SLE 3 - Viscosity, Density, Buoyancy& Compressibility

3. Investigate and compare the properties of gases and liquids: and relate variations in their viscosity,density, buoyancy and compressibility to the particle model of matter• investigate and compare fluids, based on their viscosity and flow rate, and describe the effects oftemperature change on liquid flow• observe the mass and volume of a liquid, and calculate its density using the formula d m/v [Note: Thisoutcome does not require students to perform formula manipulations or solve for unknown terms other thanthe density.]

compare densities of materials: and explain differences in the density of solids, liquids and gases, usingthe particle model of matter• describe methods of altering the density of a fluid, and identify and interpret related practicalapplications• describe pressure as a force per unit area by using the formula p F/A, and describe applications ofpressure in fluids and everyday• investigate and compare the compressibility of liquids and gases

Topic 4 - Flow Rate and Viscosity

Viscosity -

Flow rate -

Viscosity and ProductsThe viscosity of different products is very important.List some fluid products you use and ask yourself;Would I still use this product if the viscosity were higher/lower?

Product Higher Lower

Ketchup ProblemYou and your group members are food scientists employed by a consumer report magazine. Yournext assignment is to test a variety of ketchups to determine which is the best brand in terms ofviscosity.

Your task is to design an experiment which will test the viscosity and flow rates of several brands ofketchup. In your experiment and write up you must include:

a question for investigation (related to viscosity and flow rate)—0. a hypothesis— a list of variables (including controlled, manipulated, and responding)

— a list of materials and equipment— a procedure for the experiment

a plan for data collection

Once you have designed your experiment and done the pre-lab write-up work, your group willconduct your investigation, record your results, and each member will write a concluding blog postsharing their findings with the group. The post will be one to two paragraphs briefly describing howyou conducted your experiment, a description of your results, and your recommendations on whichbrand had the best viscosity.

You will have:—two classes to do the pre-lab work (including learning any pertinent information,

designing your experiment, and doing the pre-lab write up)

—0. one class to conduct the experiment, record observations, and begin writing yourconclusion

—one class to complete your conclusion, and create your blog post

C

Changes n Viscosfty

The viscosity of a liquid, decreases as it is heated and increases as it iscooled.

The viscosity of a gas increases as it is heated and decreases as it iscooled.

Temperature has the opposite effect on the viscosity of gases as onliquids.

The particle model can help explain several important points about the density ofsubstances:

• different substances have different sized particles:the size of the particlesdetermines how many particles can11fit into” a given space

• the particle model helps us visualize the empty space between particles of matter

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Topic 5 - berisityUsing everyday words, density can be described as the“crowdedness” of the particles that make up matter.

In scientific terms, density is the mass per unit volume ofa substance

States of Matter

1iqud soLd

• the spaces around the particles reduce the density of the substance

bensity of Solids, Liquids and GasesHow is the density of a substance related to the substances physical state?

States of Matter

gas liquid Bolid

According to the particle model:

• solids are more dense than liquids and gases• liquids are more dense than gases• gases have the most space between particles so they are the least dense

iY

____

()(®)

____

solid liquid gas

• the particles of a substance are the same size, but the spaces between theparticles differ depending on the state of matter they are in.

What is density?Helium gas is very light. It is lighter than air.But “lighter” isn’t really the right word to usehere. Scientists say that helium is less densethan air because it weighs less than the sameamount,or volume, of air. Another way ofdescribing helium is to say that it has a lowerdensity than air. Carbon dioxide is moredense than air, so we say it has a higherdensity than air.

One way of understanding density is toimagine three boxes, all the same size.One box is full of air, one is full of helium, andone is full of carbon dioxide. If you weighedthe boxes, the one containing carbon dioxidewould weigh the most and the one containinghelium would weigh the least.

There would be the same volume of gas ineach box, but their different densities wouldmake them have different weights.

Here are some problems for you to solve!1. The balloon contains air. Doesthe box contain helium or carbondioxide? Could the box contain air?

2. The balloons are exactly thesame size. One balloon contains air.What gas might the balloon marked? contain?

Gases all have differentdensities.Some gases, such as helium, arelighter than air. Others, such ascarbon dioxide, are heavier than air.These boxes ofgas all contain thesame volume. They are beingweighed on scales which measureveiy small amounts.

