Thermal Expansion ~ PHYSICAL SCIENCE Chapter 4 (class 10 wbbse) ( Important Short questions & Notes )

 

MADHYAMIK  PHYSICAL SCIENCE 

  SUGGESTION

Thermal Expansion  ~ PHYSICAL SCIENCE 

1. Name the three types of expansion in solid .

Ans. The three types of expansion in solid are : 

i) linear expansion

ii) Surface expansion 

iii) Volume expansion 

2. Name a substance which contracts on heating.

Ans. The volume of ice decreases on heating.

3. Define the term 'coefficient' of linear expansion.

Ans : The coefficient of linear expansion of a solid is the increase in its length per unit length per degree rise of temperature.

4. Give the SI unit of coefficient of linear expansion. 

Ans. Unit of alpha  (a) is per degree centigrade (°C-1) or S.I unit is per degree kelvin( k-1).

5. Is coefficient of linear expansion possible in the case of liquid? 

Ans. Liquids have no definite shapes of their own and take the shape of the container in which they are kept. So, the thermal expansion of the liquid is always cubicle.

6. The coefficient of linear expansion of brass is 1.8×10-5 C -1 . Whatdo you mean by this ? 

Ans. The statement means that for 1°C rise in temperature in brass its length increases by 1.8×10-5 unit per unit length.

7. What is the coefficient of apparent expansion in liquids? 

Ans. We are interested here of volume expansions of the gases with change in temperature, keeping pressure constant. Further,  the volume expansion of gases are so large in comparison to that of the containing vessel,  that expansions of the vessel is neglected and we have no apparent expansion of gases like liquids. 

8. What is the S.I unit of thermal conductivity?  

Ans. SI unit of thermal conductivity is J M-1 S -1 K -1 or W m -1k-1.

9. Explain why there are gaps between the rails.

Ans. On hot summer days, the rail lines expand and tend to increase in length. If no gap is there , the rails tends to bend sidewise due to expansion and this may cause derailment. This gap accommodates the increased length of the railway line and prevents it from bending.

10. Why is glass chimney cracked when water drops fall on it ? 

Ans. When drops of water fall on the glass  chimney of a kerosene lamp, the chimney is found to crack. When a water drop falls on the outer surface of the chimney there is cooling and that surface gets contracted,  but the inner surface is still at higher temperature and expanded. Due to such uneven expansion of the two sides of the chimney it gets cracked. 

11. Explain an experiment to show that different metals expand by different amount.

Ans. Bending of Bimetallic strip : From this simple experiment, it can be shown that different solids expand by different amounts. It is shown a composite bimetallic strip consisting of two flat bars of different metals (A.B.) revetted or welded together. When heated the straight bar is found to bend. The metal A expands more  than metal B , but as they are joined together, they can not expand independently,  the strip bends with more expanding metal bar (A) on the convex side.

12. What are the factors on which linear expansion in solid depends? 

Ans. i) Experimentally it is found that , increase in length of a rod , when heated longer the rod , more will be the increase in length of the rod for same increase in temperature. 

  ii) It is proportional to the increase in  temperature  (for a particular metal rod , more the increase in temperature,  more will be the increase in length ). 

 iii) It depends on the material of the rod (for rods of same length), if temperature is raised by same amount , aluminium rod will expand nearly twice,  compared to that of the iron rod.

13.  Define thermal conductivity?

Ans. The thermal conductivity of a material is the amount of heat which when passes normally in one second through the opposite faces of a unit cube , the difference in temperature between its two opposite faces is 1°C.

14.  Comment on the thermal conductivity of diamond, 

Ans: Diamond, an  allotrope of carbon has high value of K, indicating that it is a good conductor of heat. Thermal conductivity value of natural diamond is about 1000 W.m-1k-1, which is five times more than that of copper  ( 401 W.m-1 K-1). So , diamond wafer is used to prepare very sensitive circuit boards, where the heat produced is very quickly dissipated by the diamond. So, temperature remains constant.

15.  Define thermal conductivity.

Ans. The thermal conductivity of a material is the amount of heat which when passes normally in one second through the opposite faces of a unit cube , the difference in temperature between its two opposite faces is 1°C. 

16.  What do you mean by thermal expansion? 

Ans. Dimension of all substances generally increase with increase in temperature. This phenomenon is known as thermal expansion.

17. What is real expansion of the liquid? 

Ans. When the actual expansion of the liquid is measured by considering the expansion of the containing vessel, it is called real expansion of the liquid.

18. What is the conclusion of Hope's experiment ?  

Ans. From Hope's experiment it is proved that water at bottom which is densest at 4°C. After sufficiently long time the temperature of the lower thermometer falls slightly due to loss of heat by conduction to the upper regions. 

19.  What is molar specific heat at constant volume  (CV) ?

Ans. The  Molar specific heat of gas at constant volume is the amount of heat required to raise the temperature of one mole of the gas through one degree keeping the volume constant throughout.

20.  What is Regelation ?

Ans. The phenomenon of melting of ice under pressure and freezing again on releasing the pressure is called Regelation.

21.  What do you mean by vapour pressure of a liquid ? 

Ans. Whenever a liquid evaporates at any temperature,  the vapour exerts a definite pressure on everything in contact. This pressure is called vapour pressure of the liquid at that temperature.

22.  What is Conduction of heat ? 

Ans. It is the process of the transfer of heat through a substance without any detectable motion of the particles of the substance. 

23.  What is Radiation of heat ?

Ans. It is the transmission of heat from a hot body to a cold body without the help of any medium and without appreciable heating at the intervening medium if any. 

24.  What is perfectly black body ?  

Ans. A perfectly black body is that which absorbs completely the radiations of all the wavelengths on it.

25.  What is convection of heat ? 

Ans. It is the process by which heat is transmitted through a liquid or gas from a hotter point to a colder point due to the bodily motion of the heated particles of the substances.

26. How does you relate the coefficient of linear expansion of a rod with the length and temperature ?  

Ans. The increase in length of rod due to heating is very nearly proportional to the length of the rod and rise in temperature. 

27.  A brass disc fits tightly into the base of steel plate. Should you apply heat in order to remove the disc from the base ? 

Ans. The coefficient of linear expansion of brass is greater than that of steel. So if we apply heat then the brass disc will fit more tightly into the base.

But if we cool the concerned region, then the decrease in the diameter of the brass disc will be more than that of the base of the steel plate. 

So the disc will become loose and can be easily removed from the base. 

28.  What is weight thermometer? 

Ans. The coefficient of apparent expansion of a liquid may be determined with a thermometer, it is called as a weight thermometer. 

29.  A beaker is filled to the brim with water at 4°C. What will you notice if the temperature is decreased or increased ?  Give reasons for your answer. 

Ans. The density of water is greatest at 4°C, the volume of a given mass of water is least at that temperature. As the beaker was filled to the brim , so in both the cases water will overflow from the beaker. 

30.  Write the relation between the pressure and density of a gas at a constant pressure. 

Ans. The density of a gas at a constant temperature is proportional to pressure.