Gas has no fixed shape and volume (indicating that gas molecules can reach all corners of the container) because the thermal motion of gas molecules is particularly irregular and disorderly compared with the thermal motion of solid and liquid molecules. It is this thermal movement that makes a large number of gas molecules inevitably collide with the wall. As far as a single gas molecule is concerned, the impulse on the device wall is very small, the action time is very short, and the impulse is intermittent. However, a large number of gas molecules frequently collide with the umbrella wall, which forms a continuous and uniform pressure on the umbrella wall from a macro perspective, just as a large number of dense raindrops hit the umbrella one after another, which will produce a continuous and uniform pressure on the umbrella surface. The continuous and uniform impulse of a large number of gas molecules to the unit area of the wall is the pressure of the gas.
Therefore, the pressure of gas is not caused by the gravity of gas, or there is no difference between the pressure of gas in weightlessness and that in normal state, because the thermal motion of molecules is not affected by macroscopic mechanical motion.
According to the formation mechanism of gas pressure, gas pressure should be related to the density of gas molecules and the intensity of collision: the greater the density, the more frequent collisions, and the more continuous and uniform pressure can be formed; The greater the intensity of the collision, the greater the impulse and the greater the pressure. Among them, the density of gas molecules can be described by the number of molecules per unit volume, which can be reflected by density; The severity of collision can be described by the average velocity of gas molecules, and the average velocity can be reflected by temperature. Therefore, the gas pressure is determined by two factors:
(1) Gas density When other quantities are constant, the greater the gas density, the greater the gas pressure.
(2) The higher the gas temperature, the greater the gas pressure, with other variables unchanged.
The laws of gas experiment, such as Boyle's law and Charlie's law, all embody this law.
According to the different thermal properties of the atmosphere in the vertical direction, the atmosphere can generally be divided into five layers according to the height above the ground: (1) troposphere (2) stratosphere (3) mesosphere (4) thermosphere (5) outer layer.
Among them, the troposphere is a layer close to the ground, and its maximum thickness does not exceed 18km (its thickness varies with latitude), and 3/4 of the whole atmospheric mass and almost all water vapor and impurities are concentrated in this layer. This layer has a particularly close relationship with human beings. In non-disciplinary narrative, the atmosphere is often called this layer. Because the troposphere is closest to the ground relative to other layers, most of the heat in this layer comes directly from the ground. On the contrary, the higher the ground is, the less the atmosphere is heated and the lower the temperature is. To sum up the characteristics of tropospheric temperature in one sentence is that the temperature decreases with the increase of height. Therefore, in the troposphere, because the gas density and temperature decrease with the increase of altitude, it can be concluded that the higher the ground, the smaller the atmospheric pressure.
With the increasing altitude, when the altitude from the ground is more than 18km, but not more than 50 ~ 50~55km, it will enter the stratosphere. The temperature in the lower stratosphere does not change much with height, while the temperature in the upper stratosphere increases rapidly with height (this is because ozone in the stratosphere absorbs a lot of solar ultraviolet energy, which is the main reason for the temperature increase in the upper stratosphere). It is difficult to know the relationship between atmospheric pressure and altitude, because on the one hand, the atmospheric density decreases, on the other hand, the atmospheric temperature increases (and it rises rapidly), and it is necessary to know the specific data of the atmospheric density and temperature changes in this layer in order to draw a conclusion.
As for increasing the height, it will enter the middle layer, the thermal layer and the outer layer in turn. There are some similar complexities, so I won't go into details.
In view of the above analysis, the author doesn't quite agree with some people's statement that "the discussion of the old textbooks is more accurate and rigorous on the knowledge point that atmospheric pressure changes with height". On the contrary, the author thinks that the discussion in the new textbook is more reasonable, although not perfect. The author speculates that the reason why the new textbook revised the statement is that the current middle school textbook reform pays more and more attention to the standardization of the subject system, rather than blindly pursuing "visualization" and "popularization".