# The vacuum

What is a vacuum? This question is usually answered: "the space with thin air", or "the space within the vessel, from which the air is exhausted". What is the degree of vacuum is a vacuum? Do vacuum in communication with the atmospheric pressure?

Some background to the empirical study of a vacuum existed in the antiquity. Ancient greek scientists-mechanics created a variety of technical devices based on air vacuum. For example, the water pumps have been known in the days of Aristotle. An empirical study of a vacuum began in the 17th century, with the Renaissance and the beginning of the end of the scientific revolution of modern times. At this point, it has long been known that the suction pump can raise the water to a height of 10 meters.

In practice, highly rarefied gas called technical vacuum. In the macroscopic volumes perfect vacuum unreachable because at finite temperature all materials have a non-zero density of the saturated vapor. Furthermore, many materials (including thick metal, glass) are passed gases. In the microscopic volumes to achieve a perfect vacuum is possible in principle.

Technical vacuum called a gas in the vessel or conduit with a pressure lower than in the surrounding atmosphere. Usually between the atmospheric air and the high vacuum pump is a so-called backing pump. It creates pre-vacuum. Therefore, a partial vacuum is often called forevacuum. On further lowering of the pressure in the chamber increases the mean free path of gas molecules. Thus much more gas molecules collide with the walls than with each other. In this case we speak of a high vacuum. High vacuum in microscopic pores of some crystals is achieved at atmospheric pressure, since the diameter of the pores is much smaller than the mean free path of molecules.

Outer space has very low density and the pressure, and is a close approximation of the physical vacuum. But the vacuum of space is not really perfect. In interstellar space has several hydrogen atoms per cubic centimeter.

Assume that the air in the cylinder is evacuated to 10000 times in comparison to its density at normal atmospheric pressure, i.e. the pressure inside the container is equal to 0,076 mm.hg.

Will the vacuum in the tank? Can we continue to believe that there is a vacuum in the tank if the tank is raised to a height of 100 km above the ground, where the air pressure is only 0,007 mm. hg. Indeed, in this case, the density of air inside the container becomes 10 times larger than the outside! Then, where is to be vacuum - inside or outside the balloon?

Modern physics connects the vacuum with the mean free path of the gas molecules inside the vessel. The gas molecules are in constant random thermal motion. At room temperature the thermal velocity of air molecules is approximately 450 m/s, i.e. approaching projectile velocity. Moving in all directions, the molecules are constantly faced with each other. The denser the air, the more molecules per unit volume is and the more frequently encountered molecule.

If the air is thin out, the molecules will collide less frequently. On average, they will have to fly a longer path between two collisions, which is called the mean free path.

From a physical point of view of a vacuum - it is a vacuum in which the mean free path length is greater than the size of the vessel. If in the vessel is a vacuum, the molecular collisions are rare.

The vacuum is a good thermal insulator; heat transfer occurs in it only at the expense of thermal radiation. This property is used for thermal insulation in thermoses, consisting of tanks with double walls, the space between which is evacuated.