4                    HeatI     n Chapter 3 you learnt that woollen      and some of them are cold. Tea is hot     clothes ...
and hot water in mug C. Now dip yourleft hand in mug A and the right handin mug B. After keeping the hands inthe two mugs ...
Precautions to be observed while reading a clinical thermometer   Thermometer should be washed before and after use, prefe...
clinical thermometer. Record your            Different types of thermometersobservations as in Table 4.2.                a...
In addition to the precautions needed while reading a clinicalthermometer, the laboratory thermometer   should be kept upr...
body temperature? Obviously, it is notconvenient to use the laboratorythermometer for this purpose.                       ...
minutes. Touch the other end. Enter                                              your observation in Table 4.3.           ...
Fig. 4.10 Transfer of heat by convection in air                                                Notice that towards the top...
Day time                                      Night time                             Hot                                  ...
We often use electricity and fuels like coal and wood to keep our houses cool or warm. Is it possible to construct buildin...
What you have learnt           Our sense of touch is not always a reliable guide to the degree of hotness           of an ...
(d)    No medium is required for transfer of heat by the process of                __________.         (e)    A cold steel...
9. An iron ball at 40°C is dropped in a mug containing water at 40°C.           The heat will           (a)    flow from i...
4. Take a sheet of paper. Draw a spiral on it as shown in the Fig. 4.14. Cut out        the paper along the line. Suspend ...
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7 heat

  1. 1. 4 HeatI n Chapter 3 you learnt that woollen and some of them are cold. Tea is hot clothes are made from animal fibres. and ice is cold. List some objects you You also know that cotton clothes use commonly in Table 4.1. Mark theseare made from plant fibres. We wear objects as hot or cold.woollen clothes during winters when it Do not touch objects which are toois cold outside. Woollen clothes keep us hot. Be careful while handling awarm. We prefer to wear light coloured candle flame or a stove.cotton clothes when it is hot. These giveus a feeling of coolness. You might have We see that some objects are coldwondered why particular types of while some are hot. You also know thatclothes are suitable for a particular some objects are hotter than othersseason. while some are colder than others. How In winter you feel cold inside the do we decide which object is hotter thanhouse. If you come out in the sun, you the other? We often do it by touchingfeel warm. In summer, you feel hot even the objects. But is our sense of touchinside the house. How do we know reliable? Let us find out.whether an object is hot or cold? Howdo we find out how hot or cold an object Activity 4.1is? In this chapter we shall try to seek Take three large mugs. Label them asanswers to some of these question. A, B and C. Put cold water in mug A4.1 HOT AND COLD and hot water in mug B. Mix some coldIn our day-to-day life, we come across a Make sure that water is not so hot thatnumber of objects. Some of them are hot you burn your hand Table 4.1: Hot and cold objects (A) (B) (C)Object Cold/ C ool Cold/C Warm/ Hot Warm/HIce cream √Spoon in atea cupFruit juiceHandle of afrying pan Fig. 4.1 Feeling water in three mugs
  2. 2. and hot water in mug C. Now dip yourleft hand in mug A and the right handin mug B. After keeping the hands inthe two mugs for 2–3 minutes, put both Fig. 4.2 A clinical thermometerthe hands simultaneously in mug C your hand and examine it carefully. If(Fig. 4.1). Do both the hands get the you do not have a thermometer, requestsame feeling? a friend to share it with you. A clinical thermometer looks like the one shown in Fig. 4.2. A clinical thermometer consists of a Boojho says,“My left hand long, narrow, uniform glass tube. It has tells me that the water in mug a bulb at one end. This bulb contains C is hot and the right hand mercury. Outside the bulb, a small tells me that the same water shining thread of mercury can be seen. is cold. What should I If you do not see the mercury thread, conclude?” rotate the thermometer a bit till you see it. You will also find a scale on the thermometer. The scale we use is the Boojho’s confusion shows that we celsius scale, indicated by °C.cannot always rely on our sense of touchto decide whether an objectis hot or cold. Sometimes Boojho wondered which of the two scalesit may deceive us. shown in Fig. 4.2 he should read. Paheli told Then, how do we find him that India has adopted the celsius scaleout how hot an object really and we should read that scale. The other scaleis? A reliable measure of the with the range 94–108 degrees is thehotness of an object is its Fahrenheit scale (°F). It was in use earlier.temperature Temperature istemperature.measured by a device calledthermometer.thermometer A clinical thermometer reads4.2 MEASURING TEMPERATURE temperature from 35°C to 42°C.Have you seen a thermometer? Recall Activity 4.2that when you or someone else in yourfamily had fever, the temperature was Reading a thermometermeasured by a thermometer. The Let us learn how to read a thermometer.thermometer that measures our body First, note the temperature differencetemperature is called a clinical indicated between the two bigger marks.thermometer Hold the thermometer inthermometer. Also note down the number of divisions36 SCIENCE
