What is Vacuum?

 In vacuum science, vacuum means the state of gas below one atmosphere of pressure in a given space. People usually refer to this rarefied gas state as a vacuum state. Compared with the atmospheric state in which human beings live, this specific vacuum state mainly has the following basic characteristics;

  The earth is surrounded by the atmosphere. The troposphere is within 12km of the earth's surface in the mid-latitudes. As the height increases, it is the stratosphere, the mesosphere, the thermosphere (ionosphere), the outer atmosphere, and the interstellar space. The standard atmosphere is a mixture of various gases whose main components are 78.084% nitrogen, 20.948%, 0.934% argon, and so on.

  Vacuum is relative to the atmosphere. If the pressure in a container is lower than the surrounding atmospheric pressure, then the gas state in the container is called a vacuum state. If the atmospheric pressure of a certain place is 10⁵pa, and the pressure in the closed container is 1pa, the container is in a vacuum state, and there are 2.7*10¹⁴ gas molecules per cubic centimeter. It can be seen that the vacuum space is not empty, but full of gas molecules. With various modern pumping methods, a vacuum degree of 10^-11 ~ 10^-13pa can be obtained. Even in such a thin gas state, there are still Dozens to hundreds of gas molecules.

1. The gas pressure in a vacuum state is lower than one atmospheric pressure. Therefore, in various vacuum containers on the surface of the earth, they will be affected by atmospheric pressure, and the magnitude of the pressure difference is determined by the pressure difference inside and outside the container. Since the atmospheric pressure acting on the earth's surface is about 10135N/m2, when the pressure in the container is very small, the atmospheric pressure on the container can reach atmospheric pressure.
   
2. Due to the thinness of the gas in a vacuum state, the number of gas molecules per unit volume, that is, the molecular density of the gas is less than that of the atmospheric pressure. Therefore, the number of collisions between molecules, between molecules and other particles (such as electrons, ions, etc.), and between molecules and various surfaces (such as walls) is relatively reduced, which increases the molecular free path of the gas.