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4, , Heat, , I, , n Chapter 3 you learnt that woollen, clothes are made from animal fibres., You also know that cotton clothes, are made from plant fibres. We wear, woollen clothes during winters when it, is cold outside. Woollen clothes keep us, warm. We prefer to wear light coloured, cotton clothes when it is hot. These give, us a feeling of coolness. You might have, wondered why particular types of, clothes are suitable for a particular, season., In winter you feel cold inside the, house. If you come out in the sun, you, feel warm. In summer, you feel hot even, inside the house. How do we know, whether an object is hot or cold? How, do we find out how hot or cold an object, is? In this chapter we shall try to seek, answers to some of these questions., , 4.1 HOT, , AND, , COLD, , In our day-to-day life, we come across a, number of objects. Some of them are hot, , and some of them are cold. Tea is hot, and ice is cold. List some objects you, use commonly in Table 4.1. Mark these, objects as hot or cold., Do not touch objects which are too, hot. Be careful while handling a, candle flame or a stove., We see that some objects are cold, while some are hot. You also know that, some objects are hotter than others, while some are colder than others. How, do we decide which object is hotter than, the other? We often do it by touching, the objects. But is our sense of touch, reliable? Let us find out., , Activity 4.1, Take three small tubs/containers. Label, them as A, B and C. Put cold water in, container A and hot water in container, Make sure that water is not so hot that, you burn your hand, , Table 4.1: Hot and cold objects, Object, Ice cream, , Cold/Cool, , (A), , (B), , (C), , Warm/Hot, , √, , Spoon in a, tea cup, Fruit juice, Handle of a, frying pan, , Fig. 4.1 Feeling water in three containers, , 2020-21

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B. Mix some cold and hot water in, container C. Now dip your left hand in, container A and the right hand in, container B. After keeping the hands in, the two containers for 2–3 minutes, put, both the hands simultaneously in, container C (Fig. 4.1). Do both the, hands get the same feeling?, , Boojho says,“My left hand, tells me that the water in mug, C is hot and the right hand, tells me that the same water, is cold. What should I, conclude?”, , Fig. 4.2 A clinical thermometer, , your hand and examine it carefully. If, you do not have a thermometer, request, 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, long, narrow, uniform glass tube. It has, a bulb at one end. This bulb contains, mercury. Outside the bulb, a small, shining thread of mercury can be seen., If you do not see the mercury thread,, rotate the thermometer a bit till you see, it. You will also find a scale on the, thermometer. The scale we use is the, celsius scale, indicated by °C., , Boojho’s confusion shows that we, cannot always rely on our sense of touch, to decide whether an object, is hot or cold. Sometimes, Boojho wondered which of the two scales, it 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 scale, out how hot an object really, and we should read that scale. The other scale, is? A reliable measure of the, with the range 94–108 degrees is the, hotness of an object is its, Fahrenheit scale (°F). It was in use earlier., temperature. Temperature is, measured by a device called, thermometer., A clinical thermometer reads, , 4.2 MEASURING TEMPERATURE, , temperature from 35°C to 42°C., , Have you seen a thermometer? Recall, that when you or someone else in your, family had fever, the temperature was, measured by a thermometer. The, thermometer that measures our body, temperature is called a clinical, thermometer. Hold the thermometer in, , Activity 4.2, Reading a thermometer, Let us learn how to read a thermometer., First, note the temperature difference, indicated between the two bigger marks., Also note down the number of divisions, , 36, , SCIENCE, , 2020-21

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Precautions to be observed while using 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, marks. Suppose the bigger marks read, one degree and there are five divisions, between them. Then, one small division, can read, , 1, = 0.2 °C ., 5, , Wash the thermometer, preferably, with an antiseptic solution. Hold it, firmly and give it a few jerks. The jerks, will bring the level of mercury down., Ensure that it falls below 35°C. Now, place the bulb of the thermometer, , under your tongue. After one minute,, take the thermometer out and note the, reading. This is your body temperature., The temperature should always be, stated with its unit, °C., What did you record as your body, temperature?, The normal temperature of human, body is 37°C. Note that the temperature, is stated with its unit., , 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, thermometer, HEAT, , 37, , 2020-21