79

bensity of Solids, Liquids and GasesLiquids• you cant walk on water because your foot pushes the particles of water aside• the particles of a liquid move apart easily, allowing a dense, solid object, such as your foot,to pass through the liquid• The attractive forces between liquid particles are not strong enough to prevent your footpushing them apart

Gases• gases are even less dense than liquids• when you move through air, you are moving through mostly empty space• there are less particles to move out of your way than in liquids

Solids• Particles in a solid cannot be pushed apart• the particles in solids are very strongly attracted to each other

7.-

J .,

bensity: How are mass and volume related?To measure the density of a substance, you first need to know howmuch of the substance occupies a certain space.• Mass is the amount of matter in a substance• Volume is a measurement of the amount of space occupied by asubstance

‘ Mass and weight are not the same thingWeight is the force of gravity exerted on an object

The mass to volume ratio is the relationship between mass andvolume expressed as a quantity of the mass (m) divided by itsvolume (V).

b (density) m

V

Basically, density is the amount of matter (mass) in a certainspace (volume), or the “crowdedness” of a substance.

You can find the density of a substance by calculating the mass-to-volume ratio.

m

V

for example: the density of an object having a mass of 10 g and a volume of2 cm3 can be found the following way:

bzlOg

2cm3

tz5g/cm3

• as long as the temperature and pressure stay the same, the mass-tovolume ratio, or density, of any pure substance is a constant, which meansit does not change.

Part F: How can we use math to help us in science?

As with many things in science, we can use math to figure out the density of a fluid. Thefollowing mathematkal formula (problem) can be solved to find density.

‘I’—

I,—

VDensfly equals Mass dMded by Volume

As long as we know the mass (how heavy It Is) and the volume (how much space ii takesup), we can figure out the density (how tightly packed the particles are).

Use the density formula to help you calculate (figure out) the density of each ofthe following fluids and then answer the question below.

.Calculation:

Answer:

Calculation:

Answer:

Statement:

The density of water is g/mI.

Statement:

The density of syrup is g/mL

1. How does the density of water compare to the density of syrup?

2. What does that tell you about the buoyancy (ability to float) of objects in syrupcompared to the buoyancy of those same objects in water?

Copyright © &source Development Services, Edmonton Public Schools, 2003. Grade 8 • \4ix and Flow oF Matter

14

.

Student Exploration: Density

Vocabu1a: density, mass, matter, volume

Prior Knowledge Questions (Do these BEFORE using the Gizmo.)

1. List three objects that you think would sink in water.

Gizmo Warm-up

1. In the Gizmo sect an object and drag it onto the scale. Mas isthe amount of matte’, or “stuff,” in an object.

A. Which object did you choose?

___________________________

B. What unit of measurement is used for mass?

______________

C. What is the object’s mass?

_______________________________

2. Drag the object into the graduated cylinder. The number above thecylinder gives the volum, the amount of space the object takes up.

A. What unit of measurement is used for volume?

____________

B. Which object did you choose?

___________________________

C. What is your object’s volume?

___________________________

D. Drop the object into the beaker of water. Does it sink or float? —

Graduated Cylinder

Name:

______________________________

Date:

11

2. List three objects that you think would float in water.

3. Why do you think some things float and some things sink?

C 2007 ExporeLerning. AI rights reserved.

—Get the Gizmo ready:Activity A:

Sink or float?• Replace all objects on the shelf. ). Be sure the liquid in the beaker is Water.

I L..

Question: How do mass and volume affect sinking and floating?

1. Predict: Which objects will float in water? Which will sink? Record your predictions below.

Prediction ResultObject Mass Volume(sink or float?) (sink or float?)Ping pong ball

Golf ball

Apple

Chess piece

Penny

Rock

2. Experiment: Use the Gizmo to find the mass and volume of each object and whether it floatsor sinks. Record your results in the table.

3. Analyze results: Look at the data in your table.

A. Can you use mass alone to predict whether an object will sink or float? Explain.

B. Can you use volume alone to predict whether an object will sink or float? Explain.

4. Draw conclusion: Can you use mass and volume to predict whether an object will sink orfloat in water? Explain your thinking.

5. Apply: Measure the mass and volume of the toy soldier: Mass

_________

Volume

Will it float or sink?

__________________

Use the Gizmo to test your prediction.

2007 ExploreLearning. AU righb reserved.

4A

Get the Gizmo ready:Activity B:

• Replace the objects on the shelves.Calculating densitt

• Be sure the liquid in the beaker is Water.

