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The Fluid States
Section 13.1
Physics
Objectives
Describe
how fluids create pressure and
relate Pascal’s principle to some everyday
occurrences.
Apply Archimedes’ and Bernoulli’s
principles.
Explain how forces within liquids cause
surface tension and capillary action, and
relate the kinetic model to evaporation and
condensation.
Properties of Fluids
Fluids:
materials that flow and have no
definite shape of their own.
For the most part, Newton’s Laws and the
Laws of Conservation can be applied to
fluids.
When we apply a force to a fluid, what do
we call it?
Pressure
We
call a force applied to a fluid:
pressure.
Pressure is represented by the following
equation:
P = F/A
Pressure equals force divided by area.
Pressure
The
force is assumed to be perpendicular
to the affected surface area.
Pressure is measured in Pascals, Pa.
The Pascal: 1N/m².
Which has a greater pressure?
The area under an elephant’s foot or the area
under a woman’s high heel shoe?
The area under a high heel shoe.
Pressure from a Gas?
It
is easy to see how solids and fluids exert
pressure on a surface.
But how does a gas exert pressure on an
area?
The tiny gas particles are constantly hitting
surfaces and applying a pressure value.
Pressure from a Gas
Are
we under pressure right now?
Yep.
In fact, on every square centimeter of
Earth’s surface at sea level, the
atmosphere exerts a force of
approximately 10 N = 1 kg.
Blue Barometer Demo.
http://www.cabelas.com/cabelas/en/content/Item/61/60/81/i616081sq02.jpg
Practice Problems
Pg
1-4
303
Fluids at Rest
Water
also exerts pressure just as the
atmosphere exerts pressure.
Think of a diver, the pressure felt when
under water does not depend on whether
the diver is upright, sideways, or up-sidedown.
Pascal’s Principle accounts for this
pressure.
Pascal’s
Principle: any change in
pressure applied at any point on a
confined fluid is transmitted undiminished
throughout the fluid.
How does toothpaste demonstrate this
principle?
http://www.gauweiler.net/photos/art/toothpaste.jpg
Pascal’s Principle
Pascal’s Principle
So
why is this important?
There are more important reasons than
that of toothpaste.
Hydraulic systems use this principle
Force Exerted by lift = F = (F1A2)/A1
Practice Problem
Pg
5
304
Swimming Under Pressure
When
you dive under water, your body
feels increasing pressure as you dive
deeper.
The downward pressure of water is
illustrated by the following equation.
P = ρhg
Rho; density of fluid, h; depth, g; gravity.
Swimming Under Pressure
While
swimming, you may also notice the
upward force from the water you are
immersed in.
This force is the buoyant force.
Fbuoyant = ρVg
Rho; density, V; volume, g; gravity.
The net upward force is equal to the
weight of the fluid displaced by the object.
Archimedes’ Principle
The
buoyant force and weight of displaced
fluid relationship was discovered by the
Greek scientist Archimedes.
Archimedes’ Principle: states that an
object immersed in a fluid has an upward
force on it equal to the weight of the fluid
displaced by the object; it does not depend
on the weight of the object.
So Will It Sink Or Float?
When
an object is placed in a fluid, it has
the force of gravity pressing down and the
buoyant force pressing up.
The difference between the two
determines if the object will sink or float.
Fapparent = Fg – Fbuoyant
A negative value for Fapparent indicates the
object will float.
Practice Problems
Pg
309
6-7
http://home.earthlink.net/~divegeek/archimedes_files/buoyancy.gif
Fluids in Motion
Ok,
we have looked at static fluids, but
fluids do not just stay still.
Bernoulli’s Principle: describes the
relationship between the velocity and
pressure exerted by a moving fluid.
Bernoulli’s Principle states that as the
velocity of a fluid increases, the pressure
exerted by that fluid decreases.
Fluids in Motion
Most
airplanes get part of their lift by
utilizing this principle.
The curvature of the top wing is greater
than that of the bottom.
As the wing travels through the air, ;the air
moving over the top surface travels faster
than air moving past the bottom surface.
Fluids in Motion
The
decreased air pressure created on the
top surface results in a net upward
pressure that produces an upward force
on the wings, or lift, which helps hold the
plane up.
Race cars use this same principle…How?
Forces Within Liquids
All
of the fluids examined thus far have
been ideal fluids.
In real liquids, particles exert
electromagnetic forces of attraction on
each other.
These forces affect the behavior of liquids.
Forces Within Liquids
These
forces are cohesive forces.
Cohesive forces are the forces of
attraction between particles of a fluid.
These forces account for water’s surface
tension.
Forces Within Liquids
Another
force that affects the way a fluid
behaves is adhesion.
Adhesive forces are the attraction between
particles of a fluid and the particles of
another surface.
This is why capillary action occurs.