Heat - Science Textbook PDF

3 Heat Y ou know that woollen clothes are and some of them are cold. Tea is hot made from animal fibres. You and ice is cold. List some objects you also know that cotton clothes are use commonly in Table 3.1. Mark these made from plant...

3 Heat Y ou know that woollen clothes are and some of them are cold. Tea is hot made from animal fibres. You and ice is cold. List some objects you also know that cotton clothes are use commonly in Table 3.1. Mark these made from plant fibres. We wear woollen objects as hot or cold. clothes during winters when it is cold Do not touch objects which are too outside. Woollen clothes keep us warm. hot. Be careful while handling a We prefer to wear light coloured cotton candle flame or a stove. clothes when it is hot. These give us a feeling of coolness. You might have We see that some objects are cold wondered why particular types of while some are hot. You also know that clothes are suitable for a particular some objects are hotter than others season. while some are colder than others. How In winter you feel cold inside the do we decide which object is hotter than house. If you come out in the sun, you the other? We often do it by touching feel warm. In summer, you feel hot even the objects. But is our sense of touch inside the house. How do we know reliable? Let us find out. whether an object is hot or cold? How do we find out how hot or cold an object Activity 3.1 is? In this chapter we shall try to seek Take three small tubs/containers. Label answers to some of these questions. them as A, B and C. Put cold water in 3.1 HOT AND COLD container A and hot water in container In our day-to-day life, we come across a Make sure that water is not so hot that number of objects. Some of them are hot you burn your hand Table 3.1: Hot and cold objects (A) (B) (C) Object Cold/Cool Warm/Hot Ice cream  Spoon in a tea cup Fruit juice Handle of a frying pan Fig. 3.1 Feeling water in three containers Reprint 2024-25 B. Mix some cold and hot water in container C. Now dip your left hand in container A and the right hand in Fig. 3.2 A clinical thermom- container B. After keeping the hands in eter the two containers for 2–3 minutes, put your hand and examine it carefully. If both the hands simultaneously in you do not have a thermometer, request container C (Fig. 3.1). Do both the a friend to share it with you. A clinical hands get the same feeling? thermometer looks like the one shown in Fig. 3.2. A clinical thermometer consists of a long, narrow, uniform glass tube. It has Boojho says,“My left hand a bulb at one end. This bulb contains tells me that the water in mug mercury. Outside the bulb, a small C is hot and the right hand shining thread of mercury can be seen. tells me that the same water If you do not see the mercury thread, is cold. What should I rotate the thermometer a bit till you see conclude?” 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 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. 3.2 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 3.2 MEASURING TEMPERATURE temperature from 35°C to 42°C. Have you seen a thermometer? Recall Activity 3.2 that when you or someone else in your family had fever, the temperature was Reading a thermometer measured by a thermometer. The Let us learn how to read a thermometer. thermometer that measures our body First, note the temperature difference temperature is called a clinical indicated between the two bigger marks. thermometer. Hold the thermometer in Also note down the number of divisions HEAT 25 Reprint 2024-25  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. 3.3).  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 the one 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. 5 What did you record as your body Wash the thermometer, preferably temperature? with an antiseptic solution. Hold it The normal temperature of human firmly and give it a few jerks. The jerks body is 37°C. Note that the temperature will bring the level of mercury down. is stated with its unit. Ensure that it falls below 35°C. Now place 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 3.3 Measure the body temperature of some of your friends (at least 10) with a Table 3.2: Body temperature of some persons Name Temperature (°C) Fig. 3.3 Correct method of reading a clinical thermometer 26 SCIENCE Reprint 2024-25 clinical thermometer. Record your Different types of thermometers observations as in Table 3.2. are used for different purposes. Is the body temperature of every The maximum and minimum person 37°C? temperatures of the previous The temperature of every person may day, reported in weather not be 37°C. It could be slightly higher reports, are measured by a or slightly lower. Actually, what we call thermometer called the normal temperature is the average body maximum -minimum temperature of a large number of healthy thermometer. persons. The clinical thermometer is designed this thermometer. Look at it to measure the temperature of human carefully and note the highest and body only. The temperature of human the lowest temperature it can body normally does not go below 35oC measure. The range of a or above 42 o C. That is the reason laboratory thermometer is that this thermometer has the range generally from –10°C to 110°C 35oC to 42oC. (Fig. 3.4). 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 3.4 Do not use a clinical thermometer for Take some tap water in a beaker measuring the temperature of any or a mug. Dip the thermometer object other than the human body. in water so that the bulb is Also avoid keeping the thermometer in immersed in water but does not the sun or near a flame. It may break. touch the bottom or the sides of 3.3 LABORATORY THERMOMETER the container. Hold the thermometer vertically (Fig. 3.5). How do we measure the temperature of Observe the movement of mercury other objects? For this purpose, there in the thermometer. Wait till the are other thermometers. One such mercury thread becomes steady. thermometer is known as the laboratory thermometer. The teacher will show you Fig. 3.4 A laboratory thermometer HEAT 27 Reprint 2024-25  In addition to the precautions to be taken while reading a clinical thermometer, the laboratory thermometer  should be kept upright not tilted. (Fig. 3.5)  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 3.5 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. 3.5 Measuring temperature of water with a laboratory thermometer the thermometer is in water. You may recall that while taking your Note the reading. This is the temperature own temperature, you have to take the of water at that time. thermometer out of your mouth to note Compare the temperature of water the reading. Can you then use the recorded by each student in the class. laboratory thermometer to measure your Boojho now understands 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? 