Electrical conductivity of metals

When a metal is exposed to an electric (or magnetic) field (or temperature difference), flows of charged particles and energy appear in it.

The occurrence of these flows or currents is usually called kinetic effects or transfer phenomena, otherwise transport effects, meaning the impact of stationary fields on stationary conductors. In this case, the current or flux is proportional to the potential difference (or temperature difference), and the proportionality coefficient is determined only by the geometric dimensions of the conductor and the physical properties of the metal itself.

For unit geometric dimensions, this coefficient depends only on the properties of a given metal and is its fundamental physical characteristics, which is called the kinetic coefficient. When a conductor is in an alternating field, the currents arising in it depend not only on the geometric dimensions and kinetic coefficient, but also on the frequency variable field, conductor shapes, relative position elements of the electrical circuit.

Conductor resistance at alternating current significantly depends on its frequency, caused by the spin effect - displacement of current from the center of the conductor to the periphery. Of many possible kinetic phenomena two are most well known in technology: electrical conductivity - the ability of a substance to conduct constant electric current under the influence of an electric field that does not change in time, and thermal conductivity is similar in relation to the temperature difference and heat flow. Both of these phenomena are expressed (quantitatively) by Ohm's and Fourier's laws, respectively:

j = γ E; ω = k T.

where j is the current density, A/m;

γ - kinetic coefficient of electrical conductivity);

E - electric field strength V/m;

ω - thermal current density;

T – temperature difference;

k – thermal conductivity coefficient.

In practice, electrical resistivity or simply resistivity, Ohm m

However, for conductors it is allowed to use the non-system unit of measurement Ohm mm2/m, or it is recommended to use the equivalent SI unit μOhm/m. Transition from one unit to another in this case: 1 Ohm m = 10 6 μOhm m = 10 6 Ohm mm2/m.

Resistance of a conductor of arbitrary dimensions with a constant cross section will be determined:

where l is the length of the conductor, m;

S – conductor area, m2.

Metals are usually characterized as plastic substances with a characteristic “metallic” luster that are good conductors of electric current and heat.

For the electrical conductivity of metals, the following are typical: low resistivity at normal temperature, a significant increase in resistance with increasing temperature, quite close to direct proportionality; when the temperature drops to temperatures close to absolute zero, the resistance of metals decreases to very small values, amounting to 10-3 for the purest metals or even a smaller fraction of the resistance at normal, + 20 0C, temperatures.

They are also characterized by the presence of a connection between electrical conductivity and thermal conductivity, which is described by the empirical Wiedemann–Franz law as the ratio k / γ is approximately the same for different materials at the same temperature. The quotient of k/γ divided by absolute temperature T (L0 = k / (γ T)). called the Lorentz number, is (for all metals) a value that differs little at all temperatures.

The theory of kinetic phenomena in metals can explain the shape of the dependences of kinetic coefficients on temperature, pressure and other factors, and with its help it is also possible to calculate their values. To do this, consider the internal structure of metals.

The fundamental idea of ​​this branch of physics arose at the turn of the 19th and 20th centuries: metal atoms are ionized, and the valence electrons separated from them are free, i.e., they belong to the entire crystal.

The ions are strictly ordered and form a regular crystal lattice; their interaction with a negatively charged cloud free electrons such that makes the crystal a stable, stable formation.

The presence of free electrons well explains the high electrical conductivity of metals, and their delocalization provides high plasticity. This means that the most characteristic feature internal structure metal conductors is the presence of itinerant electrons, which confirms their electronic structure. In her the simplest model a collection of itinerant electrons is explained as electron gas, in which the particles are in chaotic thermal motion.

Equilibrium is established (if we neglect collisions between electrons) due to the collision of electrons with ions. Since thermal motion is not completely ordered, then, despite the charge of the electrons, no current (macroscopic) is observed in the circuit. If an external electric field is applied to a conductor, then the free electrons, having received acceleration, line up into an ordered component, which is oriented along the field.

