Vacuum

Vacuum, defined strictly, space that has all matter removed from it. It is impossible to create a perfect vacuum in the laboratory; no matter how advanced a vacuum system is, some molecules are always present in the vacuum area. Even remote regions of outer space have a small amount of gas. A vacuum can also be described as a region of space where the pressure is less than the normal atmospheric pressure of 760 mm (29.9 in) of mercury.

To create a vacuum, matter is removed by pumping air out of the vacuum chamber (see Vacuum Technology). As air is removed, fewer air molecules are present that can push on the vacuum chamber walls, so the pressure within the vacuum chamber is reduced. In scientific laboratories, vacuum chambers must be very strong, because when the pressure is very low within the vacuum chamber, the atmospheric pressure outside can crush the apparatus. Scientists can create vacuums that have a pressure as low as 10^-14 mm of mercury. Even at this very low pressure, in 1 cc (0.64 cu in) of gas there are still thousands of molecules. At normal atmospheric pressure, however, there are about 10²³ molecules in 1 cc of air.

Vacuums have many practical uses. A vacuum cleaner creates a vacuum within the body of the machine. Atmospheric pressure pushes air through the vacuum hose and into the body of the machine; dirt and other material is brought in with the air. As another example, a light bulb has a vacuum inside to prevent air molecules from reacting with the hot filament and burning the filament out. Also, since a vacuum provides excellent insulation against heat flow, thermos bottles have a vacuum between their inner and outer walls.