28 SCIENCE Reprint 2024-25 body temperature? Obviously, it is not convenient to use the laboratory thermometer for this purpose. Paheli asks: “Does it mean Why does the mercury not fall or rise that heat will not be in a clinical thermometer when taken transferred if the temperature of out of the mouth? two objects is the same?” Observe a clinical thermometer again. Do you see a kink near the bulb in all cases heat flows from a hotter (Fig. 3.6). object to a colder object. What is the use of the kink? It How does heat flow? Let us prevents mercury level from falling on investigate. its own. Activity 3.6 Take a rod or flat strip of a metal, say of aluminium or iron. Fix a few small wax Fig. 3.6 A clinical thermometer has a kink in it pieces on the rod. These pieces should be at nearly equal distances (Fig. 3.7). There is a lot of concern over Clamp the rod to a stand. If you do not the use of mercury in find a stand, you can put one end of thermometers. Mercury is a the rod in between bricks. Now, heat toxic substance and is very the other end of the rod and observe. difficult to dispose of if a What happens to the wax pieces? Do thermometer breaks. These these pieces begin to fall? Which piece days, digital thermometers falls the first? Do you think that heat is are available which do not use mercury. 3.4 TRANSFER OF 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 Fig. 3.7 Flow of heat through a metal strip removed from the fire, it slowly cools transferred from the end nearest to the down. Why does it cool down? The heat flame to the other end? is transferred from the pan to the The process by which heat is surroundings. So you can understand transferred from the hotter end to the that in both cases, the heat flows from colder end of an object is known as a hotter object to a colder object. In fact, conduction. In solids, generally, the HEAT 29 Reprint 2024-25  minutes. Touch the other end. Enter your observation in Table 3.3. 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 Fig. 3.8 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 3.8 conduction. Take a round bottom flask (if flask is Do all substances conduct heat not available, a beaker can be used). Fill easily? You must have observed that it two-thirds with water. Place it on a the metallic pan for cooking has a plastic tripod, or make some arrangement to or wooden handle. Can you lift a hot place the flask in such a way that you pan 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 is Activity 3.7 still. Place a crystal of potassium 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. 3.8). Wait for a few notebook and also draw a picture of Table 3.3 what you observe (Fig. 3.9). 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 rises 30 SCIENCE Reprint 2024-25  Fig. 3.10 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. 3.9 Convection of heat in water The people living in the coastal areas and water from the sides moves down. experience an interesting phenomenon. This process continues till the whole During the day, the land gets heated water gets heated. This mode of heat faster than the water. The air over the transfer 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 towards which 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 to and rises. The air from the sides comes complete the cycle (Fig. 3.11). The air in to take its place. In this way the air from the sea is called the sea breeze. To gets heated. The following activity receive the cooler sea breeze, the windows confirms this idea. of the houses in coastal areas are made Activity 3.9 to face the sea. At night it is exactly the reverse. The water cools down more Light a candle. Keep one hand above the slowly than the land. So, the cool air flame and one hand on the side of the from the land moves towards the sea. flame (Fig. 3.10). Do your hands feel This is called the land breeze. equally hot? If not which hand feels When we come out in the sun, we hotter? And why? feel warm. How does the heat from the Be careful. Keep your hands at a safe sun reach us? It cannot reach us by distance from the flame so that they conduction or convection as there is no do not get burnt. medium such as air in most part of the space between the earth and the sun. HEAT 31 Reprint 2024-25  Day time Night time Hot Cool Hot Cool Fig. 3.11 Sea breeze and Land breeze From the sun the heat comes to us by are you advised to use an umbrella when another process known as radiation. you go out in the sun? The transfer of heat by radiation does not require any medium. It can take 3.5 KINDS OF CLOTHES WE WEAR IN place whether a medium is present or SUMMER AND WINTER not. When we sit in front of a room You know that in summer we prefer heater, we get heat by this process. A light-coloured clothes and in winter we hot utensil kept away from the flame usually wear dark-coloured clothes. Why cools down as it transfers heat to the is it so? Let us find out surroundings by radiation. Our body Activity 3.10 too, gives heat to the surroundings and receives heat from it by radiation. Take two identical tin cans. Paint the All hot bodies radiate heat. When outer surface of one black and of the this heat falls on some object, a part of other white (Fig. 3.12). Pour equal it is reflected, a part is absorbed and a amounts of water in each and leave them part may be transmitted. The in the mid-day sun for about an hour. temperature of the object increases due Measure the temperature of water in to the absorbed part of the heat. Why both the cans. Do you find any difference in the temperatures? In 32 SCIENCE Reprint 2024-25  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. 3.12 Containers with black and white reflect most of the heat that falls on them surface and, therefore, we feel more comfortable which can is the water warmer? You wearing them in the summer. can feel the difference even by touching Woollen clothes keep us warm water in the two cans. in winter Activity 3.11 In the winter, we use woollen clothes. Fill the two cans used in Activity 3.10 Wool is a poor conductor of heat. with the same amount of hot water at Moreover, there is air trapped in between the same temperature (say, at 60oC). the wool fibres. This air prevents the flow Leave the cans in a room or in a shade. of heat from our body to the cold Note the temperature of water after surroundings. So, we feel warm. 10–15 minutes. Does the temperature Suppose you are given the choice in of water in both the cans fall by the same winter of using either one thick blanket amount? 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 layer wear 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 Convection HEAT 33 Reprint 2024-25  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 temperature. 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 more heat 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 ______________. 34 SCIENCE Reprint 2024-25  (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. 3.13. Mark where the heat is being transferred by conduction, by convection and by radiation. Fig. 3.13 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 HEAT 35 Reprint 2024-25  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. 36 SCIENCE Reprint 2024-25  4. Take a sheet of paper. Draw a spiral on it as shown in the Fig. 3.14. Cut out the paper along the line. Suspend the paper as shown in Fig. 3.14 above a lighted candle. Observe what happens. Think of an explanation. Fig. 3.14 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. HEAT 37 Reprint 2024-25