Pressure
1.7.
David Raju Vundi
Pressure
 Pressure is the amount of force applied to a known
area.
 We can calculate it using the equation
 Pressure = Force (N) / Area (m2)
We can reduce pressure by reducing the size of the
force or increasing the size of the area.
Pressure and Density
 Density = Mass/Volume
 A property of the material.
 Pressure = Force/Area
 Depends on the height of the fluid.
 Same in all directions.
 Units are:



Force/Area = N/m2.
Pascals  1 Pa = 1 N/m2.
Atmosphere  1 atm = 1.013 X 105 N/m2.
3
Pressure is due to the net force of the
molecules in a fluid colliding with the walls.
A very large number of
collisions happen each
second.
Each collision exerts a tiny
net force on the wall.
How to Measure Pressure
Barometer
– measures atmospheric pressure
vacuum
Patm
PHg
Aneroid Barometer
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
vacuum
mercury (Hg)
air
pressure
barometer: device to measure air pressure
Barometer
Empty space
(a vacuum)
Hg
Weight of the
mercury in
the column
Weight of the
atmosphere
(atmospheric
pressure)
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 401
Water column
(34.0 ft. high
or 10.4 m)
Barometer
 Mercury filled
760 mm = 1 atm
Atmospheric
pressure
 Water filled
10400 mm = 1 atm
Mercury column
(30.0 in. high
or 76 cm)
The barometer measures
air pressure
Barometers
Mount Everest
Sea level
Sea level
On top of Mount Everest
Pressure Experiment
100g
I think the
bigger
mass will
sink
further
I think a
bigger area
will stop the
mass
sinking
SAND
Torricelli Barometer
 The mercury in the tube pushes
Weight of
mercury




down with its weight.
The bottom of the tube is open to
the atmosphere.
The air pushes on the open
surface of the mercury.
On an average day, the pressure
of the air equals the pressure
exerted by a column of mercury
760 mm high.
Above 760 mm, there is a
vacuum in the tube.
Barometer Detail
Why doesn’t the diameter of the column of
Hg make a difference?
Recall that Pressure = force/area.
The “force” is the weight of the mercury, but
the pressure that results is that weight
divided by the area of the column. So … a
bigger column weighs more but also has a
proportionally bigger area, and the two
factors cancel one another out.
The pressure caused by the column of
mercury pressing down is independent of
the diameter of the column.
Manometer
PHg
 A manometer is comprised
of a bulb containing a gas
and a U-shaped tube.
 The U-shaped tube is
partially filled with mercury.
The weight of the mercury
puts pressure on the gas.
 If the U-tube is OPEN there
is also air pressure acting
on the gas.
 The gas molecules put
pressure on the mercury.
Closed Manometers
 There is a balance
vacuum
PHg
between the weight of the
mercury on the left (PHg)
and the pressure of the
gas on the right (Pgas).
 The difference between
the heights of the mercury
on each side of the tube is
a measure of the pressure
of the gas.
Pgas = Dh
Open Manometers
 When gas pressure is greater
Pair
PHg
than atmospheric pressure, the
mercury is pushed toward the
open end.
 The balance is between the gas
on the right, and the air plus
mercury on the left.
Pair + PHg = Pgas
 The weight of the mercury is
measured as the height
difference:
PHg = Dh
So
Pgas = Pair + Dh
Open Manometers
 When gas pressure is less than
Pair
PHg
atmospheric pressure, the
mercury is pushed toward the
gas reservoir.
 The balance is between the air
on the left and the gas plus
mercury on the right:
Pair = Pgas + PHg
 The weight of the mercury is
measured as the height
difference:
PHg = Dh
So Pair = Pgas + Dh
Or
Pgas = Pair- Dh
Sample Problems
1.
Pair = 790 mm
Dh =
20 mm
2.
Dh = 13 mm
PAIR = 753 mm
3.
PAIR = 765 mm
Dh = 27 mm
Find the pressure of the gas in each manometer.
Pay attention to whether the manometer is open or closed!
Sample Problem Answers
1. Pgas+ Dh = Pair
Pgas= 790 mm - 20 mm = 770 mm Hg
2. Pgas = vacuum + 13 mm = 13 mm Hg
3. Pgas = Pair + Dh
Pgas = 765 + 27 = 792 mm Hg