Question: How does density tell you whether an object will sink or float?

1. Calculate: Densi is the amount of mass in a certain volume. To find the density of anobject, divide its mass by its volume. Density is recorded in units of grams per milliliter(gImL). What is the density of an object if its mass is 100 g and its volume is 50 mL?

2. Record data: In the Gizmo, find mass and volume of the objects listed below. Then calculateeach object’s density and record it. Finally, test whether each one sinks or floats in water.

Object Density Sink or Float?

Chess piece

Rock

Toy soldier

Apple

3. Draw conclusion: The density of water is 1.0 g/mL. Look at the data in your table. How canyou use the density of an object to predict whether it will sink or float?

4. Apply: In the Gizmo, either Crown I or Crown 2 issolid gold (but not both) Find the density of thegold nugget and of each crown. (Hint: You will .. .

probably need a calculator to do this.) Lcwr12

A. Density of the gold nugget:

____________________

B. Density of Crown 1:

__________________________

C. Density of Crown 2:

__________________________

D. Which crown is pure gold?

____________________

2007 EpIorLearnincj AW rights rv d

4Activity C: Get the Gizmo ready:

Egg-speriment • Replace all the objects on the shelf.

Question: How does an object behave in different liquids?

1. Observe: Use the Gizmo to explore whether the egg sinks or floats in different liquids.Record what you find in the table below.

2. Draw conclusion: Which liquids are denser than the egg? Which are less dense? Explainyour reasoning.

3. Extend your thinking: Observe the egg in each liquid again.

A. In which liquid does the egg float the highest?

_______________________________

B. In which liquid does the egg sink the fastest?

_______________________________

C. Which liquid do you think is the densest? Least dense? Explain.

_______________

4. Challenge yourself: Using the objects in the Gizmo to help you, list the liquids from densestto least dense. Discuss your answer with your teacher and classmates. (Hint: Comparewhere objects float within each liquid.)

2007 ExpIoreLearnngAH rights eserd.

Topic 6: BuoyancyBIN Nye on Buoyancy Part 1...

Part 2...

Part 3...

Buoyancy is the tendency for materials to rise or float ina fluid.

Buoyant Force is the upward force acting on objectssubmerged in fluids.

• if the buoyant force is less than the weight of an object, the object will sink• if the buoyant force is greater than the weight of an object, the object will rise• if the buoyant force is the same as the weight of an object, the object will float

• Buoyant forces oppose the force of gravity. Gravity pullsthings down to the center of the earth. Buoyancy pushesthings up away from earth.

• Both gravity and buoyancy are measured in Newtons (N).

. Water can support objects that have a greater density thanwater as long as the design of the object in the water distributesthe weight across a large enough area.

bemonstration

Investigation - Build a bensity Tower

p.60

bemonstration

Find Out Activity - Cartesian biverp.61

• Average density: the total mass of all substances divided bythe total volume. Large ships can float based on this principle.

total massAverage bensty =

total volume

• Submarines, blimps and fish also rise and descend in waterbased on changes to their average density.

• Neutral buoyancy occurs when gravity equals buoyancy: anobject is NOT sinking or rising.

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Archimedes principle: The buoyant force acting on an objectequals the weight (force of gravity) of the fluid displaced by theobject.

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• The buoyant force does not depend on the weight of thesubmerged fluid, but on the weight of the displaced fluid.

;i Explç ning

Date:

Student Exploration: Archimedes’ Principle

Vdcabu1ai: Archimedes’ principle, buoyant force, density, displace, mass, volume, weight

Prior Knowledge Questions (Do these BEFORE using the Gizmo.)

1. Why does a small pebble sink in water?

________________________

2. A motorboat is a lot heavier than a pebble. Why does the boat float?

Gizmo Warm-upWhen you place an object in liquid, the downward pullof gravity causes it to start to sink. As the object sinks,the liquid pushes back up on the object with a forcethat opposes gravity.

In the Archimedes’ Principle Gizmo, you will seehow these forces cause objects to either sink or float.

1. Check that the Width, Length, and Height of theboat are set to 5.0 cm. Drag one of the green 50-gcubes into the rectangular “boat.”

What happens?

________________________

fr—

Boat’—b’i +

2. Add cubes until the boat sinks. What mas of cubes causes the boat to sink?

(Note: In this Gizmo, the mass of the boat itself is insignificant.)

