atoms in the nodes of its crystal lattice increase, which leads to the fact that the resistivity coefficient increases.

That is why in all reference tables the value of this parameter is given taking into account a temperature of 20 degrees.

How to calculate the total resistance of a conductor?

Knowing what the resistivity is equal to is important in order to carry out preliminary calculations of the parameters of electrical equipment during its design. In such cases, the total resistance of the conductors of the designed device, which have certain sizes and shapes, is determined. Having looked at the value of the resistivity of the conductor according to the reference table, having determined its dimensions and cross-sectional area, it is possible to calculate the value of its total resistance using the formula:

This formula uses the following notation:

• R is the total resistance of the conductor, which must be determined;
• p is the specific resistance of the metal from which the conductor is made (determined according to the table);
• l is the length of the conductor;
• S is the area of ​​its cross section.

As we know from Ohm's law, the current in the circuit section is in the following relationship: I=U/R. The law was derived as a result of a series of experiments by the German physicist Georg Ohm in the 19th century. He noticed a pattern: the current strength in any section of the circuit directly depends on the voltage that is applied to this section, and vice versa - on its resistance.

Later it was found that the resistance of the section depends on its geometric characteristics as follows: R=ρl/S,

where l is the length of the conductor, S is the area of ​​its cross section, and ρ is a certain coefficient of proportionality.

Thus, the resistance is determined by the geometry of the conductor, as well as such a parameter as resistivity (hereinafter referred to as c.s.) - this is what this coefficient was called. If you take two conductors with the same cross section and length and put them in a circuit in turn, then by measuring the current strength and resistance, you can see that in two cases these indicators will be different. Thus, specific electrical resistance- this is a characteristic of the material from which the conductor is made, and to be even more precise, the substance.

Conductivity and resistance

W.s. indicates the ability of a substance to block the passage of current. But in physics there is also an inverse value - conductivity. It shows the ability to conduct electricity. It looks like this:

σ=1/ρ, where ρ is the resistivity of the substance.

If we talk about conductivity, then it is determined by the characteristics of charge carriers in this substance. So, in metals there are free electrons. There are no more than three of them on the outer shell, and it is more profitable for the atom to "give away" them, which happens when chemical reactions with substances from the right side of the periodic table. In a situation where we have a pure metal, it has a crystalline structure in which these outer electrons are common. They carry a charge if an electric field is applied to the metal.

In solutions, charge carriers are ions.

If we talk about substances such as silicon, then by its properties it is semiconductor and works in a slightly different way, but more on that later. In the meantime, let's figure out how such classes of substances differ, such as:

1. conductors;
2. semiconductors;
3. Dielectrics.

Conductors and dielectrics

There are substances that almost do not conduct current. They are called dielectrics. Such substances are able to polarize in an electric field, that is, their molecules can rotate in this field, depending on how they are distributed in them. electrons. But since these electrons are not free, but serve to bond between atoms, they do not conduct current.

The conductivity of dielectrics is almost zero, although there are no ideal ones among them (this is the same abstraction as an absolutely black body or an ideal gas).

The conditional boundary of the concept of "conductor" is ρ<10^-5 Ом, а нижний порог такового у диэлектрика - 10^8 Ом.

Between these two classes there are substances called semiconductors. But their selection into a separate group of substances is associated not so much with their intermediate state in the "conductivity - resistance" line, but with the features of this conductivity in various conditions.

Dependence on environmental factors

Conductivity is not exactly constant. The data in the tables, from where ρ is taken for calculations, exists for normal environmental conditions, that is, for a temperature of 20 degrees. In reality, it is difficult to find such ideal conditions for the operation of the circuit; actually u.s. (and therefore, conductivity) depend on the following factors:

1. temperature;
2. pressure;
3. the presence of magnetic fields;
4. light;
5. state of aggregation.

Different substances have their own schedule of changes in this parameter under different conditions. So, ferromagnets (iron and nickel) increase it when the direction of the current coincides with the direction of the magnetic field lines. As for temperature, the dependence here is almost linear (there is even the concept of the temperature coefficient of resistance, and this is also a tabular value). But the direction of this dependence is different: for metals, it increases with increasing temperature, while for rare earth elements and electrolyte solutions it increases - and this is within the same state of aggregation.

For semiconductors, the dependence on temperature is not linear, but hyperbolic and inverse: as the temperature rises, their conductivity increases. This qualitatively distinguishes conductors from semiconductors. This is how the dependence of ρ on the temperature of the conductors looks like:

Here are the resistivities of copper, platinum and iron. A slightly different graph for some metals, for example, mercury - when the temperature drops to 4 K, it loses it almost completely (this phenomenon is called superconductivity).