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clinical thermometer. Record your, observations as in Table 4.2., Is the body temperature of every, person 37°C?, The temperature of every person may, not be 37°C. It could be slightly higher, or slightly lower. Actually, what we call, normal temperature is the average body, temperature of a large number of healthy, persons., The clinical thermometer is designed, to measure the temperature of human, body only. The temperature of human, body normally does not go below 35oC, or above 42 o C. That is the reason, that this thermometer has the range, 35oC to 42oC., , Different types of thermometers, are used for different purposes., The maximum and minimum, temperatures of the previous, day, reported in weather, reports, are measured by a, thermometer called the, maximum-minimum, thermometer., this thermometer. Look at it, carefully and note the highest and, the lowest temperature it can, measure. The range of a, laboratory thermometer is, generally from –10°C to 110°C, (Fig. 4.4). Also, as you did in the, case of the clinical thermometer,, find out how much a small, division on this thermometer, reads. You would need this, information to read the, thermometer correctly., Let us now learn how this, thermometer is used., , Boojho got a naughty idea. He, wanted to measure the temperature, of hot milk using a clinical, thermometer. Paheli stopped him, from doing so., CAUTION, , Activity 4.4, , Do not use a clinical thermometer for, measuring the temperature of any, object other than the human body., Also avoid keeping the thermometer in, the sun or near a flame. It may break., , Take some tap water in a beaker, or a mug. Dip the thermometer, in water so that the bulb is, immersed in water but does not, touch the bottom or the sides of, the container. Hold the, thermometer vertically (Fig. 4.5)., Observe the movement of mercury, in the thermometer. Wait till the, mercury thread becomes steady., , 4.3 LABORATORY THERMOMETER, How do we measure the temperature of, other objects? For this purpose, there, are other thermometers. One such, thermometer is known as the laboratory, thermometer. The teacher will show you, , Fig. 4.4 A laboratory thermometer, , 38, , SCIENCE, , 2020-21

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In addition to the precautions to be taken while reading a clinical, thermometer, 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, , Fig. 4.5 Measuring temperature of water with a, laboratory thermometer, , Note the reading. This is the temperature, of water at that time., Compare the temperature of water, recorded by each student in the class., , 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, the thermometer is in water., You may recall that while taking your, own temperature, you have to take the, thermometer out of your mouth to note, the reading. Can you then use the, laboratory thermometer to measure your, , Boojho now understands why, clinical thermometer cannot be, used to measure high, temperatures. But still wonders, whether a laboratory thermometer, can be used to measure his body, temperature., , Boojho wonders why the level, of mercury should change at, all when the bulb of the, thermometer is brought in contact, with another object?, , HEAT, , 39, , 2020-21

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body temperature? Obviously, it is not, convenient to use the laboratory, thermometer for this purpose., Why does the mercury not fall or rise, in a clinical thermometer when taken, out of the mouth?, Observe a clinical thermometer, again. Do you see a kink near the bulb, (Fig. 4.6)., What is the use of the kink? It, prevents mercury level from falling on, its own., , Fig. 4.6 A clinical thermometer has a kink in it, , There is a lot of concern over, the use of mercury in, thermometers. Mercury is a, toxic substance and is very, difficult to dispose of if a, thermometer breaks. These, days, digital thermometers, are available which do not, use mercury., , 4.4 TRANSFER, , OF, , Paheli asks: “Does it mean, that heat will not be, transferred if the temperature of, two objects is the same?”, , in all cases heat flows from a hotter, object to a colder object., How does heat flow? Let us, investigate., , Activity 4.6, Take a rod or flat strip of a metal, say of, aluminium or iron. Fix a few small wax, pieces on the rod. These pieces should, be at nearly equal distances (Fig. 4.7)., Clamp the rod to a stand. If you do not, find a stand, you can put one end of, the rod in between bricks. Now, heat, the other end of the rod and observe., What happens to the wax pieces? Do, these pieces begin to fall? Which piece, falls the first? Do you think that heat is, , HEAT, , You might have observed that a frying, pan becomes hot when kept on a flame., It is because the heat passes from the, flame to the utensil. When the pan is, removed from the fire, it slowly cools, down. Why does it cool down? The heat, is transferred from the pan to the, surroundings. So you can understand, that in both cases, the heat flows from, a hotter object to a colder object. In fact,, , Fig. 4.7 Flow of heat through a metal strip, , transferred from the end nearest to the, flame to the other end?, The process by which heat is, transferred from the hotter end to the, colder end of an object is known as, conduction. In solids, generally, the, , 40, , SCIENCE, , 2020-21

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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., The water and air are poor, conductors of heat. Then, how does the, heat transfer take place in these, substances? Let us find out., , Fig. 4.8 Conduction of heat by different, materials, , heat is transferred by the process of, conduction., Do all substances conduct heat, easily? You must have observed that, the metallic pan for cooking has a plastic, or wooden handle. Can you lift a hot, pan by holding it from the handle, without getting hurt?, , Activity 4.8, , Take a round bottom flask (if flask is, not available, a beaker can be used). Fill, it two-thirds with water. Place it on a, tripod, or make some arrangement to, place the flask in such a way that you, can heat it by placing a candle below it., Wait till the water in the flask is, still. Place a crystal of potassium, Activity 4.7, permanganate at the bottom of the flask, Heat water in a small pan or a beaker., gently using a straw. Now, heat the water, Collect some articles such as a steel, by placing the candle just below the, spoon, plastic scale, pencil and divider., crystal., Dip one end of each of these articles in, Write your observation in your, hot water (Fig. 4.8). Wait for a few, notebook and also draw a picture of, Table 4.3, what you observe (Fig. 4.9)., When water is heated,, Article, Material with, Does the other, the water near the flame gets, which the article, end get hot, hot. Hot water rises up., is made of, Yes/No, The cold water from the, Steel spoon, Metal, Yes, sides moves down towards, the source of heat. This, water also gets hot and rises, HEAT, , 41, , 2020-21