3. Click Reset. Experiment with different boat dimensions until you create a boat that holds themost cubes without sinking.

A. What are the boat’s dimensions? Width: Length:

_______

Height:

_______

Name:

B. How much mass can the boat hold without sinking?

Get the Gizmo ready:Activity A:

• Click Reset.Displaced liquid • Set the Width, Length, and Height to 5.0 cm.

.__Be_sure_the_Liquid_density_is_set_to_1.0_g/mL.

Question: How does the mass of the boat relate to the amount of displaced liquid?

1. Observe: Place several of the 50-g cubes into the boat. What happens to some of the liquid

in the tank?

The liquid that is pushed into the graduated cylinder is called dipiace4 liquid.

2. Predict: How do you think the mass of the boat will relate to the amount of displaced liquid?

3. Observe: Click Reset. Drag two cubes into the boat, yielding a total mass of 100 grams.

How much water is displaced into the graduated cylinder? (Units are mL.)

______________

4. Experiment:Click Reset. Choose a new set of boat dimensions. Add cubes to the boat andrecord the volume of displaced liquid. (If the boat sinks, try a larger set of dimensions.)Record your findings for three boats in the table (include units). Leave the last column blank.

Width Length Height Boat mass Volume of displaced Mass of displaced(cm) (cm) (cm) (g) liquid (mL) liquid (g)

5. Calculate: Deisitj is equal to mass per unit volume. To calculate density, divide an object’smass by its volume.

If the liquid’s density is I gram per milliliter (the density of water), the mass in grams is equalto the volume in milliliters. Use this information to fill in the last column of your data table.

6. Draw conclusions: What is the relationship between the mass of the boat and the mass of

the displaced liquid?

________________________________________________

umo%

Get the Gizmo ready: IActivity B:i

--• Click Reset.

IHow low does it• Be sure the Liquid density is set to 1.0 g/mL. :go?• Set the Height of the boat to 10.0 cm.

Introduction: In activity A, you learned that, for floating boats, the mass of the boat is equal tothe mass of displaced liquid. You can use this knowledge to predict how deep a boat will sink.

Question: How far will a boat sink in water?

1. periment: Turn on Magnify waterline. Experiment with several different sets of boatdimensions and loads. In the table, record each boat’s width, length, and mass; the depth towhich it sinks, and the volume of displaced liquid. Leave the last column blank.

Width Length Boat Sinking Volume of displaced(cm) (cm) mass (g) depth (cm) water (mL)

2. Calculate: Label the last column in your table Volume underwater. To calculate the volumeof the boat that is underwater, multiply the width, length, and depth of the boat. Record theunderwater volume of each boat. The units of volume are cm3 and mL (1 cm3 = 1 mL).

3. Analyze: What is the relationship between a boat’s mass, the volume of displaced water,

and the volume of the boat that is under water?

_____________________________________

4. Make a rule: If you know the width, length, and mass of a boat, how can you calculate how

deep it will sink in water?

________________________________________________________

5. Practice: Based on what you have learned, calculate how deep each of the following boatswill sink. Use the Gizmo to check your answers.

Boat Width Length Boat Sinking depth Sinking depthmass (calculated) (actual)

A 8.0cm 5.0cm bOg

B 6.0cm 5.0cm 150g

(Activity B continued on next page)

Activity B (continued from previous page)

7. Predict: Not all liquids have the same density as water. How do you think increasing thedensity of the liquid will change each of the following?

A. How far the boat sinks into the liquid:

________________________________________

B. The volume of displaced liquid:

_____________________________________________

C. The mass of displaced liquid:

______________________________________________

8. Observe: Set the Width, Length, and Height of the boat to 5 cm. Add one cube to the boat.Move the Liquid density slider back and forth.

What do you notice?

____________________________________________________________

9. Gather data: Measure how far the boat sinks into liquids with each density listed below.Click Reset between each trial. Calculate the volume and mass of displaced liquid. (Note:The mass of the displaced liquid is equal to the volume of the liquid multiplied by its density.)

Boat mass

Liquid Sinking Volume of displaced Mass of displaced

density depth (cm) liquid (mL) liquid (g)

50g 0.5g/mL

50g 1.OgImL

50g 2.Og/mL

10. Analyze: In the first part of this activity, you discovered that when a boat is placed in water,the volume of displaced water is equal to the mass of the boat. What is true now?

