1. Fluids & Pressure Objectives:
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

2. Basic Definitions fluid  anything that flows liquid
 definite volume, indefinite shape
gas
 indefinite volume, indefinite shape

3. 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 = kPa = 14.7 psi = 760 mm Hg
- increases if force increases or area decreases

4. P = F ÷ A Pressure Equation #1 key: P = pressure (in Pa)
F = force (in N)
A = area (in m2)

5. Pressure and Depth in a fluid, depends on depth
more fluid above increases the force
 pressure is exerted in all directions

6. P = rgh Pressure Equation #2
key: P = pressure (in Pa)  = density (in kg/m3) g = 9.81 m/s2 h = depth (in m)

7. 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

8. P1V1 = P2V2 Boyle’s Law Equation
Key: P1 = pressure at beginning V1 = volume at beginning P2 = pressure at end V2 = volume at end

9. 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

10. 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
• 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 meters because of air pressure

11. Mercury Barometer

12. 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

13. 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