gr7 - heat transfer & insulation.pdf

Unit 1 Heating as a transfer of energy When you touch a metal spoon that has been left in a hot saucepan, it burns your hand. When wate r is heated over a flame, the wate r begins to boil. You also cann ot sit too close to a fire as your skin becomes too hot. Wha t you have experienced are heat tran...

Unit 1 Heating as a transfer of energy When you touch a metal spoon that has been left in a hot saucepan, it burns your hand. When wate r is heated over a flame, the wate r begins to boil. You also cann ot sit too close to a fire as your skin becomes too hot. Wha t you have experienced are heat transfers in everyday situations. You have already learnt that heat energy can be transformed into something that you can feel. In Grade 6 you learnt that different appliances can transfer energy to their surroundings. In the case of heat energy, appliances such as stoves, kettles and geysers transfer heat energy. Heat transfer can change the phase of a substance. When heated, some substances that are solid will melt to form a liquid. Heat is removed from a liquid when it cools. It may go-ba ck to being a solid. This concept was covered in Grade 4. 8 • Topic 11 1. Name a substance that will melt easily when heated. 2. What is meant by a substance 'melting'? 3. How do the particles in a solid behave? 4. Why are the particles in a liquid able to move abou t more freely? 5. Why does a metal spoon left in a hot saucepan become hot but a wooden one does not? Heat transfer Lesson 1 Heat is transferred from hotter parts to cooler parts Heating is the process of transferring heat energy from one object to another. Heat energy always travels from a hotter object to a cooler one. In many cases, if left alone, the two objects will end up being the same temperature. This is called thermal equilibrium. When heat is transferred into an object, the object is said to become heated. When heat is transferred out of an object, the object is said to become therma·1 equilibrium: the point at which two objects, which started with different temperatures, .reach an equal temperature cooled. heat energy is transferred: There are three ways in which "I • • • conduction convection radiation. Figure 11.1 The three ways in which heat en.ergy is transferred .f MiWNhi~GMW■• I© I 1. Explain haw a cup of hat water becomes cold. 2. 3. lndlvldual · What is thermal equilibrium? An ice block is put into a glass of warm water. Which way does the energy transfer occur - from the ice to the water or from the water to the ice? 4. Explain your answer to question 3. · conduction: the transfer of heat energy between solid objects that are in direct physical contact with each other insulator: a material that does not conduct heat well Lesson 2 Conduction Conduction is the transfer of heat energy between solid objects that are in direct contact with each other. A metal pot will get hot if it is heated on the stove. The pot becomes hot because of heat transfer by conduction. Heat can also be transferred from a hotter object to another object which is cooler, but only when these objects touch each other. So heat energy can travel in one object from a hotter to a cooler region, or it can travel between two different objects th<:rt, are touching each other. Materials that easily conduct heat are called conductors. Metals such as steel and iron are examples of materials that are good con~uctors of heat. For this reason they are used to make cooking pots and frying pans. · Some materials do not conduct heat well. For example, plastic, . paper and wood are poor conductors of heat. A poor conductor of heat is called an ins_ ulator. Insulators prevent or slow down the conductl_on of heat energy. Insulating materials are used to make cool boxes to keep food inside the box cold. · Why does heat travel from hotter to colder .objects? Heat energy increases the kinetic energy of particles in a material. The hotter the object, the more kinetic energy the particles have and the faster they can move about. These particles bump into the particles nearest them, causing these neighbouring particles to move more. Their kinetic energy is increased and this makes the object Hot hotter. This is what happens when a metal spoon is left in · water . a hot saucepan. The heat transfer starts at the bottom of Figure 11.2 Heat