11. Summarize: If you know the length, width, and mass of the boat as well as the density of theliquid, how would you calculate how far the boat sinks into the liquid?

12. Practice: A rectangular boat has a width of 5 cm, a length of 8 cm, and a mass of 150 g.How far will the boat sink into liquid with a density of 1.2 gImL? Check your answer.

imas

Activity C: Get the Gizmo ready:

Weight and • Click Reset, and turn off Magnify waterline.

buoyancy • Set the Width, Length, and Height to 10.0 cm.

Introduction: When a boat is placed in liquid, two forces act on the boat. Gravity pulls the boatdown with a force equal to the wei&j! of the boat. Weight is measured in newtons (N). Tocalculate the weight of a boat, multiply its mass in grams by 0.00982.

As the boat sinks into the llquid, the liquid pushes back. The force of the liquid pushing up onthe boat is called the buoyartforc.

Question: How do gravity and the buoyant force affect a boat?

1. Observe: Turn on Show data. Place four cubes in the boat.

A. What is the Boat weight?

________

B. What is the Buoyant force?

________

C. What is the Net force on the boat?

2. Analyze: Try dragging the boat up or down. Pay attention to the Buoyant force.

A. What happens to the buoyant force when the boat is pulled down?

_______________

B. Why do you think this happens?

____________________________________________

C. What happens to the buoyant force when the boat is lifted up?

_____________

D. Why do you think this happens?

_______________________________________

3. Explore: Answer the following questions by dragging the boat up or down in the liquid.

A. What happens to the boat when its weight is greater than the buoyant force?

B. What happens to the boat when its weight is less than the buoyant force?

C. What happens to the boat when its weight is equal to the buoyant force?

(Activity C continued on next page)

‘iLmOS

Activity C (continued from previous page)

4. Observe: Click Reset. Set the Liquid density to 1.0 gImL. Add a 50-g cube to the boat.

A. What is the weight of the boat?

______________

B. What is the mass of the displaced liquid in the graduated cylinder?

______________

C. What is the weight of the displaced liquid?

______________

(Hint: If the mass is measured in grams, w = m 0.00982.)

D. What is the Buoyant force on the boat?

_____________

5. Predict: What do you think is the relationship between the buoyant force and the weight of

displaced liquid?

6. Collect data: As you add cubes to the boat, record the boat’s weight, the mass of displacedliquid in the graduated cylinder, the weight of displaced liquid, and the buoyant force.

Number of Boat Mass of displaced Weight of Buoyant forcecubes weight (N) liquid (g) displaced liquid (N) (N)

2

3

4

7. Analyze: What do you notice?

8 Make a rule Archlmede& principl states that an object is pushed up by a buoyant force

that is equal to the

______________________

of the displaced liquid.

9. Aly: A hollow ball weighs 40 newtons. In a water tank, it displaces 15 newtons of water.

A. What is the buoyant force on the ball?

________________________________________

B. Will the ball float or sink? Explain your reasoning.

______________________________

iimos

0

The Amazing Egg Hydrometerfind out activity page 66

Table 1: Buoyancy of Egg Based on Solution

solution observations

water

salt water

sugar water

epsom salts

sodium bicarbonate

How Buoyancy and bensity are RelatedThe buoyant force of an object does not depend on physical

state, but rather on density.

This means that something with a lower density will sit on top of(or float) on something with a higher density

• You saw in the Egg Lab that salt waterholds an object better than fresh water.This is because salt water has a higherdensity than fresh water.

• This means the particles are closertogether so salt water can supportmore weight per unit volume thanfresh water.

hydrometer: an instrument designed to measure liquid density.

A hydrometer will extend further out of a liquid if the liquid has a higherdensity, and will sink lower if the liquid has a lower density.

Density vs. Buoyancy

Consider: Does density affect buoyancy and, if so, how?

Important Information: The density of water is “one.” When objects are the same size, theones that are more dense usually feel heavier. Objects that aremore dense are made up of more particles packed closer together.

Hypothesis: (check the one you agree with)

D objects that are less dense than the fluid they are placed in will float

D objects that are more dense than the fluid they are placed in will float

Materials:

• 5 small objects that are the same size but have different weights

• 5 clear (glass) containers filled with two cups of water

Procedure/Analysis: (record information in the chart below)

1. Place each object in the water (one at a time).

2. Record whether each object floats or sinks.

Object: Object: Obiect:

Analysis/Conclusion:

1. Which objects had a density greater than water? How do you know?

2. Which objects had a density less than water? How do you know?

3. Now, answer the original question using evidence/examples from your investigation ORprove why your hypothesis was correct.

