In semiconductors, electrical conductivity depends significantly on temperature. At temperatures close to absolute zero, they turn into insulators, and at high temperatures their conductivity becomes significant. Unlike metals, the number of conduction electrons in semiconductors is not equal to the number of valence electrons, but constitutes only a small part of it. The sharp dependence of the conductivity of semiconductors on temperature indicates that conduction electrons appear in them under the influence of thermal motion.
7.Formulate and write down Brewster’s law. Explain your answer with a drawing.
If the tangent of the angle of incidence of the beam on the interface between two dielectrics is equal to the relative refractive index, then the reflected beam is completely polarized in a plane perpendicular to the plane of incidence, that is, parallel to the interface between the media
tg a B = n 21.
Here a B is the angle of incidence of light, called the Brewster angle, n 21 - relative indicator refraction of the second medium relative to the first
8. What is the essence of Heisenberg's uncertainty relations?
x* p x >=h
y* p y >=h
z* p z >=h
E* t>=h
Δx, y, z - inaccuracy in determining the coordinate
Δp – inaccuracy in determining the impulse
Phys. meaning: it is impossible to accurately measure position and momentum at the same time.
9. How will the frequency of free oscillations in the oscillatory circuit change if the inductance of the coil is increased by 4 times and the electrical capacity of the capacitor is reduced by 2 times?
Answer: will decrease by half
10. Indicate the product of the nuclear reaction Li+ H He+?
11.What is the inductive reactance of a coil with an inductance of 2 mH at a current oscillation frequency of n = 50 Hz?
R L =wL=2πνL=0.628 (Ohm). Answer: R L =0.628 (Ohm)
If the absolute refractive index of a medium is 1.5, then what is the speed of light in this medium?
n= c/v 2*10 8
13. Wavelength of gamma radiation is nm. What potential difference U must be applied to the X-ray tube to produce X-rays with this wavelength?
14. The de Broglie wavelength for a particle is 2.2 nm. Find the mass of the particle if it moves with speed .
m= = 6, 62*10 -34 /2, 2*10 -9 *10 5 =3, 01*10 -30 ;
As a result of photon scattering by a free electron, the Compton shift turned out to be equal to 1.2 pm. Find the scattering angle.
16. The oscillatory circuit contains a capacitor with an electrical capacity of 50nF and an inductance coil of 5/(4) μH. Determine the wavelength of the radiation
17. The work function of an electron leaving platinum is . What is the maximum kinetic energy of photoelectrons ejected from platinum by light with a wavelength of 0.5 microns?
18. Distance between strokes diffraction grating d = 4 µm. Light with a wavelength of = 0.6 µm. What is the maximum highest order does this lattice give?
d=4μm, , dsinj = nl, sinj=1,n= =
Poppy. order - 6
19. What is the half light absorption layer d 1/2, if when light passes through a 30 mm layer of substance, the light intensity decreases 8 times? , , , , , , 
,
20. In Young’s experiment, the holes were illuminated with monochromatic light of wavelength = 6 10 -5 cm, the distance between the holes is 1 mm and the distance from the holes to the screen is 3 m. Find the position of the first light stripe .
Option 18
1. A magnetic field is called uniform if... the magnetic induction vector is the same at all points. example (permanent magnet)
2. What oscillations are called forced?
Forced oscillations are oscillations that occur in any system under the influence of a variable external influence. The nature of forced oscillations is determined both by the properties of the external influence and by the properties of the system itself.
3.What is called the external photoelectric effect?
The external photoelectric effect is the ejection of electrons from a substance under the influence of electromagnetic radiation. External photoelectric effect is observed mainly in conductors
4. What is called a perfect black body?
A body capable of completely absorbing at any temperature all radiation of any frequency incident on it is called black. Consequently, the spectral absorption capacity of a black body for all frequencies and temperatures is identically equal to unity ()
5. Formulate and write Lambert’s law
Bouguer - Lambert - Beer law - physical law, which determines the attenuation of a parallel monochromatic beam of light as it propagates in an absorbing medium.
where is the intensity of the incoming beam, l is the thickness of the layer of substance through which the light passes, is the absorption index
Themes Unified State Exam codifier : semiconductors, intrinsic and impurity conductivity of semiconductors.
Until now, speaking about the ability of substances to conduct electric current, we have divided them into conductors and dielectrics. The resistivity of ordinary conductors is in the Ohm m range; The resistivity of dielectrics exceeds these values on average by orders of magnitude: Ohm m.
