solvent is heated to change it into a example, cm3 the ...grade7byng.pbworks.com/w/file/fetch/15069788/S7 Heat Transfer TEXT... · direction of energy transfer the ... radiation (EMR)

consumers organisms that eat thefood made by producers; can beeither herbivores, carnivores, oromnivores

continental drift a theory aboutEarth's structure; according to thistheory, the continents have slowlychanged their positions over time;the slow movement of continents

continental shelf the gradual, slop-ing edge of a continent that extendsout beneath the ocean

contract of substances, to shrink ordecrease in volume

control in a scientific experiment, astandard to which the results arecompared; often necessary in orderto draw a valid conclusion; ensures afair test

controlled variable in an experi-ment, a condition that is notallowed to change

convection current continuous cir-culation of a fluid (either a liquid ora gas), in which thermal energy istransferred from hotter, less densefluid to colder, more dense fluid

convergent boundary an area onEarth's crust where two plates arepushing against each other

co-ordinate graph a grid that hasdata points named as ordered pairsof numbers; for example (4,3)

core the innermost part of Earth;made of iron and nickel in solid andliquid form

corrugated [KOHR-ruh-gae-ted] ofmaterials, to be made of layers, witha middle layer that is folded into aseries of triangles to providestrength

counterweight a device used to bal-ance potentially dangerous forces onan object or a structure

criteria a set of standards or expec-tations; specifications for a design

crust the thin, outermost layer ofEarth

crystal the building block of miner-als; crystals occur naturally and havestraight edges, flat sides, and regularangles

cubic units the units used to reportthe volume of a substance; forexample, cm3

cycle concept map an events chainmap in which .a series of eventsdoes not produce a final outcome;this type of concept map has nobeginning and no end

Odata facts or information

database an organized or sorted listof facts or information, usuallygenerated by computer

dead load the weight of a structureupon itself

decomposers organisms that breakdown the cells of dead or wastematerials and absorb their nutrients;many bacteria and fungi are decom-posers

dependent (or responding) variablein an experiment, a condition that ischanged as a result of changes toindependent variables

deposition the process in whicheroded material is deposited inanother area

desalination [dee-sal-i-NAE-shuhn]a process for removing the salt fromsalt water

desertification [de-zuhrt-i-fi-KAE-shuhn] the process in whichdeserts are formed through theerosion of nutrient-rich topsoil;after desertification the soil is nolonger able to support plant life

dew point the temperature at whichair becomes saturated with watervapour, causing precipitation

dilute to weaken the strength of asolution by increasing the amountof solvent

dilute solution a solution thatcontains relatively little solute

direction of energy transfer thetendency of energy to move from aconcentrated source; for example,thermal energy always moves fromhotter objects to cooler ones

dissolving mixing a solute com-pletely with a solvent to form asolution; the distinct properties ofeach of the materials combine intoone set of properties

distillation a process for separatingthe parts of a liquid solution; thesolvent is heated to change it into a

gas, then converted back to a liquidstate through condensation

divergent boundary an area ofEarth's crust where two plates arepulling apart from each other

diversity a measure of how manydifferent species live in an ecosys-tem; an ecosystem with manyspecies has greater diversity than anecosystem with only a few species

dormant of a volcano, a stage whenno eruption is occurring

double cantilever a design used tosupport a great deal of weight; con-sists of beams that are braced onboth ends and held up by a strongcolumn at the centre

Eq

earthquake a disturbance andmovement of Earth's crust due to abuild-up of stress

ecologist [ee-KOL-oh-jist] a scien-tist who studies interactionsbetween the abiotic and biotic partsof the environment

ecology the study of how organismsinteract with each other and theirenvironment

ecosystem all the interacting partsof a biological community and itsenvironment

elastic energy a type of potentialenergy that is stored in an object,the shape of which is changed bybending, twisting, or compressing

electromagnetic radiation (EMR)energy that is transferred in theform of electromagnetic waves;examples of EMR include radiowaves, X rays, and microwaves

clement a type of pure substance(made of one type of particle oratom) that cannot be broken clown

into simpler parts by chemicalmeans and that has a unique set ofproperties

energy the ability to do workand to cause change (chemical orphysical)

energy flow the movement ofenergy, which originally comes fromthe Sun, from one organism toanother

502 Glossary

9-1 Methods of "'thermalEnergy Transfer

Imagine holding your hand near a light bulb or in front of a hot fire. You can feelthe warmth. Your skin warms up because it receives thermal energy from the bulb.The light bulb is an energy source: an object or material that can transfer itsenergy to other objects. In this section, you will study three ways in which energycan be transferred: radiation, conduction, and convection.

