Understanding Gas Pressure and Atmospheric Pressure
Transcript of 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.