But there are also substances that, in their electrical conductivity, occupy an intermediate position between conductors and dielectrics. This semiconductors: their resistivity at room temperature can take values in a very wide range of Ohm m. Semiconductors include silicon, germanium, selenium, and some others chemical elements and compounds (Semiconductors are extremely common in nature. For example, about 80% of the mass earth's crust accounts for substances that are semiconductors). The most widely used are silicon and germanium.
The main feature of semiconductors is that their electrical conductivity increases sharply with increasing temperature. The resistivity of a semiconductor decreases with increasing temperature approximately as shown in Fig. 1 .
Rice. 1. Dependence for a semiconductor
In other words, at low temperatures semiconductors behave like dielectrics, and at high temperatures they behave like fairly good conductors. This is the difference between semiconductors and metals: the resistivity of a metal, as you remember, increases linearly with increasing temperature.
There are other differences between semiconductors and metals. Thus, illumination of a semiconductor causes a decrease in its resistance (and light has almost no effect on the resistance of the metal). In addition, the electrical conductivity of semiconductors can change greatly with the introduction of even minute amounts of impurities.
Experience shows that, as in the case of metals, no transfer of substance occurs when current flows through a semiconductor. Therefore, the electric current in semiconductors is caused by the movement of electrons.
A decrease in the resistance of a semiconductor when it is heated indicates that an increase in temperature leads to an increase in the number of free charges in the semiconductor. Nothing like this happens in metals; therefore, semiconductors have a different mechanism of electrical conductivity than metals. And the reason for this is different nature chemical bond between metal and semiconductor atoms.
Covalent bond
The metallic bond, as you remember, is provided by a gas of free electrons, which, like glue, holds positive ions at the nodes of the crystal lattice. Semiconductors are structured differently - their atoms are held together covalent bond. Let's remember what it is.
Electrons located in the outer electronic level and called valence, are weaker bound to the atom than the remaining electrons, which are located closer to the nucleus. In the process of forming a covalent bond, two atoms contribute one of their valence electrons “to the common cause.” These two electrons are shared, that is, they now belong to both atoms, and therefore are called shared electron pair(Fig. 2).
Rice. 2. Covalent bond
A socialized pair of electrons is what holds the atoms near each other (using the forces of electrical attraction). A covalent bond is a bond that exists between atoms due to shared electron pairs. For this reason, a covalent bond is also called pair-electronic.
Crystal structure of silicon
Now we are ready to take a closer look at the internal structure of semiconductors. As an example, consider the most common semiconductor in nature - silicon. The second most important semiconductor, germanium, has a similar structure.
The spatial structure of silicon is shown in Fig. 3 (picture by Ben Mills). The balls represent silicon atoms, and the tubes connecting them are channels of covalent bonds between atoms.
Rice. 3. Crystal structure silicon
Note that each silicon atom is bonded to four neighboring atoms. Why does this happen?
The fact is that silicon is tetravalent - there are four valence electrons on the outer electron shell of the silicon atom. Each of these four electrons is ready to form a shared electron pair with the valence electron of another atom. This is what happens! As a result, the silicon atom is surrounded by four atoms docked to it, each of which contributes one valence electron. Accordingly, there are eight electrons around each atom (four of our own and four of others).
We see this in more detail on a flat diagram of a silicon crystal lattice (Fig. 4).
Rice. 4. Silicon crystal lattice
Covalent bonds are depicted as pairs of lines connecting atoms; These lines contain common electron pairs. Each valence electron located on such a line spends most of its time in the space between two neighboring atoms.
However, valence electrons are by no means “tightly tied” to the corresponding pairs of atoms. Overlap occurs electronic shells everyone neighboring atoms, so that any valence electron is the common property of all neighboring atoms. From some atom 1, such an electron can go to its neighboring atom 2, then to its neighboring atom 3, and so on. Valence electrons can move throughout the crystal - they are said to belong to the entire crystal(and not any one atomic pair).
However, silicon's valence electrons are not free (as is the case in the metal). In a semiconductor, the bond between valence electrons and atoms is much stronger than in a metal; Silicon covalent bonds do not break at low temperatures. The electron energy turns out to be insufficient to ensure that, under the influence of external electric field begin an orderly movement from a smaller potential to a larger one. Therefore, with enough low temperatures ah, semiconductors are close to dielectrics - they do not conduct electric current.
Self conductivity
If you connect a semiconductor element to an electrical circuit and start heating it, the current in the circuit increases. Therefore, the resistance of the semiconductor decreases with increasing temperature. Why is this happening?
