Fluids & Pressure

Objectives: 1. define fluid, liquid, gas, pressure, Pascal, barometer, aneroid 2. write two equations for pressure 3. state Boyle's Law 4. describe some examples of pressure

Basic Definitions

fluid • anything that flows liquid • definite volume, indefinite

shape gas • indefinite volume, indefinite

shape

Pressure • force per unit of area - measured in pascals (Pa) pounds per square inch (psi) torr atmospheres (atm) millimeters of mercury (mm Hg) kilopascals (kPa) millibars (mb) - conversion factors 1 atm = 760 torr = 1013 mb = 101.3 kPa = 14.7 psi = 760 mm Hg = 29.9 in Hg

- increases if force increases or area decreases

Pressure Equation #1

P = F ÷ A

key: P = pressure (in Pa) F = force (in N) A = area (in m2)

Pressure and Depth

• in a fluid, depends on depth • more fluid above increases the force • pressure is exerted in all directions

Pressure Equation #2

P = ρgh key: P = pressure (in Pa) ρ = density (in kg/m3) g = 9.81 m/s2

h = depth (in m)

Boyle’s Law

• for an enclosed gas, an increase in pressure will decrease the volume

• if the pressure is doubled, the volume is cut in half

Boyle’s Law Equation

P1V1 = P2V2

Key: P1 = pressure at beginning V1 = volume at beginning P2 = pressure at end V2 = volume at end

Real World Applications Air Pressure • air in atmosphere has a weight (3 square feet = roughly 220,000 pounds) • measured with a barometer • Two Types • wet - tube of liquid inverted in a dish increases in outside pressure force

more liquid up the tube • aneroid - partial vacuum can increase in outside pressure causes can to collapse

more

Barometers • invented by Evangelista Torricelli in 1643 • filled tube with mercury • inverted tube in dish of mercury • column of mercury fell so that it was

760 mm tall (29.9 inches) • the area above the column of mercury

is a vacuum • if he used water, the tube would have

been 10.3 m tall • maximum useful straw length is 10.3 m • water will not rise more than 10.3

meters because of air pressure

Mercury Barometer

Water Pressure

• pressure increases with depth • submarines made of steel to withstand the

pressure on all sides • caissons for Brooklyn Bridge needed to be

filled with compressed air to prevent being crushed by the water around them • dams are wider at the bottom to resist increased

pressure

Boyle’s Law

• air bubbles expand as they rise through water • divers do not hold their breath as they rise or

their lungs will explode

Farting Underwater