Radiation Transfers Energy

AoA( ak ca>7

Figure 9 .2 The ripples in this pond are evidence of energy transfer.

Figure 9 .1 A tsunamicarries enormous amountsof energy from its source,an underwater earthquake,across thousands ofkilometres of ocean. Whenthe wave hits land, theenergy can devastate

buildings and the naturalenvironment, as well assometimes costingthousands of lives.

If you toss a stone into a pond, energy is transferred from the stone to the water. A

fast-moving stone has a great deal of kinetic energy. When the stone hits stillwater, it slows down - it loses energy. Some of the stone's energy is transferred tothe water, so the water gains kinetic energy. As ripples spread across the water, thekinetic energy of the stone (the energy source) is transferred to, and across, thewater. Ripples, like all waves in water, air, or even empty space, transfer energyfrom one place to another.

Of course, water waves can transfer energy through water. Radiation is thetransfer of energy in a special form of wave that can travel through many materialsor empty space. Energy that is transferred in this way is called radiant energy andit is carried by electromagnetic radiation (EMR). There are many differentforms of EMR, including radio waves, microwaves, visible light, and X rays. Ifthe energy source is a warm object, such as the Sun or even your body, some of itsthermal energy is transferred as a type of EMR called infrared radiation (IR)

or "heat radiation." All of the different forms of radiant energy share severalcharacteristics:• They behave like waves.• They can be absorbed and reflected by objects.• They travel across empty space at the same very high speed: 300 000 km/s.

242 Thermal Energy and Heat Technology

Conducting Energy ThroughSolidsIn solids, where particles are close together, thermalenergy can be transferred directly from one particleto the next. Thermal conduction is the process oftransferring thermal energy through direct collisionsbetween particles. Study Figure 9.3 to see how con-duction transfers energy.

Figure 9 .3A Particles near the heat source absorb energy fromit and begin moving more rapidly.

Figure 9.3B The fast-moving particles bump into neighbouringp9rticles, increasing their energy and motion.

® ®J ®1

® ® S

®J

Figure 9.3C In this way, thermal energy is transferredthroughout the material.

Most metals, especially gold and copper, are excel-lent heat conductors. A hot stove burner touchingone part of a copper saucepan, for example, soonheats the entire pan. Other solids, such as glass andwood, are much less efficient at transferring thermalenergy by conduction. Poor conductors are calledheat insulators. Foam plastic cups and containers,the puffy inner layers of winter clothing, and theinsulation in walls are all poor conductors. Wheninsulators are wrapped around an object, they slowdown the transfer of thermal energy to or from thesurroundings. The object stays warm or cold longer.

Find Out

The Super StirrerCan you predict how well a substance will con -duct heat, so you can choose the most suit-able material for a particular use? Try it. Findthe material that will make the best stir stick.

What You Need

equal-length pieces of plastic from a pen,pieces of copper wire, long iron nails, woodencraft sticks, or wooden pencils

plastic cup of very hot water

Safety Precautions 1 MG

Handle the hot water with care.

13

What to Do

1. Predict which of your sample stir stickswill be the

(a) best conductor

(c) best insulator

(b) worst conductor (d) worst insulator

2. Place one end of each sample in the hotwater. Wait 1 min.

3. Touch the inside of your wrist to the topof each sample to identify the warmestone (the best conductor). Remove it fromthe cup and record which materials madethe best conducter.

4. Wait another minute. Then repeat step 3 tofind the second-best conductor. Continueto repeat step 3 until you have ranked allof the samples in order, from the best tothe worst conductor.

What Did You Find Out?