Since the ions at lattice sites are stationary, order in the movement of electrons will manifest itself as a macroscopic electric current. Specific conductivity in this case can be expressed taking into account medium length free path λ of an electron in an accelerating field of strength E:

λ = e E τ / (2 m) as γ = e2 n λ / (2 m vτ),

where e is the electron charge;

n is the number of free electrons per unit volume of metal;

λ is the average free path of an electron between two collisions;

m is the electron mass;

v τ- average speed thermal motion of a free electron in a metal.

Subject to the provisions quantum mechanics

γ = K p2/3 / λ,

where K is a numerical coefficient.

The range of resistivity of metal conductors at normal temperature is only three orders of magnitude. For different metals, the speeds of chaotic thermal motion of electrons at a certain temperature are approximately the same.

The concentrations of free electrons differ slightly, so the value of resistivity mainly depends on the mean free path of electrons in a given conductor, and it is determined by the structure of the conductor material. All pure metals with the most regular crystal lattice have minimum values specific resistance. Impurities, distorting the lattice, lead to an increase in resistivity

The temperature coefficient of resistivity or the average temperature coefficient of resistivity is expressed as

α = 1 / ρ (dρ / dt); α` = 1 / ρ (ρ2 - ρ1) / (T2 – T1),

where ρ1 and ρ2 are the conductor resistivities at temperatures T1 and T2, respectively, at T2 > T1.

Technical reference books usually give the value α`, with which you can approximately determine ρ at an arbitrary temperature T:

ρ = ρ1 (1 + αρ` (T - T1)).

This expression gives exact value specific resistance p only for the linear dependence ρ(T). In other cases, this method is approximate; the narrower the temperature interval used to determine αρ`, the narrower it is.

The resistivity of most metals, which increase in volume when melted, decreases their density. For metals that reduce their volume during melting, the resistivity decreases; These metals include gallium, antimony and bismuth.

The resistivity of alloys is always greater than that of pure metals. This is especially noticeable if, when fused, they form solid solution, i.e. crystallize together during solidification and the atoms of one metal enter the lattice of the other.

If an alloy of two metals creates separate crystallization and a solidified solution - a mixture of crystals of each of the components, then the specific conductivity γ of such an alloy changes with a change in composition almost linearly. In solid solutions, this dependence (on the content of each metal) is not linear and has a maximum corresponding to a certain ratio of alloy components.

Sometimes, at a certain ratio between the components, they form chemical compounds(intermetallic compounds), while they do not have a metallic conductivity, but are electronic semiconductors.

The temperature coefficient of linear expansion of conductors is determined in the same way as for dielectrics using the formula

ТКl = α(l) = l / l (dl / dТ), (3.1)

where TKl = α(l) - temperature coefficient linear expansion K-1

This coefficient must be known in order to be able to evaluate the performance of conjugated materials in various designs, as well as to prevent cracking or disruption of the vacuum connection between metal and glass or ceramics when the temperature changes. In addition, it is included in the calculation temperature coefficient electrical resistance of wires

ТКR = α(R) = α(ρ) - α(l).

ThermoEMF of conductors

ThermoEMF occurs when two different conductors (or semiconductors) come into contact if the temperature of their junctions is not the same. If two different conductors come into contact, then a contact difference potentials. For metals A and B

Ucb - Uc + K T / e ln(n0с / nob),

where U c and U b are the potentials of the contacting metals; electron concentration in the corresponding metals;

K is Boltzmann's constant;

T - temperature;

e - absolute value electron charge.

If the temperature of the metal junctions is the same, then the sum of the potential difference in a closed circuit is zero. If the temperatures of the layers are different (T2 and T1, for example), then in this case

U = K / e (T1 - T2) ln(nc / nb). (3.2)

In practice, expression (3.2) is not always observed, and the dependence of thermoEMF on temperature may be nonlinear. Wire made up of two insulated wires different metals or alloys, is called a thermocouple and is used to measure temperatures.

