The Fluid StatesThe Fluid States

Section 13.1Section 13.1

PhysicsPhysics

ObjectivesObjectives Describe how fluids create pressure and Describe how fluids create pressure and

relate Pascal’s principle to some everyday relate Pascal’s principle to some everyday occurrences.occurrences.

Apply Archimedes’ and Bernoulli’s Apply Archimedes’ and Bernoulli’s principles.principles.

Explain how forces within liquids cause Explain how forces within liquids cause surface tension and capillary action, and surface tension and capillary action, and relate the kinetic model to evaporation and relate the kinetic model to evaporation and condensation.condensation.

Properties of FluidsProperties of Fluids

FluidsFluids: materials that flow and have no : materials that flow and have no definite shape of their own.definite shape of their own.

For the most part, Newton’s Laws and the For the most part, Newton’s Laws and the Laws of Conservation can be applied to Laws of Conservation can be applied to fluids.fluids.

When we apply a force to a fluid, what do When we apply a force to a fluid, what do we call it?we call it?

PressurePressure

We call a force applied to a fluid: We call a force applied to a fluid: pressurepressure..

Pressure is represented by the following Pressure is represented by the following equation:equation: P = F/AP = F/A Pressure equals force divided by area.Pressure equals force divided by area.

PressurePressure

The force is assumed to be perpendicular The force is assumed to be perpendicular to the affected surface area.to the affected surface area.

Pressure is measured in Pascals, Pa.Pressure is measured in Pascals, Pa. The Pascal: 1N/mThe Pascal: 1N/m².². Which has a greater pressure?Which has a greater pressure?

The area under an elephant’s foot or the area The area under an elephant’s foot or the area under a woman’s high heel shoe?under a woman’s high heel shoe?

The area under a high heel shoe.The area under a high heel shoe.

Pressure from a Gas?Pressure from a Gas?

It is easy to see how solids and fluids exert It is easy to see how solids and fluids exert pressure on a surface.pressure on a surface.

But how does a gas exert pressure on an But how does a gas exert pressure on an area?area?

The tiny gas particles are constantly hitting The tiny gas particles are constantly hitting surfaces and applying a pressure value.surfaces and applying a pressure value.

Pressure from a GasPressure from a Gas

Are we under pressure right now?Are we under pressure right now? Yep.Yep. In fact, on every square centimeter of In fact, on every square centimeter of

Earth’s surface at sea level, the Earth’s surface at sea level, the atmosphere exerts a force of atmosphere exerts a force of approximately 10 N = 1 kg.approximately 10 N = 1 kg.

Blue Barometer Demo.Blue Barometer Demo.

http://www.cabelas.com/cabelas/en/content/Item/61/60/81/i616081sq02.jpg

Practice ProblemsPractice Problems

Pg 303Pg 303 1-41-4

Fluids at RestFluids at Rest

Water also exerts pressure just as the Water also exerts pressure just as the atmosphere exerts pressure.atmosphere exerts pressure.

Think of a diver, the pressure felt when Think of a diver, the pressure felt when under water does not depend on whether under water does not depend on whether the diver is upright, sideways, or up-side-the diver is upright, sideways, or up-side-down.down.

Pascal’s Principle accounts for this Pascal’s Principle accounts for this pressure.pressure.

Pascal’s PrinciplePascal’s Principle

Pascal’s Principle:Pascal’s Principle: any change in any change in pressure applied at any point on a pressure applied at any point on a confined fluid is transmitted undiminished confined fluid is transmitted undiminished throughout the fluid. throughout the fluid.

How does toothpaste demonstrate this How does toothpaste demonstrate this principle?principle?

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Pascal’s PrinciplePascal’s Principle

So why is this important?So why is this important? There are more important reasons than There are more important reasons than

that of toothpaste.that of toothpaste. Hydraulic systems use this principleHydraulic systems use this principle Force Exerted by lift = F = (FForce Exerted by lift = F = (F11AA22)/A)/A11

Practice ProblemPractice Problem

Pg 304Pg 304 55

Swimming Under PressureSwimming Under Pressure

When you dive under water, your body When you dive under water, your body feels increasing pressure as you dive feels increasing pressure as you dive deeper. deeper.

The downward pressure of water is The downward pressure of water is illustrated by the following equation.illustrated by the following equation.

