# Class 11 Physics Chapter 11 Important Questions Thermal Properties of Matter

Q 1:- When are two objects set to be in thermal equilibrium? Ans:- Two objects are said to be in thermal equilibrium if their temperatures are exactly

It is important for the students that all the concepts should be very clear for better marks in future. Here, we are providing important conceptual questions and answers for class 11 physics chapter 11 Thermal Properties of Matter. In this lesson, students will learn about Thermal Properties of Matter. This will not only help the students to know the important questions but will also help them during revision.

Q 1:- When are two objects set to be in thermal equilibrium?
Ans:- Two objects are said to be in thermal equilibrium if their temperatures are exactly equal.

Q 2:- What is the basic principle of a thermometer?
Ans:- Thermometer works on a principle of variation of a particular property of a substance or a group of substance.

Q 3:- How do you define a calorie? How is a calorie related to joule?
Ans:- Calorie is the amount of heat required to rise the temperature of 1 gram of water from 14.5 ℃ to 15.5 ℃.

Q 4:- In modern thermometry, how many standard fixed point are taken? name them.
Ans:- In modern thermometeric devices we consider only one standard fixed point. This standard point is the triple point of water having a value of 273.16 K.

Q 5:- Is the value of coefficient of thermal expansion of a solid constant?
Ans:- No, the value of coefficient of thermal expansion of a solid changes with temperature.

Q 6:- Which substance has maximum value of specific heat capacity amongst solids and liquids? What is the value?
Ans:- Water has maximum value of specific heat capacity among solids and liquids specific heat capacity for water is 4186 J/ kg-K.

Q 7:- What material has maximum value of specific heat capacity in nature what is the value?
Ans:- Hydrogen gas has maximum value of specific heat capacity in nature having a value of 1.01 × 10000 J/ kg-K under constant volume condition.

Q 8:- A metal disc has a hole in it. What happens to the size of the holes if the disc is heated?
Ans:- On heating the metal disc the size of whole also increases.

Q 9:- Why is heat required for change of state of a substance from solid to liquid or liquid to vapour called latent heat?
Ans:- Because it cannot be measured by the use of a thermometer or because temperature remains unchanged during the change of state.

Q 10:- What is the function of heat supplied to a substance for its change of state from solid to liquid or from liquid to vapour state?
Ans:- To heat supplied is used to increase the potential energy of molecules of given substance so that the molecules can flow.

Q 11:- How does boiling point of a liquid change with pressure?
Ans:- Boiling point of a liquid increases on increasing the pressure and decreases if the pressure is reduced.

Q 12:- How is thermal capacity of a body related to its specific heat capacity?
Ans:- Thermal capacity of a given body is equal to the product of its mass and the specific heat capacity of the material of the body.

Q 13:- Why are metallic bodies good conductors of heat?
Ans:- Because a metallic body contains an extraordinarily large number of mobile free (conduction) electrons, which can easily transport heat energy through the metallic body.

Q 14:- Why do we feel warm in woolen clothes?
Ans:- The woolen clothes trap a large number of air pockets inside them and air is an insulator of heat. Hence, when we wear woolen clothes, air pockets in them prevent the loss of heat from our body to the surroundings. As loss of heat from our body is prevented, we feel warm.

Q 15:- What qualities does a cooking utensil possess from thermal point of view?
Ans:- Specific heat capacity of the material of cooking utensil should we low but it's thermal conductivity should be high.

Q 16:- What is an ideal black body?
Ans:- An ideal black body is that which absorbs all the radiations incident on it and does not reflect or transmit any radiation. Again, an ideal body is the best source of radiation.

Q 17:- What is the value of Stefan's constant in SI system?
Ans:-The value of Stephens constant Sigma is 5.67 x 10-8 Wm-2K-2.

Q 18:- On which factor does the colour of a Star depends?
Ans:- Colour of a Star depends on its surface temperature.

Q 19:- Which law can be employed to estimate the surface temperature of the Sun?
Ans:- We can employ other Stefan's law or wien's displacement law.

