Understanding Gas Pressure

ACTIVITY

GAS PRESSURE AND ATMOSPHERIC PRESSURE

GAS PRESSURE & ATMOSPHERIC PRESURE

Gas pressure based on the kinetic theory of gases

Atmospheric pressure in terms of the weight of the atmosphere

Instruments for measuring gas pressure: c) Manometer d) Bourdon gauges

Atmospheric pressure at sea level = 105 Nm-2 or 105 Pa = 760 mm Hg

Instruments for measuring atmospheric pressure: c) Mercury barometer d) Fortin barometer e) Aneroid barometer

Problems involving atmospheric pressure and gas pressure

b)

Applications: Rubber suckers c) Straws d) Syringes e) Siphons f) Lift pumps

Gas molecules

GAS PRESSURE Gas molecules

m u

Newton’s 2nd law

v

Newton’s 3rd law

Wall of container

• Based on the Kinetic molecular theory • The force per unit area exerted by the gas particles as they collide with the walls of their container

GAS PRESSURE The mass of the gas

Depends on

The rate of impact of gas molecules on the wall Depends on

Density and temperature of the gas

THE TOTAL PRESSURE = ATMOSPHERIC PRESSURE + hpg

INSTRUMENTS OF MEASURING GAS PRESSURE •

MANOMETER A

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B

C

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The pressure can be measured by calculating the difference in height between the two columns in liquids When the manometer is not connected to any gas supply, only atmospheric pressure acts on both surfaces. When the manometer is connected to a gas supply, the gas would exert a pressure on the liquid at point B. If the pressure is greater than the atmospheric pressure, the pressure difference will force down the liquid level at point B. At equilibrium, the pressure must be same at all points along the horizontal level ( pb = pc )

ATMOSPHERIC PRESSURE • The force per unit area exerted against a surface by the weight of the air molecules above that surface • Pressure at sea level is 1 atmosphere • 1 atm = 105 Nm-2 • = 105 Pa or 76 cm Hg ( mercury ) • Acts equally in all directions • Decreased with altitude, at higher altitudes, the density and the temperature of the air are lower. As the result, the frequency of collision of the molecules is lower. Hence, the atmospheric pressure is lower.

THE EXISTENCE OF ATMOSPHERIC PRESSURE Magdeburg hemispheres

• The experiment performed by Otto Van Guericke in 1654. • When the air inside two Magdeburg hemisphere was sucked out to form vacuum, • The outside atmospheric pressure compress the two hemispheres together • Two teams of 8 horses each were unable to separated the two hemispheres until air had been readmitted.

INSTRUMENTS FOR MEASURING ATMOSPHERIC PRESSURE MERCURY BAROMETER

ANEROID BAROMETER

The aneroid barometer has a metal drum, any change in pressure causes the box to be squashed or to expand.  If the air pressure increases, the metal drum is squeezed down slightly. If the air pressure decreased, the metal drum will expand and the spring will pull up the top of the metal drum. The small movement of the box is magnified to a larger motion on the scale pointer by means of levers

APPLICATIONS OF ATMOSPHERIC PRESSURE Drinking Straw When drinking from a straw, one tends to suck the straw. This causes the pressure in the straw to decrease. The external atmospheric pressure, which is greater, will then act on the surface of the water in the glass, causing it to rise up the straw.

Rubber sucker • When the rubber sucker is pressed onto a smooth surface, usually a glass or tiled surface, the air in the rubber dish is forced out. This causes the space against the surface to have low pressure • The contact between the rubber dish and the smooth surface is airtight • The external atmospheric pressure, which is much higher, acts on the rubber dish, pressing it surely against the wall

Vacuum cleaner • A vacuum cleaner, applies the principle of atmospheric pressure to remove dust particles. • The fan sucks out the air and then becomes a partial vacuum. • The atmospheric pressure outside,( is greater ) then forces air and dust particles into the filter bag. This traps the dust particles but allows the air to flow through an exit at the back.

SOLVING PROBLEMS • • •

96

20 10

c

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Why is the space above the mercury level a vacuum? Determine the pressure at points A, B, and C,in units of cm Hg Calculate the atmospheric pressure, in units of Pa. ( take atmospheric pressure = 76 cm Hg; density of mercury = 1.36 x 104 kg m-3 ; gravitational field strength = 10 N kg-1 ) If the vacuum space in the tube is filled with gas at a pressure of ¼ atm, what will the new positions be for points A and B, based on the levels in the figure?

Solution a) The vacuum space is created when the mercury column drops to a height of 76 cm above the mercury surface. This is the maximum height at which the atmospheric pressure can provide support. b) As the space above the mercury column in the tube is a vacuum, thus: Pressure at A = 0 Pressure at B = 96 – 20 = 76 cm Hg Pressure at C = 96 – 10 = 86 cm Hg

c) Atmospheric pressure = hpg = ( 0.76 m )(1.36 x 104 kg m-3 )( 10 Nkg-1 ) = 103 360 N m-2 = 1.03 x 105 Pa d) ¼ atm = ¼ ( 76 cm Hg ) = 19 cm Hg New pressure at A = 0 + 19 = 19 cm Hg New pressure at B = 76 + 19 = 95 cm Hg New position of A = 96 – 19 = 77 cm new position of B = 96 – 95 = 1 cm

2. Which of the following places has the highest atmospheric pressure? b)On the roof top of a building c) On the top of Gunung Tahan d)Inside a room e)Inside the parking basement of a building.

HOMEWORK 1. What height of a column of mercury would exert the same pressure as 680 cm oil ? ( The density of oil = 800 kg m-3) (density of mercury = 13.6 x 103 kgm-3)

vacuum

2. A simple is used to measure the atmospheric pressure: b) What is the purpose of having the vacuum space above the mercury column? c) Calculate the value, h, of the column if the mercury is replaced by water of density 1000 kgm-3 d) What would be the effect on the height of the column of mercury if some air were trapped within the top of the mercury column? e) Explain what you would expect to see if a wider tube replaced the glass tube. f) Give two reasons why a simple barometer is not suitable as an altimeter

RECAP • What is the pressure distribution inside the cup? • The weight of liquid • What is the pressure distribution which allows the card to be retained on the bottom of the cup? • The atmospheric pressure

THE END !

Kinetic molecular theory • • • • •

Gases are made of tiny, individual particles. The volume of the particles themselves is insignificant compared with the volume occupied by the gas; therefore gases are mostly empty space. Gas particles move rapidly and randomly in straight-line motion. Particles collide with one another and with the walls of the container in elastic collisions (no overall loss or gain of energy) Individual particles are far apart and have very little attraction for each other. Particles are considered to move independently of each other. The average kinetic energies of particles of different gases are equal at a given temperature. The average kinetic energies of gas particles increase as the temperature increase.

Newton’s 2nd law

The force, F, exerted on the wall = the rate of change of momentum F = mv – mu t

Newton’s 3rd law

The force exerted by the molecule hitting the wall = opposite to the force exerted by the wall on the molecule

Temperature T

a) Gas pressure = p

Temperature T1 > T

b) Gas pressure > p because velocity of molecules increases at higher temperature

Temperature T

c) Gas pressure > p because more molecules collide with walls of container.

Gas pressure in a container is due to the collisions of gas molecules with the walls.