Chapter 3Behaviour of Fluids

Text Book: pp. 65 to 84

Related Concepts: particles, pressure,

Fluids• A fluid is a substance that can

flow, and can take the shape of its container.– Liquids are fluids.– So are gases!

• Particles (that is the molecules) of fluids are held together by weak forces, so they can move easily

• Compressible fluids (gases)– The volume of a compressible fluid can be

reduced by squeezing its container.

• Incompressible fluids (liquids)– The volume cannot be reduced. It is

impossible to squeeze a liquid into a smaller space using normal means!

Two types of fluids

Plugged syringe

Gas in syringe

Liquid in syringe

Pressure

• Pressure is a force applied to a surface– Fluids can exert a force on the inside of their

containers– This force is a form of pressure.

• The formula for pressure is:– Where:

• P=pressure in Pascals (Pa)• F= force in newtons (N)• A= area of the surface (m2)

AFP

At The Molecular Level:

• Pressure in fluids is caused by particles (molecules) colliding (or bumping) against the sides of the container.

The harder and faster the molecules collide, the greater the pressure inside the container!

Effects of Water Pressure

• If you swim deep under water, you will feel a force inside your ears. The deeper you go, the stronger this force feels. If you swim too deep, your ears will begin to hurt.

Your ears hurt because of water pressure.

The pressure comes from the weight of all the water above you pressing on all your surfaces (like your eardrums)

Atmospheric Pressure• The air in the atmosphere also has

weight. Atmospheric pressure is caused by the weight of the air pressing on us.

• Air is not very heavy, but the 100km of atmosphere above us adds up to a pressure of over 100 kilopascals– That’s the equivalent of one kilogram

for every square centimetre.– Or nearly 15 pounds per square inch

This is a barometer. The mercury in the tube is being held

up by air pressure.

Effect of Air Pressure

• If you fly in an airplane, at a high altitude, sometimes your ears will hurt.

This is the result of low air pressure around you. The higher air pressure in your lungs puts force on your eardrum.

Pressure and Your Eardrum

High pressure outside

Pressure Inside

Assignments

• Read Textbook pages 65 to 72– Copy any definitions that you missed during

the presentation into your notebook.– Add any additional notes you think are

necessary• Do Workbook pages 39 to 42

Mechanisms Related to Variations in Fluid Pressures

• First Principle: A fluid will flow from an area of high pressure to an area of low pressure.

High

Pressure

LowPressure

Pressure isDistributed

• Second Principle: Pressure applied to the surface of a fluid inside a closed container is distributed to every part of the fluid.

• Called “Pascal’s Principle”• It means that pressure is evenly distributed inside a

closed container or hydraulic system.• A puncture or weak spot will release pressure from

the whole container or system.

A leak releases pressure from the whole system

• Third Principle: A transfer of pressure in a fluid can be used to increase the force involved.

• This is called the “hydraulic principle”• It is used to make car lifts and other heavy

equipment work.• It can also be used in hydraulic brakes, where

pushing the brake pedal puts pressure on a fluid, which then puts pressure on the brake calipers.

Piston

piston

How Hydraulics Works

FluidSmall Force

LargeForce

Narrow cylinder

Wide cylinder

Other uses of hydraulics

• Lifting heavy weights

• Shock absorbers

• Making cars bounce

Hydrauliccylinders

Hydrauliccylinders

Hydraulic cylinders hidden over front wheels, hydraulic pump in trunk.

Natural Uses of Pressure and Hydraulics.

Your Heart acts like a Hydraulic Pump!

1. Blood from the body collects in the vena cava and right atrium.

2. Blood moves into right ventricle

3. Right ventricle closes, pushing blood towards lungs

4. Blood from lungs collects in left atrium

5. Blood moves through valve into left ventricle

6. Ventricle closes, pushing blood towards the body, and creating blood pressure.

valve va

lve

RightVentricle

Right Atrium

LeftAtrium

LeftVentricle

Your Lungs Use Pressure to Work

• When you inhale, you create low pressure in your lungs. This draws air into your body.

• When you exhale, you create high pressure in your lungs, forcing air out of your body.

Wind is Caused by Pressure.

• Air (a fluid) moves from areas of high pressure to areas of low pressure

• The flow of air causes wind.

Wind

Wind

• Textbook: Read pages 73 to 84• Workbook pages 43 to 48