Download - 38936975 3 3 Gas and Atmospheric Pressure
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3.3 UNDERSTANDINGGAS PRESSURE AND
ATMOSPHERIC PRESSURE
Learning Outcome
Explain gas pressure
Explain atmospheric pressureDescribe the applications of atmospheric pressure
Solve problems involving atmospheric and gas pressure
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Hands-on activity 3.3 pg 56
Aim : To gain an idea of gas pressure
Discussion
1. When air is pumped into a balloon, what
do you feel when you squeeze it ?
2. Why is there an opposing force acting on
your fingers ?
Refer text book pg 86 for the answer.
Idea of gas Pressure
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Gas Pressure
Billions of air molecules insidethe balloon moves in random motion.
Air molecules collide with the innerwall of the balloon.
Force is exerted on the surface
results in air pressure in the balloon.Air pressure is the force
exerted on a surface by air
molecules per unit area of the
surface
PrForce
essureArea
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Hands-on activity 3.4 pg 57
Aim : To study the existence of atmospheric
and gas pressure
Activity 1. Hot water in a plastic bottle.
Activity 2. Cardboard on the mouth of an
inverted glass.
Activity 3. Pulling two intact sink pump.Activity 4. Removing water using straw.
Can you explain each of the observation ?
Idea of atmospheric Pressure
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Atmospheric Pressure
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Atmospheric Pressure
Atmospheric pressure is the
force exerted on the Earths
surface by air molecules of the
atmosphere per unit area of thesurface
PrForce
essureArea
Atmospheric pressure decreases with altitude.
Reason :
1.At a higher altitude, the density of the air is lower.
2. At a higher altitude, the temperature of the air is
lower, hence less collisions.
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Barometer
a. Aneroid barometer
INSTRUMENTS FOR MEASURING
ATMOSPHERIC PRESSURE
b. Fortin barometer
It can be used as
an altimeter
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INSTRUMENTS FOR MEASURING
GAS PRESSURE
Bourdon Gauge Manometer
Gas pressure
= atmospheric pressure
+ pressure due to h cm
of mercury
mercury
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VARIOUS UNITS OF
ATMOSPHERIC PRESSURE
760 mm Hg
StandardAtmospheric
pressure (1 atmosphere)
0.76x 13600 x 10 Pa
= 103360 Pa
= 103360 N m-2
= 1.0336 x 105 Pa
= 1.0336 bar of pressure
10.336 m water
Mercury barometer
At sea level,
h = 760 mm
760 mm Hg
1 atm.
105 Pa
105 N m-2
10 m
water
76 cm Hg
1 bar pf
pressure
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Worked Example pg 90
The reading of a mercury barometer at a
meteorological station is 745 mm mercury.
What is the value of the atmospheric pressure
at the meteorological station in pascal ?(Density of mercury = 13 600 kg m-3)
solution
Atmospheric pressure = 745 mm Hg
= 0.745 x 13 600 x 10
= 101 320 Pa
hg
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Worked Example pg 90
The atmospheric pressure is 760 mmHg and the difference in themercury level of a manometermeasuring the pressure of a gas is20 mm. What is the pressure of thegas in mm Hg ?
solution
Gas pressure= Atmospheric pressure + 20 mm Hg
= 760 mm Hg + 20 mm Hg
= 780 mm Hg
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Calculate the total pressure
acting on the body of a diver at
a depth of 450 m underwater.
Worked Example Exploring pg 173
(Take density of sea water = 1200 kg m-3, g = 10 N kg-1and atmospheric Pressure = 105N m-2)
solutionTotal pressure
= Atmospheric pressure + 450 m water= 105 N m-2 + 450 (1200)(10) Nm-2
= 105 + 5400000
= 5500000 N m-2
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APPLICATIONS OF
ATMOSPHERIC PRESSURE
Drinking Straw Rubber Sucker
Syringe Siphon
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APPLICATIONS OF
ATMOSPHERIC PRESSURE
Vacuum Cleaner Lift Pump
Pouring milk
from can
Lifting big piece of
glass (pg 90)
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EVALUATION
1. If 10 cm3 of
mercury is added
into the bowl, what
happen to the
value of h ?
A > 76 mmB < 76 mm
C 76 mm
h = 76 mm
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A P
B P and Q
C Q and R
D P, Q and R
EVALUATION
2. A barometer is placed in an
region where the atmospheric
pressure is 76 mm Hg. Which
of the diagrams shows thecorrect level of the mercury.
h = 76 mm
h = 76 mm
vapour
h > 76 mm
P Q R
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EVALUATION
3. Figure shows a soft toy hung on a suction cup.
The soft toy does not fall to
the floor because thepressure in
A P > Q
B P = Q
C P < Q
D Q = 0
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Home work
Mastery practice 3.3
text book pg 91
Check your progress 3.3
Exploring pg 179