Air Pressure
p. 107
Air Lab
• Problem:
– Does Air Have Mass?
• Hypothesis:
– Air has mass.
– Air doesn't have mass.
• Materials:
– Stopwatch
beaker
Graduated cylinder
metric ruler
balance
Air Lab
• Procedure:
– Write a Step by Step Procedure
– Start with a verb.
• Data/Obs: (Record Obs)
• Conclusion:
– Identify the Problem, Hypothesis, explain if data
supported it or not.
Balloon In Jar
• Problem: What are ways the
balloon can be inflated in the
jar?
• Hypothesis:
– Blowing in tube B will inflate
the balloon.
– Pulling air out of tube A will
inflate the balloon.
Balloon In Jar (Observations)
Demonstrations
Empty Water Bottle
Peeps
Shaving Cream
Empty Water Bottle
• Problem:
– What will happen to the water
bottle in the vacuum?
• Hypothesis:
• Observations:
– The water bottle expanded.
Peep Show
• Problem: What will happen
to the peeps in the vacuum?
• Hypothesis:
• Observations:
– The peeps grew larger.
Shaving Cream
• Problem:
– What will happen to the shaving
cream in the vacuum?
• Hypothesis:
• Observations:
– The shaving cream grew and
expanded upward.
Properties of Air
• Air has the following Properties:
Air Molecule Model
• Gases consist of molecules which have:
– mass and occupy a volume.
• Therefore: they have density.
• These molecules move randomly and bump
into each other.
Density
• The amount of mass in a given volume of air.
• Density=mass/volume
Air Molecule Model
Calculate the density in each box.
Air Molecule Model Situation 1
Calculate the density for each side.
Draw a line to show where the wall should
move?
Air Molecule Model Situation 1
• The density is greater on the left, therefore
pressure is higher.
• There are more molecules on the left side and
there will be more molecules striking the wall.
– As a result, the wall will move to the right.
Air Molecule Model Situation 2
Calculate the density for each side.
Draw a line to show where the wall will move?
Air Molecule Model Situation 2
• The density of air is higher on the right,
therefore the pressure is greater.
• The molecules on the right side will hit the
wall more frequently,
– therefore the wall will move to the left.
Pressure
• The force on an area or surface.
• More mass in a given volume = a higher
density. As a result pressure will be greater.
(Air) Pressure
• The result of the
weight of a column
of air pushing down
on an area.
• The weight of a column
of air above the desk =
the weight of a large
school bus.
Desk Vs Air Pressure
• Why isn’t the desk crushed?
– Molecules of air push in all directions.
– The pressure is balanced by the air pushing up on
the desk.
Balloon In Jar (Explanation)
Balloon In Jar
Measuring Air Pressure
• A barometer is used to measure air pressure.
– Mercury Barometers
– Aneroid Barometers
Mercury Barometer
• The first barometers.
• Pressure pushes on the
surface of the mercury.
• When pressure increases
the mercury rises up the
tube.
• When pressure
decreases the mercury
falls in the tube.
Feeling the Pressure
• Which barometer
shows a higher
pressure?
• Which barometer
shows a lower
pressure?
Aneroid Barometer
• “without liquid”
• Uses an air tight metal chamber.
• The chamber is sensitive to changes in air
pressure.
Units of Air Pressure
• Weather Reports use inches of mercury.
• National Weather Service Maps use
millibars.
• 1 inHg = 33.87 millibars
Air Pressure and Density
Altitude Affects Pressure
• Altitude – elevation
• Air pressure is the
weight of a column of
air pushing down on an
area.
• Where is the pressure
greater/less?
Altitude Affects Pressure
• Less weight at the topso there is lower air
pressure.
• Sea-level has the
weight of the whole
atmosphere on it.Pressure is greatest
here.
Altitude Affects Density
• As you increase in
altitude, the density of
air decreases.
• Air pressure decreases as
altitude increases.
• As air pressure decreases
so does density.
Altitude Affects Density
• As you go up, there are
fewer molecules.
• However: The percent of
a gas in the atmosphere
always stays the same at
any altitude.
– 78% Nitrogen
– 21% Oxygen
Isobar Maps
Rules for Drawing Isobars
• Lines connect areas of __________pressure.
Sunny
(Happy)
Weather
Cloudy
(Lousy)
Weather
• Set an interval that • Make sure all points
on one side of a line
is appropriate for
are higher and lower
the change in
on the other.
pressure.
Practice Drawing Isobars
. 1008
. 1007
. 1008
. 1005
. 1007
. 1004
. 1002
. 1007
. 1006
. 1008
. 1004
. 1005
. 1002
. 1006
. 998
. 1005
. 1004
. 1001
. 1003
. 1003
. 1002
Practice Drawing Isobars
Draw Isobars on the diagram below. Mark the center of low pressure and high pressure with an H and L.
Interval = ___________________
.1004
.1000
.1005
.996
.1000
.1000
.1007
.998
.1012
.1009
.1013
.1010
.1015
.1020
Drawing Isobars
Isobar Maps on RHW
• Isobars connect lines of equal pressure.
Cloudy
(Lousy)
Weather
Sunny
(Happy)
Weather