N.B:  
•
At sea level the atmospheric pressure supports approximately 76 cm of  
mercury column or approximately 10 m of water column  
•
This difference in height column between mercury and water is that “Mercury  
is much denser than water”  
Example:  
1. The air pressure at the base of a mountain is 75 cm of mercury while at the top  
is 60 cm of mercury. Given that the average density is 1.25 kg/m3 and density  
of mercury is 13 600 kg/m3. Calculate the height of the mountain.  
Solution:  
Density of air, 흆ퟏ = ퟏ. ퟐퟓ kg/m3  
Density of mercury,  
흆ퟐ = ퟏퟑ 600 kg/m3  
∆ℎ2 = 풉풃풂풔풆 − 풉풕풐풑 = ퟕퟓ − ퟔퟎ = ퟏퟓ 풄풎 = ퟎ. ퟏퟓ 풎  
Height of mountain (h1) =?  
From:  
Pressure difference due to column of air = Pressure difference due to mercury column  
흆 풉  
ퟐ ퟐ  
ퟎ.ퟏퟓ ×ퟏퟑퟔퟎퟎ  
흆 풉 = 흆 풉  
→ 풉  
=
=
= ퟏퟔퟑퟐ 풎  
ퟏ
ퟏ
ퟐ
ퟐ
ퟏ
ퟏ.ퟐퟓ  
흆
ퟏ
Atmospheric Pressure  
•
Is the pressure within the atmosphere of the earth  
OR  
•
Is the pressure exerted by the weight of the atmosphere  
N.B:  
•
•
Atmospheric Pressure decreases with the increase in altitude  
At high altitudes, where the pressure of the air is less , nose – bleeding may  
occur due to the greater excess pressure of the blood which causes blood  
capillaries to burst  
How Gas exerts Pressure?  
•
G
as exerts pressure when its molecules are continually colliding with each  
other and with the walls of the container causing a small force on the wall. The  
pressure exerted by the gas is due to the sum of all these collision forces. The  
more particles that hit the walls, the higher the PRESSURE  
Atmospheric pressure can be observed in several areas.  
Example  
:
-
•
•
In a glass tumbler  
In a crashing can  
Experiments to demonstrate effects of Atmospheric Pressure  
(a) In a glass tumbler  
(b) In a crashing can  
In a glass tumbler  
•
Fill the glass tumbler with water and place the card firmly on top of the glass so  
that there is no air between the glass and the glass  
With your hand on the card, gently turn the tumbler upside down , then remove  
your hand  
•
Observations  
•
The card holds on the tumbler when it is turned upside down. This is because  
the atmospheric pressure acting upwards on the card is greater than  
downward pressure of water acting on the card  
In a crashing can  
•
Put a little water in a can. Boil the water while the can is open in order to  
drive off the air (fig.(a))  
•
•
Remove the heating source and quickly close the hole tightly  
Poor cold water over the can  
Observations  
•
When the can is closed and the cold water is poured on it, the can collapses.  
This is because the steam is condensed into water leaving the inside of the can  
with partial vacuum. The outside atmospheric pressure crushes the can  
Barometer  
•
I
s an instrument used to measure atmospheric pressure  
Simple Barometer  
•
•
•
This is the most fundamental of the other types of barometer.  
It uses mercury instead of water because mercury is denser than water  
It has a height of 76 cm at sea level. Therefore, the atmospheric pressure at  
sea level is 76 cm of mercury (76 cmHg)  
•
Atmospheric pressure is given by: P = ρ h g  
Fortin Barometer  
•
Fortin Barometer is a modified simple barometer. It is a very accurate type of  
mercury barometer for measuring air (atmospheric) pressure.  
