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

- 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 = ghkey:

P = pressure (in Pa)

= 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 dishincreases 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 • 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