Transfer of Heat
9th Class Physics Chapter 9 Exercise Short Questions with Answers
Q 9.2 : Why are metals good conductors of heat ?
Metal are good conductors of heat .
Because they contain free electrons which transfer heat , from one end to other end
Q 9.3 : Explain why :
(a) a metal feels colder to touch than wood kept in a cold place ?
- Because Metals are good absorber as well as good emitter . Therefore , it lose heat quickly .
- But Wood is insulator . So ,it is a bad absorber and bad emitter . Therefore , it does not lose heat quickly .
(b) land breeze blows from land towards sea ?
- At night , the land cools faster than the sea .
- Therefore , air above the sea is warmer , rises up and the cold air from land begins to move towards the sea .
- It is called land breeze .
(c) double walled glass vessel is used in thermos flask ?
Because vacuum between double walled glass vessel stop the flow of heat due to conduction and convection .
(d) deserts soon get hot during the day and soon get cold after sunset ?
In deserts , the specific heat capacity of sand is low .
- So, during day time, their temperature rises quickly and
- During night time , they lose heat quickly and become cool easier .
Q 9.4 : Why conduction of heat does not take place in gases ?
Conduction of heat does not take place in gases .
Reason : Gas molecules are at greater distance .
- Conduction is transfer of heat by the vibration / collision of adjacent molecules .
- So , conduction occurs only in solids , where molecules are closely packed .
Q 9.5 : What measures do you suggest to conserve energy in houses ?
Measures to conserve energy :
- Hot water tanks are insulated by plastic or foam lagging .
- Wall cavities are filled with plastic foam or wool .
- Ceiling of rooms is covered by insulating materials .
- Double glazed window panes are used . These window panes have air between glass sheets that provides good insulator .
Q 9.6 : Why does transfer of heat in fluids take place by convection ?
- Because , the distance between liquid molecules is greater . So , transfer of heat cannot be done by conduction.
- In liquids heat flows due to convection process only .
Q 9.7 : What is meant by convection current ?
Convection Current :
- It is a process of transfer of heat in liquids and gases .
- In this process , molecules move on the basis of density difference . i.e, Lighter (less dense ) , warm molecules rises while Heavier ( more dense ) , cool molecule move downward and take place of lighter molecules .
- This continuous flow of molecules is called Convection Current .
Q 9.8 : Suggest a simple activity to show convection of heat in gases not given in the book .
- Geysers :
- Geysers in our houses work on convection .
- Water in the boiler is heated by the process of convection and it reaches to upper part of tank .
Q 9.9 : How does heat reach us from the Sun ?
Heat reaches us from the sun through Radiation Process .
→ Radiation is the mode of transfer of heat from one place to another in the form of waves called electromagnetic waves .
Q 9.10 : How various surfaces can be compared by a Leslie cube ?
Various surfaces are compared by a Leslie cube on the basis of their ability to absorb heat through different surfaces .
- Hot water is filled in Leslie’s cube and is placed with one of its face towards a radiation detector .
- The observation are given as :
Q 9.11 : What is greenhouse effect ?
- ” The Green House Effect is a process that occurs when gases in Earth’s atmosphere trap the Sun’s heat .”
- This process makes Earth warmer . This is called Global Warming .
Q 9.12 : Explain the impact of green-house effect in global warming .
- Earth’s atmosphere contains Carbon dioxide and water . It absorbs heat and cause Green House Effect.
- It has caused an increase in the average temperature of the Earth by trapping more heat due to greenhouse effect . This phenomenon is known as Global Warming .
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