HEAT
                                                            POINTS TO REMEMBER

  • So many hot and cold objects are there, that we use in our daily life.

  • By changing temperature, we can change the temperature of the object from cold to hot and hot to cold.

  • Temperature: It is a reliable measure of the hotness of an object.

  • Thermometer: It is a device used to measure temperature.

  • Clinical thermometer: The thermometer that measures the temperature of our body is called a clinical thermometer.

  • A clinical thermometer reads temperature from 35°C to 45°C.

  • The normal temperature of a human body is 37°C.

  • The range of laboratory thermometer is generally from -10°C to 110°C.

  • Heat always flows from its higher level to lower level, i.e., towards colder objects.

  • Conduction: The process of transfer of heat from the hotter end to the colder end of an object is known as conduction. In solids, generally, the heat is transferred by the process of conduction.

  • Conductors: The materials which allow heat to pass through them easily are conductors of heat. Examples are aluminium, iron and copper etc.

  • Insulators: 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.

  • Pure water and air are the poor conductors of heat.

  • Convection: It is the process of transfer  of heat in liquid medium, from  higher to its lower level.
  • Radiation: Radiation is a process of transfer of heat in which no medium is required.

  • From the sun, heat comes to us by radiation.

  • Dark-coloured objects absorb heat better than the light-coloured objects. That is the reason we feel more comfortable in light-coloured clothes in the summer.

  • Woolen 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.

  • Celsius scale: When the distance between the freezing point and the boiling point of water is equally divided into one hundred divisions, each division is termed as one degree an is written as 1°C.

  • Sea breeze and land breeze: In the coastal areas, during the day time, the land air becomes hot and rises up. The cool air from the sea rushes towards the land. The hot air move from land to sea to complete the cycle such phenomenon is called sea breeze.

  • At night, the above phenomenon is exactly reverse. The water cools down move slowly than the land, as a result, the cool air move from land to sea. This phenomenon is called land breeze.
 
NCERT TEXTBOOK EXERCISES
 
  Q. 1. State similarities and differences between the laboratory thermometer and the clinical thermometer.
  Ans.  Similarities:
  1. both consist of a long, narrow, uniform glass tube.
  2. both have a bulb at one end.
  3. both contain mercury scale on the glass tube.
  4. both contain celsius scale on the glass tube.
           Differences:
  1. A clinical thermometer reads temperature from 35°C to 42°C, while a laboratory thermometer ranges between -10°C to 110°C.
  2. Clinical thermometer has a kink near the bulb while there is no kink in laboratory thermometer.
 
     Q. 2. Give two examples each of conductors and insulators of heat.
  1. Conductors: Copper, iron.
  2. Insulators: Plastic, wood (dry).
        
      Q. 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 …………..
                 (d) No medium is required for transfer of heat by the process of ………..

                 (e) A cold steel spoon is dipped in cup of hot milk. It transfers heat to its other end by the process of ……….. .

                 (f) Clothes of ………… colours absorb heat better then clothes of light colours.
 
       

Ans.
        (a) temperature                (c) celsius                          (e)  conduction
                   (b) clinical                       (d)  radiation                       (f)   dark.
 
Q. 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
  

Ans.        (i) Land breeze blows during                                       (d) night
                  (ii) Sea breeze blows during                                         (c) day
                 (iii)  Dark coloured clothes are preferred during           (b) winter
           (iv) Light coloured clothes are preferred during           (a) summer.
 
 Q. 5. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
 
Ans: Wearing more layers of clothing during winter keeps us warmer than wearing
Just one think piece clothing because the air trapped between the two layers of      clothes. This air prevents the of the heat from our body to cold surroundings.
 
Q.6. Look at fig.4.1 mark where the heat is being transferred by conduction,  convection and by radiation.
 
 
                                          Fig. 4.1. Heat transferred by radiation, conduction and convection


Ans: In this figure, the heat is transferred at three point in different ways:
  1. By radiation: from flame to the bottom of pan
  2. By conduction: from lower surface to upper surface of the pan.
  3. By convection: from the base to the upper region of pan water.
 
Q.7. In places of hot climate it is advised that the outer walls of houses be    painted white. Explain.

Ans. White colour reflects most of the heat thatfall on it. When the outer walls of the houses in the places of hot climate are painted white, they reflect the heat falling on them and thus help and thus help in keeping the houses cool.
 
Q.8 One litre of water at 30ºC is mixed with one litre of water at 50ºC. The temperature of the mixture will be                

        a)  80ºC                                        b) more than 50ºC but less than 80ºC
        c)  20ºC                                        c) between 30ºC and 50ºC

Ans:
d)between 30ºC and 50ºC.
 
