Basic introduction Chinese name: polarity mbth: polarity pinyin: jíxìng criterion: generally defined according to the dielectric constant of a substance: the concept that an object shows its nature or strength, the polarity of a cell, the polarity of a valence bond, the polarity of a molecule, the criterion, the solubility, the melting point, the application, the geomagnetic polarity, the polarity of a transistor, and the concept 1: The object shows the opposite in the opposite position or direction. 2. Orientation or attraction to a specific thing (such as inclination, feeling or thought); A tendency or trend in a particular direction. 3. The specific positive and negative states of two poles or charged (such as objects). 4. In chemistry, polarity refers to the inhomogeneity of charge distribution in valence bonds or valence molecules. If the charge distribution is uneven, the bond or molecule is said to be polar; If uniform, it is called nonpolar. Some physical properties of a substance (such as solubility, melting point, etc.). ) are related to the polarity of molecules. Cell polarity refers to the phenomenon that cells, cell groups, tissues or individuals show along one direction, and the opposite ends of each part have some different morphological or physiological characteristics. Polar molecules are about morphological polarity, for example, in glandular epithelial cells, the nucleus is located near the base and the centrosome is located near the surface; In the mature eggs of amphibians, the nucleus is close to the animal pole, the surface pigment layer is distributed in the animal hemisphere, and the yolk particles are mostly in the plant hemisphere. Physiological and cytochemical polarity, such as redox energy, oxygen consumption, SH group, RNA concentration gradient, etc. In the morphological composition, polarity plays a more important role in the dynamic sense. For example, when the cutting body of the worm is regenerated, the head is regenerated from the section facing the original front end and the tail is regenerated from the section facing the original rear end. The separation blocks of hydromedusae often show front and back polarity, hydromedusae regenerates from the front end, and hydromedusae stems regenerate from the back end. The polarity of an egg is closely related to the morphological axis of the embryo it forms (see egg axis). Sometimes the polarity of a cell is influenced by the environment inside and outside the cell. For example, the polarity of fucus fuciformis eggs can be controlled by pH gradient, temperature gradient and light irradiation. There are many invertebrate eggs, and their polarity is determined by the position of the egg cell and the egg single wall when the egg is formed. The polarity of valence bond * * * The polarity of valence bond is caused by the difference in electronegativity of two bonding atoms. Atoms with high electronegativity will "pull" the * * electron pair to their own side, making the charge distribution uneven. This forms a set of dipoles, and such bonds are polar bonds. The atom with high electronegativity is a negative dipole, which is denoted as δ-; The atom with low electronegativity is a positive dipole, denoted as δ+. The polarity degree of triangular boron trifluoride molecular bond can be measured by the difference of electronegativity between two atoms. The difference between 0.4 and 1.7 is a typical polar valence bond. When two atoms are exactly the same (of course, the electronegativity is also exactly the same), the difference is 0, and then the atoms form a nonpolar bond. On the contrary, if the difference exceeds 1.7, the bond between these two atoms is mainly ionic. Molecular polarity The molecule with valence of * * * is polar, indicating that the charge distribution in this molecule is uneven, or the positive and negative charge centers are not coincident. The polarity of a molecule depends on the polarity of each bond in the molecule and their arrangement. In most cases, polar molecules contain polar bonds, and nonpolar molecules contain nonpolar bonds or polar bonds. Water is a polar compound. However, all nonpolar molecules can be composed of polar bonds. As long as the molecule is highly symmetrical, the positive and negative charge centers of each polar bond are concentrated in the geometric center of the molecule, thus eliminating the polarity of the molecule. This molecule is usually linear, triangular or tetrahedral. Influence of molecular polarity on properties: There is no recognized and accurate quantitative standard to judge molecular polarity, which is usually judged according to the dielectric constant of substances (especially liquids and solids). For some simple molecules, whether they are polar or not can also be judged according to their own structures (for example, carbon dioxide is a linear molecule and a nonpolar compound, but sulfur dioxide is a polar molecule because its molecular structure is V-shaped). The polarity of soluble molecules has a great influence on the solubility of substances. Polar molecules are easily soluble in polar solvents, while nonpolar molecules are easily soluble in nonpolar solvents, that is, "nearly miscible". Polar molecules such as sucrose, ammonia and sodium chloride plasma compounds are easily soluble in water. Organic compounds with long carbon chains, such as petroleum and petroleum, are mostly insoluble in water, but soluble in nonpolar organic solvents. At the same molecular weight with melting point and boiling point, polar molecules have higher boiling points than nonpolar molecules. This is because the orientation force between polar molecules is greater than the dispersion force between nonpolar molecules. It can be used for column chromatographic analysis and crystallization separation of substances by using the usual molecular polarity. Generally speaking, the order of common solvent polarity (from small to large) is petroleum ether, cyclohexane, carbon tetrachloride, benzene, toluene, dichloroethane, dichloromethane, trichloroethylene, diphenyl ether, chloroform, n-butyl ether, ether, DME, nitrobenzene, dioxane, trioctylamine, tetrahydrofuran, ethyl acetate, etc. Glycerol (glycerol), acetonitrile, DMF, methanol, hexamethylphosphoramide, formic acid, DMSO, trifluoroacetic