Rough automatic draft, low quality: expect mistakes and awkward wording. It will be replaced by a better translation. Original →
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give the properties of the crystal (for example, the periodicity of the internal structure), while in others they do not. These are liquid crystals. For the combination of such seemingly incompatible properties as fluidity and orderliness, they received the name mezofaz (mezo means intermediate; i.e., intermediate phases).
Long polymer molecules can form another class of states, which include, for example, aspic or rubber. In these states, long molecules are combined into branched chains or networks. As a result, a peculiar gland-like body will appear, which is called a “gel”. Conditions of this type are also extremely common in nature.
Finally, let us dwell very briefly on some sense of extreme states of matter.
When a gas is heated, the kinetic energy of its molecules increases and may be on the order of the atomic infusion energy. Then, when molecules collide with each other, the atoms can become intertwined, and we get a mixture of neutral and loaded (positively and negatively) particles. It is very important that the amount of positive and negative contaminants is always the same, so that in general the gas is electrically neutral. This is plasma, a completely special state of matter with a unique property.
And in conclusion, let’s turn to…
to the stars. A star is a giant cloud of gas and dust that tends to shrink under the influence of gravitational attraction. As a result of this decline, the temperature in the core of the star increases, and at some point a thermonuclear reaction is ignited: hydrogen nuclei merge and turn into helium. The energy released in this case further transforms the star into scatia, and the star stabilizes (it is at this stage of stellar evolution that the Sun is located). But gradually the hydrogen burns out, and tell us the possibility. The colossal pressure that arises in this case crushes the atoms. A state arises in which electrons seem to float freely in the field of bare nuclei. If the mass of the star is not too large (less than 1.25 solar masses), then the specific scaling that exists between the electrons transforms it further into compression (this repulsion is not associated with electrical forces, but is purely quantum in nature). As a result, a completely special state arises with enormous density (about 60 t/cm³). Stars arranged in this way are called white dwarfs (due to their light blue glow and small size). If the mass of the star is greater (more than 1.5-2 solar masses), then electrons cannot resist gravitational shrinkage. As a result, they (electrons) move into nuclei and, merging with protons, form neutrons. A substance appears, consisting not of atomic nuclei, but of neutrons, with a very fantastic high density (≈2·10⁹ t/cm³). These are neutron stars.
We’ll probably stop there. Although, frankly speaking, the list of amazing states of matter that exist in nature is far from exhausted.
E. E. Gorodetsky
A few changes regarding the photo effect
In the school physics course, you became acquainted with the phenomenon of the photoelectric effect, i.e. the emission of electrons by a substance under the influence of light, and its laws (“Physics 10”, chapter 10).
What is the main thing in the theory of photoelectric effect? Of course, the hypothesis of light quanta - photons. The photoelectric effect can be represented as the result of two successive processes: 1) absorption of a light quantum by an electron, 2) emission of an electron outside the substance. If it happens
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Both processes and the phenomenon are more correctly called external photoeffect. If the absorption of photons does not lead to the emission of electrons from the substance, but changes its electrical conductivity, then they speak of an internal photoelectric effect (usually observed in semiconductors). The actual photoelectric effect is often called the act of absorption of a photon by an electron.
The question arises: can the photoelectric effect occur on a single free electron? At first glance - why not? After all, we say: the photon is absorbed by the electron. What does the substance have to do with it?
Let’s take an electron, shine a flashlight on it, and it begins to “swallow” photons and accelerate them! It turns out that nothing will work out. A free electron cannot absorb a single photon. True, it will move from its place, but the reason will not be absorption, but scattering of photons. This is also a very interesting process - the well-known Compton effect, in which the quantum properties of light are clearly manifested, but… not the photo effect.
Why can’t a photon be absorbed by a free electron? Let us carry out, as mathematicians say, a proof “by contradiction”. Let an electron (at rest or moving) absorb a photon incident on it, and at the same time its speed changes. It turns out that such a process is prohibited by the laws of conservation of energy and momentum. This becomes very serious if you choose an inertial reporting system in which the electron ends up dead after the photoelectric effect. See for yourself. What do we have in the final state? An electron at rest and nothing else. And in the initial state? Moving an electron and a photon to boot. Real energy is not conserved.
This means that the photo effect “according to all laws” is possible only in the presence of a third participant. In half-shaped metals, this role is played by ions of the crystal lattice. But when calculating energy (for example, in Einstein’s equations for the photoelectric effect), we do not take them into account because, due to their large mass, they quickly take up a very small part of the energy (playing an important role in the law of conservation of momentum).