Q.9. An iron ball at 40ºC is dropped in a mug containing water at 40ºC. The heat
  1. Flow from iron ball to water.
  2. Not flow from iron ball to water or from water to iron ball.
  3. Flow from water to iron ball.
  4. Increase the temperature of both.
Ans: 2. not flow from iron ball to water or from water to iron ball.
 
Q.10. A wooden spoon is dipped in a cup of ice cream. Its other end
  1. Become cold by the process of conduction.
  2. Become cold by the process of convection
  3. Become cold by the process of radiation.
  4. Does not become cold.
Ans:  4. Does not become cold.
 
Q.11. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that
  1. Copper bottom makes the pan more durable.
  2. Such pans appear colourful.
  3. Copper is a better conductor of heat than the stainless steel.
  4. Copper is easier to clean than the stainless steel.
Ans: 3. Copper is better conductor of heat than the stainless steel.
 
 
 
ADDITIONAL IMPORTANT QUESTIONS

Q. 1. What is heat?


Ans.
Heat is a form of energy. It enables us to feel hotness or coldness.
 
Q.2 What is temperature?

Ans:
Temperature is the measurement of hotness of an object.

Q.3. By which device temperature of an object can be measured?


Ans:
Thermometer.
 
Q.4. why are dark coloured clothes preferred in winters?

Ans:
Because dark coloured clothed absord most of the heat falling on them, thus keep us warm.
 
Q.5. Name the type of thermometer that is used to measure the body temperature.

Ans:
Clinical thermometer.
 
Q.6. Which metal is used in the bulb of a thermometer?

Ans:
Mercury.
 
Q.7. What is the unit of temperature?

Ans:
SI unit of temperature is Kelvin (K). Other units are celsius (ºC) and fahrenheit (ºF).
 
Q.8. What is the range of clinical thermometer?

Ans:
35ºC to 42ºC.
 
Q.9. What is the normal temperature of the human body?

Ans:
37ºC.
 
Q.10. The temperature of man on celsius scale is 37ºC. What will it be on fahrenheit scale?

Ans:
98.6ºF.
 
Q.11. What is the range of laboratory thermometer?

Ans:
-10ºC to 110ºC.
 
Q.12. What is the use of a kink present near the bulb of a clinical thermometer.

Ans:
 It prevents mercury level from falling on its own.
 
Q.13.What kind of cloth we prefer in summer?

Ans:
In summer we prefer light coloured clothes.
 
Q.14.In which method of transfer of heat no medium is required?

Ans:  
Radiation.
 
Q.15. What is the method of transfer of heat in the most of solids?

Ans:
Conduction.
 
Q.16. Water and air are the poor conductors of heat. Then how does the heat transfer take place in these substances.

Ans:
By convection.
 
Q.17. By which method we get heat from the sun?

Ans:
Radiation.
 
Q.18. Give the name of any three kinds of thermometers?

Ans:
   i) Clinical thermometer
           ii) laboratory thermometer
          iii) Maximum-minimum thermometer.
 
Q.19. Name three methods of transfer of heat.

Ans:
   i) Conduction.
           ii) Convection
          iii) Radiation.
 
Q.20. Is the body temperature of every person 37ºC? Why?

Ans:
The temperature of every person may not be 37ºC. It could be slightly higher or slightly lower. Actually what we call normal temperature is the average body temperature of a healthy person.
 
Q.21. Why is the range of a clinical thermometer kept between 35ºC to 42ºC?

Ans:
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 42ºC. That is the reason that this thermometer has the range 35ºC to 42ºC.
 
Q.22. Why does the mercury not fall or rise in a clinical thermometer when taken out from the mouth?

Ans:
There is a kink near the bulb of a clinical thermometer. It prevents mercury level from falling on its own.
 
Q.23. What is conduction?

Ans:
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 heat is transferred by conduction.
 
Q.24. Why is it more comfortable to wear white or light coloured clothes in the summer and dark coloured clothes in the winter?

Ans:
Dark surface absorb more heat and, therefore, we feel comfortable with dark coloured clothes in the winter. Light coloured clothes reflect most of the heat that fall on them and, therefore, we feel more comfortable wearing them in the summer.
 
Q.25. How woollen clothes keep us warm in the winter?

Ans:
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 surrounding. So, we feel warm.
 
Q.26. Why is the handle of a metallic kettle covered with strips of cane?

