Heat Class 7th Science Lesson 4

NOTES PREPARED BY 
ASHAQ HUSSAIN BHAT 
TEACHER SCHOOL 
EDUCATION DEPARTMENT 
JAMMU AND KASHMIR

'HEAT'

SOME IMPORTANT DEFINITIONS

1. Thermometer. It is a device which is used to measure temperature of an object.

2. Temperature. The degree of hotness or coldness of an object is called its temperature.

3. Conductor. The material which allows heat to flow through it is called conductor.

4. Insulator. The material which does not allow heat to pass through it, is called insulator.

5. Conduction. The process in which heat flows from body at a higher temperature to a body at a lower temperature without the actual motion of the particles of the material. Solids are generally heated by this method.

6. Convection. It is the method of heating in which heat is transferred from hotter part of the body to its colder part by actual motion of the particles of the medium. Liquids are generally heated by this process.

7. Radiation. It is mode of transmission of heat in which heat is directly transferred from hot body to a cold body without heating the medium in between

8. Land-breeze. The movement of cool air from land to wea at night when land cools down more rapidly than sea water is called land-breeze,

9. Sea-breeze. The movement of cold air from sea to land is called sea-breeze,

TEXT BOOK EXERCISES

Q. 1. State similarities and differences between the laboratory thermometer and the clinical thermometer. Similarities between

Ans. the laboratory thermometer and clinical thermometer.

(i) Both are used to note the temperature.

(ii) Both are made of long narrow uniform glass tube with a bulb at one end.

(iii) Mercury is used in both thermometers.

(iv) Both are marked in Celsius scale.

Differences between the laboratory thermometer and clinical thermometer.

Laboratory thermometers are larger and broader in size as compared to clinical thermometers.

(ii) Laboratory thermometers are used to note the temperature of different objects in different states while clinical thermometers are used only to note the temperature of human beings.

(iii) Laboratory thermometers are marked from--10°C to 110°C while clinical thermometers are marked between 35°C to 42°C only.

(iv) While noting readings of thermometer the laboratory thermometer has to be in touch with the object whose temperature is to be noted but in clinical thermometer the temperature it is seen when outside the body.

(u) There is no kink in laboratory thermometer but there is open kink in clinical thermometer in order to prevent mercury level from falling on its own due to its weight.

Q. 2. Give two examples each of conductors and insulators of heat.

Ans. Examples of: 

(i) Conductors : Aluminium, Iron, Copper.

(ii) Insulators : Water, Air.

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..................thermometer

(c) Temperature measure in degree is....... 

(d) No medium is required for transfer of heat by the  process of............. 

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

(f)Clothes of ..........colours absorb heat better than clothes of light colour.

Answer. 

(a) temperature 

(b) clinical 

(c) of hotness

(d) radiation 

(e) conduction 

(f) dark.

Q. 4. Match the following:

(i) Land-breeze blows during (a) Summer

(ii) Sea-breeze blows during. (6) Winter

(ii) Dark coloured clothes are (c) Day

preferred during

(iv) Light coloured clothes are  (d) Night

preferred during

Answer 

(i) Land-breeze blows during. (d) Night

(ii) Sea-breeze blows during: (c) Day

(iii) Dark coloured clothes are (b) Winter

preferred during

(iv) Light coloured clothes are (a) Summer

preferred during.

Q. 5. Discuss why wearing more layers of clothing during winter keeps us warm than wearing just one thick piece of clothing.

Ans. Different layers of clothing trap air inside them. Air is a poor conductor of heat so it prevents the flow of heat from our body to the cold surrounding and makes our body feel warmth.

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

Ans :- Fig

Q. 7. In places of hot climate, it is advised that the outer walls of houses be painted white. Explain.

Ans. Light coloured or shiny surfaces reflect most of heat that falls on it with the result the surface remains cool. So, it is advised to paint outer walls of houses white in places of hot climate.

Q. 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 (6) more than 50°C but less than 80°C

(c) 20°C (d) between 30°C and 50°C.

Ans. (d) between 30°C and 50°C.

Q. 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.

(6) 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.

Ans. (6) 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
(a) becomes cold by the process of conduction.
(6) becomes cold by the process of convection.
(c) becomes cold by the process of radiation.
(d) does not become cold.

Ans. (d) does not become cold.

Q.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.
(6) Such pans appear colourful.
(c) Copper is better conductor of heat than the
stainless steel.
(d) Copper is easier to clean than the stainless steel.

Ans. (c) Copper is better conductor of heat than the stainless steel

ADDITIONAL SHORT ANSWER QUESTIONS

Q. 1. When cold milk is added to hot tea, how does the heat transfer take place ?

Ans. When cold milk is added to hot tea, the two come in contact with each other. The transfer of heat energy takes place from hot molecules of tea to cold molecules of milk by the process called conduction 

Q. 2. What are the main conditions necessary for the flow of heat ?

Ans. Conditions for the flow of heat. The following are three conditions responsible for the flow of heat from one place to another :

1. The two substance ; should be in contact with each other.

2. The temperature of both the substances should be different.

3. The heat flows from hot to cooler substance.

Q. 3. If a spoon is held over fire, it becomes hot after sometime. Describe the process of heating up.

Ans. We know that in solids the molecules are packed close together. When one end of the spoon is held over the fire, the molecules of material of spoon absorb the heat energy and begin to vibrate rapidly. These molecules, in turn, cause their neighbouring molecules to vibrate. This process continues along the length of the spoon and energy is transferred from the hotter part to the colder part. After sometime, the other end of the spoon also becomes hot.

