# 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 .​

Reason :

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 ?

Reason :

• 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 ?

Reason :

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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