In such cases, they try to use materials that have a large and stable thermoEMF coefficient. to measure high temperatures, sometimes it is necessary (especially when measuring temperatures in aggressive environments) to use thermocouples with lower thermoEdS coefficients, but withstanding high temperatures and not oxidizing in aggressive environments.

Thermocouple alloys have various combinations, including one electrode can be made of pure metal. The most common are nickel and copper-nickel alloys. For temperatures in the range of 1000 - 1200 0C, chromel - alumel (TCA) thermocouples are used; at higher temperatures, platinum - platinum-rhodium electrodes are used; in these alloys, rhodium ranges from 6.7 to 40.5%. The brands of such thermocouples are as follows: PlRd-7, PlRd-10, PlRd-30, PlRd-40.

The classical theory of electrical conductivity of metals originated at the beginning of the twentieth century. Its founder was the German physicist Karl Rikke. He experimentally established that the passage of a charge through a metal does not involve the transfer of conductor atoms, unlike liquid electrolytes. However, this discovery did not explain what exactly the carrier electrical impulses in the structure of the metal.

The experiments of scientists Stewart and Tolman, conducted in 1916, allowed us to answer this question. They were able to establish that the smallest charged particles - electrons - are responsible for the transfer of electricity in metals. This discovery formed the basis of the classical electron theory electrical conductivity of metals. From this moment it began new era research of metal conductors. Thanks to the results obtained, today we are able to use household appliances, production equipment, machines and many other devices.

How does the electrical conductivity of different metals differ?

The electronic theory of electrical conductivity of metals was developed in the research of Paul Drude. He was able to discover such a property as resistance, which is observed when electric current passes through a conductor. In the future, this will allow us to classify different substances by conductivity level. From the results obtained, it is easy to understand which metal is suitable for the manufacture of a particular cable. This is very important point, since incorrectly selected material can cause a fire as a result of overheating from the passage of excess voltage current.

Silver metal has the highest electrical conductivity. At a temperature of +20 degrees Celsius, it is 63.3 * 104 centimeters-1. But making wiring from silver is very expensive, since it is quite rare metal, which is used primarily for the production of jewelry and decorative items or bullion coins.

Metal that has the most high electrical conductivity Among all the elements of the ignoble group - copper. Its indicator is 57*104 centimeters-1 at a temperature of +20 degrees Celsius. Copper is one of the most common conductors used for household and industrial purposes. It withstands constant electrical loads well, is durable and reliable. High temperature melting allows you to work without problems for a long time in a heated state.

In terms of abundance, only aluminum can compete with copper, which ranks fourth in electrical conductivity after gold. It is used in low-voltage networks, as it has almost half the melting point of copper and is not able to withstand extreme loads. The further distribution of places can be found by looking at the table of electrical conductivity of metals.

It is worth noting that any alloy has much lower conductivity than pure substance. This is due to the merging of the structural network and, as a consequence, disruption of the normal functioning of electrons. For example, in production copper wire material is used with an impurity content of no more than 0.1%, and for some types of cable this indicator is even stricter - no more than 0.05%. All given indicators are the electrical conductivity of metals, which is calculated as the ratio between the current density and the magnitude of the electric field in the conductor.

Classical theory of electrical conductivity of metals

The basic principles of the theory of electrical conductivity of metals contain six points. First: a high level of electrical conductivity is associated with the presence large number free electrons. Second: electric current arises by external influence on a metal, in which electrons move from random motion to ordered motion.

Third: the strength of the current passing through a metal conductor is calculated according to Ohm's law. Fourth: different number elementary particles in the crystal lattice leads to unequal resistance of metals. Fifth: electric current in the circuit occurs instantly after the start of the action on electrons. Sixth: as the internal temperature of the metal increases, the level of its resistance also increases.

The nature of the electrical conductivity of metals is explained by the second point of the provisions. In a quiet state, all free electrons rotate chaotically around the nucleus. At this point, the metal is not able to reproduce on its own. electric charges. But you just have to connect external source impact, as electrons instantly line up in a structured sequence and become carriers of electric current. With increasing temperature, the electrical conductivity of metals decreases.

