3.3 Gas Pressure Intensive

7/31/2019 3.3 Gas Pressure Intensive

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3.1 Gas Pressure &

Atmospheric Pressure
http://d/Bahan%20fizik/Question/CD/MENU%20UTAMA%20CD.pptx


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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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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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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:


= .. 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.


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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3.3 Gas Pressure Intensive