Fluids

1
Fluids

• Review Defining a Fluid
• Fluid Density
• Fluid Pressure and Pascals Principle
• Buoyancy and Archimedes Principle
• Bernoullis and Torricellis Principles
• Viscosity and Turbulence
• Cohesion, Adhesion, Surface Tension

2
States of Matter

• Solids are defined as having a definite volume
  and definite shape.
• Crystalline solids have a regular repeating
  pattern like salt.
• Amorphous solids do not have an orderly
  arrangement like glass.
• Liquids have a definite volume, but not a
  definite shape. They are virtually
  incompressible.
• Gases have neither a definite volume nor definite
  shape, but can easily be compressed.
• Plasma is an ionized gas.

3
Fluids

• Both liquids and gases exhibit a unique property.
  Both have the ability to flow.
• Hence the Definition A fluid is a material that
  has the ability to flow. (gas or a liquid)
• An increase in thermal energy of a fluid causes
  its particles to move faster and spread out.

4
Density

• Density is defined as mass per unit volume.
• The symbol for density is rho.
• The metric unit for density is kg/m3.
• FYI Water has a density of about 1 g/mL or 1 kg/
  m3. (The density varies slightly with different
  temperatures and pressures.)

5
Pressure

• Pressure is force per unit area.
• Pressure is measured in pounds per square inch
  (psi), atmospheres (atm), or torrs (millimeter of
  mercury.)
• The metric unit for pressure is the pascal, Pa.
• 1 Pa 1 N/m2.
• FYI At sea level, atmospheric pressure is
  normally,
• 1 atm 101,000 Pa 760 torr 14.7 psi
• FYI The deepest ocean trench has pressure of
  about 110 million pascals.

6
Density/Pressure Practice

• 1. A book measures 20 cm x 32 cm x 3 cm and has a
  mass of 1.25 kg. What is the density of the book?
• 2. What are the three possible pressures that the
  book could apply to a table?

D m/V 1.25 kg / (0.20m x 0.32m x 0.03 m)
650 kg/m3
F ma 1.25 kg(9.8 m/s2) 12.3 N P1 F/A
12.3 N / (0.20m x 0.32m) 192 Pa P2 F/A
12.3 N / (0.32m x 0.03 m) 1281 Pa P3 F/A
12.3 N/ (0.20m x 0.03 m) 2050 Pa
7
Pressure in a Fluid

• Pressure in a fluid acts in all directions.
• What is the force that is exerted on your
  desktop?
• F PA (14.7 lbs/in2)(4 ft2)(144 in2/ft2)
  8467 lb
• Because the air pushes equally up on your desk as
  it pushes down, the desk does not collapse.
• Why dont you crush under the pressure of the
  air?
• The pressure inside you is same as the air
  pressure.
• The pressure at a given depth in a fluid is the
  same throughout.

8
Pressure/Density Questions
1. Why do snowshoes keep you from sinking into
the snow? 2. Why do swimmers float better in the
ocean than in a lake? 3. Why dont they make
longer snorkels so that people could dive deeper
without scuba gear? 4. Which is denser, Earth or
the sun?
The snowshoes increase the area which decreases
the pressure.
Seawater is 2.5 denser making the buoyant force
greater.
The pressure 6 m below the surface would be
enough to keep the diver from expanding his lungs.
On average, Earth is more dense, but the core of
the sun is much denser than the earth.
9
Pressure and Freezing

• The freezing point of a liquid increases if its
  pressure is increased (Water is the exception.)
• i.e. It is easier to freeze a liquid if it is
  under pressure.
• To turn a liquid into a solid the molecules need
  to slow down or come closer together.
• Water expands as the temperature decreases from
  4C to 0C. Thus, it is harder for water to
  freeze if more pressure is applied.
• Due to pressure, an ice skater melts the ice
  temporarily as her blade passes over.

10
Pressure and Boiling

• The boiling point of a liquid increases if its
  pressure is increased.
• i.e. It is easier to boil a liquid if the
  pressure is low.
• Molecules need more kinetic energy to break free
  from other molecules. Less pressure aids this
  process.
• Water boils below 100C in the mountains.(At
  10,000ft it boils at 90C.) It takes food longer
  to cook because the temperature is not as hot.
• Water can remain a liquid up to 120C in a
  pressure cooker (high pressure) which can cook
  food faster.

11
Freezing of Solutions

• The freezing point of a solution, such as salt
  water is lower than the freezing point of just
  water.
• The higher the concentration of the salt, the
  lower the freezing point.
• The molecules of the solute (salt) interfere with
  the molecules of the solvent as they try to form
  a structure.

12
Boiling of Solutions

• The boiling point of a solution, such as salt
  water is higher than the freezing point of just
  water.
• In order for water to boil the vapor pressure of
  the water must be equal to the air pressure. The
  hotter the liquid the greater the vapor pressure.
• Ions from the salt takes up space at the surface
  and with fewer water molecules at the surface the
  lower the pressure.

13
Suction

• Suction is a force that causes a fluid to be
  drawn into a certain space because of a
  difference in internal and external pressure.
• A vacuum cleaner works by creating a low pressure
  inside which allows the air pressure on the
  outside to flow in (with dirt.)
• A suction cup sticks to surfaces because there is
  little air in between and the air pressure
  pushes on the dart.
• You are able to draw pop up a straw by creating a
  low pressure inside. Air pressure pushes the pop
  up the straw.

14
Pressure Formula

• The pressure of a fluid depends on the density,
  depth, and gravitational pull on the fluid.
• Pressure is less in the mountains and greater in
  the depths of the ocean.

15
Formula Proof

• Imagine a box under water with the top at the
  surface. The pressure at the bottom is greater
  because of the weight of the water above it.
• P F/A (mwater g )/ A
• (mwater g h )/ Ah
• (mwater g h )/ V
• (mwater /V)g h
• rwater g h
• The total pressure in the sum of the atmospheric
  pressure and water pressure. (P PA r g h)

16
Pressure Depends on DepthNot Shape

• All the containers are the same height.
  Therefore, the pressure at the bottom of each is
  the same.

• Note The pressure inside the fluid is the same.
  The pressure exerted by the container on the
  table is different.

17
Phase Changes

• Evaporation Liquid to Gas
• A volatile liquid is one that evaporates quickly.
• Examples gasoline, turpentine, moth balls,
  acetone (nail polish remover)
• Condensation Gas to Liquid
• Melting Solid to Liquid
• Freezing Liquid to Solid
• Sublimation Solid to Gas
• Example Dry Ice (frozen CO2)