  3. 3. Precautions to be observed while reading a clinical thermometer Thermometer should be washed before and after use, preferably with an antiseptic solution. Ensure that before use the mercury level is below 35°C. Read the thermometer keeping the level of mercury along the line of sight. (See Fig. 4.3). Handle the thermometer with care. If it hits against some hard object, it can break. Don’t hold the thermometer by the bulb while reading it.(shown by smaller marks) between these under your tongue. After one minute,marks. Suppose the bigger marks read take the thermometer out and note theone degree and there are five divisions reading. This is your body temperature.between them. Then, one small division The temperature should always be 1 stated with its unit, °C.can read 0.2 C . What did you record as your body 5 Wash the thermometer, preferably temperature?with an antiseptic solution. Hold it The normal temperature of humanfirmly and give it a few jerks. The jerks body is 37°C. Note that the temperaturewill bring the level of mercury down. is stated with its unit.Ensure that it falls below 35°C. Nowplace the bulb of the thermometer Paheli measured her body temperature. She got worried as it was not exactly 37°C. Let us try to assure Paheli that there is nothing wrong with her. Activity 4.3 Measure the body temperature of some of your friends (at least 10) with a Table 4.2: Body temperature of some persons Name Temperature (°C) Fig. 4.3 Correct method of reading a clinical thermometerHEAT 37
  4. 4. clinical thermometer. Record your Different types of thermometersobservations as in Table 4.2. are used for different purposes. Is the body temperature of every The maximum and minimumperson 37°C? temperatures of the previous The temperature of every person may day, reported in weathernot be 37°C. It could be slightly higher reports, are measured byor slightly lower. Actually, what we call a thermometer called thenormal temperature is the average body maximum-minimumtemperature of a large number of healthy thermometer.persons. The clinical thermometer is designed this thermometer. Look at itto measure the temperature of human carefully and note the highest andbody only. The temperature of human the lowest temperature it canbody normally does not go below 35oC measure. The range of aor above 42 o C. That is the reason laboratory thermometer isthat this thermometer has the range generally from –10°C to 110°C35oC to 42oC. (Fig. 4.4). Also, as you did in the case of the clinical thermometer, find out how much a small Boojho got a naughty idea. He division on this thermometer wanted to measure the temperature reads. You would need this of hot milk using a clinical information to read the thermometer. Paheli stopped him thermometer correctly. from doing so. Let us now learn how this thermometer is used. CAUTION Activity 4.4Do not use a clinical thermometer for Take some tap water in a beakermeasuring the temperature of any or a mug. Dip the thermometerobject other than the human body. in water so that the bulb isAlso avoid keeping the thermometer in immersed in water but does notthe sun or near a flame. It may break. touch the bottom or the sides of4.3 LABORATORY THERMOMETER the container. Hold the thermometer vertically (Fig. 4.5).How do we measure the temperature of Observe the movement of mercuryother objects? For this purpose, there in the thermometer. Wait till theare other thermometers. One such mercury thread becomes steady.thermometer is known as the laboratorythermometer. The teacher will show you Fig. 4.4 A laboratory thermometer38 SCIENCE
  5. 5. In addition to the precautions needed while reading a clinicalthermometer, the laboratory thermometer should be kept upright not tilted. (Fig. 4.5) bulb should be surrounded from all sides by the substance of which the temperature is to be measured. The bulb should not touch the surface of the container. Are there any variations in the readings? Discuss the possible reasons. Let us try to answer this question. Activity 4.5 Take some hot water in a beaker or a mug. Dip the thermometer in water. Wait till the mercury thread becomes steady and note the temperature. Now take out the thermometer from water. Observe carefully what happens now. Do you notice that as soon as you take the thermometer out of water, the level of mercury begins to fall. This means that the temperature must be read while Fig. 4.5 Measuring temperature of water with an laboratory thermometer the thermometer is in water. You may recall that while taking yourNote the reading. This is the temperature own temperature, you have to take theof water at that time. thermometer out of your mouth to note Compare the temperature of water the reading. Can you then use therecorded by each student in the class. laboratory thermometer to measure your Boojho now understand why clinical thermometer cannot be used to measure high Boojho wonders why the level temperatures. But still wonders of mercury should change at whether a laboratory thermometer all when the bulb of the can be used to measure his body thermometer is brought in contact temperature. with another object?HEAT 39
