Heat4In Chapter 3 you learnt that woollen

clothes are made from animal fibres.You also know that cotton clothes

are made from plant fibres. We wearwoollen clothes during winters when itis cold outside. Woollen clothes keep uswarm. We prefer to wear light colouredcotton clothes when it is hot. These giveus a feeling of coolness. You might havewondered why particular types ofclothes are suitable for a particularseason.

In winter you feel cold inside thehouse. If you come out in the sun, youfeel warm. In summer, you feel hot eveninside the house. How do we knowwhether an object is hot or cold? Howdo we find out how hot or cold an objectis? In this chapter we shall try to seekanswers to some of these questions.

4.1 HOT AND COLD

In our day-to-day life, we come across anumber of objects. Some of them are hot

We see that some objects are coldwhile some are hot. You also know thatsome objects are hotter than otherswhile some are colder than others. Howdo we decide which object is hotter thanthe other? We often do it by touchingthe objects. But is our sense of touchreliable? Let us find out.

Activity 4.1

Take three large mugs. Label them asA, B and C. Put cold water in mug Aand hot water in mug B. Mix some cold

Table 4.1: Hot and cold objects

Object Cold/Cool Warm/Hot

Ice cream √

Spoon in a

tea cup

Fruit juice

Handle of a

frying pan Fig. 4.1 Feeling water in three mugs

(A) (B) (C)

and some of them are cold. Tea is hotand ice is cold. List some objects youuse commonly in Table 4.1. Mark theseobjects as hot or cold.

Do not touch objects which are toohot. Be careful while handling acandle flame or a stove.

Make sure that water is not so hot thatyou burn your hand

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your hand and examine it carefully. Ifyou do not have a thermometer, requesta friend to share it with you. A clinicalthermometer looks like the one shownin Fig. 4.2.

A clinical thermometer consists of along, narrow, uniform glass tube. It hasa bulb at one end. This bulb containsmercury. Outside the bulb, a smallshining thread of mercury can be seen.

If you do not see the mercury thread,rotate the thermometer a bit till you seeit. You will also find a scale on thethermometer. The scale we use is thecelsius scale, indicated by °C.Boojho’s confusion shows that we

cannot always rely on our sense of touchto decide whether an objectis hot or cold. Sometimesit may deceive us.

Then, how do we findout how hot an object reallyis? A reliable measure of thehotness of an object is itstemperature. Temperature ismeasured by a device calledthermometer.

4.2 MEASURING TEMPERATURE

Have you seen a thermometer? Recallthat when you or someone else in yourfamily had fever, the temperature wasmeasured by a thermometer. Thethermometer that measures our bodytemperature is called a clinicalthermometer. Hold the thermometer in

Fig. 4.2 A clinical thermometer

A clinical thermometer readstemperature from 35°C to 42°C.

Activity 4.2

Reading a thermometerLet us learn how to read a thermometer.First, note the temperature differenceindicated between the two bigger marks.Also note down the number of divisions

Boojho says,“My left handtells me that the water in mugC is hot and the right handtells me that the same water

is cold. What should Iconclude?”

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 boththe hands simultaneously in mug C(Fig. 4.1). Do both the hands get thesame feeling?

Boojho wondered which of the two scalesshown in Fig. 4.2 he should read. Paheli toldhim that India has adopted the celsius scale

and we should read that scale. The other scalewith the range 94–108 degrees is the

Fahrenheit scale (°F). It was in use earlier.

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Precautions to be observed while reading a clinical thermometer

Thermometer should be washed before and after use, preferably with anantiseptic 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 canbreak.Don’t hold the thermometer by the bulb while reading it.

(shown by smaller marks) between thesemarks. Suppose the bigger marks readone degree and there are five divisionsbetween them. Then, one small division

can read 1

0.2 C5= ° .

Wash the thermometer, preferablywith an antiseptic solution. Hold itfirmly and give it a few jerks. The jerkswill bring the level of mercury down.Ensure that it falls below 35°C. Nowplace the bulb of the thermometer

Fig. 4.3 Correct method of reading a clinicalthermometer

under your tongue. After one minute,take the thermometer out and note thereading. This is your body temperature.The temperature should always bestated with its unit, °C.

What did you record as your bodytemperature?

The normal temperature of humanbody is 37°C. Note that the temperatureis stated with its unit.

Let us try to assure Paheli that thereis nothing wrong with her.

Activity 4.3

Measure the body temperature of someof your friends (at least 10) with a

Table 4.2: Body temperature ofsome persons

Name Temperature (°C)

Paheli measured her bodytemperature. She got worried as

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clinical thermometer. Record yourobservations as in Table 4.2.