transfer the spoo'n and is conducted up towards the cooler handle. by conduction Eventually the whole spoon will be hotter than before. . i. • e Topic 11 Heat transfer ' - Lesson 3 Convection What is convection? Convection describes the way heat moves in liquids and · gases. When liquids or gases are heated, ttae particles get more kinetic energy (movement energy). ·The particles move faster and further apart. Liquids and gases·exJ!)and when they are heated. The particles take up more space as the distance between them increases. The heated liquid or gas ✓.. ~ • Topic 11 Heat transfer Word bank convection: the transfer of heat energy through the mov81Tlent of particles in liquids and gases · Energy and Change ' ..current: a flow . of particles from one place to another condenses: . when a gas changes to a liquid as it cools is less dense than liquid or gas in cooler areas. The heo~ed · particles r;nove upwards into cooler areas. As the particles move further away from the heat source, they lose kinetic energy and slow down. This cools the liquid or gas. The particles move closer together again. The liquid or gas becomes heavier as a result and sinks. As the particles .get to the bottom again, they are near to the heat source. They get re-heoted and the process starts again. This sets up the cycle of currents that are called convection currents. Convection _currents in water ' Figure 11A Convection in water When water is warmed, as is shown in Figure 11.4, the water particles will rise. • This happens because heated liquids expand and become less dense. When the hot liquid reaches the surface, it is pushed to the sides by more rising hot water below. The water particles at the surface cool down, have less kinetic energy and become packed closer together again. The cooled water sinks to the bottom of the · beaker where it gets heated once again. Convection currents in water in nature The water cycle in nature works using convection. Water in dams, ponds and oceans is warmed by the Sun. The water evaporates and rises up into the atmosphere. It is much colder higher in the atmosphere, so the water condenses. The water becomes dense and eventually falls back to the Earth as rain. f. .. Currents in the·ocean are also caused by convection. Hot water rises to the top of the sea while the cold water sinks to the bottom. The movement of water at different temperatures causes convection currents in the sea. At the _ e quator the water is warm. ~ • Topic 11 Heat transfer • This warm wa ter flows tow ard s the cooler poles. There the wa rm wa ter becomes cooler and sinks, pushing the cold wa ter along the bot tom of the ocean. This set s up con vection currents in the oceans which are imp ort ant for all life on Earth. Convection currents in the home Knowledge of convection helps you to ma ke the bes t use of appliances used for heating or cooling a room. He ate rs are more effective if placed close to the floor (Figure 11.5). The air in fro nt of the hea ter rises as it warms up, and spr eads thr oug hou t the room. If the heater wa s placed too high in a room, the ho t air would be tra ppe d aga ins t the ceiling. The lower air would still. be cool and woud tak e much longer to warm up. Air conditioners which are used to cool roo ms, are more effi cie nt if pla ced higher up on a wall (Figure 11.5). As the heavier, coo ler air leaves the air conditioner, it sinks down tow ard s the floor. Lighter, wa rm er air is pushed upwards and will be cooled as it com es into con tac t wit h the cooler air ' from the air conditioner. If the air conditioner were placed too low in the room, the cooled air would sta y near the floor and no t circulate. ' 1. 8 • Topic 11 Heat transfer Figure 11.5 A bar heo teio nd on air conditioner need to be pla ced in differen t places to effectively coo l or hea t a room. ,-ihi➔ i+iul-iiMWF\ ·- - 1. . Using Your knowledge of convection currents, I © 1· . ·. · dra w a diag ram and label it to explain why , hea ters wor k bes t when placed near the floor. 1" d ividua i_ ~ 2. ·· Using you r knowledge of convection currents, -·_.