This is due to the fact that they are weakening molecular bonds in a crystal lattice, elementary particles begin to rotate in an even more chaotic order, so building electrons into a circuit becomes more complicated. Therefore, it is necessary to take measures to prevent overheating of the conductors, as this negatively affects their performance properties. The mechanism of electrical conductivity of metals cannot be changed due to current laws physics. But it is possible to neutralize negative external and internal influences that interfere with the normal course of the process.

Metals with high electrical conductivity

Electrical conductivity alkali metals is on high level, since their electrons are weakly attached to the nucleus and easily line up in the desired sequence. But this group is characterized by low melting points and enormous chemical activity, which in most cases does not allow their use for the manufacture of wires.

Metals with high electrical conductivity when opened are very dangerous for humans. Touching a bare wire will result in an electrical burn and a powerful shock to all areas. internal organs. This often results in instant death. Therefore, special insulating materials are used for the safety of people.

Depending on the application, they can be solid, liquid or gaseous. But all types are designed for one function - isolating the electric current inside the circuit so that it cannot affect outside world. The electrical conductivity of metals is used in almost all fields modern life human beings, so ensuring safety is a top priority.

Electronic conductivity of metals

At the beginning of the 20th century, the classical electronic theory of conductivity of metals was created (P. Drude, 1900, H. Lorenz, 1904), which provided a simple and visual explanation of most of the electrical and thermal properties of metals. Let's consider some provisions of this theory.

Free electrons

The metal conductor consists of:

1) positively charged ions oscillating around the equilibrium position, and

2) free electrons capable of moving throughout the entire volume of the conductor.

Thus, electrical properties metals are due to the presence of free electrons in them with a concentration of the order of 1028 m–3, which approximately corresponds to the concentration of atoms. These electrons are called conduction electrons. They are formed by removing them from metal atoms valence electrons. Such electrons do not belong to any particular atom and are able to move throughout the entire volume of the body. In a metal, in the absence of an electric field, conduction electrons move chaotically and collide, most often with ions of the crystal lattice (Fig. 1). The collection of these electrons can be approximately considered as a kind of electron gas, obeying the laws ideal gas. The average speed of thermal motion of electrons at room temperature is approximately 105 m/s.

Figure 1

Electric current in metals

The ions of the metal crystal lattice do not take part in the creation of current. Their movement during the passage of current would mean the transfer of matter along the conductor, which is not observed. For example, in the experiments of E. Riecke (1901) the mass and chemical composition conductor did not change when current passed for a year.

Experimental proof The fact that the current in metals is created by free electrons was demonstrated in the experiments of L.I. Mandelstam and N.D. Papaleksi (1912, the results were not published), as well as T. Stewart and R. Tolman (1916). They discovered that when a rapidly rotating coil suddenly stops, an electric current arises in the coil conductor, created by negatively charged particles - electrons.

Consequently, electric current in metals is the directional movement of free electrons.

Since electric current in metals is formed by free electrons, the conductivity of metallic conductors is called electronic conductivity.

Electric current in metals arises under the influence of an external electric field. Conduction electrons located in this field are affected by electric force, giving them acceleration directed in the direction opposite to the field strength vector. As a result, the electrons acquire a certain additional speed (it is called drift). This speed increases until the electron collides with an atom in the metal crystal lattice. During such collisions, electrons lose their excess kinetic energy, transferring it to ions. Then the electrons are again accelerated by the electric field, again decelerated by ions, etc. The average electron drift speed is very small, about 10–4 m/s.

Current propagation speed and drift speed are not the same thing. The speed of current propagation is equal to the speed of propagation of the electric field in space, i.e. 3⋅108 m/s.

When colliding with ions, conduction electrons transfer part of the kinetic energy to the ions, which leads to an increase in the energy of motion of the ions of the crystal lattice, and, consequently, to heating of the conductor.

