Fluids
States of Matter
Phase Changes
Density
Pressure
Pascal’s Principle
Buoyant Force
Archimedes’ Principle
Bernoulli’s Principle
Torricelli’s principle
Viscosity
Turbulence
Cohesion
Adhesion
Surface Tension
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Fluids - Physics - Lecture Slides, Slides of Physics
In these Physics Lecture Slides, following major aspects of physics have been discussed : Fluids, States of Matter, Phase Changes, Density, Pressure, Pascal’S Principle, Buoyant Force, Archimedes’ Principle, Surface Tension, Adhesion
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2012/2013
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Fluids
States of Matter
Phase Changes
Density
Pressure
Pascal’s Principle
Buoyant Force
Archimedes’ Principle
Bernoulli’s Principle
Torricelli’s principle
Viscosity
Turbulence
Cohesion
Adhesion
Surface Tension
States of Matter Matter comes in a variety of states: solid, liquid, gas, and plasma.
- The molecules of solid are locked in a rigid structure and can only vibrate. (Add thermal energy and the vibrations increase.) Some solids are crystalline , like table salt, in which the atoms are arranged in a repeating pattern. Some solids are amorphous , like glass, in which the atoms have no orderly arrangement. Either way, a solid has definite volume and shape.
- A liquid is virtually incompressible and has definite volume but no definite shape. (If you pour a liter of juice into several glasses, the shape of the juice has changed but the total volume hasn’t.)
- A gas is easily compressed. It has neither definite shape nor definite volume. (If a container of CO 2 is opened, it will diffuse throughout the room.)
- A plasma is an ionized gas and is the most common form of matter in the universe, since the insides of stars are plasmas. Docsity.com
Fluids
The term fluid refers to gases and liquids. Gases and
liquids have more in common with each other than they
do with solids, since gases and liquids both have atoms/
molecules that are free to move around. They are not
locked in place as they are in a solid. The hotter the fluid,
the faster its molecules move on average, and the more
space the fluid will occupy (if its container allows for
expansion.) Also, unlike solids, fluids can flow.
Density
Density is given by: (^) =
m
V
The symbol for density is “rho.” Density is simply mass
per unit volume. Water, for example, has a density of
about 1 gram per milliliter. (It varies slightly with
temperature and pressure.) The S.I. unit for density is the
kg / m 3. For water:
1 g
mL
1 mL
1 cm 3
· (^) ·
(100 cm) 3
m 3
1 kg
1000 g
·
1000 kg
m 3
Pressure / Density Example
Tofu
(^) Cookbook
Schmedrick uses his 6 lb tofu recipe book to teach his little brother Poindexter about density and pressure. He sets the book on the table and calculates the pressure on the table, which depends on the book’s orientation. The book’s density is 6 lb / (9” · 14” · 3”) = 0.0159 lb / in 3. Note the pressures are very small compared to atmospheric pressure.
Tofu Cookbook 14”
9”^ 3”
P = 6 lb / (9” · 14” ) = 0.0476 lb / in 2
P = 6 lb / (9” · 3” ) = 0.222 lb / in 2
P = 6 lb / (3” · 14” ) = 0.143 lb / in 2
Pressure in a Fluid
Unlike the cookbook on the table, the pressure in a fluid acts in all directions, not just down. The force on a 4 ft 2 desktop due to the air is:
F = (4 ft 2 ) (144 in 2 / ft 2 ) (14.7 lb / in 2 ) = 8467.2 lb!
The desk doesn’t collapse since the air pushes up just as hard from below.
The reason we are not crushed by our atmosphere is because the pressure inside our bodies is the same as the pressure outside.
Pressure in a fluid is the result of the forces exerted by
molecules as they bounce off each other in all directions.
Therefore, at a given depth in a liquid or gas, the pressure
is the same and acts in every direction.
Pressure & Freezing
For most liquids—but not water—the freezing point increases if
its pressure is increased, i.e., it’s easier to freeze most liquids if
they’re subjected to high pressures. In order to turn a liquids into
a solid, the molecules typically must get close enough together to
form a crystal. Low temps mean slow moving molecules that are
closer together, but high pressure can squeeze the molecules closer
together, even if they’re not moving very slowly.
Water is an exception to this because, due to its molecular shape,
it expands upon freezing. (Most other substances occupy more
space as liquids than as solids.) So, squeezing water makes
freezing it harder. The pressure on ice due to a passing skater can
actually melt a small amount of the ice.
Pressure & Boiling
The lower the pressure on a liquid, the easier it is to make it boil, i.e., as pressure increases, so does the boiling pt. This is because in order for a liquid to boil, molecules need enough kinetic energy to break free from the attraction of the molecules around it. (Molecules with this much energy are in a gaseous state.) It’s harder for a liquid to vaporize when subjected to high pressure, since gases take up more space than liquids.
Water, for example, boils at temps below 100 ºC up in the mountains where the air pressure is lower. (Water boils at 90 ºC at 10,000 ft.) It takes longer to cook food in boiling water at high altitudes because the boiling water isn’t as hot. In a vacuum water will boil at any temp, since there is no pressure at the surface to prevent the water from vaporizing. At high pressure water boils at a high temp. In a pressure cooker water can remain liquid up to 120 ºC, and the hotter water can cook food faster.
Boiling of Solutions
If you’re in a hurry and you need to bring water to boil on a
stove, should you add salt to it? answer:
No, salt actually increases the boiling point of water,
thereby increasing your wait. In order for water to boil, the
vapor pressure of the water must match to air pressure
around it. The hotter the water, the higher the vapor
pressure will be. Ions from the dissolved salt take up space
near the surface of the water. With fewer water molecules
exposed to the air, the vapor pressure is reduced. This
means that salt water must be greater than 100 ºC in order
to boil.
Suction
Suction is a force that causes a fluid or solid to be drawn into a space or to adhere to a surface because of the difference between the external and internal pressures. A vacuum cleaner creates a low pressure region inside itself. The higher pressure external air rushes into the low pressure region, taking dirt with it.
A dart with a suction cup tip sticks to a wall because there is very little air between the wall and the suction cup, so the greater pressure on the outside forces it into the wall. This increases the frictional force enough to support the dart’s weight. Eventually air seeps in, and the pressure difference diminishes until the dart falls.
Pressure Depends on Depth, not Shape
All these containers are the same height. Therefore, the
pressure at the bottom of each is the same. The shape
matters not! (See upcoming slides for further explanation.)
Note: We’re talking about the pressure inside the fluid, not the pressures exerted by the containers on the table, which would greater for a cylinder than a cone of the same height & base.
Pressure at a Given Depth is Constant
At a given depth, pressure must be the same. If it weren’t,
the fluid would have to be moving to the right, left, or back &
forth, which doesn’t happen with a fluid in equilibrium.
Imagine submersing a container of water in the shape of a
rectangular prism (a box).
A B
If the pressure at A were greater
than at B, then there would be a
net force on the container to the
right, since the area is the same
at each side.
Barometers
h
vacuum
mercury
The pressure at A is the same as the pressure of the surrounding air, since it’s at the surface. A and B are at the same pressure, since they are at the same height. The pressure at C is zero, since a vacuum has no pressure. The pressure difference from B to C is g h (where is the density of mercury), which is the pressure at B, which is the pressure at A, which is the air pressure. Thus, the height of the barometer directly measures air pressure. At normal air pressure, h 30 inches (760 mm), which is 760 torr. The weight of the column of mercury is balanced by the force exerted at the bottom due to the air pressure. Since mercury is 13.6 times heavier than water, a water barometer would have to A B be 13.6 times longer.
C
Pascal’s Principle