# Thermal Properties of Matter

## 9th Class Physics Chapter 8 Exercise Short Questions with Answers

Q 8.2 : Why does heat flow from hot body to cold body ?

Reason :

Heat flows from hot body to cold body to attain the condition of thermal equilibrium .

• The atoms of higher kinetic energy tries to move and collide with the atoms of low kinetic energy .
• Thus , heat transfers from a hot body toa cold body .

Q 8.3 : Define the term heat and temperature .

Heat :

Heat is the form of energy that is transferred from one body to another in thermal contact with each other as a result of the difference of temperature between them .”

Temperature :

” Temperature of a body is the degree of hotness or coldness of the body .”

Unit : Its SI unit is Kelvin (K).

Q 8.4 : What is meant by internal energy of a body ?

Internal energy of a body :

” The sum of kinetic energy and potential energy associated with the atoms , molecules and particles of a body is called its internal energy .”

Formula : I.E = K.E. + P.E.

Q 8.5 : How does heating affect the motion of molecules of a gas ?

By heating the gas , its molecules get high kinetic energy and start to collide more randomly and motion of gas molecules is increased by heating .

So , pressure and volume of gas molecules is increased by heating .

Q 8.6 : What is thermometer ? Why mercury is preferred as a thermometric substance ?

Thermometer :

” A Thermometer is a device which is used to measure the temperature of a body . “

• Mercury as thermometric substance : Mercury is preferred as thermometric substance due to following properties .
1. It is easily visible .
2. It has uniform thermal expansion .
3. It has low freezing point and high boiling point .
4. It has a small specific heat capacity .

Q 8.7 : Explain the volumetric thermal expansion .

Volumetric Thermal Expansion :

” Fractional change in volume per unit temperature change is called volumetric thermal expansion .”

Formula :

Q 8.8 : Define specific heat . How would you find the specific heat of a solid ?

Specific Heat :

” The specific heat of a substance is the amount of heat required to raise the temperature of 1 kg mass of that substance through 1 K . ”

Q 8.9 : Define and explain latent heat of fusion .

Latent Heat of Fusion :

” Heat energy required to change unit mass of a substance from solid to liquid state at its melting point without change in its temperature called Latent Heat of Fusion .”

Formula :

Symbol : Hf

Unit : Its SI unit is Jkg-1 ( Joule per kilogram ).

Q 8.10 : Define Latent heat of vaporization .

Latent Heat of Vaporization :

” The quantity of heat that changes unit mass of a liquid completely into gas at its boiling point without any change in its temperature is called its Latent Heat of Vaporization .”

Symbol : It is denoted by Hv .

Formula :

Unit : Its SI unit is Jkg-1 .

Q 8.11 : What is meant by evaporation ? On what factors the evaporation of a liquid depends ? Explain how cooling is produced by evaporation .

Evaporation :

” The process of changing of liquid into gas at any temperature is called evaporation .”

Factors :

• Temperature
• Surface Area
• Wind
• Nature of Liquid

Cooling by evaporation :

During evaporation , molecules having greater kinetic energy escape out from the surface of a liquid , while the molecules having lower kinetic energies are left behind .

Th this way , evaporation produces cooling by lowering the average kinetic energy and the temperature of molecules of a liquid .

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