Fluids & PressureChapter 7 Section 1

Objectives:•Describe how fluids exert pressure

•Analyze how atmospheric pressure varies with depth

•Explain how depth and density affect water pressure

•Give examples of fluids flowing from high to low pressure

Refresher on Gas Laws:

•Charles Law: If the pressure does not change, the volume of a gas will increase as the temperature increases

•Boyle’s Law: If the temperature does not change, the pressure of a gas will increase as the volume decreases

http://mw.concord.org/modeler1.3/mirror/chemistry/piston.gif

Vocabulary:

•Fluid

•Pressure

•Pascal (Pa)

•Atmospheric pressure

Fluid:

•Any material that can flow and that takes the shape of its container

•Liquids and gases

•Particles move quickly past each other

Fluids Exert Pressure:

•Air pressure, water pressure (air and water are fluids)

•Pressure: the amount of force exerted on a given area

•Calculating pressure: Pressure = force ÷ area

•SI unit: pascal (1Pa = 1N/m²)

Math Focus:

•Textbook page 181

•Pressure, force, area

•Now its your turn…#1, #2…

Pressure & Bubbles:

•Fluids exert pressure evenly in all directions

•When you blow a bubble, it spreads out in all directions to create a sphere

Atmospheric Pressure:• The pressure caused by the weight of the

atmosphere

• It is exerted on everything on Earth, including us!

• At sea level: 101,300Pa; 10N (~2lbs) on every square cm of your body

• Fluids in your body exert pressure, so you don’t feel this atmospheric pressure

Atmospheric Pressure:

http://www.asc-csa.gc.ca/images/spacesuit_pressure.jpg

Variation in Atmospheric Pressure:•150km above the Earth’s surface pressure

is okPa

•80% of gases are found within the first 10km

•At the top, pressure ~ 0 Pa

•Gas particles are far apart at top

Atmospheric Pressure and Depth:•Pressure increases as you approach the

surface of the Earth (as the atmosphere gets “deeper”)

•Fluids: pressure varies according to depth

Pressure Changes & Your Body:•Fluids in your body have to adjust to maintain

equal pressure as the outside pressure changes

•“Popping” in your ears when you are in an airplane or driving up a mountain side

Water Pressure:•Water exerts pressure like air in the

atmosphere does

•Pressure increases as depth increases

•There more water above, the higher the pressure

•The atmosphere also pulls down on the water (total pressure is atmospheric + water pressure)

Water Pressure:

http://www.nasa.gov/audience/forstudents/k-4/home/F_Oceans_K-4.html

Water Pressure & Depth: •Depends on depth, not the amount of fluid

present

•A swimmer 3m below the surface in a pool experiences the same pressure a person 3m below in the ocean does

•A diver feels twice the pressure at 10m than at the surface

Depth by the numbers:

•500m below the surface = 5,000kPa (pressure suits required)

•Wreck of the Titanic 3,660m below = 36,600kPa

•Viper fish live 8,000m below = 80,000kPa

•Trieste (1960) went to 11,000m = 110,000kPa

Density Making a Difference: •Water is about 1,000x more dense than air!

•Density = the amount of matter in a given volume

•d = m ÷ v

•Ex: Climb a 10m tree or dive 10m underwater…which experiences more pressure??

Pressure Differences & Fluid Flow:•Fluids flow from an area of high pressure

to an area of low pressure

•Ex: drinking from a straw▫How do you decrease the pressure inside

the straw?▫By removing some of the air inside the

straw by sucking air in

Pressure Differences & Breathing:•Breathing:

▫Inhale: chest cavity expands, pressure decreases▫Air rushes in from high pressure (outside) to

lower pressure inside ▫Exhale: chest cavity contraction, pressure

increase▫Air rushes out from an area of high pressure to

lower pressure (outside the body)

Pressure Differences & Tornadoes:•Air pressure inside a tornado is very low

•Air rushes into the tornado, acting like a giant vacuum cleaner

•Objects get pushed into the tornado resulting in destruction

Quick Quiz:

•What do liquids and solids have in common?

•Why does pressure increase with depth?