The document discusses atmospheric pressure and how it is caused by the weight of air above the Earth's surface. It explains that atmospheric pressure decreases with increasing altitude as there is less air above. Instruments like mercury barometers, aneroid barometers, and manometers can be used to measure atmospheric pressure. Gas pressure inside a container is also explained using kinetic molecular theory, where gas molecule collisions with the container walls exert pressure.

1. Atmospheric PressureAtmospheric Pressure

2. Atmospheric PressureAtmospheric Pressure
Earth Atmosphere
Surface of
the Earth
Atmospheric pressure exerted on the
surface of the Earth (or at the sea
level) as well as objects on Earth.

3. Atmospheric PressureAtmospheric Pressure
 The gasThe gas
molecules havemolecules have
weight.weight.
 AtmosphericAtmospheric
Pressure isPressure is
caused by thecaused by the
weight (force) ofweight (force) of
the thick layer ofthe thick layer of
air above theair above the
earth’s surface.earth’s surface.

4. HOW does a human
body withstand the
pressure on its surface?
Living cells maintain an
internal pressure that is
about the same as 1 atm,
which is similar to the
atmospheric pressure. So,
we don’t feel high
atmospheric pressure
exerted to us.

5. Surface of bottle
Internal pressure
External
pressure
External
pressure
Internal
pressure
Internal Pressure = External pressure

6. Atmospheric Pressure acts equally in allAtmospheric Pressure acts equally in all
direction.direction.

7. Characteristics of AtmosphericCharacteristics of Atmospheric
PressurePressure
 It is not affected by the surface area.It is not affected by the surface area.
 The pressure exerted by the air molecules at
sea level is 1.013 × 105
Pa.
 At sea level, PAt sea level, Patmatm == 1.013 × 105
Pa (Nm-2
)
= 76 cmHg= 76 cmHg
= 10mH= 10mH22OO

8. Atmospheric Pressure decreases with altitudeAtmospheric Pressure decreases with altitude
because air is gets thinner as altitude increases.because air is gets thinner as altitude increases.

10. Peter goes to the top of mountain
and feels breathing difficulty. The
number of air molecules
_________. The collision rate of air
molecules _________ and he
encounters ______ atmospheric
pressure. So, he has to breath
_____ when he goes up to
mountain ( _____ altitude).
John enters to the underground
mines and feels breathing
difficulty. The number of air
molecules _________. The
collision rate of air molecules
_________ and he encounters
______ atmospheric pressure. So,
he has to breath _____ when he
goes down mines ( ____ altitude).
decreases
decreases
lower
more
high
increases
increases
higher
less
low

11. Existence of AtmosphericExistence of Atmospheric
PressurePressure

12. At normal condition, the
rate of collision on inner
wall is same with on outer
wall. So, the air pressure
inside and outside of the
bottle is the same.
After the air has been pumped out,
there are almost no more air
molecule inside the bottle to
balance the force of collision of air
molecule outside the bottle. The
external force due to atmospheric
pressure crashes the bottle.

13. Existence of AtmosphericExistence of Atmospheric
PressurePressure

14. Glass
filled by
water
Atmospheric
pressure
Cardboard
Why the cardboard
does not fall from the
mouth of the inverted
glass filled with water?
The atmospheric pressure
that presses the
cardboard against the
glass produce a force that
is strong enough to
support the weight of the
water in the glass.

15. Existence of AtmosphericExistence of Atmospheric
PressurePressure
syringesyringe BarometerBarometer

16. Application of
atmospheric pressure
Pouring the condensed
milk from its can
A rubber suction cap
on a smooth surface
Drinking with a straw
Removing dust with
a vacuum cleaner
Putting plastic sticker
onto the inner surface
windscreen
Measuring blood
pressure

17. The presence of a
second hole is to
enable air flow into
the can. So, the air
pressure inside the
can same as
atmospheric
pressure. It will
force the milk out of
the lower hole.
Pouring the condensed milk from its can
Milk
Air flows
into the can
Atmospheric
pressure

18. A rubber suction cap on a smooth surface
When air is forced
out of the suction
cup, a partial
vacuum is created
in the space
between the cup
and the smooth
surface. The
surrounding
atmospheric
pressure forces
the cup tightly
against the
smooth surface.

19. Removing dust with
a vacuum cleaner
When the vacuum cleaner
switches on, the motor will work
and the fan blades turn, force the
air forward towards the exhaust
port.
The pressure level in the area
behind the fan drops below the
pressure level outside the vacuum
cleaner.
This creates partial vacuum inside
the vacuum cleaner.
The dust will flows into the
vacuum cleaner through the
intake port because the air
pressure inside the vacuum
cleaner is lower than the pressure
outside.

20. Drinking with a straw
Straw
Atmospheric
pressure
forces the
drink into
the straw
Air being
sucked up
and
creating a
partial
vacuum in
the straw
When the air is being
sucked from the straw,
a partial vacuum is
created in the straw.
The surrounding
atmospheric pressure
will force the drink into
the straw and enable it
to be moved into the
mouth.

