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Why
do we need to learn about a vacuum?Vacuum technology is widely used in a
variety of industries. Here are some applications you would have certainly
heard about:
I) An early application of
vacuum technology came around 1900 when the first major industrial use was
for light bulbs and TV tube production (later on). It has been shown that
filaments emit electrons under vacuum which is the major property used in
television technology.
II) The second major application is in
the electronic industry. Many processes that occur in a semiconductor
fabrication facility require vacuums of different levels, including the
deposition of thin films of material on computer chips.
III) Another major
application is in space technology. The main issue in space technology is
how to design the space station or shuttle in order to maintain a
pressurized cabin. Also, it is important to design safe space-suits to
protect astronauts during their missions in open space. These are examples of how vacuum
technology helps us. Now we will proceed to learn about vaccum and how it
is created and measured. In order to learn about vacuum, you will first
need to understand the concept of pressure.
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What is pressure?
Figure 1: Closed container with air
inside.
Imagine a closed container
with air inside (see Figure 1). Air, as a gas, is composed of molecules
that you can imagine as round elastic balls. Molecules move in straight
lines until they collide with neighboring molecules or the container wall.
Molecules of gas hitting the wall impose a force on the wall. The amount
of this impact force per area of the container inner walls is called
pressure. The mathematical definition of pressure can be written as:
where F is the force of
impact of molecules on the walls and A is the area of the walls. The unit
for pressure in the SI-system (International System of units) is the
Pa (Pascal) and it is named after the famous French scientist who
clarified principles related to pressure measurements. Strictly speaking,
the Pa is the only valid unit. However, there are some other
pressure units which are still in use: bar, atm, and
Torr. For example, the pressure at sea level is 101300 Pa at
standard temperature, and the most common name for this specific pressure
is an atmosphere or atm. The closest match to the atmosphere is 1
bar = 100000 Pa. The ratios between different units are
given in Table 1.
Table 1: Ratios between
different pressure units.
| Pressure unit |
Pa |
Bar |
Atm |
Torr |
| Pa |
1 |
0.00001 |
9.869*10-6 |
7.501*10-3 |
| Bar |
100000 |
1 |
9.869*10-1 |
7.501*102 |
| Atm |
101325 |
1.01325 |
1 |
760 |
| Torr |
113.22 |
0.001133 |
1.316*10-3 |
1 | |
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It is important to
understand that pressure defined above is a property of gases. In the case
of liquids the pressure at the certain position in liquid is created by
the weight of the fluid column above (remember that as you dive deeper
into a ocean, pressure is higher). The pressure imposed by the height of
water is called the hydrostatic pressure and is directly proportional to
the height of the fluid above and its density. The mathematical definition
of hydrostatic pressure is:
where h is the height of the
fluid,  is fluid density and g is
gravitational acceleration.
What
is vacuum?
Imagine a closed container
with nothing inside (see Figure 3). The space of the container with
nothing in it is called a vacuum. The word comes from the Latin word
vacuus which means empty. Vacuum is the most prevalent state in the
Universe, and on the average, most space qualifies as a very good vacuum.
However, around the objects with sufficient gravity one can find a trapped
gas mixture, and thus, the enormous variation of pressure exists.
Therefore, vacuum is usually divided into ranges with somewhat arbitrary
cutoffs.
The change of pressure from
the surface of the Earth to high altitudes is illustrated in Figure 3:
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Figure 3: The change of
pressure with height.
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