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3.1 Gas Pressure &
Atmospheric Pressure
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Learning Outcomes
explain gas pressure
explain atmospheric pressure .
describe applications of atmosphericpressure .
solve problems involving atmosphericpressure and gas pressure .
1
2
3
4
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ACTIVITY 1:Idea of Gas Pressure
Use a bicycle pump to pump air into adeflated ball. When air is pumped into
a ball, what changes do you observe in
the ball?
How would you know when there isenough air in the ball?
The ball will be fully inflated and it
expands
It will become hard when you press the ball.
Feel an opposing force acting on your finger
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When you squeeze a ball filled with air,you will feel an __________ force acting
from the inside of the ball.
The force comes from billions andbillions of air molecules striking theinner wall of the ball.
The molecules are in constant ________motion.
When the molecules _________ withthe inner wall of the ball, the force ofcollisions _________ the wall outwards.
opposing
random
collide
pushes
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The __________ exerted on the inner wall of
the ball results in the air / gas pressure in the
ball.
force
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Explain Gas Pressure
The gas pressure in a container is caused bythe collision of gas molecules with the walls
of the container.
Gas/Air pressure is the force exerted on asurface by air molecules per unit area of the
surface.
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Atmospheric Pressure
Have you ever wondered how plastic stickerscan be attached on the windscreen of a car
without dropping off?
The stickers are
actually pressed on the
glass surface by the
surrounding ATMOSPHERIC PRESSURE
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Atmospheric Pressure
The surface of the Earth is in fact the bottomof an ocean of air known as the atmosphere.
The air which is a fluid exerts a force that
pushes our body inwards just like waterpressure in a swimming pool.
Most of the fluids in our bodies are
approximately at the same pressure as the airaround us, so we are not crushed by this
huge pressure.
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Explain atmospheric pressure
Atmospheric pressure is the pressure caused
by the weight of the air above us.
Atmospheric pressure acts equally in all
directions.
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Explain atmospheric pressure
Atmospheric pressure varies with the heightof the object above sea level.
It decreases with the altitude or the height
above sea level.
At higher altitudes, the density and the
temperature of the air are lower. As a result,
the frequency of collisions of the molecules is
lower.
Hence, atmospheric pressure is lower.
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ACTIVITY 2: Experiments to show the
existence of the atmospheric pressure
Fill an empty plastic bottle with hot water
until it is about a quarter full. Tighten the
cap immediately and wait for a few seconds.
Observation The bottle has been crushed
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The _________ from the boiling water drivesout the air from the plastic bottle.
The cap prevents outside _______ fromflowing into the bottle.
When cold water is poured on it, the steaminside the bottle __________ to water and it
forms partial __________ inside the bottle. The atmospheric pressure outside which is
________ then compress the bottle.
steam
air
condense
vaccuum
higher
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Application:
the resultant force caused by theatmospheric pressure acts on the surface of
the cardboard is greater than the weight of
the water in the glass.
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Application:
When we suck through a straw, the airpressure in the straw is lowered. Then the
pressure of the atmosphere acting on the
surface of the drink in the glass pushes the
water up the straw and into our mouth.
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When the sucker is
pressed into place, mostof the air behind it issqueezed out. The suckeris held in position by the
pressure of atmosphereon the outside surface ofthe rubber. If the sealbetween the sucker and
the surface is airtight ,the sucker will stickpermanently.
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Vacuum Cleaner
The fan inside the vacuum cleaner blows air
out of the vents. Which less air inside , the airpressure there drops. The atmospheric
pressure outside then pushes air up the
cleaner hose ,carrying dust and dirt with it.
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Mercury
Barometer The barometer is made
by filling a long glasstube with mercury. The
tube is then turnedupside down in a bow ofmercury. The mercurylevel in the tube will dropuntil a level h = 76 cm
above the mercury levelin the bowl.
(a)Why the level does not
drop further?
__________________________Atmospheric pressure
support the mercury
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Mercury
Barometer(b) Pressure at A is due to
the _______________
(b) Pressure at B is due tothe ________________
(d) Pressure at C = ____
(d) Compare the pressureat A and B.
PA PB
Atmospheric pressure______ Mercury Pressure
Atmospheric pressure
Mercury column
0
=
equal
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Mercury
BarometerAtmospheric pressure
Equal to Mercury Pressure
(f) If the density of the
mercury is 1.36 x 104 kg m-3
and the acceleration due to
gravity is 10 ms-2, calculate
the atmospheric pressure in
pascal.
P = 1.36 x 104 x 10 x 0.76
= 1.03 x 105 Pa
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Mercury
BarometerWhat is the value of theatmospheric pressure in cm
Hg?
What is the pressure at pointx in cm Hg?
The density of water is 1000kg m-3. If a barometer ismade using water instead ofmercury, and a very longtube, how high is the watercolumn.
76 cm Hg
76 + 14 = 90 cm Hg
h =
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Mercury
Barometer At sea level:
Atmospheric pressure
= .. Pa
= .. cm Hg
= . m water
= . Atm
1.03 x 105
76
10
1
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Question 2
If a mercury barometer were carried up a
mountain, how would you expect the height
of the mercury column to change? Explain.
Decreases, atmospheric pressure
decreases as the altitude increases
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Question 3
The diagram shows a
mercury barometer. Gas
X is trapped in a glass
tube. If the vertical
height of the mercury
level is 40 cm of Hg andthe atmospheric
pressure is 75 cm Hg,
calculate the pressureexerted by the gas X
inside the tube.75 40 = 35 cm Hg
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A manometer
consists of a U-tube
that is filled with a
liquid like water, oil
or mercury.
In figure (a) whenboth ends of the
tube is exposed to
__________pressure, both levels
are the same.
Atmospheric pressure
Manometer
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In figure (b) when one end is connected
to a gas supply, the difference in level, h
gives the pressure of the gas, P.
Manometer
M
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What is the total
pressure a point
W?
Compare the
pressure at W and the
gas pressure.
Write an equationthat relate between
Patmopherice , Pmercuryand P
gas
.
P W = PgasAt the same level
P W = Patm + Pmerkuri
P gas = Patm + Pmerkuri
Manometer
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Manometer
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Manometer
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Manometer
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P W = PgasAt the same level P W = Patm + Pmerkuri
P gas = Patm + Pmerkuri
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[Patmospheric = 76 cm Hg, g = 10 Nkg-1
density of mercury = 1.36 x 104 kg m-3]
P gas = 32 + 76
= 108 cm Hg
What is the gas pressure in Pa?
P gas = gh
1.36 x 104 x 10 x 1.08
= 1.46 x 105 Pa
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