Copyright © Resource 1)evelopment Services, Edmonton Public Schools, 2003. Giade 8 • Mix and llow of Matter

11

• ‘‘1 4 1/ •‘

‘ (,_ I• ‘.

• •

.Playing with Buoyancy

Consider: How does adding salt to water affect the buoyant force?

Important Information: Buoyant force is how hard it pushes “up” to keep objects floating.Remember what you learned about The Particle Theory of Matter—the salt particles are now taking up extra space in the fluid.

Hypothesis: (check the one you agree with)

D adding salt to water will increase the buoyant force (make more objects float)

D adding salt to water will decrease the buoyant force (make more objects sink)

Materials:

• five small objects used in Investigation 4

• clear (glass) container of water

• I cup of salt (may not need all of it)

• spoon (or something else to stir with)

Procedure: (record information in the chart below)

1. Mix salt into water one spoonful at a time until no more will dissolve.

2. Repeat procedure from Investigation 4.

oObject: I Object: Object: Object: Object:

Analysis/Conclusion:

1. How did adding salt affect the density of water? How do you know?

2. Now, answer the original question using evidence/examples from your investigation ORprove why your hypothesis was correct.

_ __ _ __1•

Copyright © Resource Development Services, Edmonton Public Schools, 2003. Grade 8 • Mix and Flow of Matter

12

Topic 7: Fluid PressurePressure is a measure of the force acting perpendicular

to a unit area.

Pressure (P): Force (F) or P £Area (A) A

Force is measured in Newtons (N) and area is measured insquare meters Cm2).

• The unit of pressure is Newtons/rn2also known as Pascals(Pa), after Blaise Pascal.

kilopascal (kPa) is equal to 1000 Pa.

Compression of a GasHow can a gas be compressed??

The gas must be enclosed in a sealed container with sturdy walls.Gas molecules distribute themselves equally throughout the container.

• There is so much space between particles that even after theparticles are squeezed closer together, they are still far enoughapart to behave as a gas.

• An external or outside force is applied to the enclosed gas to pushthe particles closer together.

• Gases are compressible because their particles can be squeezedcloser together into a smaller volume.

The spaces in liquids and solids are already almost as small aspossible.

. When force is applied to a solid or a liquid, the particles cannot movemuch closer together and are incompressible.

The particles themselves do not move in the direction of theforce because they have no space to move.

Instead of changing the volume of solids and liquids, an outside forceis passed from one particle to the next, throughout the substance.

Uses of Compression

• Gases exert a force back (a counterforce) when they arecompressed. This property can be used to cushion shocks.

For example,• car tires push back against the force exerted by the car• air bags in cars use compressed nitrogen gas to protect a driver in acollision

‘.

—

II tho *rn,,,t In WatIoa in ollinn dntiWhen y.wr compeler rn.nhnn, an dr bag In ,rtlnwz4

.0 bOO wont bang your bead in tnttcflaIIoo.

Atmospheric Pressure

• Air has mass. If that is the case, then why do we not feelthe weight of the air on us all the time?

Earths atmosphere is in balance with your body, bothinside and out. Air pressure is hardly noticed.

Iretjr

.—---J ;JfEH-

Measuring Air PressureBarometers are used to measure air pressure.

Barometer-—

latex balloon

Lsoda strawselastic band

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‘‘erury AScare

80 Whatisairpressure?Find out more by looking at Look out of your window. Is the air moving? It is probablypages 66—67 moving enough to make the branches on the trees sway

slightly. The weight of this air pressing down all around theearth produces air pressure. If you live high up above sea leveor even if you go up to the top of a tall skyscraper, the airpressure is less. This is because there is less air above you, anctherefore less air pressure.

On the surface of the earth, or near it, air always moves fromareas of high pressure to areas of low pressure. Temperaturehas a great effect on air pressure. As warm air rises, it producean area of low pressure near the ground. As the air pressurechanges from place to place, and from day to day, it helps movair across the earth’s surface all the time. As cooler air movesdown, it leaves behind an area of high pressure.

Cool airis heavier than warm air.Cool air sinks down from an area of high pressure

high pressure, while warm airrisesfrom an area of lowpressure.

