Vacuum is a space that contains little matter. A vacuum holds far fewer molecules than does the same volume of air at atmospheric pressure. One cubic centimeter of air at atmospheric pressure and room temperature has roughly 25 billion billion molecules. The best artificial vacuums contain less than 1,000 molecules per cubic centimeter. Interstellar space, the space between the stars, has only about one atom per cubic centimeter.
Scientists measure the quality of a vacuum in terms of the pressure exerted by the matter it contains. The less matter, the lower the pressure. The international standard unit of pressure is the pascal, which equals approximately one hundred thousandth of atmospheric pressure at sea level. That is, 101,325 pascals equal 1 atmosphere. Pressure can also be measured in torr. One atmosphere equals 760 torr.
Properties
of a vacuum depend on the density of molecules it contains, and thus on the pressure. In a relatively high-pressure vacuum, the molecules are more densely packed and so collide with one another frequently. Such vacuums behave in much the same way that liquids or gases do at atmospheric pressures. Scientists describe such vacuums as having “viscous flow.” In a low-pressure vacuum, molecules are less densely packed, colliding with the walls of their container more often than with one another. Such vacuums are said to have “molecular flow,” with each molecule behaving independently.
Because of the low density of molecules, particles can move undisturbed for longer distances in a vacuum than they can through air. Some televisions have picture tubes that make use of this property. Within the tubes, electrons travel through a vacuum toward the screen. If the tubes contained air, the electrons would scatter, distorting the image.
In a vacuum, water and other liquids evaporate at temperatures much lower than normal. Thus, people can use a vacuum to remove moisture from a substance quickly without burning it. Workers prepare freeze-dried food in this way.
Because vacuums hold relatively few molecules, they provide clean environments for manufacturing. To help avoid contamination, they are often used in the making of such sensitive electronic devices as computer chips.
Producing a vacuum.
People create artificial vacuums using seals that prevent molecules from flowing into a vacuum region. Without such boundaries, the random motions of molecules would equalize the pressure between neighboring regions, preventing the formation of a vacuum. Engineers have developed vacuum chambers made from stainless steel or other materials and sealed with metal or rubber gaskets. Such chambers are used in scientific research and in manufacturing.
The pressure of a vacuum is typically lowered by either displacement or entrapment. In displacement, also called transfer, a pump or other means removes molecules from the vacuum. A simple example of displacement is sucking on a straw. The sucking action removes molecules from the straw, lowering the pressure inside. The higher pressure of air outside pushes fluids up the straw and into the mouth. Entrapment involves trapping molecules in an isolated region of a volume. One type of entrapment pump, called a cryogenic pump or cryopump, traps gases by freezing them onto cold surfaces.
In particle physics.
Scientists have discovered that no space can ever truly be empty. This knowledge comes from quantum mechanics, a branch of physics that describes the behavior of atoms and subatomic particles (pieces of matter smaller than atoms). According to the principles of quantum mechanics, particles can appear from nowhere as long as they disappear again in a sufficiently short time. Such particles are known as virtual particles.
The presence of virtual particles gives even empty space some energy, called vacuum energy. Scientists have observed the influence of vacuum energy through an occurrence known as the Casimir effect, named for the Dutch physicist Hendrik Brugt Gerhard Casimir. The Casimir effect occurs when two parallel metal plates are separated by an extremely short distance in a vacuum. The space between the plates has a lower density of virtual particles than does the space outside the plates. This difference in density results in a pressure on the plates, pushing them together.
