Vacuum. How a vacuum works

Vacuum is a space that has no matter in it However, there is no such thing as a complete vacuum because no one has ever been able to remove all the air molecules in a space. A vacuum may also be described in terms of the pressure of the air or another gas that remains in a partially evacuated container. In this sense, a vacuum is any enclosed space in which pressure is less than normal atmospheric pressure. Normal atmospheric pressure is the pressure of the atmosphere at sea level— 14.696 pounds per square inch (101.325 kilopascals).

Scientists speak of high or low vacuums, depending on how much gas has been removed from a container. The fewer the number of gas molecules in a container, the less the pressure is. The highest vacuum measured so far, approximately 0.0000000001 pascal, is only about one-quadrillionth of normal atmospheric pressure. Even at this extremely low pressure, 1 cubic inch (16 cubic centimeters) of gas contains about 540,000 molecules. In contrast, 1 cubic inch of air at normal atmospheric pressure contains about 410 billion billion gas molecules.

How a vacuum works is shown in these illustrations of a meat baster. When the bulb is squeezed, left, air is forced from the tube. When the bulb is released, right, the greater air pressure outside pushes liquid into the tube to fill the vacuum

Various kinds of pumps are used to produce different degrees of vacuums. For example, a diffusion pump is used to attain high vacuums. This device sprays streams of vapor that sweep gas molecules out of the enclosed space. Lower vacuums can be produced by means of mechanical pumps equipped with rotors and valves.

Vacuums have many practical uses. Gases and liquids tend to flow from areas of high pressure to those of lower pressure-that is, into a vacuum. Drinking liquid through a straw involves this principle. Sucking on the straw produces a partial vacuum inside the mouth and in the top part of the straw. The greater pressure of the air outside pushes the liquid up the straw. A vacuum cleaner operates on the same basic principle. The cleaner has a fan that produces a vacuum inside of it Air outside the machine rushes in to fill the vacuum, carrying with it particles of dust and dirt.

A vacuum conducts heat poorly, and so it is an effective insulator. A vacuum bottle used to keep liquids hot or cold consists of a double-walled container with a vacuum in the space between the walls. A similar type of bottle called a Dewar flask is used in laboratories to store extremely cold liquefied gases.

In a vacuum, water and many other liquids evaporate rapidly at a temperature much lower than their normal boiling point For this reason, vacuum chambers are used in drying operations where it is essential to remove moisture from a substance quickly without burning it Such operations include sugar refining and the freeze-drying of food.

Many kinds of electronic devices require a vacuum. The picture tube of a television set and the visual display of a computer system are familiar examples of vacuum tubes. The vacuum in such a tube allows beams of electrons to travel directly to the screen, where they form a picture. If too many air molecules were present in the tube, the electrons would collide with the molecules and scatter, destroying the picture.

Other kinds of devices, such as those employed in industry and scientific research, use vacuum tubes or chambers for a similar reason. For example, cyclotrons, synchrotrons, and other particle accelerators used to increase the energy level of atomic particles require extremely high vacuums.