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AP Unit II A 1

AP Unit II A 1. Hydrostatic Pressure. II A 1. Hydrostatic Pressure Ref. Chapter 9.1-9.2. Students should understand the concept of pressure as it applies to fluids, so they can: a) Apply the relationship between pressure, force, and area.

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AP Unit II A 1

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  1. AP Unit II A 1 Hydrostatic Pressure

  2. II A 1. Hydrostatic PressureRef. Chapter 9.1-9.2 • Students should understand the concept of pressure as it applies to fluids, so they can: • a) Apply the relationship between pressure, force, and area. • b) Apply the principle that a fluid exerts pressure in all directions. • c) Apply the principle that a fluid at rest exerts pressure perpendicular to any surface it contacts.

  3. d) Determine locations of equal pressure in a fluid. • e) Determine the values of absolute and gauge pressure for a particular situation. • f) Apply the relationship between depth and pressure in a liquid ΔP = ρ g Δh

  4. Pressure • Pressure = Force (N) / Area (m2) • P = F/A • It is measured in Newtons per meter squared (N/m2) or more simply Pascals (Pa) named after the scientist Pascal.

  5. Units of Pressure • It may sometimes also be measured in N/cm2, pounds per square inch(psi), atmospheres(atm), inches or centimeters of mercury(Hg) , bars or milliBars or Torr or milliTorr • 1 atmosphere = 1 x 105 Pa = 100 kPa = 1 Bar = • 1 000 milliBars = 76 cm Hg = 760 mm Hg = 760 milliTorr = 39” Hg = 15 psi ~ 360” Water Head Pressure.

  6. Atmospheric Pressure • This governs our weather and is measured with a barometer. • In the old days the barometer was a tube filled with mercury and upended in a dish of mercury. • The normal atmospheric pressure could only support 76 cm of Mercury so if the tube was long a vacuum was created in the top of the tube. • This vacuum was called the Torricelean vacuum after Signor Torricelli. • If we used water the column would be 10 meters (30’) high.

  7. Nowadays we use an aneroid barometer, which is a partial vacuum in a sealed metal can which expand or contracts with changes in atmospheric pressure. • Usually low pressure means bad weather and high pressure means good weather.

  8. Hydrostatic pressure • A fluid exerts pressure in all directions. • As you increase depth in a fluid the pressure increases due to the weight of the fluid above you. • For example if you think of a column of fluid of density ρ, height Δh and cross sectional area A, then the weight of the column is mass x gravity (mg). Since m = density x volume= ρ x Δh x A this becomes weight = ρ x Δh x A x g. • Since pressure = force/area and force is the weight then the hydrostatic pressure ΔP = (ρ x Δh x A x g)/A = ρ x Δh x g or ΔP = ρΔhg.

  9. Calculations • 1. Calculate the gauge pressure at the bottom of a 10 meters of water in both Pascals and atmospheres. What is the absolute pressure at this depth? • 2. Calculate the gauge pressure at the bottom of 30m. • 3. If the height of the air above us is 100 km, calculate its density on a normal day. • 4. Calculate the density of mercury. • 5. How would you demonstrate that a fluid exerts pressure in all dimensions. • Homework HAPIIA1: q14 p246, q20p248,q10-18ch9p299

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