As the temperature rises, the thermal vibrations of silicon atoms become more intense, and the energy of the valence electrons increases. For some electrons, the energy reaches values sufficient to break covalent bonds. Such electrons leave their atoms and become free(or conduction electrons) - exactly the same as in metal. In an external electric field, free electrons begin to move in an orderly manner, forming an electric current.
The higher the silicon temperature, the greater the electron energy, and the greater the number of covalent bonds that fail and break. The number of free electrons in a silicon crystal increases, which leads to a decrease in its resistance.
The breaking of covalent bonds and the appearance of free electrons is shown in Fig. 5 . At the site of the broken covalent bond, a hole- vacant place for an electron. The hole has positive charge, since with the departure of a negatively charged electron, an uncompensated positive charge of the nucleus of the silicon atom remains.
Rice. 5. Formation of free electrons and holes
The holes do not stay in place - they can wander around the crystal. The fact is that one of the neighboring valence electrons, “traveling” between atoms, can jump to the resulting vacant place, filling the hole; then the hole in this place will disappear, but will appear in the place where the electron came from.
In the absence of an external electric field, the movement of holes is random, because valence electrons wander randomly between atoms. However, in an electric field it begins directed movement of holes. Why? This is not difficult to understand.
In Fig. Figure 6 shows a semiconductor placed in an electric field. On the left side of the picture is the initial position of the hole.
Rice. 6. Motion of a hole in an electric field
Where will the hole go? It is clear that the most likely electron > hole jumps are in the direction against field lines (that is, to the “pluses” that create the field). One of these jumps is shown in the middle part of the figure: the electron jumped to the left, filling the vacancy, and the hole, accordingly, shifted to the right. The next possible electron jump caused by the electric field is depicted on the right side of the figure; As a result of this jump, the hole took a new place, located even further to the right.
We see that the hole as a whole is moving towards field lines - that is, where positive charges are supposed to move. Let us emphasize once again that the directed movement of a hole along the field is caused by jumps of valence electrons from atom to atom, occurring predominantly in the direction against the field.
Thus, in a silicon crystal there are two types of charge carriers: free electrons and holes. When an external electric field is applied, an electric current appears, caused by their ordered counter motion: free electrons move opposite to the field strength vector, and holes - in the direction of the vector.
The generation of current due to the movement of free electrons is called electronic conductivity, or n-type conductivity. The process of orderly movement of holes is called hole conductivity,or p-type conductivity(from the first letters of the Latin words negativus (negative) and positivus (positive)). Both conductivities - electron and hole - are collectively called own conductivity semiconductor.
Each electron leaving a broken covalent bond generates a “free electron–hole” pair. Therefore, the concentration of free electrons in a pure silicon crystal is equal to the concentration of holes. Accordingly, when the crystal is heated, the concentration of not only free electrons, but also holes increases, which leads to an increase in the intrinsic conductivity of the semiconductor due to an increase in both electron and hole conductivity.
Along with the formation of free electron–hole pairs, the reverse process also occurs: recombination free electrons and holes. Namely, a free electron, encountering a hole, fills this vacancy, restoring the broken covalent bond and turning into a valence electron. Thus, in a semiconductor it is established dynamic equilibrium: the average number of ruptures of covalent bonds and the formation of electron-hole pairs per unit time is equal to the average number of recombining electrons and holes. This state of dynamic equilibrium determines the equilibrium concentration of free electrons and holes in the semiconductor under given conditions.
Changes in external conditions shift the state of dynamic equilibrium in one direction or another. In this case, the equilibrium value of the charge carrier concentration naturally changes. For example, the number of free electrons and holes increases when the semiconductor is heated or illuminated.
At room temperature, the concentration of free electrons and holes in silicon is approximately equal to cm. The concentration of silicon atoms is on the order of cm. In other words, there is only one free electron per silicon atom! This is very little. In metals, for example, the concentration of free electrons is approximately equal to the concentration of atoms. Respectively, intrinsic conductivity of silicon and other semiconductors at normal conditions small compared to the conductivity of metals.
Impurity conductivity
The most important feature of semiconductors is that their resistivity can be reduced by several orders of magnitude as a result of the introduction of even a very small amount of impurities. In addition to its own conductivity, a semiconductor has a dominant impurity conductivity. It is thanks to this fact that semiconductor devices have found such wide application in science and technology.