Is the body temperature of everyperson 37°C?

The temperature of every person maynot be 37°C. It could be slightly higheror slightly lower. Actually, what we callnormal temperature is the average bodytemperature of a large number of healthypersons.

The clinical thermometer is designedto measure the temperature of humanbody only. The temperature of humanbody normally does not go below 35oCor above 42oC. That is the reasonthat this thermometer has the range35oC to 42oC.

Different types of thermometersare used for different purposes.The maximum and minimumtemperatures of the previousday, reported in weatherreports, are measured bya thermometer called them a x i m u m - m i n i m u mthermometer.

CAUTION

Do not use a clinical thermometer formeasuring the temperature of anyobject other than the human body.Also avoid keeping the thermometer inthe sun or near a flame. It may break.

this thermometer. Look at itcarefully and note the highest andthe lowest temperature it canmeasure. The range of alaboratory thermometer isgenerally from –10°C to 110°C(Fig. 4.4). Also, as you did in thecase of the clinical thermometer,find out how much a smalldivision on this thermometerreads. You would need thisinformation to read thethermometer correctly.

Let us now learn how thisthermometer is used.

Activity 4.4

Take some tap water in a beakeror a mug. Dip the thermometerin water so that the bulb isimmersed in water but does nottouch the bottom or the sides ofthe container. Hold thethermometer vertically (Fig. 4.5).Observe the movement of mercuryin the thermometer. Wait till themercury thread becomes steady.

Fig. 4.4 A laboratory thermometer

4.3 LABORATORY THERMOMETER

How do we measure the temperature ofother objects? For this purpose, thereare other thermometers. One suchthermometer is known as the laboratorythermometer. The teacher will show you

Boojho got a naughty idea. Hewanted to measure the temperature

of hot milk using a clinicalthermometer. Paheli stopped him

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Are there any variations in thereadings? Discuss the possible reasons.

Let us try to answer this question.

Activity 4.5

Take some hot water in a beaker or amug. Dip the thermometer in water.Wait till the mercury thread becomessteady and note the temperature. Nowtake out the thermometer from water.Observe carefully what happens now.Do you notice that as soon as you takethe thermometer out of water, the levelof mercury begins to fall. This meansthat the temperature must be read whilethe thermometer is in water.

You may recall that while taking yourown temperature, you have to take thethermometer out of your mouth to notethe reading. Can you then use thelaboratory thermometer to measure your

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 thetemperature is to be measured. The bulb should not touch the surface of thecontainer.

Fig. 4.5 Measuring temperature of water with alaboratory thermometer

Boojho now understand whyclinical thermometer cannot be

used to measure hightemperatures. But still wonders

whether a laboratory thermometercan be used to measure his body

temperature.

Note the reading. This is the temperatureof water at that time.

Compare the temperature of waterrecorded by each student in the class.

Boojho wonders why the levelof mercury should change at

all when the bulb of thethermometer is brought in contact

with another object?

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body temperature? Obviously, it is notconvenient to use the laboratorythermometer for this purpose.

Why does the mercury not fall or risein a clinical thermometer when takenout of the mouth?

Observe a clinical thermometeragain. Do you see a kink near the bulb(Fig. 4.6).

What is the use of the kink? Itprevents mercury level from falling onits own.

in all cases heat flows from a hotterobject to a colder object.

How does heat flow? Let usinvestigate.

Activity 4.6

Take a rod or flat strip of a metal, say ofaluminium or iron. Fix a few small waxpieces on the rod. These pieces shouldbe at nearly equal distances (Fig. 4.7).Clamp the rod to a stand. If you do notfind a stand, you can put one end ofthe rod in between bricks. Now, heatthe other end of the rod and observe.

What happens to the wax pieces? Dothese pieces begin to fall? Which piecefalls the first? Do you think that heat is

Fig. 4.6 A clinical thermometer has a kink in it

4.4 TRANSFER OF HEAT

You 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 isremoved from the fire, it slowly coolsdown. Why does it cool down? The heatis transferred from the pan to thesurroundings. So you can understandthat in both cases, the heat flows froma hotter object to a colder object. In fact,

Paheli asks: “Does it meanthat heat will not be

transferred if the temperature oftwo objects is the same?”

transferred from the end nearest to theflame to the other end?

The process by which heat istransferred from the hotter end to thecolder end of an object is known asconduction. In solids, generally, the

Fig. 4.7 Flow of heat through a metal strip

There is a lot of concern overthe use of mercury inthermometers. Mercury is atoxic substance and is verydifficult to dispose of if athermometer breaks. Thesedays, digital thermometersare available which do notuse mercury. © N

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heat is transferred by the process ofconduction.