1. Explain which of your samples would bethe best for making

(a) a stir stick

(b) the bottom of a frying pan

(c) the handle of a frying pan

(d) a container for delivering hot pizza

2. How might the particles in your bestinsulator differ from the particles in aconducting material?

Energy Transfer Systems 245

11

Keeping in the Warmth

Fibreglass building insulation

Insulators that are used in building construction

are rated by their RSI value. This value describes

the resistance of a 1 cm thickness of a material

At Home

to heat conduction. Materials with higher values

are better insulators, Some typical values are

given in the table below.

Material RSI per cm

blue plastic foam panels 0.35

white plastic foam panels 0.29

fibreglass 0.24

vermiculite 0.16

plywood 0.087

glass 0.017

Extra thickness increases the RSI value. For

example, a 3 cm thickness of fibreglass would

have an RSI value of 3 x 0.24 = 0.72. Only

2 cm of blue plastic foam would provide

about the same resistance to heat conduction

(2 x 0.35 = 0.70).

What to Do l

Try to find out what type of insulation is used in

your home and how thick it is. Then calculate its

total RSI value. If the material is not listed in the

table, check with a building materials store, in the

library, or on the Internet to find its RSI value.

Convection, Energy on the Move

Thermal energy can be transferred in a third way

by fluids: materials that can be poured or that flow

from place to place. A hot, fluid may force its way

up through a colder fluid. In convection, the warm

fluid, itself, moves from place to place, carrying the

thermal energy with it. The moving fluid is called a

convection current. Study Figure 9.5 to identify

the different parts of a convection current. Then

read on to learn the details of how a convection

current operates.Figure 9 .4 Smoke trails in this

apparatus show how air moves

in a convection current.

246 Thermal Energy and Heat Technology

Why do fluids, at different temperatures, rise,sink, and create convection currents? Rememberthat materials expand as they warm up. Their parti-cles move farther apart. Each section of the warmed

Qmaterial is left with fewer particles than when it wascold, so each section is a bit lighter than it used to

heat sourcebe. In other words, the warmed material becomes

Iless dense. Colder, denser fluid sinks down and

---o warm airpushes nearby warmer fluid upward. Then this cold

P. cool airfluid, too, is warmed and pushed upward.

As warm fluid rises and moves away from theheat source, it cools. It contracts as its particles

Q Warmed air

Q The cool, denser

move closer together. It becomes denser and sinks

particles expand.

air sinks.

back down toward the heat source, where it is

Q Less dense, warmer

The cool air moves

warmed and forced upward. As the whole process

air rises.

in to replace the

Q The rising air cools

rising warm air.repeats, a continuous movement - a convection

current - forms.

and contracts.

Convection currents occur throughout Earth'soceans and atmosphere, distributing solar energy around the planet. As you will

Figure 9.5 All convectionsee in Unit 4, scientists believe that convection currents in molten rocky material

currents display thewithin Earth carry entire continents slowly from one location to another. Much

features shown here.

smaller-scale convection currents are important in the operation of home heatingsystems, air conditioners, and even ovens in stoves.

Career CONNEC FSetting the Standard

Steve Reid knows that convection currents cannot warma house unless the heat source is working. Steve is agas appliance technician. His job is to install and repair

any type of furnace, boiler, stove, dryer, or other appli-ance that uses natural gas as its fuel.

A mistake could turn a safe appliance into a hazardous

one, so Steve's work must be done with great care. Totrain for his work, Steve completed a college course in

heating, refrigeration, and air conditioning. Then he passed a government test in order to receive agas fitter's licence. Without this licence, Steve could not legally work on gas appliances.

To ensure everyone's safety, the government has set standards that must be met by people whowant to work in occupations that are potentially dangerous to the worker or the public. In someother occupations, an organization of people who already work in the occupation create a test thatothers must pass to become a registered or certified member. This process ensures that everyone

who works in the occupation is well qualified.

With a partner, brainstorm at least three occupations that may require people to be licensed, regis-tered, or certified. Start by thinking about occupations that could involve danger to the worker orthe public. You and your partner could each choose one of the occupations, and, after checkingwith your teacher, contact someone in that field of work for comments on how licensing is done

and its importance to the occupation.

Energy Transfer Systems 247

DidYouKnow?

