3.3 UNDERSTANDING GAS PRESSURE AND ATMOSPHERIC PRESSURE

Learning OutcomeExplain gas pressureExplain atmospheric pressureDescribe the applications of atmospheric pressureSolve problems involving atmospheric and gas pressure

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

Gas Pressure

• Billions of air molecules inside the balloon moves in random motion.

• Air molecules collide with the inner wall 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

Hands-on activity 3.4 pg 57Aim : 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

Atmospheric Pressure

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Atmospheric Pressure

Atmospheric pressure is the force exerted on the Earth’s surface by air molecules of the atmosphere per unit area of the surface

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.

Barometer

a. Aneroid barometer

INSTRUMENTS FOR MEASURING ATMOSPHERIC PRESSURE

b. Fortin barometer

It can be used as an altimeter

INSTRUMENTS FOR MEASURING GAS PRESSURE

Bourdon Gauge

Manometer

Gas pressure

= atmospheric pressure + pressure due to h cm of mercury

mercury

VARIOUS UNITS OF ATMOSPHERIC PRESSURE

760 mm Hg Standard Atmospheric

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

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

Worked Example pg 90

The atmospheric pressure is 760 mm Hg and the difference in the mercury level of a manometer measuring the pressure of a gas is 20 mm. What is the pressure of the gas in mm Hg ?

solution

Gas pressure = Atmospheric pressure + 20 mm Hg= 760 mm Hg + 20 mm Hg= 780 mm Hg

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-1 and atmospheric Pressure = 105 N m-2)

solutionTotal pressure = Atmospheric pressure + 450 m water= 105 N m-2 + 450 (1200)(10) Nm-2

= 105 + 5400000= 5500000 N m-2

APPLICATIONS OF ATMOSPHERIC PRESSURE

Drinking Straw Rubber Sucker

Syringe Siphon

APPLICATIONS OF ATMOSPHERIC PRESSURE

Vacuum Cleaner Lift Pump

Pouring milk from can

Lifting big piece of glass (pg 90)

EVALUATION

1. If 10 cm3 of mercury is added into the bowl, what happen to the value of h ?

A > 76 mm

B < 76 mm

C 76 mm

h = 76 mm

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 the correct level of the mercury.

h = 76 mmh = 76 mm

vapour

h > 76 mm

P Q R

EVALUATION

3. Figure shows a soft toy hung on a suction cup.

The soft toy does not fall to the floor because the pressure in

A P > Q

B P = Q

C P < Q

D Q = 0

Home work

Mastery practice 3.3 text book pg 91

Check your progress 3.3 Exploring pg 179