  6. 6. body temperature? Obviously, it is notconvenient to use the laboratorythermometer for this purpose. Paheli asks: “Does it mean Why does the mercury not fall or rise that heat will not bein a clinical thermometer when taken transferred if the temperature ofout of the mouth? two objects is the same?” Observe a clinical thermometeragain. Do you see a kink near the bulb in all cases heat flows from a hotter(Fig. 4.6). object to a colder object. What is the use of the kink? It How does heat flow? Let usprevents mercury level from falling on investigate.its own. Activity 4.6 Take a rod or flat strip of a metal, say of aluminium or iron. Fix a few small waxFig. 4.6 A clinical thermometer has a kink in it pieces on the rod. These pieces should be at nearly equal distances (Fig. 4.7). There is a lot of concern over the use of mercury in Clamp the rod to a stand. If you do not thermometers. Mercury is a find a stand, you can put one end of the rod in between bricks. Now, heat toxic substance and is very difficult to dispose of if a the other end of the rod and observe. What happens to the wax pieces? Do thermometer breaks. These these pieces begin to fall? Which piece days, digital thermometers are available which do not falls the first? Do you think that heat is use mercury.4.4 TRANSFER OF HEATYou might have observed that a fryingpan becomes hot when kept on a flame.It is because the heat passes from theflame to the utensil. When the pan is Fig. 4.7 Flow of heat through a metal stripremoved from the fire, it slowly cools transferred from the end nearest to thedown. Why does it cool down? The heat flame to the other end?is transferred from the pan to the The process by which heat issurroundings. So you can understand transferred from the hotter end to thethat in both cases, the heat flows from colder end of an object is known asa hotter object to a colder object. In fact, conduction In solids, generally, the conduction.40 SCIENCE
  7. 7. minutes. Touch the other end. Enter your observation in Table 4.3. The materials which allow heat to pass through them easily are conductors of heat. For examples, aluminum, iron and copper. The materials which do not allow heat to pass through them easily are poor conductors of heat such as plastic and wood. Poor conductors are known as insulators. insulators The water and air are poor conductors of heat. Then, how does the Fig. 4.8 Conduction of heat by different heat transfer take place in these materials substances? Let us find out.heat is transferred by the process of Activity 4.8conduction. Take a round bottom flask (if flask is Do all substances conduct heat not available, a beaker can be used). Filleasily? You must have observed that it two-thirds with water. Place it on athe metallic pan for cooking has a plastic tripod, or make some arrangement toor wooden handle. Can you lift a hot place the flask in such a way that youpan by holding it from the handle can heat it by placing a candle below it.without getting hurt? Wait till the water in the flask isActivity 4.7 still. Place a crystal of potassium permanganate at the bottom of the flaskHeat water in a small pan or a beaker. gently using a straw. Now, heat the waterCollect some articles such as a steel by placing the candle just below thespoon, plastic scale, pencil and divider. crystal.Dip one end of each of these articles in Write your observation in yourhot water (Fig. 4.8). Wait for a few notebook and also draw a picture of Table 4.3 what you observe (Fig. 4.9). Article Material with Does the other When water is heated, which the article end get hot the water near the flame gets is made of Yes/No hot. Hot water rises up. The cold water from the Steel spoon Metal Yes sides moves down towards the source of heat. This water also gets hot and risesHEAT 41