Resistance of metals

The resistance of metals is explained by collisions of conduction electrons with ions of the crystal lattice. In this case, obviously, the more often such collisions occur, i.e., the shorter the average free travel time of an electron between collisions τ, the greater the resistivity of the metal.

In turn, time τ depends on the distance between the lattice ions, the amplitude of their vibrations, the nature of the interaction of electrons with ions and the speed of thermal motion of electrons. As the temperature of the metal increases, the amplitude of ion vibrations and the speed of thermal motion of electrons increase. The number of crystal lattice defects also increases. All this leads to the fact that as the temperature of the metal increases, collisions of electrons with ions will occur more often, i.e. time τ decreases, and the resistivity of the metal increases.

The experiment of Mandelstam and Papaleksi in elucidating the motion of the electron

If an electron has mass, then its mass, or ability to move by inertia, should be manifested everywhere, not just in an electric field. Russian scientists L. I. Mandelstam (1879-1949; founder of the school of radiophysicists) and N. D. Papaleksi (1880 - 1947; the largest Soviet physicist, academician, chairman of the All-Union Scientific Council on Radiophysics and Radio Engineering at the USSR Academy of Sciences) carried out an original experiment in 1913. They took a coil of wire and began to twist it in different directions.

They will spin, for example, clockwise, then abruptly stop and then back.

They reasoned something like this: if electrons really have mass, then when the coil suddenly stops, the electrons should continue to move by inertia for some time. The movement of electrons along a wire is an electric current. It happened as we planned. We connected a telephone to the ends of the wire and heard a sound. Since sound is heard in the phone, therefore, current flows through it.

The experience of Mandelstam and Papaleksi was repeated in 1916 by American scientists Tolman and Stewart. They also twisted the coil, but instead of a telephone, they connected a device to its ends to measure the charge. They managed not only to prove the existence of electron mass, but also to measure it. Tolman and Stewart's data were then checked and refined many times by other scientists, and now you know that the mass of the electron is 9.109 10-31 kilograms.

When setting up these experiments, we proceeded from the following thought. If there are free charges in a metal that have mass, then they must obey the law of inertia. A rapidly moving conductor, for example, from left to right, is a collection of metal atoms moving in this direction, which carry free charges along with them. When such a conductor suddenly stops, the atoms included in its composition stop; free charges, by inertia, must continue to move from left to right until various obstacles (collisions with stopped atoms) stop them. The occurring phenomenon is similar to what is observed when a tram suddenly stops, when “loose” objects and people not attached to the car continue to move forward for some time by inertia.

Thus, short time after the conductor stops, the free charges in it must move in one direction. But the movement of charges in a certain direction is an electric current. Consequently, if our reasoning is correct, then after a sudden stop of the conductor we should expect the appearance of a short-term current in it. The direction of this current will allow us to judge the sign. Charge. If they move in this direction negative charges, then a current should be observed in the direction from right to left and vice versa. The resulting current depends on the charges and the ability of their carriers to maintain their movement by inertia for more or less a long time, despite interference, i.e., on their mass. Thus, this experiment not only allows us to verify the assumption of the existence of free charges, but also to determine the charges themselves, their sign and the mass of their carriers (more precisely, the charge-to-mass ratio elm).

In the practical implementation of the experiment it turned out to be more convenient to use not progressive, but rotational movement conductor. A diagram of such an experiment is shown in Fig. 2.

Figure 2

A wire spiral 1 is mounted on a coil into which two semi-axes 00 are isolated from each other. The ends of the spiral are soldered to both halves of the axis and, using sliding contacts 2 (“brushes”), are connected to a sensitive galvanometer 3. The coil is driven in fast rotation and then suddenly slowed down. The experiment actually revealed that in this case an electric current arose in the galvanometer. The direction of this current showed that negative charges were moving by inertia. By measuring the charge carried by this short-term current, it was possible to find the ratio of the free charge to the mass of its carrier. This ratio turned out to be equal to e/m=l.8 1011 C/kg, which coincides well with the value of this ratio for electrons determined by other methods.