And for semiconductors, this dependence will be something like this:

During the transition to the liquid state, the ρ of the metal increases, but then they all behave differently. For example, in molten bismuth it is lower than at room temperature, and in copper it is 10 times higher than normal. Nickel exits the line chart at 400 degrees, after which ρ drops.

But in tungsten, the temperature dependence is so high that it causes incandescent lamps to burn out. When turned on, the current heats the coil, and its resistance increases several times.

Also at. With. alloys depends on the technology of their production. So, if we are dealing with a simple mechanical mixture, then the resistance of such a substance can be calculated on the average, but it is the same for a substitutional alloy (this is when two or more elements are added into one crystal lattice) will be different, as a rule, much larger. For example, nichrome, from which spirals for electric stoves are made, has such a figure for this parameter that this conductor, when connected to the circuit, heats up to redness (which is why, in fact, it is used).

Here is the characteristic ρ of carbon steels:

As can be seen, when approaching the melting temperature, it stabilizes.

Resistivity of various conductors

Be that as it may, ρ is used in calculations under normal conditions. Here is a table by which you can compare this characteristic for different metals:

As can be seen from the table, the best conductor is silver. And only its cost prevents its massive use in cable production. W.s. aluminum is also small, but less than that of gold. From the table it becomes clear why the wiring in houses is either copper or aluminum.

The table does not include nickel, which, as we have already said, has a slightly unusual y curve. With. from temperature. The specific resistance of nickel after raising the temperature to 400 degrees does not begin to grow, but to fall. It behaves interestingly in other substitutional alloys as well. This is how an alloy of copper and nickel behaves, depending on the percentage of both:

And this interesting graph shows the resistance of zinc-magnesium alloys:

High-resistance alloys are used as materials for the manufacture of rheostats, here are their characteristics:

These are complex alloys consisting of iron, aluminum, chromium, manganese, nickel.

As for carbon steels, it is approximately 1.7 * 10 ^ -7 Ohm m.

The difference between u. With. different conductors determines their application. Thus, copper and aluminum are widely used in the production of cables, and gold and silver are used as contacts in a number of radio engineering products. High-resistance conductors have found their place among electrical appliance manufacturers (more precisely, they were created for this).

The variability of this parameter depending on the environmental conditions formed the basis of such devices as magnetic field sensors, thermistors, strain gauges, and photoresistors.

Copper is one of the most common wire materials. Its electrical resistance is the lowest of the affordable metals. It is less only in precious metals (silver and gold) and depends on various factors.

What is electric current

On different poles of a battery or other current source, there are oppositely named electric charge carriers. If they are connected to a conductor, charge carriers begin to move from one pole of the voltage source to the other. These carriers in liquids are ions, and in metals they are free electrons.

Definition. Electric current is the directed movement of charged particles.

Resistivity

Electrical resistivity is a quantity that determines the electrical resistance of a reference material sample. The Greek letter "r" is used to denote this value. Formula for calculation:

p=(R*S)/ l.

This value is measured in Ohm*m. You can find it in reference books, in tables of resistivity or on the Internet.

Free electrons move through the metal inside the crystal lattice. Three factors influence the resistance to this movement and the resistivity of the conductor:

• Material. Different metals have different atomic densities and the number of free electrons;
• impurities. In pure metals, the crystal lattice is more ordered, so the resistance is lower than in alloys;
• Temperature. Atoms do not sit still in their places, but oscillate. The higher the temperature, the greater the amplitude of oscillations, which interferes with the movement of electrons, and the higher the resistance.

In the following figure, you can see a table of the resistivity of metals.

Interesting. There are alloys whose electrical resistance drops when heated or does not change.

Conductivity and electrical resistance

Since the dimensions of the cables are measured in meters (length) and mm² (section), the electrical resistivity has the dimension of Ohm mm² / m. Knowing the dimensions of the cable, its resistance is calculated by the formula:

R=(p* l)/S.

In addition to electrical resistance, some formulas use the concept of "conductivity". This is the reciprocal of resistance. It is designated "g" and is calculated by the formula:

Conductivity of liquids

The conductivity of liquids is different from the conductivity of metals. The charge carriers in them are ions. Their number and electrical conductivity increase when heated, so the power of the electrode boiler increases several times when heated from 20 to 100 degrees.

Interesting. Distilled water is an insulator. Conductivity is imparted to it by dissolved impurities.

Electrical resistance of wires

The most common wire materials are copper and aluminum. The resistance of aluminum is higher, but it is cheaper than copper. The specific resistance of copper is lower, so the wire size can be chosen smaller. In addition, it is stronger, and flexible stranded wires are made from this metal.

The following table shows the electrical resistivity of metals at 20 degrees. In order to determine it at other temperatures, the value from the table must be multiplied by a correction factor that is different for each metal. You can find out this coefficient from the relevant reference books or using an online calculator.

