NCERT Solutions for Class 7th: Ch 4 Heat Science
Page No: 45
Exercises
1. State similarities and differences between the laboratory thermometer and the clinical thermometer.
Answer
Similarities:
(i) both made up of uniform glass tube.
(ii) both contain mercury.
(iii) both have bulb at one end.
(iv) both generally have celcius scale.
Differences:
(i) Range of laboratory thermometer is from -10°C to 110°C while range of clinical thermometer is from 35°C to 42°C.
(ii) Laboratory thermometer is used to take the reading of temperature in laboratory while clinical thermometer is used to measure the temperature f human body.
(iii) Mercury level falls when removed from the source in case of laboratory thermometer while in case of clinical thermometer
2. Give two examples each of conductors and insulators of heat.
Answer
Examples of conductors : Iron, Copper
Examples of insulators : Plastic, Wood
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 a cup of hot milk. It transfers heat to its other end by the process of ______________.
(f ) Clothes of ______________ colours absorb heat better than clothes of light colours.
Answer
(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.
Page No: 46
4. Match the following :
| Column I | Column II |
| (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
| Column I | Column II |
| (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. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
Answer
We wear more layers of clothing during winter to keeps us warmer than wearing just one thick piece of clothing because because air gets trapped in between the two layers of blankets. This air prevents the flow of heat from our body to the cold surroundings as air is bad conductor of heat.
6. Look at Fig. 4.13. Mark where the heat is being transferred by conduction, by convection and by radiation.
Answer
Conduction: From pan to water.
Convection: Heat within water.
Radiation: From burner to pan.
7. 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 because white colour is absorb least heat. It reflects most of the heat and hence keeps the house cooler.
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
Answer
► (d) between 30°C and 50°C
Heat gained by cold water will be equal to the lost by hot wate.
Page No: 47
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.
Answer
► (b) not flow from iron ball to water or from water to iron ball.
Both have the same temperature.
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.
Answer
► (d) does not become cold.
Wood is a bad conductor of heat.
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.
Answer
► (c) copper is a better conductor of heat than the stainless steel.
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