-, -· :· ·: dra w a diag ram and label it to explain why air con ditio ners wor k bes t when placed near a ceiling . . • , . . : 1 Challenge ,. · · • ·· ' Given wha t you now know abo ut convection, do you thin k convection can take place in space? Explain you r answer. : ·.: · •. Do con vec tion currents alw ays travel upwards? Tes tr t~is by making.an· ice cube out of·coloure~ water. · ·~. Pta·c e the coloured ice cub e into glass of tap wat er. , Obs erve wha t hap pen s as the.ice~stdyts mel t: ;. ,i Dra w a diag ram to show wha t·. happens and explain • · . wha t ~bs eive in term s of convection. • a to you · Lesson 4 Radiation I t:. ... • Rad iatio n is ano ther way tha t hea t energy can be radiation: . . the trans fer of :· · transferred. Rad iatio n doe s not involve the tran sfer of energy heat energy by '. · : using moving part icle s. In radiation, energy is transfer waves, witho ut . red by . cont act or · · ·. · · waves similar to ligh t and microwaves. Radiation is the only .. mov eme nt of ;· : :· · way tha t hea t can be tran sfer red thro ugh an emp ty spa ce or parti cles vacuum. Unlike con duc tion and convection, radiatio n doe s not need a mat eria l for the tran sfer of hea t energy. Rad iatio n of hea t energy from the Sun occ urs through spa ce and reaches the Earth. The Sun prod uce s the energy tha t enables life on Ear th to exist. In Topic 15 you will learn more abo ut the Sun as the cen tre of the Solar Sys tem . Hea t trans fer Energy and Change Figure 11.7 An electric ligh t bulb rad iate s hea t and light. ene rgy . This is cal led All ob jec ts giv e off an d tak e in he at em pty spa ce as waves. rad ian t he at. Ra dia tion tra vel_s acr oss mo re rad ian t he at ene rgy Th e ho tte r an ob jec t bec om es, the ent ligh t bul b is a goo d rad iat es off its sur fac e. An inc and esc t bul b is sw itch ed off, the re exa mp le (Figure 11.7). Wh en the ligh duc ed and the gla ss is is no ligh t or he at ehe rgy bei ng pro el of rad iati on is so low coo l to tou ch. In oth er wo rds , the lev b is sw itch ed on, the as to be neg ligi ble. On ce the ligh t bul es ver y ho t and rad iate s gla ss sur rou ndi ng the ele me nt bec om t energy. no tice ab le am ou nts of ligh t and hea rad ian t hea t: hea t energy tha t can be transferred by rad iati on c cur ren t tha t Th e sou rce of this ene rgy is the ele ctri b. Ele ctri cal ene rgy is flow s thr oug h the ele me nt in the bul . When you pu t you r tra nsf orm ed int o hea t and ligh t energy ligh t bulb, wit hou t ha nd nea r the sid e of the sw itch ed on ng rad iate d tow ard s tou chi ng it, you will feel the hea t bei thr oug h the air. you r ski n. The hea t ene rgy is moving Concentrated radiation •tllillhb ► Figure 11.8 Concentrate~ rad iati on increases heatmg be He at ene rgy rad iate d fro m the Sun can effe ct. con cen tra ted to inc rea se the hea ting s of the An eas y wa y to con cen tra te the ray fyin g Sun is to use a cur ved lens. A ma gni rks well. gla ss or a pai r of rea din g glasse~ wo g the m Curved lenses ben d ligh t ray s bringin to a poi nt. o dry Dir ect ing the se con cen tra ted ray s ont the gra ss gra ss or a scr ap of pa pe r will cau se ful fac t or pap er to cat ch alig ht. This is a use any to rem em ber wh en you do no t hav e . ma tch es an d you nee d to sta rt a fire Energ y and Chan ge Lesson 5 Reflection and absorption of heat energy When light and heat radiated from the Sun strike the Earth's surface, most energy is reflected back into space. Different surfaces can reflect or absorb different amoun ts of heat energy. Shiny surfaces are good reflectors of heat energy and dark surfaces absorb heat energy. Shiny surfaces can also reflect light energy and dark surfaces can absorb it. You will learn about this in Grade 8 and in higher grades. Prac tical activ ity 4 Investigating the amou nt of radian t heat absorbed by different coloured surfaces v v :: ©. © :: ©. Group Aim To investigate the amount of radiant heat absor bed by different colooured surfaces You will need: sheets of different coloured paper - white (or yellow), • matt black and a sheet of aluminium foil • the.rmometers • a stop watch or timing device Method 1. 2. 