Sunday, 19 June 2011

Heat : Lesson 4. NCERT Class VII (7th) Science

Exercises & Solutions :: Questions & Answers
1.Question :  State similarities and differences between the laboratory thermometer and the clinical thermometer.

Similarities : They are used to measure the temperature. Both type of thermometers uses calibrated glass tubes with a fine passage inside, along the length of tube for mercury expansion . They have glass tube with a closed end on one side. Both type have a bulb for containing Mercury at other end.   Both types measure temperature in degree Celsius



Differences : A clinical thermometer is used for measuring the temperature of Human Body where as a laboratory thermometer is used for measuring the temperature of other substances other than human being. A clinical thermometer reads temperature from 35°C to 42°C  as he normal temperature of human body is 37°C. The range of a
laboratory thermometer is generally from –10°C to 110°C. A clinical thermometer has a kink  to prevent mercury from being fallen back on of its own where as a laboratory thermometer does not have one.  A laboratory thermometer should be kept upright not tilted for taking temperature reading.

2. Question : Give two examples each of conductors and insulators of heat.
Answer :
Conductors : Iron, Copper, Brass
Insulators :  Wood, Wool, Plastic

3.Question :  Fill in the blanks :

(a) The hotness of an object is determined by its temperature.
(b) Temperature of boiling water cannot be measured by a clinical  thermometer.
(c) Temperature is measured in degree Celsius.
(d) No medium is required for transfer of heat by the process of Radiation.
(e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of conduction.
(f ) Clothes of dark colours absorb heat better than clothes of light colours.

4. Question : 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

Answer :
(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


5.Question :  Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing .
Answer : Wearing more layers of clothing during winter keeps us warmer because in between the layers there is trapped air. As air is bad conductor of heat so the out side low temperature do not get transferred to body as well as prevent our body heat to get transferred to colder surrounding out side. Hence more layers of cloths keep us warmer during cold winter.

6.Question :  Look at Fig. 4.13. Mark where the heat is being transferred by conduction, by convection and by radiation
  • Water inside the pan is getting heat through  Convection.
  • Heat is being transferred through Steel  body of pan by conduction.
  • Steel Pan is getting heat from burner through Radiation 
Transfer of heat by
radiation, conduction and convection
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7. Question :  In places of  hot climate it is advised that the outer walls of houses be painted white. Explain.

Answer : In places of  hot climate it is advised that the outer walls of houses be painted white becuase a light colour absorb very less heat radiation so a less amount of heat is absorbed by the wall and transfered inside the house. And we feel comfortable inside such houses due to lower temprature.

8.Question :  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
Answer : (d) between 30°C and 50°C

9. Question : 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.
Answer : ( b) not flow from iron ball to water or from water to iron ball.

10.Question :   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.

Answer : (d) does not become cold.

11. Question :  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.

Answer : (c) copper is a better conductor of heat than the stainless steel.

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