Suppose, for example, that a little pentavalent arsenic is added to the silicon melt. After crystallization of the melt, it turns out that arsenic atoms occupy places in some nodes of the formed silicon crystal lattice.
The outermost electronic level of the arsenic atom has five electrons. Four of them form covalent bonds with their nearest neighbors - silicon atoms (Fig. 7). What is the fate of the fifth electron not occupied in these bonds?
Rice. 7. N-type semiconductor
And the fifth electron becomes free! The fact is that the binding energy of this “extra” electron with the arsenic atom located in the silicon crystal is much less than the binding energy of valence electrons with silicon atoms. Therefore, already at room temperature, almost all arsenic atoms, as a result of thermal movement, remain without a fifth electron, turning into positive ions. And the silicon crystal, accordingly, is filled with free electrons that have been detached from the arsenic atoms.
Filling a crystal with free electrons is not new to us: we saw this above when it was heated clean silicon (without any impurities). But now the situation is fundamentally different: the appearance of a free electron leaving an arsenic atom is not accompanied by the appearance of a mobile hole. Why? The reason is the same - the bond of valence electrons with silicon atoms is much stronger than with the arsenic atom in the fifth vacancy, therefore the electrons of neighboring silicon atoms do not tend to fill this vacancy. The vacancy thus remains in place; it is, as it were, “frozen” to the arsenic atom and does not participate in the creation of current.
Thus, the introduction of pentavalent arsenic atoms into the silicon crystal lattice creates electronic conductivity, but does not lead to the symmetrical appearance of hole conductivity. The main role in creating current now belongs to free electrons, which in this case are called main carriers charge.
The mechanism of intrinsic conductivity, of course, continues to work even in the presence of an impurity: covalent bonds are still broken due to thermal motion, generating free electrons and holes. But now there are much fewer holes than free electrons, which in large quantities provided by arsenic atoms. Therefore, in this case the holes will be non-major media charge.
Impurities whose atoms give up free electrons without the appearance of an equal number of mobile holes are called donor. For example, pentavalent arsenic is a donor impurity. If there is a donor impurity in a semiconductor, the majority charge carriers are free electrons, and the minority charge carriers are holes; in other words, the concentration of free electrons is much higher than the concentration of holes. Therefore, semiconductors with donor impurities are called electronic semiconductors, or n-type semiconductors(or simply n-semiconductors).
And how much, interestingly, can the concentration of free electrons exceed the concentration of holes in an n-semiconductor? Let's do a simple calculation.
Let us assume that the impurity is , that is, there is one arsenic atom per thousand silicon atoms. The concentration of silicon atoms, as we remember, is of the order of cm.
The concentration of arsenic atoms, accordingly, will be a thousand times less: cm. The concentration of free electrons given up by the impurity will also be the same - after all, each arsenic atom gives up an electron. Now let us remember that the concentration of electron-hole pairs that appear when silicon covalent bonds are broken at room temperature is approximately equal to cm. Do you feel the difference? The concentration of free electrons in this case is greater than the concentration of holes by orders of magnitude, that is, a billion times! Accordingly, the resistivity of a silicon semiconductor decreases by a billion times when such a small amount of impurity is introduced.
The above calculation shows that in n-type semiconductors the main role is indeed played by electronic conductivity. Against the background of such a colossal superiority in the number of free electrons, the contribution of hole movement to the overall conductivity is negligible.
On the contrary, it is possible to create a semiconductor with predominant hole conductivity. This will happen if a trivalent impurity is introduced into a silicon crystal - for example, indium. The result of such implementation is shown in Fig. 8 .
Rice. 8. P-type semiconductor
What happens in this case? The outermost electronic level of the indium atom contains three electrons that form covalent bonds with the three surrounding silicon atoms. For the fourth neighboring silicon atom, the indium atom no longer has enough electron, and a hole appears in this place.
And this hole is not simple, but special - with a very high binding energy. When an electron from a neighboring silicon atom gets into it, it will “get stuck in it forever,” because the attraction of the electron to the indium atom is very strong - more than to silicon atoms. The indium atom will turn into a negative ion, and a hole will appear in the place where the electron came from - but now an ordinary mobile hole in the form of a broken covalent bond in the silicon crystal lattice. This hole will begin to wander around the crystal in the usual way due to the “relay race” transfer of valence electrons from one silicon atom to another.
And so, each impurity indium atom generates a hole, but does not lead to the symmetric appearance of a free electron. Such impurities, the atoms of which “tightly” capture electrons and thereby create a mobile hole in the crystal, are called acceptor.