3· Fold the sheets of paper and the aluminium foil so as to make little envelope shapes. Take the temperature reading on the therm omete r at the start of the investigation. Put th e bulb end of a therm omete r inside one of the envelopes. Place the envelope in direct sunlig ht. • • • • Hea ting is a pro ces s in whi ch ene rgy is tran sfer red from a hot ter bod y to a coo ler bod y. Hea t ene rgy will be tran sfer red unti l two obje cts are at the sam e tem per atu re - .ther mal equ ilibr ium . Hea t ene rgy can be trqn sfer red by con duc tion , · con vec tion or rad iatio n. sfer s hea t betw een soli d obj ect s tran tion duc Con . tha t are in con tac t with eac h oth er or bet wee n coole,r and ·h otte r par ts of an obje ct. t eria.l move-fa ster whe n the y are . Par ticle s in q _ma_ hea ted and tran·:sfer -hea t ene rgy to oth er par ticle s. nMet als are goo d .c ond ucto rs of hea t whe rea s no_ met als are usually poo r con duc tors of hea t. . Con vec tion is the tran sfer of hea t by the mov eme nt . • • • • ·• -• • • • < of liquid or gas part icle s. Con vec tion results in .c urre nts in wat er and in air. Rad iatio n is the tran sfer of hea t thro ugh em pty spa ce - . it doe s not require con tac t of sub stan ces or mov eme nt of par ticle s. · The Sun is:a sou rce of rad iatio n . . Shin y· surf ace s refle ct radiatior) ·and so do not hea t up quic kly . . d iatio n and so hea t up Dar k surf ace s abs orb ·ra_ quickly. Unit 1 Using insulating ma ter ial s Wha t you alrea dy know 511 Some materials, for example metals, can easily cond uct heat · and so will be quick to warm up. They can also lose heat rapidly. Other materials are poor cond ucto rs of heat . When you buy a take -awa y meal you are usually given the meal in a polystyrene container. If you buy take -awa y coffe e or tea, it is given to you in a polystyrene or pape r cup, with a plas tic lid on it. All these materials are used as they keep the food and drink warm for longer. This is because they are poor cond ucto rs of heat so will not transfer heat energy well. These mate rials are called insulators. Insulators include materials such as plas tic, polystyrene and wood. 1. cnecK Why are matches made of wood? 2. · Why are polystyrene cups used for take -awa y hot drinks? 3. How does having a lid on the cup help to keep the drink warm? 4. Homeless people often use newspaper to try and keep warm. Why do you think this works? 5. Bird feathers are used to make warm beds prea ds and sleeping bags. Explain why they are light yet very efficient at keeping heat in. Lesson 1 Hea t loss and hea t gain There are three ways of transferring heat energy · Th ese are condu r c ion, convection and radiation. ·Heat ener alw ays . gy moves from the hotte r object to th e cooIer obJe ct. When an Energy and Change object experiences a loss of heat we say that it has become cooled. When an object experiences a gain in heat, then it has become heated or warmed. For example, when a person first sits on a chair, the chair feels cool. The body heat of the person will warm the chair up as heat energy passes from the person to the cooler seat by conduction. When the person stands up, the c~air will start to lose its heat energy to the cooler surroundings by radiation. However, if another person sits on the chair straight away, they will feel a nice warm seat. Electric geysers use a heating element to warm water by conduction, convection and .radiation. We need warm water every day so that we can wash our clothes, our dishes and our bodies. Using an electric geyser · to heat water takes a lot of electricity and . costs a lot of money. We can use the Sun's energy to do the jo.b for us. Heating water using a solar water heating system can save at least 50% of a household's energy Figure 12.1 An electric water geyser costs. The initial cost of installing solar geysers is expensive, but it will eventually pay for itself in savings. Solar water heating systems Word bank passive: a process that does not need energy to make it happen actively: using energy There are five major parts in solar water heating systems: • Collector(s) - captures solar energy • Circulation system - moves water between the collectors to a storage tank - this may be done by passive convection or actively , by using a small ·pump • Insulated storage tank - holds the heated water until needed • Backup heating system - in case of prolonged cloudy weather • Control system - regulates the overall system operation. • Topic 12 Insulation and energy soving G@ How a solar wat er hea ter works A solar wat er hea ter uses collector pipes to cap ture rad iant energy from the Sun and use it to hea t water. These pipes are often loca ted on a