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Chapter 5 – The Properties of Fluids

Key Ideas• Knowledge of the properties of fluids is important in technology• An object immersed in a fluid will experience pressure• Forces can be transferred through confined fluids• Pressure, temperature, and volume of a fluid affect each other• Machines and other devices that use fluids can make work and

movement easier

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5.4 – Fluids Under Pressure

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PRESSURE – is the amount of force per unit area Use the following formula to determine pressure

Pressure = force/area –or – p=F/A *Remember* Force is measured in Newtons (N) and

area is measured in cm2 or m2

As you can see from the above formula – the larger the area the force is applied to, the lower the pressure will be

Pressure

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Pressure

Can you explain how these snowshoes help this person travel across the snow?

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Pressure – Sample Problem

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Pressure

Pressure is measured in pascals Pascal (Pa)– the unit of measure for

pressure; equivalent to one Newton per square metre (N/m2) A pascal is a relatively small force so we use

kilopascals (kPa) 1 kPa = 1000 Pa

So 1kPA = 1000 (N/m2)

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Atmospheric Pressure

Atmospheric Pressure– the weight of the air pushing down on itself and on the Earth's surface

The average pressure of Earth’s atmosphere is about 100 kPa – this will change with weather conditions and elevation

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How is temperature related to Pressure?

Now, here's where it all comes together. Since the atmosphere is warmed from the ground up, and since the air is at its most dense near the surface of the earth, the air near the surface is going to be able to retain much more heat than the air at higher elevations due to the increased amounts of air molecules; higher elevations have fewer air molecules and consequently can't retain as much heat. So, even though you would be much closer to the sun if you were standing on top of a mountain, the air temperature would be considerably less than it would be at sea level.

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Atmospheric Pressure

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5.5 – Pressure in Confined Fluids

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Pressure in Confined Fluids

A confined fluid means that a fluid can move around but cannot leave the system

Blood is a confined fluid (unless you get cut) and air in a tire is confined

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Pressure in Confined Fluids

There are two types of systems that use confined fluids to transmit forces from one location to another

Hydraulic System– is a confined, pressurized system that uses moving liquids.

Pneumatic System– is a confined, pressurized system that uses moving air or other gases.

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Pressure in Confined Fluids

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From our syringe exercise we learned: both plungers moved the same distance, but in

opposite directions one plunger moved down and the other plunger

moved up the force pushing down on one plunger was the

same as the force pushing up on the other plunger To make a hydraulic system more effective

you need to change the relative sizes of the cylinders

Pressure and Forces in a Hydraulic System

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The force on the smaller cylinder is multiplied in the larger cylinder.

The amount that the force is multiplied by is determined by the ratio of the areas of the larger and smaller pistons. For example, if the area of the larger piston is nine times

larger than the area of the smaller piston, the force on the larger cylinder is nine times larger than the force on the smaller cylinder.

To do the same amount of work, however, the smaller piston must move a greater distance

Pressure and Forces in a Hydraulic System

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Solution

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Practice

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Using The Kinetic Molecular Theory

Using the KMT we know that the spaces between gases are much greater than the spaces between liquids

Gases must be compressed before they can transfer force

Compression- action where an external force pushes particles together reducing volume

Compressible- characteristic of a substance where its volume can be reduced by external pressure

**If enough pressure is added or taken away a change of state can occur**

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5.8 – Fluid Power at Work

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Fluid Power @ Work

Uses for Fluids1. To Entertain Us2. Rescue3. Training4. Construction