Existence of Gas pressure based
on the kinetic theory
 gas molecules move freely and randomly.
 The gas molecules collide with one another and
also collide with the walls of their container.
 The collision of gas molecules with one another is
an elastic collision
 The collision of gas molecules with the wall of the
container produces change of momentum or
impulsive force
 So the gas molecules exert a pressure on the
inside of the container because pressure is force
per unit area
(P=F)
A
………………………………………………………………………………………..
The conclusion about gas
pressure
 Gas pressure is the force per unit area exerted
by the gas molecules as they collide with the
walls of their container.
What happen if the volume inside
a container decreases?
 The rate of collisions between the gas
molecules with the walls of the container
increase
 Therefore the pressure of the gas increases
 ……..Volume decreases, pressure increases……..
( what law ??????)
What happen if the container is
heated?
 The velocity of the gas molecules
increase
 The kinetic energy of the gas molecules
increase
 The collision rate between the gas molecules
with the walls of container increase
 So the force of collisions will increase
 Therefore the pressure of the gas increases
…..Temperature increases, pressure increases…
( what law???????)
Measuring Gas Pressure
1.Manometer
manometer
P gas = P atmosfera + h
Pgas = Patmosfera - h
P gas = P atmosfera
Example 1
 Calculate the pressure of the gas supply in the units
 (i) cm Hg
 (ii) Pa
 [ Density of mercury = 1.36 x 104 kg m-3 and
Atmospheric pressure = 76 cm Hg ]
2. Bourdon Gauge
Bourdon gauge
The another use of U-Tube
 The U-tube can also be used to determine the density
of a liquid.
Pressure P1 = Pressure P2
h11g = h22g
h11 = h22
Example 2
The figure shows a U-tube
use to determine the density
of a liquid K. When liquid K
is poured into one arm, the
water level in the other arm
rises.
If the density of water is 1 000 kg
m-3, determine the density of
liquid K
.
Existance of Atmosheric
 The gas molecules in the air possess mass and
experience the pull of gravity.
 The result is that gases have weight.
 The weight of the gas molecules will produce force and
as a result will exert pressure on you because pressure
is force per area ( P = F )
A
 The pressure is called the
pressure.
atmospheric
Characteristics of Atmospheric
Pressure
 acts equally in all directions
 pressure on any object is not dependent on the surface
area of the object.
 influenced by the altitude.
(Hence as the altitude increases , the atmospheric
pressure decreases because the higher it is from the
surface of the Earth , the lower is the density of air.)
-The value of atmospheric pressure at sea level is
approximately
1 atm = 1x 105 Pa
= 76 cm Hg
= 10 m of water.
Example 3
 The atmospheric pressure is 76 cm Hg.
 Calculate the atmospheric pressure in
the units Pa.
[Density of mercury = 1.36 x104 kg m-3 ].
Measuring atmospheric pressure
 The Simple Fortin barometer
 The mercury column rises or falls
according to the pressure of air on the
mercury in the dish.
 The space above the mercury column is a
vacuum so it exerts no pressure on the
top of the mercury column.
 If the vertical height of the mercury is h
cm , therefore the atmospheric pressure
reading is “ h cm mercury ”.
BAROMETER
THINK !!!!!
What happen to a column
of mercury if a barometer
is taken to the top of a
mountain and why?
What happen to a column
Of mercury if a barometer
is taken to the bottom of
the sea and why?
Example 4
 The figure shows a mercury barometer is placed in a
school laboratory where the atmospheric pressure is
75 cm Hg.
 If the density of mercury is 1.36 x 104 kgm-3
and the density of water is 1 x 103 kgm-3,
determine
(i) the atmospheric pressure in the units Pa
(ii) the value of h if the mercury is replaced
by water.
(iii) the value of h if the barometer is
submerged in water at depth of 40.8 cm.
Aneroid Barometer
 When the atmospheric pressure decreases , the
container will expand.
 When the atmospheric pressure increases, the
container will constrict.
 The slight movement of the box is magnified
by a lever system which is connected to a
pointer.
 The Aneroid barometer can be used as an
altimeter by mountaineers or in an aeroplane
to determine its altitude.
BAROMETER ANEROID
BAROMETER ANEROID
Applications of Atmospheric
Pressure
 (i) Drinking straw
 (ii) Rubber sucker
 (iii) Syringe
THE END