  8. 8. Fig. 4.10 Transfer of heat by convection in air Notice that towards the top, the air gets heated by convection. Therefore, the hand above the flame feels hot. On the sides, however, there is no convection and air does not feel as hot as at the top. Fig. 4.9 Convection of heat in water The people living in the coastal areasand water from the sides moves down. experience an interesting phenomenon.This process continues till the whole During the day, the land gets heatedwater gets heated. This mode of heat faster than the water. The air over thetransfer is known as convection. land becomes hotter and rises up. The How does the heat travel in air? In cooler air from the sea rushes in towardswhich direction does the smoke go? the land to take its place. The warm air The air near the heat source gets hot from the land moves towards the sea toand rises. The air from the sides comes complete the cycle. The air from the seain to take its place. In this way the air is called the sea breeze To receive the breeze.gets heated. The following activity cooler sea breeze, the windows of theconfirms this idea. houses in coastal areas are made to faceActivity 4.9 the sea. At night it is exactly the reverse (Fig. 4.11). The water cools down moreLight a candle. Keep one hand above the slowly than the land. So, the cool airflame and one hand on the side of the from the land moves towards the sea.flame (Fig. 4.10). Do your hands feel This is called the land breeze Fig. 4.11 breeze.equally hot? If not which hand feels shows this phenomenon.hotter? And why? When we come out in the sun, we Be careful. Keep your hands at a safe feel warm. How does the heat from the distance from the flame so that they sun reach us? It cannot reach us by do not get burnt. conduction or convection as there is no medium such as air in most part of the42 SCIENCE
  9. 9. Day time Night time Hot Cool Hot Cool Fig. 4.11 Land breeze and sea breezespace between the earth and the sun. are you advised to use an umbrella whenFrom the sun the heat comes to us by you go out in the sun?another process known as radiation radiation.The transfer of heat by radiation does 4.5 KINDS OF CLOTHES WE WEAR INnot require any medium. It can take SUMMER AND WINTERplace whether a medium is present or You know that in summer we prefernot. When we sit in front of a room light-coloured clothes and in winter weheater, we get heat by this process. A usually wear dark-coloured clothes. Whyhot utensil kept away from the flame is it so? Let us find outcools down as it transfers heat to the Activity 4.10surroundings by radiation. Our bodytoo, gives heat to the surroundings and Take two identical tin cans. Paint thereceives heat from it by radiation. outer surface of one black and of the All hot bodies radiate heat. When other white (Fig. 4.12). Pour equalthis heat falls on some object, a part of amounts of water in each and leave themit is reflected, a part is absorbed and a in the mid-day sun for about an hour.part may be transmitted. The Measure the temperature of water intemperature of the object increases due both the cans. Do you find anyto the absorbed part of the heat. Why difference in the temperatures? InHEAT 43
  10. 10. We often use electricity and fuels like coal and wood to keep our houses cool or warm. Is it possible to construct buildings, that are not affected much by heat and cold outside? This can be done by constructing outer walls of buildings so that they have trapped layers of air. One way of doing this is to use hollow bricks, which are available these days. the summer and dark-coloured clothes in the winter? Dark surfaces absorb more heat and, therefore, we feel comfortable with dark coloured clothes in the winter. Light coloured clothes Fig. 4.12 Containers with black and white reflect most of the heat that falls on them surface and, therefore, we feel more comfortablewhich can is the water warmer? You wearing them in the summer.can feel the difference even by touching Woollen clothes keep us warmwater in the two cans. in winterActivity 4.11 In the winter, we use woollen clothes.Fill the two cans used in Activity 10 with Wool is a poor conductor of heat.the same amount of hot water at the Moreover, there is air trapped in betweensame temperature (say, at 60oC). Leave the wool fibres. This air prevents the flowthe cans in a room or in a shade. Note of heat from our body to the coldthe temperature of water after 10–15 surroundings. So, we feel warm.minutes. Does the temperature of water Suppose you are given the choice inin both the cans fall by the same winter of using either one thick blanketamount? or two thin blankets joined together. Do these activities suggest to you the What would you choose and why?reason why it is more comfortable to Remember that there would be a layerwear white or light-coloured clothes in of air in between the blankets. Keywords Celsius scale Insulator Sea breeze Conduction Land breeze Temperature Conductor Radiation Thermometer Convection44 SCIENCE