P = P = ρρhghg Rho; density of fluid, h; depth, g; gravity. Rho; density of fluid, h; depth, g; gravity.

Swimming Under PressureSwimming Under Pressure

While swimming, you may also notice the While swimming, you may also notice the upward force from the water you are upward force from the water you are immersed in.immersed in.

This force is the This force is the buoyant forcebuoyant force.. FFbuoyant buoyant = = ρρVgVg

Rho; density, V; volume, g; gravity.Rho; density, V; volume, g; gravity. The net upward force is equal to the The net upward force is equal to the

weight of the fluid displaced by the object.weight of the fluid displaced by the object.

Archimedes’ PrincipleArchimedes’ Principle

The buoyant force and weight of displaced The buoyant force and weight of displaced fluid relationship was discovered by the fluid relationship was discovered by the Greek scientist Archimedes.Greek scientist Archimedes.

Archimedes’ Principle:Archimedes’ Principle: states that an states that an object immersed in a fluid has an upward object immersed in a fluid has an upward force on it equal to the weight of the fluid force on it equal to the weight of the fluid displaced by the object; it does not depend displaced by the object; it does not depend on the weight of the object.on the weight of the object.

So Will It Sink Or Float?So Will It Sink Or Float?

When an object is placed in a fluid, it has When an object is placed in a fluid, it has the force of gravity pressing down and the the force of gravity pressing down and the buoyant force pressing up. buoyant force pressing up.

The difference between the two The difference between the two determines if the object will sink or float.determines if the object will sink or float.

FFapparentapparent = F = Fgg – F – Fbuoyantbuoyant

A negative value for FA negative value for Fapparent apparent indicates the indicates the

object will float.object will float.

Practice ProblemsPractice Problems

Pg 309Pg 309 6-76-7

http://home.earthlink.net/~divegeek/archimedes_files/buoyancy.gif

Fluids in MotionFluids in Motion

Ok, we have looked at static fluids, but Ok, we have looked at static fluids, but fluids do not just stay still.fluids do not just stay still.

Bernoulli’s Principle:Bernoulli’s Principle: describes the describes the relationship between the velocity and relationship between the velocity and pressure exerted by a moving fluid.pressure exerted by a moving fluid.

Bernoulli’s Principle states that as the Bernoulli’s Principle states that as the velocity of a fluid increases, the pressure velocity of a fluid increases, the pressure exerted by that fluid decreases.exerted by that fluid decreases.

Fluids in MotionFluids in Motion

Most airplanes get part of their lift by Most airplanes get part of their lift by utilizing this principle.utilizing this principle.

The curvature of the top wing is greater The curvature of the top wing is greater than that of the bottom.than that of the bottom.

As the wing travels through the air, ;the air As the wing travels through the air, ;the air moving over the top surface travels faster moving over the top surface travels faster than air moving past the bottom surface.than air moving past the bottom surface.

Fluids in MotionFluids in Motion

The decreased air pressure created on the The decreased air pressure created on the top surface results in a net upward top surface results in a net upward pressure that produces an upward force pressure that produces an upward force on the wings, or lift, which helps hold the on the wings, or lift, which helps hold the plane up.plane up.

Race cars use this same principle…How?Race cars use this same principle…How?

Forces Within LiquidsForces Within Liquids

All of the fluids examined thus far have All of the fluids examined thus far have been ideal fluids.been ideal fluids.

In real liquids, particles exert In real liquids, particles exert electromagnetic forces of attraction on electromagnetic forces of attraction on each other.each other.

These forces affect the behavior of liquids.These forces affect the behavior of liquids.

Forces Within LiquidsForces Within Liquids

These forces are These forces are cohesive forcescohesive forces.. Cohesive forces are the forces of Cohesive forces are the forces of

attraction between particles of a fluid.attraction between particles of a fluid. These forces account for water’s surface These forces account for water’s surface

tension.tension.

Forces Within LiquidsForces Within Liquids

Another force that affects the way a fluid Another force that affects the way a fluid behaves is behaves is adhesionadhesion..

Adhesive forces are the attraction between Adhesive forces are the attraction between particles of a fluid and the particles of particles of a fluid and the particles of another surface.another surface.

This is why This is why capillary actioncapillary action occurs. occurs.