2. Air Pressure
• Air pressure is the force
exerted on you by the weight
of tiny particles of air.
• Though, the particles are
invisible, they still have
weight.
• We are under high pressure
but we do not realized
because we are so used to it.
3. Air Pressure
• Atmospheric pressure is the force per unit area.
(P = F/A)
• Pressure decreases as altitude increases.
• Atmospheric pressure is measured by a “barometer”.
4. How heavy is air?
• At sea level, the atmosphere
exerts pressure on the Earth at
a force of 14.7 pounds per
square inch.
• This means a column of air 1-
inch square, extending from
the surface up to the upper
atmospheric limit, weighs
about 14.7 pounds.
1 atm = 14.7 lb/in2 = 29.92 in Hg = 760 mm Hg = 105 N/m2 = 104 kg/m2 = 1013.2 mb
(1 bar = 100 /m2)
5. Measurement of atmospheric pressure
The barometer measures the height of a
column of mercury inside a glass tube.
A section of the mercury is exposed to
the pressure of the atmosphere, which
exerts a force on the mercury.
An increase in pressure forces the
mercury to rise inside the tube; as
pressure drops, mercury drains out of
the tube, decreasing the height of the
column.
This type of barometer is typically used
in a lab or weather observation station,
is not easily transported, and is a bit
difficult to read.
6. Aneroid Barometer
The aneroid barometer contains a closed
vessel, that contracts or expands with
changes in pressure.
The aneroid cell attaches to a pressure
indicator with a mechanical linkage to
provide pressure readings.
It is important to note that due to the
linkage mechanism of an aneroid barometer,
it is not as accurate as a mercurial
barometer.
Standard sea level pressure is defined as
29.92 in. Hg. at 59°F (15°C) or 1013.2 Typical millibar pressure readings range
millibars. from 950.0 to 1040.0 millibars.
7. Effect of altitude on atmospheric pressure
As altitude increases, pressure
diminishes, as the weight of the
air column decreases.
On average, with every 1,000
feet of altitude increase, the
atmospheric pressure decreases
1 inch of mercury.
(every altitude +11m
, pressure -1 mm Hg)
8. • When altitude increases, pressure decreases, and water may
boil at a temperature below 100°C
• On every 27 mm Hg of
atmospheric pressure
increase, the B.P will
decrease by 1°C
9. Pressure VS Altitude
P = 760 – (H/11)
H = 11(760-P)
Pressure VS Boiling Point
B.P. = 100 – (760-P)/27
B.P. = 100 – (H/297)
11. Why Do My Ears Pop?
If you've ever traveled on a plane,
or taken a lift up or down a
skyscraper, you might have
experienced a weird or painful
sensation in your ears. This feeling
is known as the popping of the
ears.
But why do our ears pop?
12. drum remains the same as it was at ground
level. So, as you go up, the pressure inside
your ear drum becomes higher than the air
pressure outside. Your ears then try to reduce
the pressure inside. By doing so, the air inside
presses against your ear drums, making you
feel as if your ear drums are going to burst.
On the other hand, if you descend rapidly, such
as when an airplane is touching down, the
pressure of air outside the ear will be higher
than that of the air inside. The opposite will
happen. The air outside will press against your
ear drums and you may feel pain.
There is a tube inside our ears that helps to
maintain the pressure inside and outside our
ear. It is called the Eustachian tube. This tube
links the middle ear to the throat. A valve at
the end of the tube admits or releases air to
equalise the pressure on the inside and
13. Calculation
Altitude-Pressure- and Boiling Point
1) Find the altitude where the atmospheric pressure
(429 m.)
is 721 mm of Hg
2) Find the atmospheric pressure on top of a
(390 mm. Hg)
mountain with 4,070 m. high.
(92 °C)
3) Find the boiling point of water on top of a
mountain with 2,376 m. high. (95°C)
4) Find the boiling point of water at a point where the
atmospheric B.P-, H+ B.P-) is 625 mm of Hg
(H- P-, P- pressure
5) Explain the relation between A-P, P-B.P. and A-B.P.