roof where they receive maximum sunlight. The coll ecto r pipes are fitte d inside a dark, glazed, insulated box. The glazing is usually made of glass tha t helps keep the hea t in the collector. This is similar to the way tha t a car parked in the sun with all its windows closed, will get very hot inside. The glass trap s the Sun's hea t. Figure 12.2 A sola r wate r hea ter on the roof of a house Cold wat er comes in at the bott om of the tan k and is drawn up the pipes to the collector. The wat er is s, and then heated by the Sun as it as it passes through the pipe passed down to a stor age insulated stora ge hot wate r for house tank. The movement of tank inside roof the hot wat er crea tes a back up type of convection curr ent ·heatin~ _ which keeps the wat er element cold , circulating. wate rinlet -~--_. .1- tank supp ort: · stan d . • ~\~ _. ,_ ··,:· . --. ·-------- ------- '.'.' ·-- - There are two type s of solar wat er heaters: passive and acti ve. j Passive solar hea ter systems The wat er tan k is plac ed above the coll ecto r in passive sys tem s (Figure Figure 12.3 How a passive sola r wate r 12.3). Cold wat er is fed heat er works into an insulated tan k and -· m water. sinks into the collectors. Cold wat er is heavier than war Sun. As the I~ the collectors it is warmed by the radiation of the tion to the pipes become hot, the heat is transferred by conduc to . ~. water i~ the pipe~. The hot water now rises into the tan k due · - • Topi c 12 Insu latio n and ener gy savi ng Energy and Change c:onvection. A continuous flow of water through the collectors is created by these convection currents, without the need for pumps. This is known as a thermosiphon effect. the_rmosiphon effect: a method of passive heat · exchange based on natural convection currents Active solar heater systems Active solar heaters work in a s.imilar way to passive heaters. The main difference is that active heaters use a pump to get the water to the collectors and this uses energy. An advantage of having a pump though is that it allows the insulated water tanks to be placed below the collectors, which is often more convenient. Classroom activity 1 The diagram in Figure 12.4 shows an :I ©. V © I; active sola_ r water heating system: , · Pairs Compare it to the diagram of the passive solar system shown in -· -: -., · collectors Figure 12.3. Draw up a mounted on roof table in which you compare two differences that you can see · , between the two ,.:· systems. flow ... .. .. . _' Conduction, convection hot II /::< l .·:·.··· · .·. :, water and radiation are all outlet ·,:. used in solar water '. : pump ..._ cold water level heoting systems. Briefly - storage tank floor level explain where each type of transfer takes place·.· · • Figure ·12A An active solar · water heating system Give one advantage and one disadvantage of solar •1 water heaters. \ 1 Name the five parts that make up a solar heating system I and give a function for each part. I 2. ~-\ I . ,_··_ ,• ,_ _;_ , . I. ~ '' ' . . . . II . ~_,: _- · _ ..:· . ., 3. I 4. ' Lesson 3 Insulators and conductors in . everyday life vacuum: a space in whieh there are no particles The insulating and conducting properties of different materials have many useful , applications in everyday activities. People use insulators and conductors all the time to help reduce heat loss in winter or heat gain in summer. Cooking utensils are made from metals that can quickly and Figure 12.6 easily conduct heat. This helps food cook evenly. The handles of A metal frying pots and pans are usually made of wood or strong plastic. These pan with on insulating handle are poor conductors of heat so that you do not burn your hands when picking them up. Irons, . originally named for the metal from which they were made, are used to remove creases in clothes. Hot coals were placed in · the iron. Heat is conduct~d through the metal base. It spreads out evenly to warm the base of the iron. The heat, together with pressure from the iron, smoothes out creased clothes. Modern irons use electricity to produce heat and steam. . Figure 12.7 An old-fashioned cool iron . A thermos flask is a useful way to keep liquids hot. Many workers use thermos flasks to take tea or coffee to work. The flask keeps their drinks hot for many hours. The flask is r ~•W~"T..:~~-w•~,_ made of two layers of giass