Q. 4. Why is it preferred to wear bright clothes in summer and dark clothes in winter ?

Ans. Dark coloured clothes are good absorbers of heat as compared to white clothes. The dark coloured clothes absorb the heat falling on them and become warmer. So we feel warm in winter by wearing these clothes. On the other hand, white clothes reflect heat coming from the sun. That is why, we prefer to wear bright clothes in summer and dark clothes in winter.

Q.5. Define conduction. Why the handles of cooking utensils are made of insulators ?

Ans. Conduction. The mode of transfer of heat in which heat is transferred from one particle to another without the actual motion of the particles is called conduction.

The handles of cooking utensils are made up of insulating materials because they do not allow heat to flow through. them. So heat cannot reach our body and thus our hand is Prevented from  burning 

6. Define radiation.

Ans. Radiation. The process of transfer of heat from one body to another without heating the intervening medium is called radiation. No medium is needed for the transference

of heat by this process.

Q. 7. Distinguish between heat and temperature.

Ans. Differences between heat and temperature:

       

 Heat                                        

1. It is a kind of energy. .

              

2. It is the cause that helps in raising the temperature

Temperature

1. It is the degree of hotness or coldness of a body.

2. It is the effect of  in raising the  heat of a body

Q. 8. What is convection ? Give two practical applications of convection of heat.

Ans. Convection. The process of transference of heat in which heated gas or a liquid particles actually move from one place to another is called convection. All the liquids and gases are heated by the method of convection. Examples.

(1) When a stove is lighted, the air in contact with the hot stove becomes warm. On being heated, it expands and becomes lighter. As a result, it rises upwards and the cool air from its surrounding comes to take its place.

(ii) The ventilation of ordinary buildings is due to

convection.

Q.9. As soon as the tube is removed from the source of heat, the level of liquid drops. How is this avoided in the thermometer that doctors use

 

Ans. It is seen that the level of liquid falls in a narrow- tube when it is removed from the source of heat. But when the doctor's thermometer is removed from the source of heat, the level of mercury does not fall. It is because a kink

is provided in the tube of clinical thermometer which prevents mercury from falling down due to its own weight.

ADDITIONAL LONG ANSWER QUESTIONS

Q. 1. What do you understand by transfer of heat ?

Ans. When we bring two bodies at different temperatures, in contact with each other the temperature of the hot body lowers and that of cold body rises. In other words, the hot body loses heat while the cold body gains heat. In other words, heat flows from the hot body to the cold body i.e. from the body at higher  temperature to the body at lower temperature. The flow of heat from the body at higher temperature to the body at lower temperature is called the transfer of heat. This process continues until the temperature of the two bodies become same.

Q. 2. How are the sea breeze and the land breeze set up ?

Ans. Setting up of the sea breeze and the land

breeze. 

Near the coastal region, sun gives equal amount of

heat energy to the land surface as well as to the sea water. The temperature of the land rises more rapidly as compared to the sea water. Hence the air above the land becomes hot and rises up. The cooler air from the sea starts blowing towards land and thus sea breeze is set up. 

During night, the land as well as the sea water cools, but the temperature of the land falls more rapidly as compared to sea water.

Therefore, at night the temperature of the sea water is more than the land. Hence at night the wind starts blowing from the land towards the sea resulting in land breeze,

Q. 3. Give an experiment to show that black bodies are good absorbers and good radiators .

Ans. Experiment. Take two tin cans of the same size. Paint one of them black and put equal amount of water in each can. Now place them in the sun for about an hour, When we touch water in the tin cans we find that the water 

in the black tin has become warmer than that in the other tin. Since the size of the tins is the same, this shows that black body has absorbed more heat radiations than the other one.

You can also do the reverse experiment by taking equal amount of hot water in each can. Leave the cans in a shady place for 10 minutes and record the temperature in each can. You would find the water in the dark had cooled down more. Thus, we conclude that black bodies are good

absorbers and also good radiators of heat.

Q. 4. Give an experiment to show the transfer of heat by conduction.

Ans. Experiment. Take a flat aluminium rod. Fix some iron nails on this rod with the help of molten wax. Clamp this rod in a stand as shown in the figure. the Future

Heat one end of the rod with the help of spirit lamp. The iron nails drop one by one, starting from the hot end. It shows that heat is transferred slowly from one end of the rod to the other end by conduction. Conduction from the hot part of the object to its colder  parts; or from one hot object to a colder object, happens only if :

(i) the two parts are in contact, and

(ii) their temperatures are different.

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