The value of metals is directly determined by their chemical and physical properties. In the case of an indicator such as electrical conductivity, this relationship is not so straightforward. Most electrically conductive metal when measured this indicator at room temperature (+20 °C), - silver.

But high cost limits the use of silver parts in electrical engineering and microelectronics. Silver elements in such devices are used only if economically feasible.

Physical meaning of conductivity

The use of metal conductors has a long history. Scientists and engineers working in fields of science and technology that use electricity have long decided on materials for wires, terminals, contacts, etc. It helps to determine the most electrically conductive metal in the world physical quantity, called electrical conductivity.

Back conductivity concept electrical resistance. The quantitative expression of conductivity is related to the unit of resistance, which is international system units (SI) are measured in Ohms. The SI unit is siemens. Russian designation of this unit - cm, international - S. An area of ​​1 cm has an electrical conductivity electrical network with a resistance of 1 ohm.

Conductivity

A measure of the ability of a substance to conduct electric current is called. The most electrically conductive metal has the highest similar indicator. This characteristic can be determined instrumentally for any substance or medium and has numeric expression. of a cylindrical conductor of unit length and unit cross-sectional area is related to the resistivity of this conductor.

System unit conductivity is Siemens per meter - S/m. To find out which metal is the most electrically conductive metal in the world, it is enough to compare their experimentally determined conductivities. You can determine the resistivity using a special device - a microohmmeter. These characteristics are inversely dependent.

Conductivity of metals

The very concept of a directed flow of charged particles seems more harmonious for substances based on crystal lattices characteristic of metals. Charge carriers when an electric current occurs in metals are free electrons, and not ions, as is the case in liquid media. It has been experimentally established that when a current occurs in metals, there is no transfer of particles of matter between the conductors.

Metallic substances differ from others by having looser bonds at the atomic level. Internal structure metals are characterized by the presence of a large number of “lonely” electrons. which, at the slightest influence of electromagnetic forces, form a directed flow. Therefore, it is not for nothing that metals are the best conductors of electric current, and it is precisely such molecular interactions distinguished by the most electrically conductive metal. Another specific property of metals is based on the structural features of the crystal lattice of metals - high thermal conductivity.

Top best conductors - metals

4 metals having practical significance for their use as electrical conductors, they are distributed in the following order relative to the value of specific conductivity, measured in S/m:

1. Silver - 62,500,000.
2. Copper - 59,500,000.
3. Gold - 45,500,000.
4. Aluminum - 38,000,000.

It can be seen that the most electrically conductive metal is silver. But like gold, it is used to organize the electrical network only in special specific cases. The reason is high cost.

But copper and aluminum are the most common option for electrical appliances and cable products due to their low resistance to electric current and affordability. Other metals are rarely used as conductors.

Factors affecting the conductivity of metals

Even the most electrically conductive metal reduces its conductivity if it contains other additives and impurities. Alloys have a different crystal lattice structure than “pure” metals. It is characterized by a violation of symmetry, cracks and other defects. Conductivity also decreases with increasing ambient temperature.

The increased resistance inherent in alloys is used in heating elements. It is no coincidence that nichrome, fechral and other alloys are used to manufacture working elements of electric furnaces and heaters.

The most electrically conductive metal is precious silver, mostly used by jewelers, for minting coins, etc. But also in technology and instrument making, its special chemical and physical properties are widely used. For example, in addition to being used in components and assemblies with reduced resistance, silver plating protects contact groups from oxidation. The unique properties of silver and alloys based on it often make its use justified, despite its high cost.

« Physics - 10th grade"

How do electrons move in a metal conductor when there is no electric field in it?
How does the movement of electrons change when a voltage is applied to a metal conductor?

Electric current is carried out by solids, liquids and gaseous bodies. How are these conductors different from each other?

You became familiar with electric current in metal conductors and with the experimentally established current-voltage characteristic of these conductors - Ohm's law.

Along with metals, they are good conductors, i.e. substances with a large number free charged particles are aqueous solutions or melts of electrolytes and ionized gas - plasma. These conductors are widely used in technology.