Ans:
Handle of a metallic kettle is covered with strips of cane because when kettle is heated the heat does not pass through strips of cane because strips of cane are bad conductor of heat and we may hold the handle with bare hands.

Q.27. Can you use clinical thermometer to measure the temperature of a candle flame?

Ans:
No, we cannot measure the temperature of a candle frame by clinical thermometer. It is because the temperature of candle flame is more than the temperature of the upper fixed point (42ºC) of clinical thermometer. If we try to measure the temperature of candle flame by this thermometer, it will break.
 
Q.28. What are conductors and insulators? Explain giving examples.

Ans:
The materials which allow heat to pass through them easily are conductors ofheat. Examples are all metallic objects. Materials which do not allow heat to pass through them easily are poor conductors of heat such as plastic and wood. They are also known as insulators.
 
Q.29. Explain the structure of a clinical thermometer.

Ans:
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. On the glass tube we can see a temperature scale, usually celsius scale.
 
 
Q.30. How does the heat travel in water or in gases?

Ans:
When water is heated, the water near the flame gets hot. Hot water rises up. The cold water from the sides moves down towards the source of heat. This water also get hot and rises and water from sides moves down. This process continues till the whole water gets heated. This mode of heat transfer is known as convection.

Fig. 4.3. Convection of heat in water
 
Q.31. Explain the difference between heat and temperature.
Ans
 
                             Heat                         Temperature
1. t is form of energy that flow from one object to another when there is a temperature between the objects.

 

2. It is total amount of internet energy of the molecules of a body.
3. As a result of heat exchange between two bodies, total amount of heat of two bodies remains unchanged.
4. It is the cause.
5. Its S.I. unit is joule (J).
 
  1. It is a thermal condition of a body. It is the quantity that indicates whether or not, and in which direction, heat will flow from one to the other.
  2. It is proportional to average kinetic energy of the molecules of the body.
  3. As a result of heat exchange between two bodies’ sum of their temperature before and after exchange is not same.
  4. It is the effect.
  5. Its S.I. unit is Kelvin (K).
 
Q.32. What precaution should we take while reading a clinical thermometer?

Ans: The following precautions are to be taken 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 35ºC.
  • Read the thermometer keeping the level of mercury along the line of sight.
  • Handle the thermometer with care. If it hits against hard object, it will break.
  • Don’t hold the thermometer by the bulb while reading the temperature.
 
Q.33. How does the heat travel in air? Explain the sea breeze and land breeze in costal areas in this reference.

Ans:
The air near the heat source gets hot and rises. It leaves the place, which is filled by the cold air from the sides. In this way the air gets heated.
The people in coastal areas experience an interesting phenomenon. During the day, the land gets heated faster than water. The air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take it’s place. The warm air from the land moves towards the sea to complete the cycle. The air from the sea is called the sea breeze. To receive the cool 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 land. So the cool air from the land moves towards the sea. This is called the land breeze.



Fig. 4.4. Lad breeze and see breeze
 
Q.34. Write a short essay on ‘radiation’.
Ans: The solar energy cannot reach to us by the process of conduction or convection, as there is no medium such as air in most part of the space between the earth and the sun.
         The solar energy reaches to us by a process is 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 when kept away from the flame, cools down as it transfers heat to the surrounding by radiation. Our body too gives heat to the surrounding and receives heat from it by radiation.
         All hot bodies radiate heat. When this heat falls on some object, it divides into three different ways:
 
  • A part of it is reflected.
  • A part of it gets absorbed.
  • A part of it may be transmitted.
The temperature of the object increases due to the absorbed part of the heat.


Fig. 4.5. When any radiation falls on an object: a part of it is reflected, a part is absorbed and a part may be transmitted
 
 
OBJECTIVE TYPE QUESTIONS
Q.1. match the items given in columm I with those given in column II.
 
Column IColumn II
  1. Conduction
  2. Convection
  3. Radiation
  4. Conductors
  5. Insulators
  6. Land breeze
  7. Sea breeze
  8. Dark coloured surfaces
  9. Light coloured surfaces
  10. Thermometer
  1. Wood, plastic, wool, air, water
  2. Day time
  3. Solids
  4. Night
  5. Liquids
  6. Metals
  7. A device to measure the degree of hotness
  8. Poor absorber of heat\
  9. Good absorber of heat
  10. Requires no medium
 
 
Q.2 Fill in the blanks:
  1. Temperature is the measure of ………….. of an object.
  2. The thermometer used to measure human body temperature is called ………… thermometer.
  3. The normal temperature of human body is ……….ºC.
  4. A……………. near the bulb of a clinical thermometer prevents mercury level from falling of its own.
  5. In ……………generally, the heat is transferred by the process of conduction.
  6. The water and air are …………. Conductor of heat.
  7. In coastal areas, cold air flow in the day from sea to land. It is called ……….
  8. When we come out in the sun we feel ……………
  9. All hot bodies radiate…………..
  10. Wool is a …………..conductor of heat.
 