Thermogenic plants can

raise their own tempera-

ture, sometimes by an

astonishing amount.

Skunk cabbage, arum lily,

and the metre-tall voodoo

lily are all thermogenic.

On a day when the air

temperature is 20°C, a

voodoo lily can heat itself

to 35°C, only 2°C less

than human body tem-

perature! Even a tiny

crocus plant, when it is

growing rapidly, can

release enough thermalenergy to melt an open-

ing through the snow.

Figure 9 .13 Clustering together in cold weather is one way to stay warm. Individuals near the inside of

the mass are insulated and warmed by those on the outside.

Creatures that live in northern climates have developed several methods to cope

with severe winter temperatures. Special body features like thick fur or fat can

insulate and reduce heat loss. Many species of birds, andsome insects, migrate to warmer climates to avoid

46

INTERNET cgNNEcTwww.school.mcgrawhill.ca/resources/

Scientists track the seasonal migrations of birds, fish, mam-

mals, and even insects. For example, monarch butterflies make spring

and fall journeys between Mexico and the United States or Canada.

Find out more by going to the web site above. Go to the ScienceResources, then to SCIENCEPOWER 7 to know

where to go next.

winter entirely. Other creatures reduce their

energy requirements and energy losses by

hibernating through the winter. Some ani-

mals even use snow, which is a good thermal

insulator, to keep warm. By living in tunnels

under the snow, small animals such as mice,

can avoid extreme temperatures.

Insulation

To survive winter, you need to trap thermal energy in the animal, the person, or

the room that needs to stay warm. Fur, clothing, and building insulation all have

the same function - they prevent energy transfer. This means preventing conduc-

Figure 9.14 Polar bear furis excellent insulation -and more. Each hair in thefur is actually transparent.Radiant energy from theSun enters the hair,bounces down to the bear'sblack skin, and is absorbed,helping to warm the bear.Light that is reflected fromthe mass of hairs makesthe animal appear white.

tion, convection, andradiation. In addition, themovement of heated air,and especially watervapour, often needs to becontrolled so that it doesnot carry its "hiddenheat" away. Each methodof heat transfer is con-trolled in a different way.

260 Thermal Energy and Heat Technology

• Radiation Shiny silver-coloured metals or coatingsreflect radiant energy. Glass thermos bottles,lightweight survival blankets, and some sleepingbags use this principle to reduce loss of heat byradiation. Some types of building insulation have athin covering of metallic foil that does the samejob. Even window glass can be given a reflectivemetal coating, just like mirror sunglasses.

• Conduction Air is a very poor heat conductor, somany types of insulation are filled with air spacesor bubbles. Animal fur, feathers, clothing, andhouse insulation all contain air spaces thatreduce heat loss. The solid part of the insulation(usually plastic, glass fibres, or wood) is also apoor conductor, so it helps to reduce energytransfer by conduction. Metal window frames anddoor frames, which are good conductors, are oftenmade like a sandwich. The inside and outsidemetal sections are separated by a solid plastic orwood "thermal break." Since the thermal break isa poor conductor, it reduces heat transfer throughthe metal.

• Convection Air itself may be a good insulator, but large spaces filled with air arenot. Examine Figure 9.16, which shows heat loss in the space between the insideand outside walls of a house. frame. One way to stop such heat loss by convectionis to prevent motion of the air by breaking up air spaces into many little pockets.

•

That is the main reason why exterior house walls are often filled with vermi-culite, cellulose fibre, or fibreglass insulation. The insulation stops the formationof large convection currents by creating many small air spaces.

Air next to the inside wall iswarmed by conduction (A).The warmed air starts to movein a convection current (B),and ends up next to the cold

interior upstairs

II C M h IF houtside wa (). uc o t eair's thermal energy istransferred to the outer wall (D)and then dissipated in the coldoutside air (E). Then thecooled air moves back againstthe inside wall (F), or eventhrough it into the house (G),

hot air from where the furnace warms it up.

0 basement furnace

Figure 9 .16 Convection currents in a house wall contributeto heat loss.

Figure 9 .15 The outside ofthis building is mostlyglass, with a reflectivecoating to control thetransfer of radiant energy.

Energy Transfer Systems 261