Clas seven science heat Chapter 4 NCERT Solutions
Chapter 4: Heat
• Celsius scale: It is known as the centigrade scale. We always state temperature with its unit; its symbol is °C.
• Conduction: The process through which heat is transferred from the hotter part to the colder part of an object is called conduction.
• Conductor: The solids that easily allow the heat to pass through them are known as conductors.
• Insulators: The solids that do not allow heat to pass through them are called insulators.
• Convection: The method of transferring heat by the movement of the particles of substance away from the source of heat is called convection.
• Radiation: It is a process in which the heat from the sun reaches earth. In radiation, the transfer of heat does not require any medium.
• Temperature: The measure of hotness or coldness of an object.
• Thermometer: It is a device that is used to measure temperature.
• Sea Breeze: The breeze that moves from the sea surface to the land is called a sea breeze.
• Land Breeze: The breeze that moves from land towards the sea is called a land breeze.
1. State similarities and differences between the laboratory thermometer and the clinical thermometer.
(i) Both laboratory and clinical thermometers have a long narrow uniform glass tube.
(ii) Both have a bulb at one end.
(iii) Both contain mercury in bulb.
(iv) Both use the Celsius scale on the glass tube.
(i) A clinical thermometer reads the temperature in the range of 35°C to 45°C while the range of laboratory thermometer is -10°C to 110°C.
(ii) The clinical thermometer has a kink near the bulb while there is no kink in the laboratory thermometer because of this mercury does not fall down on its own in a clinical thermometer.
2. Give two examples each of conductors and insulators of heat.
Answer Aluminium and iron are examples of conductors whereas plastic and wood are examples of insulators.
3. Fill in the blanks
(a) The hotness of an object is determined by its————–.
(b) The 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.
Answers (a) temperature (b) clinical (c) Celsius (d) radiation (e) conduction
1. The land breeze blows during (c) night
2. The sea breeze blows during (a) day
3. Dark coloured clothes are preferred during (d) winter
4. Light coloured clothes are preferred during (b) summer
5. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing?
Answer More layers of clothing keep us warm in winters as they have a lot of space between them. This space gets filled up with air. Air is a bad conductor; it does not allow the body heat to escape out.
6. Look at Fig. 4.13. Mark where the heat is being transferred by conduction, by convection and by radiation.
7. In places of the hot climate, it is advised that the outer walls of houses be painted white. Explain.
Answer In places of the hot climate, it is advised that the outer wall of houses be painted white because white colour reflects most of the heat radiation falls on it which keeps the house cool from inside even if there is a hot climate outside the house.
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) The temperature of the mixture will be between 30°C and 50°C.
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) The heat will not flow from iron ball to water or from water to iron ball.
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) Its other end does not become cold.
11. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that:
(a) the 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) The reason for this could be that copper is a better conductor of heat than the stainless steel.