In vacuum electronic devices Electric current is formed by flows of electrons.

Metal conductors are widely used in the transmission of electricity from current sources to consumers. In addition, these conductors are used in electric motors and generators, electric heating devices, etc.

Except conductors And dielectrics(substances with relatively a small amount free charged particles), there is a group of substances whose conductivity is intermediate position between conductors and dielectrics. These substances do not conduct electricity well enough to be called conductors, but not so poorly as to be classified as dielectrics. That's why they got the name semiconductors.

For a long time, semiconductors did not play a significant practical role. In electrical engineering and radio engineering, exclusively various conductors and dielectrics were used. The situation changed significantly when the easily feasible possibility of controlling the electrical conductivity of semiconductors was first theoretically predicted, and then discovered and studied.

There is no universal current carrier. The table shows current carriers in different environments.

Electronic conductivity of metals.

Let's start with metal conductors. We know the current-voltage characteristic of these conductors, but so far nothing has been said about its explanation from the point of view of molecular kinetic theory.

The carriers of free charges in metals are electrons. Their concentration is high - about 10 28 1/m 3.

These electrons participate in random thermal motion. Under the influence of an electric field, they begin to move in an orderly manner with an average speed of the order of 10 -4 m/s.

Experimental proof of the existence of free electrons in metals.

Experimental proof that the conductivity of metals is due to the movement of free electrons was given in the experiments of Mandelstam and Papaleksi (1913), Stewart and Tolman (1916). The scheme of these experiments is as follows.

A wire is wound onto a coil, the ends of which are soldered to two metal disks isolated from each other (Fig. 16.1). A galvanometer is connected to the ends of the disks using sliding contacts.

The reel is brought into rapid rotation and then abruptly stopped. After a sudden stop of the coil, free charged particles move for some time relative to the conductor by inertia, and, consequently, an electric current arises in the coil. The current exists for a short time, since due to the resistance of the conductor, charged particles are slowed down and the ordered movement of particles that forms the current stops.

The direction of the current in this experiment suggests that it is created by the movement of negatively charged particles. The charge transferred in this case is proportional to the ratio of the charge of the particles creating the current to their mass, i.e. |q|/m. Therefore, by measuring the charge passing through the galvanometer during the existence of the current in the circuit, it was possible to determine this ratio. It turned out to be equal to 1.8 10 11 C/kg. This value coincided with the ratio of the electron charge to its mass e/m, found earlier from other experiments.

Movement of electrons in a metal.

Free electrons in a metal move randomly. When a conductor is connected to a current source, an electric field is created in it, and electrons begin to be affected by Coulomb force= q e . Under the influence of this force, the electrons begin to move directionally, i.e. the chaotic movement of the electrons is superimposed on the speed of the directional movement and increases for some time t 0 until a collision of electrons with ions occurs crystal lattice. In this case, the electrons lose their direction of movement, and then begin to move directionally again. Thus, the speed of directional motion of the electron varies from zero to some maximum value, equal As a result, the average speed of the ordered movement of electrons turns out to be equal, i.e., proportional to the electric field strength in the conductor: υ ~ E and, therefore, the potential difference at the ends of the conductor, since where l is the length of the conductor.

The current strength in the conductor is proportional to the speed of the ordered movement of particles (see formula (15.2)). Therefore, we can say that the current strength is proportional to the potential difference at the ends of the conductor: I ~ U.

This is qualitative explanation of Ohm's law based on the electronic theory of metal conductivity.

It is impossible to construct a satisfactory quantitative theory of the motion of electrons in a metal based on the laws of classical mechanics. The fact is that the conditions for the movement of electrons in a metal are such that classical mechanics Newton is not applicable to describe this movement. This fact confirms, for example, the dependence of resistance on temperature. According to classical theory metals, in which the movement of electrons is considered on the basis of Newton’s second law, the resistance of the conductor is proportional to the same experiment shows linear dependence temperature resistance.