acid, formamide, water, trifluoromethanesulfonic acid, anhydrous sulfuric acid, anhydrous perchloric acid and anhydrous hydrofluoric acid. Among them, strong acids such as trifluoroacetic acid, trifluoromethanesulfonic acid, anhydrous sulfuric acid, anhydrous perchloric acid and anhydrous hydrofluoric acid are not widely used in general experiments because of their strong corrosiveness. Here, only the polarities of substances are listed for comparison. Organic solvents commonly used in column chromatography are petroleum ether, cyclohexane, dichloromethane, trichloroethylene, ether, DME, dioxane, tetrahydrofuran, ethyl acetate, acetone, ethanol, acetic acid and methanol. In the crystallization separation of substances, solvents with different polarities are usually added to the solution to crystallize the desired substances. The most common method is to add ethanol to the synthesis of molar salt and blue alum to precipitate them (both of them are insoluble in ethanol). As for the recrystallization of organic matter, there are countless examples (for example, when caffeine is recrystallized, water is added to its ethanol solution to precipitate it. Geomagnetic polarity Geomagnetic polarity is a positive period at present. According to the existing knowledge, the height of Mount Everest will definitely decrease after hundreds of thousands of years. It can be seen from the records of the history of geomagnetic polarity reversal that the positive period of geomagnetic polarity rarely exceeds 6.5438+0 million years. At present, the geomagnetic positive cycle has been maintained for 780 thousand years, so the geomagnetic polarity should be reversed to the "reversal period" for more than 200 thousand years at most. This means that the height of Mount Everest should be reduced. Birds and compasses can distinguish between north and south because the earth's magnetic field is like a huge magnetic bar, and the magnetic lines at the two poles are very close to the earth's axis of rotation, which is a simple physical common sense. Little known is that in recent 150 years, the magnetic field generated by geomagnetic dipole has been declining sharply. If this development continues, the geomagnetic field will disappear completely sometime in the next Millennium. If the geomagnetic umbrella is seriously damaged, high-energy cosmic particles and solar particles will destroy satellites, and things closely related to human beings will be fatally irradiated. Fortunately, the disappearance of geomagnetic dipole is temporary, and it is a phenomenon that the magnetic pole gradually deflects to the south, which eventually leads to the compass pointing to the south pole instead of the north pole. The magnetic minerals in ancient rocks show that there have been hundreds of so-called geomagnetic polarity reversal events on the earth in the past 500 million years, but there is no way to determine the specific time of such events, so it is impossible to predict the occurrence of geomagnetic polarity reversal. Most geophysicists accept the assumption that there is a layer of molten iron flowing in the core with a thickness of 2200 kilometers, which produces the basic magnetic field of the earth. But it was not until about six years ago that scholars wrote complex computer programs to simulate the movement of the earth's core and its magnetic effects. At present, some programs can not only simulate the movement of the earth's core, but also simulate the geomagnetic polarity reversal, and some programs can be completed in only 1200 years-this is only a blink of an eye in geological times. Others look for clues from the real world about why geomagnetic polarity reversal occurs. At the beginning of 2002, Gauthier Hulot of the Institute of Geophysics in Paris and his colleagues tracked the changes of magnetic field behavior near the surface of the core through satellite measurements. They found that in the depths of the earth's surface at the southern tip of Africa, there is a small area where the geomagnetic field lines point to the center of the earth, and the mainstream geomagnetic field in this area points to the ground. There are many similar abnormal debris areas of geomagnetic lines near the North Pole. Hulot's team believes that the increase of debris area in geomagnetic inversion can explain the current attenuation of geomagnetic field. The rotation direction of the iron core in this region is opposite to that of the core mainstream, and in some computer simulations, the diffusion of this debris region will lead to the overall inversion of the geomagnetic field. As for what happens when the local magnetic field suddenly reverses, the new geophysical sci-fi horror novel "The End of the Earth" gives such a scene: birds lose their way and humans live in frequent radiation warnings. In the movie of the same name, countries all over the world jointly built a manned probe ship, which can pass through the solid rocks in the 2900-kilometer-thick mantle layer and withstand the heat of the earth's core-the temperature here can almost match the surface temperature of the sun. The mission of the probe ship is to detonate the nuclear bomb, so as to restore the natural flow of iron atoms in the center of the earth and counter the trend of geomagnetic field inversion. The existing technology is far from this jules verne-style solution, so scientists put forward other assurances: the weakening of geomagnetic dipole does not necessarily mean that the geomagnetic field will be reversed immediately. Of the 10000 natural fluctuations of the geomagnetic field, only occasionally does the geomagnetic polarity completely reverse. The recent computer simulation also shows that when the mainstream dipole geomagnetic field weakens, the magnetic field around the earth, which accounts for 10% of the whole geomagnetic field, will increase. Transistor polarity transistor polarity refers to its classification or pin polarity. According to the classification, for example, triodes are divided into silicon triodes and germanium triodes, as well as PNP and NPN. Pin polarity, for example, the triode refers to PNP or NPN type, and three corresponding electrodes are led out from the emitter (E), the base (B) and the collector (C). LED pin polarity (figure 1)