with a vacuum . I Stopper Vacuum Double glass shell Silver coating Casting Figure 12-B Internal structune Ofo thermos flo~ • between then:,. The vacuum prevents heat from moving from the liquid to the cooler surrounding air because there are no particles to conduct the heat energy. Ice cream sellers use a polystyrene cool box Figure 12.q A with lots of blocks of dry ice (frozen CO ) inside_ 2 ,uo"";,er enu·oys his n, '"' to keep their ice cream frozen. Even when it is tea break with a hot cup of tea. a very hot day, the ice cream does not melt. • Topic 12 Insulation and energy saving Energy and Change The polysty rene is a very good insulator and prevent$ the condu ction of heat from the warm air outside. The dry ice keeps the cool box at freezing temperature inside and the ice cream stays frozen. Rgure 12.10 An ice cream seller has a well insulated cool box. Figure 12.11 A worker installs insulatio n in a ceiling. This can cut down heat I s by up to Good insulation in the ceiling of homes helps prevent warm air from escaping through the roof. This cuts down heat loss throug h conduc tion or convection in winter. Practical activity 2 Test insulating properties of different materi als A : A : A ~ : ~ : ~ Group Poppy and Mbali noticed that their grandmother covers the teapot in a knitted woollen tea cosy. They asked her why she did this and she told them that it kept the tea warmer for longer. The girls started to think about insulators and how well each one works. They decided to do a simple investigation. Aim To investigate which materials are the best insulators You will need: six identical containers, either tin • cans or glass jars cotton wool • polystyrene pieces • woollen jersey • aluminium foil • newspaper • cotton dish cloths • thermometer • • watch or other timing device Figure 12.12 An insulated tea pot, covered with a woollen cosy • Topic 12 Insulation and energy savl':,:. ~ Le ss on 5 m es an d bu ildings ho in gy er en at he of n io . at rv C on se ey on el ec tri cit y fo r on m of t lo a d en sp a ric Af h ut So People in er communities! or po In . es us ho ing at he d an g in lighting, co ok which causes ai r , od wo d an al co ing us ne do tly he at in g is m os dependence gy er en is th ce du re to ne do be n pollution. M uc h ca gy efficient. by designing houses th at are ener . I • an cl im at e ric Af h ut So e th r fo s ea id gn si de Ho m e n in to consideration ke ta be to s ed ne n gio re a of e at The cli m d teqhnology. Fa ct or s an es us ho nt ie fic ef gy er en g nin when desig ction th e house re di e th e ar t un co ac to in n ke ta th at need to be ing th er m al ly us d an ht ig nl su l ra tu na of e us st faces, making th e be efficient building materials. here, th er m al ly isp m He rn he ut So e th in is a ric Af Since South ef fic ie nt houses should: ge t maximum be ne fit fro m to as so rth No g cin fa ilt bu be • llection th e Sun, an d fo r effective so la r co side of th e house h ut so e· th on s ow nd wi w fe a ly have on • sulat ion an d energy saving • • use wooden window frames as these are better insulators than metal ones have windows that are glazed with glass. Building materials thermal mass: a material that absorbs heat from a heat source, and then releases it slowly Building materials which are able to store heat during the day and release it slowlQ at night should-be-also useg!._ - . _rbesematerials a~~§ald-te-AQVa.cLbigh thermaLmass. Examples of these are hollow cement blocks, concrete and earth bricks. Earth bricks may be made locally, saving on both cost and need for transport. Roof design . The roof should have an overhang on the north side. This shades the house in summer when the Sun is high in the sky, and allows the sunlight into the house in winter when the Sun is much lower. Ceilings trap air, ensuring_E__ reduction of heat flow into or out of .