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Fig. 4.10 Transfer of heat by convection in air, , Fig. 4.9 Convection of heat in water, , and water from the sides moves down., This process continues till the whole, water gets heated. This mode of heat, transfer is known as convection., How does the heat travel in air? In, which direction does the smoke go?, The air near the heat source gets hot, and rises. The air from the sides comes, in to take its place. In this way the air, gets heated. The following activity, confirms this idea., , Activity 4.9, Light a candle. Keep one hand above the, flame and one hand on the side of the, flame (Fig. 4.10). Do your hands feel, equally hot? If not which hand feels, hotter? And why?, Be careful. Keep your hands at a safe, distance from the flame so that they, do not get burnt., , 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., The people living in the coastal areas, experience an interesting phenomenon., During the day, the land gets heated, faster than the water. The air over the, land becomes hotter and rises up. The, cooler air from the sea rushes in towards, the land to take its place. The warm air, from the land moves towards the sea to, complete the cycle (Fig. 4.11). The air, from the sea is called the sea breeze. To, receive the cooler sea breeze, the windows, of the houses in coastal areas are made, to face the sea. At night it is exactly the, reverse. The water cools down more, slowly than the land. So, the cool air, from the land moves towards the sea., This is called the land breeze., When we come out in the sun, we, feel warm. How does the heat from the, sun reach us? It cannot reach us by, conduction or convection as there is no, medium such as air in most part of the, space between the earth and the sun., , 42, , SCIENCE, , 2020-21

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Night time, , Day time, , Hot, , Hot, , Cool, , Cool, , Fig. 4.11 Sea breeze and Land breeze, , From the sun the heat comes to us by, another process known as radiation., The transfer of heat by radiation does, not require any medium. It can take, place whether a medium is present or, not. When we sit in front of a room, heater, we get heat by this process. A, hot utensil kept away from the flame, cools down as it transfers heat to the, surroundings by radiation. Our body, too, gives heat to the surroundings and, receives heat from it by radiation., All hot bodies radiate heat. When, this heat falls on some object, a part of, it is reflected, a part is absorbed and a, part may be transmitted. The, temperature of the object increases due, to the absorbed part of the heat. Why, , are you advised to use an umbrella when, you go out in the sun?, , 4.5 KINDS OF CLOTHES WE WEAR, SUMMER AND WINTER, , IN, , You know that in summer we prefer, light-coloured clothes and in winter we, usually wear dark-coloured clothes. Why, is it so? Let us find out, , Activity 4.10, Take two identical tin cans. Paint the, outer surface of one black and of the, other white (Fig. 4.12). Pour equal, amounts of water in each and leave them, in the mid-day sun for about an hour., Measure the temperature of water in, both the cans. Do you find any, difference in the temperatures? In, , HEAT, , 43, , 2020-21

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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., , Fig. 4.12 Containers with black and white, surface, , which can is the water warmer? You, can feel the difference even by touching, water in the two cans., , Activity 4.11, Fill the two cans used in Activity 4.10, with the same amount of hot water at, the same temperature (say, at 60oC)., Leave the cans in a room or in a shade., Note the temperature of water after, 10–15 minutes. Does the temperature, of water in both the cans fall by the same, amount?, Do these activities suggest to you the, reason why it is more comfortable to, wear white or light-coloured clothes in, , 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, reflect most of the heat that falls on them, and, therefore, we feel more comfortable, wearing them in the summer., , Woollen clothes keep us warm, in winter, In the winter, we use woollen clothes., Wool is a poor conductor of heat., Moreover, there is air trapped in between, the wool fibres. This air prevents the flow, of heat from our body to the cold, surroundings. So, we feel warm., Suppose you are given the choice in, winter of using either one thick blanket, or two thin blankets joined together., What would you choose and why?, Remember that there would be a layer, of air in between the blankets., , Keywords, Celsius scale, Conduction, Conductor, Convection, , Insulator, Land breeze, Radiation, , 44, , Sea breeze, Temperature, Thermometer, , SCIENCE, , 2020-21

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What you have learnt, n, , Our sense of touch is not always a reliable guide to the degree of hotness, of an object., , n, , Temperature is a measure of the degree of hotness of an object., , n, , Thermometer is a device used for measuring temperature., , n, , 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., , n, , The normal temperature of the human body is 37°C., , n, , 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., , n, , 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., , n, , The materials which allow heat to pass through them easily are, conductors of heat., , n, , The materials which do not allow heat to pass through them easily are, called insulators., , n, , Dark-coloured objects absorb more heat than the light-coloured objects., That is the reason we feel more comfortable in light-coloured clothes in, the summer., , n, , 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, , 2020-21

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(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. Heat is transferred, to its other end by the process of ______________., , (f ), , Clothes of ______________ colours absorb more 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°C, , 46, , SCIENCE, , 2020-21

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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, , 2020-21

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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., , 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, , 2020-21