atmospheric pressure

Chapter 9 Fluid Mechanics
9-2 Fluid Pressure and Temperature
Pressure – a measure of how much force is
applied over a given area.
Formula: P = F/A
Pressure = Force (N) / area (m2)
Fluids – ability to flow – gases and liquids
• Our atmosphere – “ocean of gas”
• Density of our atmosphere decreases with altitude
• The atmosphere exerts pressure –
atmospheric pressure – caused by the
weight (in Newtons) of the air.
 Barometer Measures atmospheric pressure or
“barometric pressure”
The SI unit for measuring
Pressure = pascal (Pa) = 1
N/m2
1 square meter at sea level =
100,000 N -- so 100,000
N/m2
Average atmospheric pressure
at sea level is 101.3kPa
Or 1000000 Pa
Conversions: All equivalent units
14.7 pounds per square inch (psi) = 29.92 in Hg
= 760 mm Hg = 101.3 kPa = 1.00 atm
Example: Convert 232 psi to kPa
232 p.s.i ● 101.3 kPa
14.7 psi
= 1599 kPa
Example: Convert 3.50 atm to mm Hg
3.50 atm ● 760 mm Hg
1 atm
= 2660 mm Hg
9-4 Properties of Gases
 Ideal gas Law – relates gas volume, pressure and temperature.
 For a given Volume of Gas at a given Pressure and a given
Temperature there should be a consistent # of
molecules/atoms, n
Formula:
P ● V = n ●R ● T
n = number of moles
R = universal gas constant = 8.31 J / (mol ● K)
K = SI unit of temperature Kelvin (0C + 273)
Avogadro: said that it doesn’t matter what gas it is,
1 mole = 22.4 L of gas (6.02 x 1023 particles) at STP
(standard temp/press) = 00C and 1 atm of pressure
Example:
How many moles of carbon dioxide gas (CO2) are
contained in a 6.2 L tank at 101 kPa and 30 0C?
Given: V = 6.2 L P = 101 kPa
T = 30 oC (303 K)
And,
Constant R = 8.31 kPa x L / mol x K)
n=?
Ideal Gas Law --
n = 0.25 mol
P ●V= n●R ●T
Boyles’ Law: The pressure and volume of a gas at constant
temperature are inversely proportional. Increase one –
decrease the other.
• Increasing pressure on a gas (compressible) decreases volume.
Formula:
P1 ● V1 = P2 ● V2
Boyles’ Law: A graph of an inverse relationship
P1 ● V1 = P2 ● V2
Example: A volume of gas at 1.10 atm was measured at 326
cm3. What will be the volume if the pressure is adjusted to
1.90 atm?
Given: P1 = 1.10 atm, V 1 = 326 cm3, V2 = ? , P2 = 1.90 atm
Answer V2 = 189 cm3
Charles’s Law: at constant pressure, the volume of a
gas is directly proportional to its Kelvin temperature
Increase one, increase the other.
Formula: V1T2 = V2T1 proportional to its Kelvin temperature
or
V1 = T1
V2 T2
Direct
relationship
Example: The gas in a balloon occupies 2.25 L at 298 K.
At what temperature will the balloon expand to 3.50 L?
Given: V1 = 2.25 L T1 = 298 K T2 = ? V2= 3.50 L
T2 = 464 K
“Combined Gas Law”
P1V1 = P2 V2
T1
T2
Always convert temperature to Kelvin.
Gay-Lussac’s Law:
•The pressure of a gas is directly proportional to the Kelvin
temperature if the volume is held constant.
Direct relationship
P1 / T1 = P2 / T2
Question: The air temperature at an altitude of 10 km is a chilling ---35 0C. Cabin temperatures in airplanes flying at this altitude are
comfortable because of air conditioners rather than heaters. Why?
Answer: Airliners have pressurized cabins. The process of stopping
and compressing outside air to near sea-level pressures would
normally heat the air to a roasting 55 0C (130 0F). So air conditioners
must be used to extract heat from the pressurized air.