  11. 11. What you have learnt Our sense of touch is not always a reliable guide to the degree of hotness of an object. Temperature is a measure of the degree of hotness of an object. Thermometer is a device used for measuring temperatures. Clinical thermometer is used to measure our body temperature. The range of this thermometer is from 35°C to 42°C. For other purposes, we use the laboratory thermometers. The range of these thermometers is usually from –10°C to 110°C. The normal temperature of the human body is 37°C. The heat flows from a body at a higher temperature to a body at a lower temperature. There are three ways in which heat can flow from one object to another. These are conduction, convection and radiation. In solids, generally, the heat is transferred by conduction. In liquids and gases the heat is transferred by convection. No medium is required for transfer of heat by radiation. The materials which allow heat to pass through them easily are conductors of heat. The materials which do not allow heat to pass through them easily are called insulators. Dark-coloured objects absorb radiation better than the light-coloured objects. That is the reason we feel more comfortable in light-coloured clothes in the summer. Woollen clothes keep us warm during winter. It is so because wool is a poor conductor of heat and it has air trapped in between the fibres. Exercises 1. State similarities and differences between the laboratory thermometer and the clinical thermometer. 2. Give two examples each of conductors and insulators of heat. 3. Fill in the blanks : (a) The hotness of an object is determined by its __________. (b) Temperature of boiling water cannot be measured by a _____________ thermometer. (c) Temperature is measured in degree ______________.HEAT 45
  12. 12. (d) No medium is required for transfer of heat by the process of __________. (e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of ______________. (f ) Clothes of ______________ colours absorb heat better than clothes of light colours. 4. Match the following : (i) Land breeze blows during (a) summer (ii) Sea breeze blows during (b) winter (iii) Dark coloured clothes are preferred during (c) day (iv) Light coloured clothes are preferred during (d) night 5. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing . 6. Look at Fig. 4.13. Mark where the heat is being transferred by conduction, by convection and by radiation. Fig. 4.13 7. In places of hot climate it is advised that the outer walls of houses be painted white. Explain. 8. One litre of water at 30°C is mixed with one litre of water at 50°C. The temperature of the mixture will be (a) 80°C (b) more than 50°C but less than 80°C (c) 20°C (d) between 30°C and 50°C46 SCIENCE
  13. 13. 9. An iron ball at 40°C is dropped in a mug containing water at 40°C. The heat will (a) flow from iron ball to water. (b) not flow from iron ball to water or from water to iron ball. (c) flow from water to iron ball. (d) increase the temperature of both. 10. A wooden spoon is dipped in a cup of ice cream. Its other end (a) becomes cold by the process of conduction. (b) becomes cold by the process of convection. (c) becomes cold by the process of radiation. (d) does not become cold. 11. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that (a) copper bottom makes the pan more durable. (b) such pans appear colourful. (c) copper is a better conductor of heat than the stainless steel. (d) copper is easier to clean than the stainless steel. Extended Learning — Activities and Projects 1. Go to a doctor or your nearest health centre. Observe the doctor taking temperature of patients. Enquire: (a) why she dips the thermometer in a liquid before use. (b) why the thermometer is kept under the tongue. (c) whether the body temperature can be measured by keeping the thermometer at some place other than the mouth. (d) whether the temperature of different parts of the body is the same or different. You can add more questions which come to your mind. 2. Go to a veterinary doctor (a doctor who treats animals). Discuss and find out the normal temperature of domestic animals and birds. 3. Wrap a thin paper strip tightly around an iron rod. Try to burn the paper with candle while rotating the iron rod continuously. Does it burn? Explain your observation.HEAT 47
  14. 14. 4. Take a sheet of paper. Draw a spiral on it as shown in the Fig. 4.14. Cut out the paper along the line. Suspend the paper as shown in Fig. 4.14 above a lighted candle. Observe what happens. Think of an explanation. Fig. 4.14 5. Take two similar transparent glass bottles having wide mouths. Put a few crystals of potassium permanganate or pour a few drops of ink in one bottle. Fill this bottle with hot water. Fill the other bottle with cold water. Cover the cold water bottle with a thick piece of paper such as a postcard. Press the postcard firmly with one hand and hold the bottle with the other hand. Invert the bottle and place it on top of the hot water bottle. Hold both the bottles firmly. Ask some other person to pull the postcard. Observe what happens. Explain. You can read more on the following website: www.bbc.co.uk/schools/gcsebitesize/physics/energy/ energytransferrev6.shtml Did you know? The celsius scale was devised by a Swedish astronomer, Anders Celsius in 1742. Strangely, he fixed the temperature of the boiling water as 0°C and of freezing water as 100°C. However, this order was reversed very soon.48 SCIENCE

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