Is a photo effect possible on a single atom (or molecules) - for example, in a gas? It turns out, yes. The photon is absorbed by one of the electrons of the atoms, and only the impulse is carried away by the atomic nucleus. It is interesting to note that the photoelectric effect was first discovered by G. Herme precisely in experiments with gases (1887). He studied the electrical breakdown of the air gap between the electrodes and discovered that when this gap is irradiated with light, the breakdown occurs at a lower voltage.
Why is it that only the external photo effect is discussed in detail in a school physics course? After all, with any photoelectric effect, the main quantum phenomenon occurs - the absorption of a photon by an electron. The thing is that the laws of the external photoeffect are quite simple and are relatively easy to study experimentally. In this case, the quantitative characteristics of the photoeffect can be found both from the experiments on the external photoeffect and by independent methods. Let’s talk about this in a little more detail.
Einstein’s equation
contains two directly variable quantities: the frequency of light and the maximum kinetic energy of knocked out electrons . Optical methods can create beams of light with well-extracted frequencies . For information
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o emitted electrons (the number of electrons knocked out in one second, as well as the maximum final energy) the cathode of a vacuum lamp is made from the metal under study. Since the current through the lamp is precisely knocked out by electrons, all the necessary information can be obtained from the current-voltage characteristics of the lamp. In particular, the maximum final energy of electrons is expressed in terms of the retardation voltage :
If for various metals we plot the experimental dependences of on the frequency of light , then, in accordance with Einstein’s equation, parallel straight lines are obtained (see Fig.ok). From the slope of these straight lines, one can calculate Planck’s constant , and from the points of intersection of the graphs with the wasps, one can find the work function and the limiting frequency , called the red limit of the photoelectric effect. As you know, for the first time the concept of quantization of energy (), as well as the constant , was introduced by M. Planck to explain the laws of thermal radiation. From the theory of the photoelectric effect, the same (up to experimental errors) value is obtained, which greatly strengthened the position of quantum theory.
Is it possible to change the work function in another way, besides the photo effect? The answer is directed by itself - it is necessary to somehow force electrons to leave the substance in a different way, without ordinary light. The most obvious is heating the category, causing electrons to “evaporate” from its surface. This phenomenon is called thermionic emission and is used in electron tubes - diodes, triodes, etc. The process of electron emission is actually very similar to evaporation - only the fastest electrons, the energy of which exceeds the work function, can escape.
For most metals, the work function is on the order of several electrons. Is it a lot or a little? Let us estimate the average energy of thermal motion of electrons using the formula that was obtained for a monoatomic gas:
At room temperature ( K) this value is several hundredths of an electron voltaica, i.e. hundreds of times less than the work function. This means that the number of electrons that leave the metal due to thermal motion is very small at room temperature (this phenomenon can be ignored when studying the photoelectric effect). For the “evaporation” of electrons to become a noticeable process, the category must be heated to several thousand degrees.
By studying the dependence of the number of emitted electrons on temperature, the work function can be calculated. The work function values determined in this way are in good agreement with the predictions of the theory of the external photoelectric effect, which is an important independent confirmation of the correctness of its main provisions.
A. I. Chernovchan
List of readers who sent correct solutions
(For the beginning, see p. 29)
(Diepropetrovsk) 17; M. Dorozhova (Chernovogovka village, Moscow region) 05–07.13; A. Dunaevsky (Moscow) 07.13; V. Dyadechko (Vinnitsa) 03-05,13-17; A. Efimchik (Minsk) 03; A. Zhuk (Rivne) 03—07.17; V. Zhuravlev (Donetsk) 13; V. Zavadsky (Minsk) 05.12-17; V. Zaitsev (Borisoglebsk) 05; A. Zalesky (Kharkov) 05—07.17; F. Zanin (Starey Oskol) 03.05; S. Zelensky (Semipalatinsk) 17; D. Zolotagev (Kharkov) 03.13; K. Zuev (Vologda) 03.05.13; A. Zuevov (Donetsk) 05; M. Iznatev (Slavyansk) 13; I. Iopne (Moscow) 03; S. Kazenas (Almaty) 03-07.13; V. Kamchaty (Kyiv) 03—07.17; M. Kapustin (Lviv) 05—07,13,17; A. Karpenko (Brest) 05; S. Kasamyan (Oktemberyan district of Armenian SSR) 05; D. Kirillov (Kaliningrad) 05; A. Kozhevnikov (Kaluga) 03-05.13; V. Kokolin (Sobino settlement, Komi Autonomous Soviet Socialist Republic) 13; M. Kolpakov (village of Pochet, Krasnoyarsk region) 03–05.13; D. Komisarenko (Vinnitsa) 03.13-17; A. Komnik (Starey Oskol) 03-07; V. Kondratev (Kaliningrad) 05; S. Korenov (Vasilkov) 13; A. Korshkov (Mozyr)
(For the end see page 79)