Understanding Gas Pressure and Atmospheric Pressure

7/31/2019 Understanding Gas Pressure and Atmospheric Pressure

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UNDERSTANDING GASPRESSURE &

ATMOSPHERIC PRESSURE


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Gas Pressure

200 kPa 500 kPa 400 kPa 1100 kPa+ + = ? kPa


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Gas Pressure

The gas pressure in a

container is caused bythe collision of gas

molecules with the wallof the container.


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How to measure gas

pressure?


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Bourdon Gauge

Bourdon gauge(consists of a semi-

circular or C-shapedcopper tube thattends to straighten ifmore and more gas

is pumped(compressed) intoit).


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Manometer

Manometer (consistsof a U-tube about 1

m in height. About50% of the volumeof the U-tube is filledwith liquid such as

mercury or water).


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ManometerWith both legs of a U-tube

manometer open to the

atmosphere or subjected to the

same pressure, the liquidmaintains the same level in

each leg, establishing a zero

reference.


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Manometer

With a greater pressureapplied to the left side ofa U-tube manometer, theliquid lowers in the left legand rises in the right leg.

The liquid moves until theunit weight of the liquid,as indicated by h, exactlybalances the pressure.


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Manometer


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A manometermeasures thepressure of agas in acontainer

Gas pressureis the forceexerted by thecollisions of

gas particleswith a surface


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Variations on the U-Tube Manometer


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ATMOSPHERIC PRESSURE


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Earths Atmosphere

About 10 kmthick

Consists mostlyof molecularnitrogen (N2)

and oxygen (O2)


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The air is made up of molecules.


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Gravity pulls the air

molecules toward the earth,

giving them weight. Theweight of the air molecules all

around us is called the

atmospheric pressure.The atmospheric pressure is

caused by the downward

force exerted by the air, or theweight of the atmosphere on

the Earths surface.


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1 atmosphere = 76 mm Hg = 10 mwater = 1.0 x 105Pa


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High altitudes = lower pressure

Low altitudes = higher pressure


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The greater altitude from the sea level,the smaller the atmospheric pressure.


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INSTRUMENTS FORMEASURING ATMOSPHERIC

PRESSURE


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to

measure

airpressure.

A Barometer

is

used


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The AneroidBarometer

No fragile tubes!

No toxic chemicals!

No batteries!

Never needs winding!


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An aneroid barometer

uses a cell which has

had most of the airremoved.

As the air pressure

around the cell

increases, it presseson the cell, which

causes the needle to

move.

MILLIBARS


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In 1643, Evangelista Torricelli

invented the barometer


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Torricellis barometer

used a glass column

suspended in a bowl of

mercury. The pressure

of the air molecules

pushed the mercury upinto the glass tube.

The weight of the mercury in

the tube was equal to the

weight of the air pressing

down on the mercury in the

dish.


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Asatmospheric

pressure

increases

The mercury in

the tube rises.


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The Mercury Barometer

Good: Bad:

Simple to construct

Highly accurate

Glass tube is fragile

Mercury is verytoxic!


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Example 1:

The atmospheric pressure is 760 mm Hg. What

is the value of the atmosphericpressure inPascal?

[ Density of mercury, (Hg) = 13 600 kg m-3 ]


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Changing Pressure

A rising barometer= increasing air pressure.

This usually means:

Rising barometer readings indicate that a

high pressure system is approaching.Higher atmospheric pressure is usuallyassociated with fair weather and clearingskies.


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Changing Pressure

A falling barometer= decreasing air pressure.

This usually means:

Falling barometer readings usuallyindicate the approach of an area of

low pressure. Low pressure readingsare usually associated with stormsystems. Tornadoes and hurricanescan produce very low barometricreadings.

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Understanding Gas Pressure and Atmospheric Pressure