Cable section selection

Since the wire has resistance, when an electric current passes through it, heat is generated and a voltage drop occurs. Both of these factors must be taken into account when choosing cable sizes.

Selection according to allowable heating

When current flows through a wire, energy is released. Its quantity can be calculated by the formula of electric power:

In a copper wire with a cross section of 2.5mm² and a length of 10 meters R=10*0.0074=0.074Ohm. At a current of 30A, P \u003d 30² * 0.074 \u003d 66W.

This power heats the conductor and the cable itself. The temperature to which it heats up depends on the laying conditions, the number of cores in the cable and other factors, and the permissible temperature depends on the insulation material. Copper has a higher conductivity, so the power output and the required cross section are less. It is determined by special tables or using an online calculator.

Permissible voltage losses

In addition to heating, when an electric current passes through the wires, the voltage near the load decreases. This value can be calculated using Ohm's law:

Reference. According to the norms of the PUE, it should be no more than 5% or in a 220V network - no more than 11V.

Therefore, the longer the cable, the larger its cross section should be. You can determine it from tables or using an online calculator. In contrast to the selection of the section according to the allowable heating, voltage losses do not depend on the conditions of the gasket and the insulation material.

In a 220V network, voltage is supplied through two wires: phase and zero, so the calculation is made for double the length of the cable. In the cable from the previous example, it will be U=I*R=30A*2*0.074Ω=4.44V. This is not much, but with a length of 25 meters it turns out 11.1V - the maximum allowable value, you will have to increase the cross section.

Electrical resistance of other metals

In addition to copper and aluminum, other metals and alloys are used in electrical engineering:

• Iron. The specific resistance of steel is higher, but it is stronger than copper and aluminum. Steel conductors are woven into cables intended for laying through the air. The resistance of iron is too high for the transmission of electricity, therefore, when calculating the cross section, the cores are not taken into account. In addition, it is more refractory, and leads are made from it for connecting heaters in electric furnaces of high power;
• Nichrome (an alloy of nickel and chromium) and Fechral (iron, chromium and aluminum). They have low conductivity and refractoriness. Wirewound resistors and heaters are made from these alloys;
• Tungsten. Its electrical resistance is high, but it is a refractory metal (3422 °C). It is used to make filaments in electric lamps and electrodes for argon-arc welding;
• Constantan and manganin (copper, nickel and manganese). The resistivity of these conductors does not change with changes in temperature. They are used in claim devices for the manufacture of resistors;
• Precious metals - gold and silver. They have the highest conductivity, but due to the high price, their use is limited.

Inductive reactance

The formulas for calculating the conductivity of wires are valid only in a DC network or in straight conductors at low frequency. In coils and in high-frequency networks, an inductive resistance appears many times higher than usual. In addition, the high frequency current only propagates over the surface of the wire. Therefore, it is sometimes coated with a thin layer of silver or litz wire is used.

Experience has shown that the resistance R metal conductor is directly proportional to its length L and inversely proportional to its cross-sectional area A:

R = ρ L/ A (26.4)

where coefficient ρ is called resistivity and serves as a characteristic of the substance from which the conductor is made. This is common sense: the resistance of a thick wire should be less than that of a thin wire, since electrons can move over a larger area in a thick wire. And we can expect an increase in resistance with an increase in the length of the conductor, since the number of obstacles in the path of the electron flow increases.

Typical values ρ for different materials are given in the first column of Table. 26.2. (Actual values ​​may vary depending on purity, heat treatment, temperature, and other factors.)

Silver has the lowest resistivity and is thus the best conductor; however, it is expensive. Copper is slightly inferior to silver; it is clear why wires are most often made of copper.

The specific resistance of aluminum is higher than that of copper, but it has a much lower density, and in some cases it is preferred (for example, in power lines), since 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. Conductivity is measured in units of (Ohm m) -1 .

The resistivity of a substance depends on temperature. Generally, the resistance of metals increases with temperature. This should not be surprising: as the temperature rises, the atoms move faster, their arrangement becomes less ordered, and they can be expected to interfere more with the flow of electrons. In narrow temperature ranges, the resistivity of the metal increases almost linearly with temperature:

Where ρT- resistivity at temperature T, ρ 0 - resistivity at standard temperature T 0 , and α - temperature coefficient of resistance (TCR). The values ​​of a are given in Table. 26.2. Note that for semiconductors, 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 can 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 this value is valid (for example, according to a reference book of physical quantities). If the range of temperature change is wide, then the linearity will be violated, and instead of (26.6), an expression containing terms that depend on the second and third degrees of temperature should be used:

ρT = ρ 0 (1+αT+ + βT 2 + γT 3),

where coefficients β And γ usually very small (we put T 0 = 0°C), but at high T the contribution of these members becomes significant.

