Vacuum is a pressure lower than atmospheric. Except in outer space, vacuums occur only in closed systems. In the simplest terms, any reduction in atmospheric pressure in a closed system may be called a partial vacuum. In effect, vacuum is the pressure differential produced by evacuating air from the system. To illustrate the basic concept of using the atmosphere to create a vacuum,
A vacuum pump would be used to reduce atmospheric pressure in the closed space. The same principle would apply, however. By removing air from one side of an air-tight barrier of some sort, atmospheric pressure can act against the other side. Just as with the suction cup, this action creates a pressure differential between the closed system and the open atmosphere. The pressure differential can be used to do work. For example, in liquid packaging (bottling), reducing the pressure in a bottle (the enclosed space) makes the filling operation go much faster because the liquid or other material is literally pulled into the bottle, rather than simply failing by gravity