21. Putting plastic sticker
onto the inner surface
windscreen
When the plastic is placed
on the glass surface, air is
being forced out of the
space between the sticker
and the glass surface,
creating a partial vacuum.
The surrounding
atmospheric pressure will
hold the sticker tightly on
the windscreen.
Windscreen
Sticker
Pressure
Air is being
force out

22. Instruments for measuringInstruments for measuring
Atmospheric PressureAtmospheric Pressure
Mercury barometerMercury barometer
Fortin barometerFortin barometer
Aneroid barometerAneroid barometer

23. Measuring atmospheric pressure
Simple Barometer
Made of a glass tube of about
100 cm filled with liquid
(normally mercury is used).
For mercury, the liquid level will
drop as it flows into a bowl to
until a vertical height of above
76 cm from the surface of
mercury in the bowl.
ρHg = 1.36 × 104
kg m-3
g = 9.8 N kg-1
h = 76 cm = 0.76 m
P = 1.36 × 104
× 9.8 × 0.76
= 1.103 × 105
Pa

24. Question
Liquid A has been filled into
the barometer to measure the
atmospheric pressure at sea
level. According to the figure,
find the density of the liquid A.
Solution
As we know, atmospheric
pressure, P is equal to
1.013 × 105
Pa at sea level.
So, P = 1.013 × 105
= hρg
1.013 × 105
= 1.10 × ρA × 10
ρA = 9209 kg m-3

25. Fortin BarometerFortin Barometer
 More accurateMore accurate
 unit mmHgunit mmHg

26. Fortin barometers have to be
set before each reading is
taken.
Using the screw at the bottom
adjust the mercury level, seen
in the glass reservoir, until the
surface just touches the tip of
the pointer. The barometer is
then ready for a reading to be
taken.

27. Aneroid BarometerAneroid Barometer
 More handy and mobileMore handy and mobile

28. Aneroid BarometerAneroid Barometer
• the partially
evacuated chamber
expands and
contracts in response
to changes in
atmospheric pressure

29. AneroidAneroid
BarometerBarometer
 Also an altimeterAlso an altimeter

30. Normally used by
pilot to determine the
atmospheric
pressure in sky.
Sometimes, it is also
used to measure the
height above the sea
level (altitude). So, it
called altimeter.
Aneroid BarometerAneroid Barometer

31. Gas PressureGas Pressure

32. Kinetic MolecularKinetic Molecular
TheoryTheory
Basic assumptions:Basic assumptions:
 A gas consists of a collection of small particles which are
moving rapidly and randomly in straight-line motion and
obeying Newton's Laws.
 Gas molecules are constantly colliding with one another and
the collisions is perfectly elastic (that is, no energy is gained
or lost during the collision).
 Average kinetic energy is equal to the temperature.Average kinetic energy is equal to the temperature.

33. Gas Molecule
Pressure in
gases due
to collision
of
molecules
with the
wall of a
container.

34. Gas PressureGas Pressure
 Gas molecules collide withGas molecules collide with
the wall of the container andthe wall of the container and
change velocity andchange velocity and
momentum,momentum,
 the rate of change ofthe rate of change of
momentum = Forcemomentum = Force
 The force on the wall of theThe force on the wall of the
container creates gascontainer creates gas
pressure.pressure.

35. Pressure
Rate of collisions
Number of
particles
Speed of
particles
Volume of
container

36. When number of particleWhen number of particle INCREASESINCREASES
Distance
between
particles
__________,
______ collisions
occurs, pressure
__________.
DECREASE
MORE
INCREASE

37. When volume of containerWhen volume of container INCREASEINCREASE
Particles have
______ space to
move around,
_____ collisions
occurs,
pressure
__________.
MORE
LESS
DECREASE

38. When temperature inside of containerWhen temperature inside of container
INCREASEINCREASE
P
P
P
P
P
P
Particles gain
______ energy
and move
________,
average speed
__________,
______ collisions
occurs, pressure
__________.
MORE
MORE
INCREASE
INCREASE
FASTER

39. Instruments for measuringInstruments for measuring
Gas pressureGas pressure
 ManometerManometer
 Bourdon gaugeBourdon gauge

40. Measuring gas pressureMeasuring gas pressure
Manometer
Manometer consists of a
U-tube that is filled with a
liquid, oil or mercury.
The figure shows a
manometer is not
connect of gas supply.
The atmospheric
pressure acts on both
surfaces of the liquid at
points A and B.
P0 – Atmospheric pressure

41. The figure shows one end of
manometer is connected to the
gas supply.
The gas would exert a pressure
on the liquid at point A.
If the pressure greater than
atmospheric pressure, liquid
level at point A will be forced
down.
Liquid in another end (point B)
will be forced up in equilibrium.
P = Patm + hρg

42. Question
A mercury manometer with one end attached
to a gas supply measures a difference in the
level of mercury of 32 cm as in figure.
Calculate the pressure of the gas supply in
(a) cmHg (b) Pascal
[ Patm = 76 cmHg; g = 10 Nkg-1
;
ρmercury = 1.36 × 104
kgm-3
]
Solution
(a) Pressure = Atmospheric pressure +
pressure due to mercury column
= 76 cmHg + 32 cmHg
= 108 cmHg
(b) Pressure of gas supply = hρg= 108 × 10-2
× 1.36 × 104
× 10
= 1.46 × 105
Pa

43. Question
A mercury manometer with one end attached
to a gas supply measures a difference in the
level of mercury of 10 cm as in figure.
Calculate the pressure of the gas supply in
(a) cmHg (b) Pascal
[ Patm = 76 cmHg; g = 10 Nkg-1
;
ρmercury = 1.36 × 104
kgm-3
]
Solution
(a) Pgas = PHg + Patm
= 10 cmHg + 76 cmHg
= 86 cmHg
(b) Pressure of gas supply = hρg
= 86 × 10-2
× 1.36 × 104
× 10
= 1.1696 × 105
Pa

44. Bourdon gaugeBourdon gauge
• More accurate
• Measures in unit Pascal

45. Bourdon gaugeBourdon gauge
 When gas supply is connected the pressureWhen gas supply is connected the pressure
in the gas acts to straighten the copper coilin the gas acts to straighten the copper coil
The movementThe movement
of the coil isof the coil is
transferred to thetransferred to the
lever systemlever system
which actuates awhich actuates a
pointer to movepointer to move
across a scaleacross a scale
which has beenwhich has been
calibrated to givecalibrated to give
readings ofreadings of
pressurpressur