How do barometers work?You can forecast what the weather will be like for the next fewhours by measuring the air pressure.

The instrument used to measure changes in air pressure iscalled a barometer. If you have one at home, it is probably ananeroid barometer. There’s a needle behind a glass-covereddial marked “Fine,” “Fair” and “Stormy.” If you tap the glasslightly in the morning, the needle points to the weatheryou canexpect that day.

warm air

Types of barometersInside the aneroid barometer is a metal box, or chamber, fromwhich almost all the air has been removed. The metal of thechamber is so thin that small changes in the air pressureoutside it make the metal bend. This makes the needle turnround and point to a new position on the dial.

A mercury barometer measures the air pressure moreaccurately. Along, glass tube is filled with mercury, a silveryliquid metal. The tube is sealed at one end, and placed openend down in a small dish of mercury. Some of the mercurystays in the tube. It is held there by the air pressure at thesurface of the mercury in the dish. As the air pressurechanges, the column of mercury moves up or down. A scalebeside the tube shows the pressure measurements.

column of mercury

glass tube

81

The aneroid barometer contains asmall metal chamber which issensitive to changes in airpressure.

glass

pressure

The mercury barometer is aninstrumentfor accurate forecastingof the weather.

Balanced and Unbalanced Forces

If the inside of a closed container experiences a lower airpressure than the air pressure pushing on the outside, thewalls of the container will buckle and cave in.

Measuring Mr Pressure

• Barometers are used to measure air pressure.

Barometer

latex balloon

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elastic band

empty juice cart scal

Gass Vacuum

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Part E: How do fluids behave?

Before you continue, it’s important to learn some of the key words that will be used inthe rest of this section. Read these definitions carefully and then complete theactivity on the next page.

ViscosityViscosity is a word used to describe how “thIck” a fluid is. A fluid that is thick Is very viscousand a fluid that Is thin is not very viscous. The viscosity of a fluid affects Its flow rate (how fast itmoves when it is poured).

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DensityDensity Is a word used to describe how close together, or tightly-packed, the particles (tinypieces) In a fluid (or other material) ore. The more particles there are In a small amount of fluid,the more dense it Is, Fluids with fewer particles in the same amount of space (so they are lesstightly-packed) are less dense, $4

Buoyancy +Buoyancy Is a word used to explain whether things will float or sink in a fluid. The fluid’sbuoyant force is how much it pushes “up” at objects (floating) to fight against gravity that pullthem down (sinking). Even though buoyancy really describes the fluid, we often use It todescribe how the objects behave. Things that are buoyant float and things that are notbuoyant sink.

CompressibflltyCompressiblilty Is a word used to describe whether a fluid can be “squeezed” into a smallerspace. Liquid fluids cannot be compressed—only gases can be compressed (squeezed into asmaller space).

Copyright © Resource Development Scrvices Edmonton Public Sthools 2003 Grade 8 Mix and I low of Matter

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Fill in the blanks in the following sentences with words from page eight. The blanksare there to help you check how many letters are in the missing words.

1. A fluid that is very thick is very . A fluid that is not veryviscous is very

2. How quickly a fluid flows is called its

3. is a word that describes how thick or thin a fluid is.

4. Fluids with a lot of particles in a small amount of space are very

5. is a word that describes how close together (or tightly-packed) particles are.

6. A fluid’s force describes how much it pushes “up” at objects tofight against gravity making them sink.

7. Buoyancy is a word used to describe whether an object willor

8. is a word used to describe whether a fluidcan be squeezed into a smaller space.

9. fluids can’t be compressed but can.

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10. Do you think the density of a fluid affects its buoyant force? Explain your answer.

11. How do you think viscosity affects flow rate? Explain your answer.

(:op;right © Resource 1)evelopment Senrices, Edmonton Public Schools, 2003. Grade 8 • Mix and Now of Matter

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Study this graphic and read the information below it. Then, use what you havelearned about fluids to answer the question.

Once the liquid Is added to a bottle or can of pop, carbon dioxidegas (another fluid) is dissolved in the liquid. Once a bottle or canof pop is opened and poured into a glass (like the picture above),the carbon dioxide bubbles rise to the top of the liquid and burstinto the air.

1. What does a glass of pop tell us about the density of carbon dioxide gas?Explain your answer.

Copyright © Resource Development Services, Edmonton Public Schools, 2003. Grade 8 • Mix and Flow of Matter

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