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A vacuum is the absence of matter (molecules, atoms...) in a volume of space. Normally we think of vacuum cleaners, which are actually pumps that create a partial vacuum by moving air.

The partial vacuum can be measured in Pascal or Torr, or as a percentage of atmospheric pressure using the bar or barometer scale.

Degrees of vacuum:

Atmospheric pressure = 760 torr
vacuum cleaner = around 300 torr
mechanical vacuum pump = around 10 millitorr
near earth outer space = around 1 x 10-6 torr
pressure on the moon = around 1 x 10-8 torr
interstellar space = around 1 x 10-10 torr

Creating a vacuum:

When creating a vacuum, the matter in the volume being evacuated flows differenly at different pressures based on the laws of fluid mechanics. Initally a compression pump can be used to remove the material, as the molecules are interacting with each other and will push on their neighboring molecules. when the distance between the molecules increases, the molecules interact with the walls of the chamber more often than the other molecules, and compression pumping is no longer effective.

At this stage, we have entered a state called molecular flow, where the directionality of each mloecule is basicly random. Three basic ways to remove the remaining gas are by converting the molocules of gas to their solid phase (by freezing them, called cryopumping or cryotraping), converting them to solids by electricly combining them with other materials (ion pumping) or using a specialized machine called a turbopump.

The term vacuum is also in colloquial usage as an abbreviation of vacuum cleaner.