Q 20:- Temperature of a hot body is doubled. How would its radiating power change?
Ans:- If temperature of a hot body is doubled then in accordance with Stefan's law its radiating power will become 16 times of its previous value.

Q 21:- Why does the thermal expansion of a liquid inside a glass bulb not give the true expansion of the liquid?
Ans:- Because we have not taken thermal expansion of the glass wall into account.

Q 22:- Why does a thick glass tumbler crack when boiling water or hot tea is poured in it?
Ans:- When boiling water or hot tea is poured in a thick walled glass tumbler, it's inner surface increases. However, on account of poor thermal conductivity of glass. the temperature of outer surface of glass does not rice much. Hence, the outer glass surface expends by a small amount. due to uneven expansion of the inner and outer surfaces of glass, the glass tumbler cracks.

Q 23:- A metal ball can pass through, a metal ring. however, on heating the ball, it gets stuck in the ring. What would happen if the ring, rather than ball, were hated?
Ans:- When the metal ball is heated, its size increases and hence, it gets stuck in the ring. However, if instead of metal ball, we heat up the metal ring, then metal ring will expand and hence ball will easily pass through the ring.

Q 24:- Hot water bottles are used for hot fomentation. Why?
Ans:- The specific heat of water is maximum as compared to all other liquids and solids. therefore, for a given volume and temperature, the heat content of water in a hot water bottle is maximum. Moreover, due to large specific heat, temperature of hot water will fall at a slow rate. Due to these properties for hot fomentation of the inflamed or painful part of the body we prefer a hot water bottle.

Q 25:- Burns caused by steam are more severe than those caused by boiling water. give justification
Ans:- Steam at hundred degree Celsius carries 2.26 × 105 J/ kg more heat then boiling water at 100 ℃ hence the burns caused by steam are more saver then those due to boiling water.

Q 26:- On which basic conservation law is the principle of calorimetry based?
Ans:- The principle of calorimetry is based on the conservation law of energy. In the context, according to conservation law of energy, for an isolated and closed system, total heat content of the system must remains constant. It is what the principle of calorimetry makes use off and states that heat given by hotter body in a closed system equals to heat taken by colder body.

Q 27:- Why does a gas have more than one specific heats?
Ans:- When a gaseous substance is heated, besides increase in temperature, the pressure and volume of it may also change. Therefore, under different conditions of pressure and volume, a gas may have different values of specific heat capacities. Generally, a gas has two principle specific heats. These are

• (a) specific heat capacity under constant volume condition and
• (b) specific heat capacity under constant pressure condition.

Q 28:- Why do two layers of clothes of equal thickness provided warmer covering than a single layer of cloth of double the thickness?
Ans:- When we use two layers of clothes of equal thickness, a thin layer of air is trapped between them. this trapped layer of air is very poor conductor of heat. As a result, the combination of two thin clothes of equal thickness provides warmer covering than a single layer of clothes of double the thickness.

Q 29:- A flannel keeps of ice cold but keeps a person warm. how?
Ans:- A flannel is a bad conductor of heat. moreover, it has air pockets trapped in its pores which makes it still better insulator. Thus, a piece of flannel prevents flow of heat through it. When a person uses a flannel in winter, loss of heat from his body is avoided and he feels warm. On the other hand, if ice is wrapped in a flannel piece, heat it cannot flow from the surroundings to the ice and consequently, ice does not easily melt.

Q 30:- Why is felt used for thermal insulation in preference to air?
Ans:- Although air is a bad conductor of heat but it allows heat transfer due to convection. On the other hand, felt does not allow heat flow either by conduction or by convection. Hence, felt is a better material for thermal insulation than air.

Q 31:- An iron chair appears to be colder than a wooden chair in winter but hotter in summer. why?
Ans:- We know that iron is a good conductor of heat but wood is a poor conductor. during winter, when room temperature is much less than that of a human body, if a person touches the iron chair, there is conduction of heat from his body to chair and he feels cold. On the other hand, if the person touches the wooden chair, there is very little flow of heat and thus, chair appears-less cold. Similarly, on a hot summer day, the iron chair appears hotter than the wooden chair.

