2 9-2 Fluid Pressure and Temperature Pressure – a measure of how much force is applied over a given area.Formula: P = F/APressure = Force (N) / area (m2)
3 Fluids – ability to flow – gases and liquids Our atmosphere – “ocean of gas”Density of our atmosphere decreases with altitudeThe atmosphere exerts pressure –atmospheric pressure – caused by theweight (in Newtons) of the air.
4 Barometer Measures atmospheric pressure or “barometric pressure” The SI unit for measuring Pressure = pascal (Pa) = 1 N/m21 square meter at sea level = 100,000 N -- so 100,000 N/m2Average atmospheric pressure at sea level is 101.3kPaOr Pa
5 Conversions: All equivalent units 14 Conversions: All equivalent units 14.7 pounds per square inch (psi) = in Hg = 760 mm Hg = kPa = 1.00 atmExample: Convert psi to kPap.s.i ● kPa14.7 psi= 1599 kPaExample: Convert atm to mm Hg3.50 atm ● 760 mm Hg1 atm= 2660 mm Hg
6 9-4 Properties of GasesIdeal 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, nFormula: P ● V = n ●R ● Tn = number of molesR = 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
7 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 = kPa x L / mol x K)n = ?Ideal Gas Law -- P ● V = n ● R ● Tn = 0.25 mol
8 Increasing pressure on a gas (compressible) - decreases volume. 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
9 Boyles’ Law: A graph of an inverse relationship P1 ● V1 = P2 ● V2Example: 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 atmAnswer V2 = 189 cm3
10 Charles’s Law: at constant pressure, the volume of a gas is directly proportional to its Kelvin temperatureIncrease one, increase the other.Formula: V1T2 = V2T1 proportional to its Kelvin temperatureor V1 = T1 V2 T2Direct relationship
11 Example: The gas in a balloon occupies 2. 25 L at 298 K 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 LT2 = 464 K
12 “Combined Gas Law” T1 T2 P1V1 = P2 V2 Always convert temperature to Kelvin.
13 Gay-Lussac’s Law: P1 / T1 = P2 / T2 The pressure of a gas is directly proportional to the Kelvin temperature if the volume is held constant.Direct relationshipP1 / T1 = P2 / T2
14 Question: The air temperature at an altitude of 10 km is a chilling C. 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.