Q.3. Choose the true and false statements from the following:
  1. Our sense of touch about hotness or coldness is not always reliable.
  2. Normal temperature of human body is 98.6ºC.
  3. Heat always flow from a hotter object to colder object.
  4. The solar energy reach to us by a process is known as radiation.
  5. Marking on clinical thermometer is from 0ºC to 100ºC.
  6. Shining thread in thermometer is the column of mercury.
  7. The materials which allow heat to pass through them easily are called conductors.
  8. Water and air are good conductor of heat.
  9. Woollen clothes keep us warm during winter.
  10. The maximum and minimum temperature of the day is measured by a laboratory thermometer.
 
 
Q.4 Choose the correct option in the following:


(i)   Which one of the following is a reliable measure?

a) Hotness                                         b) Coldness
c) Temperature                                 d) Heat
 

(ii)  Which device is used to measure the hotness or coldness of an object?   
   
     a) Picometer                                      b) Barometer
     c) Manometer                                    d) Thermometer
 

(iii) What is the SI unit of temperature? 

    a) Kelvin                                            b) Celsius
   c) Fahrenheit                                      d) All the these
 

(iv) What is the range of a clinical thermometer? 

   a) 0ºC to 100ºC                                 b) 35ºC to 42ºC
   c) -10ºC to 110ºC                             d) 50ºC to 80ºC
 
(v) Which one is filled in the bulb of a thermometer? 

  a) Mercury                                        b) Lead
  c) Copper                                          d) Silver
 
 
 
(vi) What is the normal temperature of a healthy person?

 a) 37ºC                                               b) 37ºF
 c) 37ºK                                               d) 27ºF

 
(vii) Heat always flow

a) from a colder object to hotter object
b) from a hotter object to colder object
c) in both the directions
d) heat never flows from one object to other

 
(viii) Conduction is the method of transfer of heat in

a) liquids                                            b) solids
c) gases                                              d) vacuum
 

(ix) Heat from the sun reaches to us by


a) radiation                                        b) conduction
c) convection                                    d) by all these methods
 

(x) Which kind of cloth will you prefer the most in summer?


a) Thin clothes with black colour     b) Thick clothes with white colour
c) Thick clothes with black colour   d) Thin clothes with white colour
 
 
 (xi)
Which one will you prefer on a cold night?

a) Two thin blankets joined together
b) One thick blanket
c) Give equal preference to both
d) Can not say
 

(xii) Which of the following shows linear expansion?

a) Copper                                         b) Petrol
c) Hydrogen gas                              d) Sand
 

(xiii) Cooking utensils are made from


a) good conductor                           b) bad conductor
c) both                                             d) insulators

 
(xiv) One calorie is equal

a) 1 J                                                b) 1.42 J
c) 4.2 J                                             d)  420 J

 
(xv) Ventilation in rooms due to

a) convection                                   b) conduction
c) radiation                                      d) all of these
 
 
ANSWER TO OBJECTIVE TYPE QUESTIONS
 
Q.1 Match the item in column I with column II:
 
Column IColumn II
  1. Conduction
  2. Convection
  3. Radiation
  4. Conductors
  5. Insulators
  6. Land breeze
  7. Sea breeze
  8. Dark coloured surface
  9. Light coloured surface
  10. Thermometer
  1. Solid
  2. Liquid
  3. Required no medium
  4. Metals
  5. Wood , plastic, wool, air, water
  6. Night
  7. Day time
  8. Good absorber of heat
  9. Poor absorber of heat
  10. A device to measure the degree of hotness
 
Q.2 Fill in the blanks:


i. Hotness            ii. Clinical       iii. 37                   iv.   Kink

v. Solid                vi. Poor           vii. Sea breeze    viii.  Warm

ix. Heat                  x. Poor
 
 
Q.3  True/False:


i. True          ii. False       iii. True         iv. True         v. False

vi. True        vii. True      viii. False        ix. True         x. False
 

Q.4. Choose the correct option:


i. (c)            ii. (d)            iii. (a)            iv. (b)            v. (a)

vi. (a)          vii. (b)          viii. (b)            ix. (a)            x. (d)

xi. (a)          xii. (a)          xiii. (a)          xiv.  (c)          xv. (a)