Do all substances conduct heateasily? You must have observed thatthe metallic pan for cooking has a plasticor wooden handle. Can you lift a hotpan by holding it from the handlewithout getting hurt?

Activity 4.7

Heat water in a small pan or a beaker.Collect some articles such as a steelspoon, plastic scale, pencil and divider.Dip one end of each of these articles inhot water (Fig. 4.8). Wait for a few

minutes. Touch the other end. Enteryour observation in Table 4.3.

The materials which allow heat topass through them easily areconductors of heat. For examples,aluminum, iron and copper. Thematerials which do not allow heat topass through them easily are poorconductors of heat such as plastic andwood. Poor conductors are known asinsulators.

The water and air are poorconductors of heat. Then, how does theheat transfer take place in thesesubstances? Let us find out.

Activity 4.8

Take a round bottom flask (if flask isnot available, a beaker can be used). Fillit two-thirds with water. Place it on atripod, or make some arrangement toplace the flask in such a way that youcan heat it by placing a candle below it.Wait till the water in the flask isstill. Place a crystal of potassiumpermanganate at the bottom of the flaskgently using a straw. Now, heat the waterby placing the candle just below thecrystal.

Write your observation in yournotebook and also draw a picture of

what you observe (Fig. 4.9).When water is heated,

the water near the flame getshot. Hot water rises up.The cold water from thesides moves down towardsthe source of heat. Thiswater also gets hot and rises

Fig. 4.8 Conduction of heat by differentmaterials

Table 4.3

Article Material with Does the otherwhich the article end get hotis made of Yes/No

Steel spoon Metal Yes

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Notice that towards the top, the airgets heated by convection. Therefore, thehand above the flame feels hot. On thesides, however, there is no convectionand air does not feel as hot as atthe top.

The people living in the coastal areasexperience an interesting phenomenon.During the day, the land gets heatedfaster than the water. The air over theland becomes hotter and rises up. Thecooler air from the sea rushes in towardsthe land to take its place. The warm airfrom the land moves towards the sea tocomplete the cycle. The air from the seais called the sea breeze. To receive thecooler sea breeze, the windows of thehouses in coastal areas are made to facethe sea. At night it is exactly the reverse(Fig. 4.11). The water cools down moreslowly than the land. So, the cool airfrom the land moves towards the sea.This is called the land breeze. Fig. 4.11shows this phenomenon.

When we come out in the sun, wefeel warm. How does the heat from thesun reach us? It cannot reach us byconduction or convection as there is nomedium such as air in most part of the

and water from the sides moves down.This process continues till the wholewater gets heated. This mode of heattransfer is known as convection.

How does the heat travel in air? Inwhich direction does the smoke go?

The air near the heat source gets hotand rises. The air from the sides comesin to take its place. In this way the airgets heated. The following activityconfirms this idea.

Activity 4.9

Light a candle. Keep one hand above theflame and one hand on the side of theflame (Fig. 4.10). Do your hands feelequally hot? If not which hand feelshotter? And why?

Fig. 4.10 Transfer of heat by convection in air

Fig. 4.9 Convection of heat in water

Be careful. Keep your hands at a safedistance from the flame so that theydo not get burnt.

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space between the earth and the sun.From the sun the heat comes to us byanother process known as radiation.The transfer of heat by radiation doesnot require any medium. It can takeplace whether a medium is present ornot. When we sit in front of a roomheater, we get heat by this process. Ahot utensil kept away from the flamecools down as it transfers heat to thesurroundings by radiation. Our bodytoo, gives heat to the surroundings andreceives heat from it by radiation.

All hot bodies radiate heat. Whenthis heat falls on some object, a part ofit is reflected, a part is absorbed and apart may be transmitted. Thetemperature of the object increases dueto the absorbed part of the heat. Why

are you advised to use an umbrella whenyou go out in the sun?

4.5 KINDS OF CLOTHES WE WEAR INSUMMER AND WINTER

You know that in summer we preferlight-coloured clothes and in winter weusually wear dark-coloured clothes. Whyis it so? Let us find out

Activity 4.10

Take two identical tin cans. Paint theouter surface of one black and of theother white (Fig. 4.12). Pour equalamounts of water in each and leave themin the mid-day sun for about an hour.Measure the temperature of water inboth the cans. Do you find anydifference in the temperatures? In

Fig. 4.11 Land breeze and sea breeze

Day time Night time

HotCool

CoolHot

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We often use electricity and fuels like coal and wood to keep our houses cool orwarm. Is it possible to construct buildings, that are not affected much by heatand cold outside? This can be done by constructing outer walls of buildings sothat they have trapped layers of air. One way of doing this is to use hollowbricks, which are available these days.

which can is the water warmer? Youcan feel the difference even by touchingwater in the two cans.