-------·-tnenouse . As a result, the house is warmer in winter and cooler in summe~. Insulating a ceiling is very efficient but does add to the Flooring should also have a high--thermal mass. Materials such as concrete, clay or bricks will all trap heat coming through the windows. ---- lt· cosC - i - • - I I W:I'B n!S.....ii a Figure 12.14 A house designed to be energy efficient Energy and Change Indigenous, traditional homes ... . In Soutn Africa, homes are traditionally built from materials that ·.,i. , are available locally such as thatching grass, wood and clay. People use these materials to build their homes because they have physical properties that make them energy efficient and that suit the climate. For example, thatching grass is often used as a roofing material in traditional indigenous homes because it is an excellent thermal insulator. This means that a thatched roof helps to keep the home warm in winter and cool in summer. In more recent times in South Africa there have been a number of projects which aimed to build ecovillages. The emphasis of these projects is on using cheaper renewable energy sources and thermally efficient house design. These ecovillages combine traditional and cultural building practices with more modern technology and they are designed to be very energy efficient. A good example of such a project is in Ivory Park o·n the outskirts of Johannesburg. ~MHi·I·Ull@ilii!Et _ ·---~~.----~-1. 2. 3. 4. 5. I I I © Which direction should houses face in this country? Individual Why should the houses face this way? What i~ the benefit of having a ceiling in the rdof? How does the position of the Sun change in winter and summer? In the drawing of the energy efficient house shown in this lesson, list the features that you can see that ood design features. are g_ -R~<--o~~\·J &,\1n() - \n:.rs floor• nc. :.. \1 \\ , \ 1) b -¼~ ,.,or\\- c:,1 1 . •\ Mc~'.:, ''lf' ~½v~ \ ,. .·) . l:_)r1rJ\ ~, 1 • ldk iiAw }liis i's, do !JD4 20241 . JrGu \,t J • • Hea t ma y be lost or gained through radiation, conduction and _convection. Ma ny homes use solar water hedting systems tha t· reduce the cost_of heating water by using the energy from the. Sun_~·· • . .- · In~ulatirig'mCteriOis which are poor conductors of hea t are·-used · minimise heat loss in winter, and hea t gain in sum.mer. · Insulatirag materials work by slowing down heat transfer. · Some materials·are better heat insulators than others. Insulating materials are used to make cool boxes, in ceilings, for clothing and for building houses. A knowledge of insulating techniques can help to conserve heat energy in homes and therefore reduce costs. ·-indigenous and traditional homes in South Africa are designed to suit our climate, for example, they have thatched roofs or large verandahs. ... .., ,._ ¥ • • • • 0 • ' , to : ~OPic to transfer Energy 13 surroundings ~!41-it·lli~i+llii4@i~Piii·IMMlt·ii!IIM!imllU·~i The picture shows the areas of potential loss of heat energy from a home. In previous lessons you learnt about the different types of energy and how they can be transferred from one form to another. In many of these energy transfers, heat was given off. This heat energy is often 'lost'. This just means that the heat energy is not used for any other. purpose and so is wasted. The house above could be efficiently heated if ways were found to prevent the loss of heat through the windows, floors and so on. In this topic you will learn about the energy efficiency of some machines and household appliances. ra• • Topic 13 Energy transfer to surroundings Un it 1 Useful and 'Wasted ' energy What you alread y know Figure 13.1 An electric drill is an examp le of an energy system . ~- In the previous units you learnt about energy systems. An energy system is a network of parts that interact together. It will have an input, process and output. For example, an electric drill is an energy system. It consists of mechanical parts that work together to perform the function of drilling. When the ' drill is supplied with electrical energy-(input), its motor will start to work (process). It changes electrical energy into kinetic energy. The drill bit turns and does work (output). At the same time some heat and sound energy are produced. Draw a flow diagram to show the energy transfers_that take place in an electric drill. Make sure that you add labels to show input, process and output, as well as the wasted energy. 2. Choose another energy system and draw a flow .. diagram to show the energy transfers. 3. What happens to the heat energy produced by energy systems? 