Trivalent indium is an example of an acceptor impurity.
If an acceptor impurity is introduced into a crystal of pure silicon, then the number of holes generated by the impurity will be much more number free electrons resulting from the breaking of covalent bonds between silicon atoms. A semiconductor with an acceptor impurity is hole semiconductor, or p-type semiconductor(or simply p-semiconductor).
Holes play main role when creating a current in a p-semiconductor; holes - main charge carriers. Free electrons - minor media charge in a p-semiconductor. The movement of free electrons in this case does not contribute significant contribution: Electric current is provided primarily by hole conduction.
p–n junction
The point of contact between two semiconductors various types conductivity (electronic and hole) is called electron-hole transition, or p–n junction. In the region of the p–n junction, an interesting and very important phenomenon occurs - one-way conductivity.
In Fig. 9 shows the contact of p- and n-type regions; the colored circles are holes and free electrons, which are the majority (or minority) charge carriers in the corresponding regions.
Rice. 9. Blocking layer of the p–n junction
Performing thermal movement, charge carriers penetrate through the interface between the regions.
Free electrons move from the n-region to the p-region and recombine there with holes; holes diffuse from the p-region to the n-region and recombine there with electrons.
As a result of these processes, an uncompensated charge of positive ions of the donor impurity remains in the electronic semiconductor near the contact boundary, and an uncompensated negative charge of the acceptor impurity ions appears in the hole semiconductor (also near the boundary). These uncompensated space charges form the so-called barrier layer, the internal electric field of which prevents further diffusion of free electrons and holes across the contact boundary.
Let us now connect a current source to our semiconductor element, applying the “plus” of the source to the n-semiconductor, and the “minus” to the p-semiconductor (Fig. 10).
Rice. 10. Inclusion in reverse direction: no current
We see that the external electric field moves the majority charge carriers further from the contact boundary. The width of the blocking layer increases, and its electric field increases. The resistance of the blocking layer is high, and majority carriers are not able to overcome the p–n junction. The electric field allows only minority carriers to cross the boundary, but due to the very low concentration of minority carriers, the current they create is negligible.
The considered scheme is called turning on the p–n junction in the opposite direction. Electric current there are no main carriers; there is only a negligible minority carrier current. In this case, the p–n junction turns out to be closed.
Now let’s change the polarity of the connection and apply “plus” to the p-semiconductor, and “minus” to the n-semiconductor (Fig. 11). This scheme is called forward switching.
Rice. 11. Switching on in the forward direction: current flows
In this case, the external electric field is directed against the blocking field and opens the way for majority carriers through the p–n junction. The barrier layer becomes thinner and its resistance decreases.
There is a massive movement of free electrons from the n-region to the p-region, and holes, in turn, rush together from the p-region to the n-region.
A current arises in the circuit caused by the movement of the majority charge carriers (Now, however, the electric field interferes with the current of the minority carriers, but this insignificant factor does not have a noticeable effect on the overall conductivity).
One-way conductivity of the p–n junction is used in semiconductor diodes. A diode is a device that conducts current in only one direction; in the opposite direction, no current passes through the diode (the diode is said to be closed). A schematic representation of the diode is shown in Fig. 12 .
Rice. 12. Diode
In this case, the diode is open in the direction from left to right: the charges seem to flow along the arrow (see it in the figure?). In the direction from right to left, the charges seem to rest against the wall - the diode is closed.
Conductor particles (molecules, atoms, ions) that do not participate in the formation of current are in thermal motion, and particles that form the current are simultaneously in thermal and directional motion under the influence of an electric field. Due to this, numerous collisions occur between particles that form the current and particles that do not participate in its formation, in which the former give up part of the energy they carry from the current source to the latter. The more collisions, the lower the speed of the ordered movement of particles that form the current. As can be seen from the formula I = enνS, a decrease in speed leads to a decrease in current. Scalar quantity, which characterizes the property of a conductor to reduce current, is called conductor resistance. From the formula of Ohm's law, resistance 
Ohm - the resistance of the conductor in which a current of strength is obtained 1 a with a voltage at the ends of the conductor of 1 V.
The resistance of a conductor depends on its length l, cross-section S and the material, which is characterized by resistivity 
The longer the conductor, the more collisions per unit time of particles that form the current with particles that do not participate in its formation, and therefore the greater the resistance of the conductor. The less cross section conductor, the denser the flow of particles that form the current, and the more often they collide with particles that do not participate in its formation, and therefore the greater the resistance of the conductor.
