14 / 218
The air
Everyentityhavingmassandspacedimensionsisdefinedmatterandismadeupofminuteparticlescalled
“molecules”
.
Matter exists in the following forms:
•
solid
, themolecules are rigidly bound, consequently the solids take on their own shape and volume;
•
liquid
, themolecules are not rigidly bound together, they possess volume and assume the shape of the container
which holds them;
•
gaseous
, themoleculesmove freely - to the point that the distance between them varies continuously, as does
their relative positions. Gases therefore have neither shape nor definitive volume.
In this section we focus on a characteristic of gas:
compression
, as a demonstration we use the example of the
bicycle pump.
Figure 1
Pos. 1:
through the hole placed at the end of the pump, the outside air is
drawn into
the cylinder (chamber) as a
result of the piston and, its volume and its shape coincidewith the size of the container, i.e. the chamber.
Pos. 2:
by closing the hole in the pump and by exerting pressure on the piston, the air, which is unable to escape,
will be forced to occupy a diminished space. As the volume that the air occupies is reduced, the air is thereby
“compressed”
.
Pos. 3:
by further increasing the pressure on the piston, the volume occupied by air decreases further, although
the number of air molecules remains constant. As themolecules are subjected to compression they are forced to
occupy ever-decreasing spaces.
Assume that the total number ofmolecules contained in thepump is900and that thepump chamber has a volume
of 150
cm³
; calculate the number of molecules present for each
cm³
:
n° of molecules / cm
3
=
tot. n° of molecules
= 900 =
6
volume
150
Reducing the volumetric space, the air is compressed and the number of molecules per
cm³
increases.
Lowering the volume from150 to100
cm³
and subsequently to60
cm³
, calculate thenumber ofmolecules present:
n° of molecules / cm
3
=
tot. n° of molecules
= 900 =
9
volume
100
n° of molecules / cm
3
=
tot. n° of molecules
= 900 =
15
volume
60
In conclusion: as the air is compressed and the number ofmolecules remains unchanged, the number of
molecules
per
cm³
increases.
Figure 1
Pos. 4:
compression has the property of influencing the temperature of the gas; the gasmoleculesmutually collide
with each other andwith thewalls of the container, by decreasing the volume, the speed and number of collisions
increases causing
the increase of temperature
.
1
2
3
4
Free air
Fig. 1
1
12
CAMOZZI
>
PHYSICS