Chapter 4 – Heat
State similarities and differences between the laboratory thermometer and the clinical thermometer.
(i) Both clinical and laboratory thermometers have long, narrow, uniform glass tubes.
(ii) The bulbs of both the thermometers have mercury in them.
(i) The temperature range of clinical thermometers is from 350C to 420C while that of laboratory thermometer is from —100C to 1100C.
(ii) Clinical thermometer is used to measure the temperature of a human body. However, laboratory thermometer cannot be used to measure the temperature of a human body.
(iii) The least count of both the thermometers differs.
(iv) Unlike clinical thermometer that can be tilted, laboratory thermometer is kept upright while reading the temperature values.
Give two examples each of conductors and insulators of heat.
Two examples of conductors of heat are:
Two examples of insulators of heat are:
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.
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|
|(i) Land breeze blows during||(a) Night|
|(ii) Sea breeze blows during||(b) Day|
|(iii) Dark coloured clothes are preferred during||(c) Winter|
|(iv) Light coloured clothes are preferred during||(d) Summer|
Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
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.
Look at Fig. 4.13. Mark where the heat is being transferred by conduction, by convection and by radiation.
(i) Transfer of heat from hot plate to pan is by conduction.
(ii) Transfer of heat within water is by convection.
(iii) Transfer of heat from hot bodies like pan, plate, burner etc. to the surroundings is by radiation.
Concept insight: There are three methods of heat transfer – conduction, convection and radiation. Conduction and convection need material medium but radiation does not need any material medium.
In places of hot climate it is advised that the outer walls of houses be painted white. Explain.
In places of hot climate, it is advised to paint the outer walls of houses as white because a light colour such as white reflects back most of the heat that falls on it. We can say that it absorbs less amount of heat and keeps the house cool.
Concept insight: White colour is a poor absorber of heat and very good reflector of heat.
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
(d) The temperature of the mixture will be between 30°C and 50°C.
Concept insight: Heat gained by cold water will be equal to the lost by hot water such that the final temperature of the mixture will be between 30°C and 50°C.
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.
(b) The heat will not flow from iron ball to water or from water to iron ball as both the substances have same temperature.
Concept insight: Heat flow takes place from a hot body to a cold body. As both water and iron ball have the same temperature, so there is no temperature difference between the two for heat flow to take place.
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.
(d) Its other end does not become cold as wood is a bad conductor of heat.
Concept insight: A bad conductor does allow heat to pass through it.
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.
(c) The reason for this is that copper is a better conductor of heat than stainless steel.
Concept insight: As copper is better conductor of heat than stainless steel so the heat from the flame is transferred quickly and more effectively through the copper bottom to the contents of the pan.