Activity 4.11

Fill the two cans used in Activity 4.10with the same amount of hot water atthe same temperature (say, at 60oC).Leave the cans in a room or in a shade.Note the temperature of water after10–15 minutes. Does the temperatureof water in both the cans fall by the sameamount?

Do these activities suggest to you thereason why it is more comfortable towear white or light-coloured clothes in

the summer and dark-coloured clothesin the winter? Dark surfaces absorbmore heat and, therefore, we feelcomfortable with dark coloured clothesin the winter. Light coloured clothesreflect most of the heat that falls on themand, therefore, we feel more comfortablewearing them in the summer.

Woollen clothes keep us warmin winterIn the winter, we use woollen clothes.Wool is a poor conductor of heat.Moreover, there is air trapped in betweenthe wool fibres. This air prevents the flowof heat from our body to the coldsurroundings. So, we feel warm.

Suppose you are given the choice inwinter of using either one thick blanketor two thin blankets joined together.What would you choose and why?Remember that there would be a layerof air in between the blankets.

Fig. 4.12 Containers with black and whitesurface

Keywords

Celsius scaleConductionConductorConvection

InsulatorLand breezeRadiation

Sea breeze

Temperature

Thermometer

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What you have learnt

Our sense of touch is not always a reliable guide to the degree of hotnessof 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. Therange of this thermometer is from 35°C to 42°C. For other purposes, weuse the laboratory thermometers. The range of these thermometers isusually 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 lowertemperature. There are three ways in which heat can flow from oneobject to another. These are conduction, convection and radiation.

In solids, generally, the heat is transferred by conduction. In liquidsand gases the heat is transferred by convection. No medium is requiredfor transfer of heat by radiation.

The materials which allow heat to pass through them easily areconductors of heat.

The materials which do not allow heat to pass through them easily arecalled insulators.

Dark-coloured objects absorb radiation better than the light-colouredobjects. That is the reason we feel more comfortable in light-colouredclothes in the summer.

Woollen clothes keep us warm during winter. It is so because wool is apoor conductor of heat and it has air trapped in between the fibres.

Exercises

1. State similarities and differences between the laboratory thermometerand 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 ______________.

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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. It transfers heatto its other end by the process of ______________.

(f ) Clothes of ______________ colours absorb heat better than clothesof 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 uswarmer than wearing just one thick piece of clothing .

6. Look at Fig. 4.13. Mark where the heat is being transferred byconduction, by convection and by radiation.

Fig. 4.13

7. In places of hot climate it is advised that the outer walls of houses bepainted white. Explain.

8. One litre of water at 30°C is mixed with one litre of water at 50°C. Thetemperature 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

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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. Thereason 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 takingtemperature 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 thethermometer at some place other than the mouth.

(d) whether the temperature of different parts of the body is the sameor different.

You can add more questions which come to your mind.

2. Go to a veterinary doctor (a doctor who treats animals). Discuss andfind out the normal temperature of domestic animals and birds.

3. Wrap a thin paper strip tightly around an iron rod. Try to burn thepaper with candle while rotating the iron rod continuously. Does it burn?Explain your observation.

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4. Take a sheet of paper. Draw a spiral on it as shown in the Fig. 4.14. Cut outthe paper along the line. Suspend the paper as shown in Fig. 4.14 abovea lighted candle. Observe what happens. Think of an explanation.

Fig. 4.14

Did you know?

The celsius scale was devised by a Swedish astronomer, Anders Celsiusin 1742. Strangely, he fixed the temperature of the boiling water as 0°Cand of freezing water as 100°C. However, this order was reversed verysoon.

5. Take two similar transparent glass bottles having wide mouths. Put afew crystals of potassium permanganate or pour a few drops of ink inone bottle. Fill this bottle with hot water. Fill the other bottle with coldwater. Cover the cold water bottle with a thick piece of paper such as apostcard. Press the postcard firmly with one hand and hold the bottlewith the other hand. Invert the bottle and place it on top of the hotwater bottle. Hold both the bottles firmly. Ask some other person to pullthe postcard. Observe what happens. Explain.

You can read more on the following website:

www.bbc.co.uk/schools/gcsebitesize/physics/energy/energytransferrev6.shtml

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