4. What other type of energy is produced by the drill? Lesson 1· Energy efficiency Energy systems, like those_f9~~9_i[I_~lectrical _9ppli9_n.c~ o_r_ · . ~echa nical de~ices~-aredes_ignedio prov~de u~eful ~ oJJ!,puts. An electric orillTsable to drill holes in materials. -~ e r in any energy system, not all of the energy in the syste m is used ----- -- - - - for its inten__ded purpose. Some __ener_gy_ escape s into the surrounding environment. i:_his is calle~w asted energy'. The most commo n fQrms of wasted e_nerg~re-be.at.. and sound. The output energy in a system is always less than the input energy because of wasted energy. Calcul ate energy efficiency The unit for measuring en.ergy is a Joule (J)orkilo joure{kJ ). One thousan d Joules is_ equal to one kilojoule. T~e efficiency of an applian ce can be calculat ed by working . out what percent age of the input energy Figure 13.2 Diagram showing the energy into the system is used and how much is changes in an electric drill. wasted . An energy efficient system is input one that has the least difference electrical energy between input and output energy. For example, 100 kJ of 100kJ electrical energy is supplied to the electric drill. If 60 kJ of this process energy are converted to kinetic electric drill energy, 20 kJ are transformed . into heat energy and 20 kJ are changed to sound energy, we can work out the efficiency of the output drill. ,:he heat and sound energy, a-total af 40 kJ, are not useful god so_tne.y are_w.as_ted. Of the 20 kJ 20 kJ 60kJ 100 kJ supplied to the drill, only Rgure 13.3 A diagram showing how 60 kJ was used for useful work to calculate the efficiency of an so the drill's efficiency is 60% electric drill. ) ( 60 X· 100. +. /! ~ - -- 100 Lesson 2 Energy efficiency of power stat ions In the last lesso n you learn t that cars are only 35% energ efficient. You will now find out abou t coal- fired power statio~. Practical acti vity 2 IE Research energy efficiency in power stati ons Aim To research ener gy effici ently in powe r stati ons lndlvldua1 answer Read the infor matio n abou t ener gy effici ency in a powe r stati on and the ques tions . ~ • Did you know? The world 's large st coal- fired powe r stati on is in Kendal in Mpu mala nga Province, Sout h Africa. t)~ .l~t♦♦♦H ♦♦♦♦♦♦♦♦-t How a pow er station uses coal Power stati ons use energy to make elect rical energy in the form of electricity. In a coal- fired powe r station, burning coal is used to heat wate r until it turns to steam . The stea m produces energy to drive a turbine. Blades inside the turbine spin. The kinet ic energy of the spinning turbine is Word bank turbine: a machine that has blades, which are turned by a force like high · pressure steam . · heat eat source - fire under boiler 1 · •~ · r -•~ Figure 13.6 Stage s in the gener ating of electr icity from coal Topic~ 3 Energy trans fer to surroundings JIii"""""" I ge ne ra to r (Figure 13_6). a by ty ci tri ec el to in d te er conv ully transformed in ef us is ~t th gy er en e th of The gr ea te r th e pe rc e~ t~ ge fired power stations are alCo is. em st sy e th nt ie a system, th e m or e ef fic means th at half the energy is Th st. be at nt ie fic ef gy only ab ou t 50% en er or other forms of energy. ht lig or , at he as ed st wa in the coal will be rs, mosphere. Large cooling towe at e th to in ed as le re is at The wa st e he nts, are used to release pla r we po of es ur at fe e bl ea th at are th e m os t no tic towers. The chimneys are ng oli co ge lar ry ve six s ha this heat. Kendal rning the coal. This is bu of lt su re e t,h e ar at th s used to release th e gase ctricity from coal because el~ ing ak m th wi s m le ob pr t one of th e bi gg es and sulphur dioxide is e xid dio on rb ca of t lo A . air these ga se s po llu te th e to greer:1house gases which add e ar e es Th e. er ph os m at e released in to th ge. the problem of cl im at e chan vironmental impact of en d n a_ y nc ie fic ef gy er en W ay s to im pr ov e th e . · ns io at e buildings in st r heat th po we to at he ste wa e th e us to . . ·on has been . t 1 es gg su ne e co~I ~1th other types of 0 bin m co ns tio sta r we po e m so th e nearby di st ric t. eff1c1ency of the power gy er en e th es ov pr im is Th as lg · ra l. tu na as h e atter from landfill sit. es as fue m fu ~ su ste wa e us to is y ilit ib ss . ' A th po the plant. no er vir lp en onmentally by easing he uld wo ns io at st r we po d Waste-fire nd · problem of overfull la fills. · d power station makes fire alQu es tio ns co a w ho in pla ex to es . _ Write three sentenc 1 electricity.