At very low temperatures, 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 go into the superconducting state below the transition temperature, which is usually a few degrees Kelvin (slightly above absolute zero). An electric current was observed in the superconducting ring, which practically did not weaken in the absence of voltage for several years.

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

Speaking about copper, it must be said that even at the dawn of the electrical era, it began to be used in the production of electrical engineering. It was used largely due to the unique properties that this alloy possesses. By itself, it is a material with high ductility properties and good ductility.

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

Advantages of using electrolytic copper

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

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

Applications

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

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

What is the resistivity?

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

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

It is also necessary to know this value in order to correctly calculate the electrical resistance conductor. When calculating, they also focus on its geometric dimensions. When making calculations, use the following formula:

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

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

Siemens \u003d 1 / Ohm.

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

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

Resistivity

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

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

There is no doubt that the expansion of the scope of this material will occur with the further development of technical progress. Although, in addition to copper, there are other materials, but still the designer uses copper to create equipment and various installations. The main reason for the demand for this material is in good electrical and thermal conductivity of this metal, which it provides at room temperature.

Temperature coefficient of resistance

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

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

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

Conclusion

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

Often in the electrical literature there is the concept of "specific copper". And involuntarily you ask yourself, what is it?

The concept of "resistance" for any conductor is continuously connected with the understanding of the process of electric current flowing through it. Since the article will focus on the resistance of copper, then we should consider its properties and the properties of metals.

When it comes to metals, you involuntarily remember that they all have a certain structure - a crystal lattice. Atoms are located at the nodes of such a lattice and make relative distances and the location of these nodes depends on the forces of interaction of atoms with each other (repulsion and attraction), and are different for different metals. Electrons revolve around the atoms in their orbits. They are also kept in orbit by the balance of forces. Only it is to the atom and centrifugal. Imagine a picture? You can call it, in a sense, static.

Now let's add dynamics. An electric field begins to act on a piece of copper. What happens inside the conductor? The electrons, torn off by the force of the electric field from their orbits, rush to its positive pole. Here you have the directed movement of electrons, or rather, electric current. But on the way of their movement, they stumble upon atoms at the nodes of the crystal lattice and electrons that still continue to rotate around their atoms. At the same time, they lose their energy and change the direction of movement. Now it becomes a little clearer the meaning of the phrase "conductor resistance"? It is the atoms of the lattice and the electrons rotating around them that resist the directed movement of the electrons torn off by the electric field from their orbits. But the concept of conductor resistance can be called a general characteristic. More individually characterizes each conductor resistivity. Medi including. This characteristic is individual for each metal, since it directly depends only on the shape and size of the crystal lattice and, to some extent, on temperature. With an increase in the temperature of the conductor, the atoms perform a more intense oscillation at the lattice sites. And the electrons revolve around the nodes at a higher speed and in orbits of a larger radius. And, naturally, free electrons encounter more resistance when moving. Such is the physics of the process.

For the needs of the electrical industry, a wide production of such metals as aluminum and copper, the resistivity of which is quite small, has been established. Cables and various types of wires are made from these metals, which are widely used in construction, for the production of household appliances, for the manufacture of tires, transformer windings and other electrical products.

For each conductor there is a concept of resistivity. This value consists of Ohms, multiplied by a square millimeter, further, divided by one meter. In other words, this is the resistance of a conductor whose length is 1 meter and the cross section is 1 mm2. The same is true of the resistivity of copper, a unique metal that is widely used in electrical engineering and power engineering.

copper properties

Due to its properties, this metal was one of the first to be used in the field of electricity. First of all, copper is a malleable and ductile material with excellent electrical conductivity properties. Until now, there is no equivalent replacement for this conductor in the energy sector.

The properties of special electrolytic copper with high purity are especially appreciated. This material made it possible to produce wires with a minimum thickness of 10 microns.

In addition to high electrical conductivity, copper lends itself very well to tinning and other types of processing.

Copper and its resistivity

Any conductor resists when an electric current is passed through it. The value depends on the length of the conductor and its cross section, as well as on the effect of certain temperatures. Therefore, the resistivity of conductors depends not only on the material itself, but also on its specific length and cross-sectional area. The easier a material passes a charge through itself, the lower its resistance. For copper, the resistivity index is 0.0171 Ohm x 1 mm2 / 1 m and is only slightly inferior to silver. However, the use of silver on an industrial scale is not economically viable, therefore, copper is the best conductor used in energy.

The specific resistance of copper is also associated with its high conductivity. These values ​​are directly opposite to each other. The properties of copper as a conductor also depend on the temperature coefficient of resistance. Especially, this applies to resistance, which is influenced by the temperature of the conductor.

Thus, due to its properties, copper is widely used not only as a conductor. This metal is used in most devices, devices and assemblies, the operation of which is associated with electric current.

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 conductive substance itself - 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 aluminum grade AB0000.
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 affordable 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.