Under the influence of an electric field, the particles that form the current move accelerated between collisions, increasing their kinetic energy due to the energy of the field. When colliding with particles that do not form a current, they transfer part of their kinetic energy to them. As a result, the internal energy of the conductor increases, which is externally manifested in its heating. Let's consider whether the resistance of a conductor changes when it is heated.
The electrical circuit contains a coil of steel wire (string, Fig. 81, a). Having closed the circuit, we begin to heat the wire. The more we heat it, the less current the ammeter shows. Its decrease occurs because when metals are heated, their resistance increases. Thus, the resistance of a hair of an electric light bulb when it is not lit is approximately 20 ohm, and when it burns (2900° C) - 260 ohm. When a metal is heated, the thermal movement of electrons and the rate of vibration of ions increases. crystal lattice, as a result of this, the number of collisions of current-forming electrons with ions increases. This causes an increase in conductor resistance *. In metals, unfree electrons are very tightly bound to ions, so when metals are heated, the number of free electrons practically does not change.
* (Based electron theory, it is impossible to derive an exact law for the dependence of resistance on temperature. Such a law is established quantum theory, in which the electron is considered as a particle with wave properties, and the movement of a conduction electron through a metal is a process of propagation of electronic waves, the length of which is determined by the de Broglie relation.)
Experiments show that when the temperature of conductors from various substances For the same number of degrees, their resistance changes differently. For example, if a copper conductor had a resistance 1 ohm, then after heating to 1°C he will have resistance 1.004 ohm, and tungsten - 1.005 ohm. To characterize the dependence of the resistance of a conductor on its temperature, a quantity called the temperature coefficient of resistance was introduced. A scalar quantity measured by the change in resistance of a conductor in 1 ohm, taken at 0° C, from a change in its temperature by 1° C, is called the temperature coefficient of resistance α. So, for tungsten this coefficient is equal to 0.005 deg -1, for copper - 0.004 deg -1. Temperature coefficient resistance depends on temperature. For metals, it changes little with temperature. For a small temperature range, it is considered constant for a given material.
Let us derive a formula that calculates the resistance of a conductor taking into account its temperature. Let's assume that R0- conductor resistance at 0°С, when heated to 1°C it will increase by αR 0, and when heated to t°- on αRt° and it becomes R = R 0 + αR 0 t°, or
The dependence of the resistance of metals on temperature is taken into account, for example, in the manufacture of spirals for electric heating devices and lamps: the length of the spiral wire and the permissible current are calculated from their resistance in the heated state. The dependence of the resistance of metals on temperature is used in resistance thermometers, which are used to measure the temperature of heat engines, gas turbines, metal in blast furnaces, etc. This thermometer consists of a thin platinum (nickel, iron) spiral wound on a porcelain frame and placed in a protective case. Its ends are connected to an electrical circuit with an ammeter, the scale of which is graduated in degrees of temperature. When the coil heats up, the current in the circuit decreases, this causes the ammeter needle to move, which shows the temperature.
The reciprocal of the resistance of a given section or circuit is called electrical conductivity of the conductor(electrical conductivity). Electrical conductivity of a conductor The greater the conductivity of a conductor, the lower its resistance and the better it conducts current. Name of electrical conductivity unit 
Conductor conductivity resistance 1 ohm called Siemens.
As the temperature decreases, the resistance of metals decreases. But there are metals and alloys, the resistance of which, at a low temperature specific for each metal and alloy, sharply decreases and becomes vanishingly small - almost equal to zero (Fig. 81, b). Coming superconductivity- the conductor has practically no resistance, and once the current excited in it exists for a long time, while the conductor is at the superconducting temperature (in one of the experiments, the current was observed for more than a year). When passing a current density through a superconductor 1200 a/mm 2 no release of heat was observed. Monovalent metals, which are the best conductors of current, do not transform into a superconducting state down to the extremely low temperatures at which the experiments were carried out. For example, in these experiments copper was cooled to 0.0156°K, gold - up to 0.0204° K. If it were possible to obtain alloys with superconductivity at ordinary temperatures, this would be of great importance for electrical engineering.
According to modern ideas, the main reason for superconductivity is the formation of bound electron pairs. At the temperature of superconductivity, exchange forces begin to act between free electrons, causing the electrons to form bound electron pairs. Such an electron gas of bound electron pairs has different properties than ordinary electron gas - it moves in a superconductor without friction against the nodes of the crystal lattice.