Q 32:- What should be the qualities of a coolant used in a chemical or a nuclear plant?
Ans:- A coolant used in a chemical or nuclear plant should have high value of specific heat capacity and high value of thermal conductivity so that it can transfer large quantity of heat quickly from the plant and is an an efficient coolant.

Q 33:- Why are ventilators provided in a room just below the ceiling?
Ans:- The ventilators are provided in a room just below the ceiling. The air exhaled by persons present in the room is warmer and contains carbon dioxide. This warmer air becomes lighter, moves up and leaves the room through the ventilators. In turns, fresh colder air, rich in oxygen, enter the room through door and windows. The convection process take place continuously and as a result, persons present in the room continue to get fresh air supply.

Q 34:- Cloudy nights are warmer than clear nights. explain why.
Ans:- During day time, the Earth's surface absorbs heat radiation coming from the sun and so the earth's temperature rises. At night, the earth radiates heat radiation and gets cooled. However, the clouds reflect back the heat radiation going upwards. As heat radiations are received back by the earth, temperature on a cloudy night does not fall appreciably, that is cloudy nights appear to be warmer.

Q 35:- Do all black coloured object should be considered black bodies?
Ans:- No, all black coloured objects cannot be considered as Black bodies. Only that bodies can be called a black body which absorb all the radiation incident on it and does not reflect or or refract at all. To make the concept clear, let us consider two examples. A shining or polished black surface although black in colour is not a black body on the other hand the sun which is a shining hot sphere behave as a perfect black body and is a black body radiator.

Q 36:- Can water be boil even at the room temperature?
Ans:- Yes, water can we boil even at room temperature provided that the pressure is reduced to an appropriate low value.

Q 37:- Which mode of heat transfer is the quickest?
Ans:- Radiation because radiant energy travels through free space as well as air with a speed of 3 × 108 m/s.

Q 38:- If air is a poor conductor of heat, why do we not we feel comfortable without clothes?
Ans:- Air is a very poor conductor of heat and it does not allow it loss from our body due to conduction. However,air allows convection to take place. Hence, in the absence of clothes, we shall feel cold in winter season on account of continuous heat loss due to convection.

Q 39:- Two thermometers are constructed in same way, except that one has a spherical bulb and the other is cylindrical bulb. which thermometer will respond quickly to temperature changes?
Ans:- We know that for a given volume, surface area is minimum for a spherical shape. Therefore, the thermometer with cylindrical bulb will respond quickly to temperature changes because on the account of largest surface area, the bulb will attain the temperature to be measured quickly.

Q 40:- Why are high quality measuring tapes prepared from Invar alloy?
Ans:- Invar is an alloy whose coefficient of linear expansion is the least amongst all the metals and alloys. Therefore, error in the measuring tape on account of expansion or contraction due to change in temperature will be extremely small and negligible, so, the measuring tape will give correct reading at all the temperatures.

Q 41:- Tea or coffee gets cold when sugar is added to it. why?
Ans:- When sugar is added to tea or coffee, it gets in dissolved and for this purpose, it takes heat from tea or coffee. As a result, the temperature of tea or coffee falls slightly.

Q 42:- In cold countries juice portals are generally place under water so as to avoid freezing. why?
Ans:- Since specific heat of water is high, the fall in temperature of the juice bottle placed under water is less. Therefore, juice inside the juice bottle does not freeze.

Q 43:- How is skating possible on snow?
Ans:- Under the pressure, on account of weight of skater, snow below the skates melts and water is formed. This water acts as a lubricant for skate. Thus, skating can be done comfortably on snow.

Q 44:- Why do not electrons contribute towards the conductivity of an insulator solid?
Ans:- In an insulator solid (generally,a non metal) there are no free electrons available, hence conduction is not possible by motion of electrons. Of course, a large number of electrons are present in the solid but these are revolving in their respective shells and are not free. Hence, they can not contribute in thermal conduction.

Q 45:- The cooling unit of a refrigerator is fitted near the top.why?
Ans:- The cooling unit of a refrigerator is fitted near the top so that the cold Air, on becoming denser, moves down and whole of the inside of the refrigerator gets cooled.