It performs functions like that of simple barometer  
•
Aneroid Barometer  
•
Is a barometer which is mostly used in aeroplanes to record the air pressure at  
a certain altitude  
•
Aneroid Barometer does not use any liquid. It consists of an evacuated metal  
box connecting a system of levers and a pointer. It is compacted and portable  
•
Aneroid barometer which is used in aircraft to show the height at which the  
plane is flying is called Altimeter  
Advantages of Aneroid Barometer over Fortin’s Barometer  
Aneroid Barometer  
•
•
•
It is used to measure air pressure in confined spaces  
It is compact and portable  
It is used in aircraft to show the height at which the plane is flying (The  
barometers used are called Altimeters)  
Disadvantages of using Fortin barometer  
a) Mercury is expensive and toxic  
b) It is not portable (it is approximately 1 m tall and contains liquid)  
c) It must be mounted in a vertical position  
NB:  
Aneroid barometer which is used in aircraft to show the height at which the  
plane is flying is called Altimeter  
Application of Atmospheric Pressure  
•
There are a variety of common and even simple devices that work under  
atmospheric pressure (working under the Principle that air exerts pressure).  
These include  
1. Siphon  
2. Lift pump  
3. Force pump  
4. Syringe  
5. Bicycle pump  
Siphon  
•
•
Is a tube or pipe that allows liquid to flow from the higher level to the lower level  
Siphon is a tube or pipe used to transfer liquid from one container to  
another container by using atmospheric pressure to make liquid flow  
•
•
The pressure on the surface of the liquid is atmospheric pressure.  
Since end C of the tube is below the surface A by height h, thus the pressure  
at C is greater than that at the surface.(ie. Pressure at C = pa + 흆풉품  
)
NB: Siphon can lift water about 10 m below the ground  
Application of Siphon in everyday life  
•
•
•
•
It is used in the toilet flushing cisterns (Chain and ball flushing tank)  
It is used in Siphon rain gauge to automatically drain out the excess water  
A siphon cup is a reservoir attached to a gun  
It is used in some drainage systems to drain water to another point  
Lift Pump  
•
•
Is used to raise water from underground water sources.  
Lift pump is a pump that is used to lift the liquid rather than force liquid up  
How it works  
•
The pump starts with the piston at the bottom of the empty cylinder and both  
valves closed (fig.(a))  
•
The pump handle is then pushed down lifting the piston upwards  
(upstrokes).The transfer valve remains closed and the intake valv opens to  
allow water from the external source to fill the lower chamber(fig.(b)). This is  
due to the low pressure region created between the valve A and the Piston  
•
•
T
he handle is then lifted upwards pushing the piston down (Down strokes).The  
intake valve now closes and the transfer valve opens (fig.(c)).This allows water  
to pass into the upper chamber  
Finally the pump handle is pushed down again lifting the piston upward.  
Transfer valve closes and Intake valve opens to allow water from the external  
source to fill the lower chamber .The water in the upper chamber is lifted and  
flows out of the spout (This process is repeated continuously)  
Limitation of Lift Pump  
✓
It can lift water up to height of 10 m  
✓
Few strokes are required  
Force Pump  
•
Is a modified of a lift pump which can be used to raise water to a height of  
more than 10 m  
Advantages of a force pump over a lift pump  
•
•
•
Force pump enables continuous flow of water  
Force pump is able to move water from greater depths than the lift pump  
Height to which water can be raised does not depend on the atmospheric pressure  
Syringe  
•
Is a simple piston pump that is used to inject fluid into or withdraw fluid from the body  
How it works  
•
It consists of a plunger that fits in a tube.  
T
he plunger is pulled and pushed  
while inside a cylindrical tube or barrel. This action enables the syringe to take  
in or expel fluid through the opening (nozzle) at the end of the tube  
Uses of the Syringe  
❖
❖
❖
Used for medical purpose e.g. Injecting vaccines  
They are used to measure liquids and gases in the laboratory  
Used to apply in a certain compound such as glue or lubricant  
Bicycle Pump  
•
•
Is a type of force pump that consists of a hollow metal cylinder and a movable piston  
It is specifically designed for inflating bicycle tyres  
Mechanism  
•
When piston is pulled out, a low pressure is created in the region just below the  
piston. The atmospheric pressure then forces air into the pump trough the space  
between the piston and the metal cylinder  
•
And when the piston is pushed in, the trapped air below the piston is compressed  
and the tube of the bicycle is then inflated using a valve  
N.B:  
Other examples (applications) of atmospheric pressure in our daily life are:-  
•
•
•
•
•
Detecting altitude of hills and mountains  
Drop ink through ink dropper  
Drinking straws  
Inflate vehicle tires  
Cleaning house using vacuum cleaners  
•
Enhance passage of air for breathing etc  
.