Transmission of Heat : Question Bank for Class 11 Physics

Get important questions of Heat and Thermodynamics for Board exams. Download or View 11th Physics important questions for exam point of view. These important questions will play significant role in clearing concepts of Physics. This question bank is designed by NCERT keeping in mind and the questions are updated with respect to upcoming Board exams. You will get here all the important questions for class 11 Physics chapter wise CBSE.

Q.1 In what respect is heat radiation different from light ?

Ans.The only difference between them is their wavelength or frequency.

Q.2 Is it necessary that all black coloured objects should be considered black bodies ?Ans. No, we can not consider a polished black surface as a black body as it reflects radiation incident on it. On the other hand, sun, which is dazzling white, is a black body.

Q.3 Glass stopper in a bottle get stuck to the neck, the neck is heated to loosen the stopper. Why?

Ans. Glass is a bad conductor, so when the mouth of bottle is heated it expands in cross – section but not the stopper. The stopper thus separates.

Q.4 In a winter night we feel warmer when clouds cover the sky than when the sky is clear. Why ?

Ans. This is because heat can not be escaped due to cloud covering as cloud is bad conductor of heat.

Q.5 Explain why we can easily boil water in a paper cap?

Ans. It is because heat is easily conducted through the paper to water, and as such the temperature reached is not sufficient for the paper to be charred.

Q.6 A piece of paper wrapped lightly on a wooden rod is found to be charred quickly when held over a flame compared to a similar piece of paper wrapped on a brass rod. Why ?

Ans. Brass being a good conductor of heat conducts away heat very rapidly such that the paper does not reach the charring temperature. But when wrapped on a wooden rod the heat can not

conduct away and so the paper soon reaches the charring temperature and get charred.

Q.7 A solid sphere of copper of radius R and a hollow sphere of the same material having internal radius r and external radius R are heated to same temperature. They are allowed to cool in the same environment. Which of them starts cooling faster ?

Ans. The solid sphere and the hollow sphere have got the same surface area. So, the rate of loss of heat is same for both. But we know that the rate of loss of heat

$=$ mass of object $\times$ sp. heat $\times$ rate of fall of Temperature

So, the hollow sphere having less mass has greater rate of fall of temperature.

Q.8 Explain why in summer a block of metal feels hotter than a block of wood while in winter reverse in the case, although temperature of both is same either in summer or in winter (what ever it is). At what temperature would the two feel equally hot or cold ? Ans. When we touch a body, the feeling of hotness depends not only upon the temperature but also on the thermal conductivity. The conductivity of metals is much higher than that of wood. In summer room temperature is more than that of human body (37°C). So, on touching heat will flow from blocks to our body. Due to large thermal conductivity metals will conduct while wood will not and as heat flows from higher temperature to lower, metal will appear hotter than wood.

In winter room temperature is lower than that of body, so on touching heat will flow from our body to blocks. However, due to large thermal conductivity of metal heat will flow from body to metal so it will appear colder than wood.

However if the temperature of room i.e., block is at 37°C (equal to that of our body) no transfer of heat will take place between our body and the blocks (whatever be their thermal conductivities) and so both will appear equal hot or cold.

Q.9 Explain why rooms are provided with the ventilators near the roof ?

Ans. It is done to remove the harmful impure air, and to replace it by the cool fresh air. The air we breathe out is warm and so it is lighter. It rises upwards and can go out through the ventilator provided near the roof. The cold fresh air from outside enter the room through the doors and windows. Thus, the convection current is set up in the air.

Q.10 The woolen blanket keeps our body warm, but on wrapping ice with the same blanket, it keeps ice cold. Why ?

Ans. The insulating action of the blanket is due to the air filled in the numerous small holes. The air is a bad conductor of heat, further convection currents can not be formed in the air surrounded by the fibres of the blanket. When we cover our body by an blanket, the air in it prevents heat transfer from our body to the surroundings. Hence our body remains warm. When a piece of ice is covered by this blanket, the same air prevents the transfer of heat from out side to ice. Hence ice remains cold.

Q.11 Can we boil water inside an earth satellite by convection ?

Ans. No, in convection the hot liquid at the bottom becoming lighter, rises up. Since the base of convection is the difference in weight and upthrust. In weightlessness this difference does not occur. So, convection is not possible in earth’s satellite.

Q.12 A blackened platinum wire, when gradually heated, appears first dull red, then yellow, then blue and finally white. Why ?

Ans. According to Wein’s displacement law,

$\lambda_{m} T=$ constant

On heating up to ordinary temperature only long wavelength (red) radiation is emitted. As the temperature rises, shorter wavelengths are also emitted in more and more quantity, hence the colour of radiation emitted by the hot wire shifts from red to yellow, then to blue and finally to white.

Q.13 Out of the three stars in the sky, one appears white, the second reddish and the third bluish. which one of these has the lowest temperatures and which one highest and why ?

Ans. As temperature of a body rises, the quantity of high energy (shorter wavelength) radiation increases in the radiant energy emitted by it. Hence the star appearing reddish has the lowest temperature and that appearing bluish has the highest temperature.

Q.14 In a burning coal oven, the cavities in the coal pieces looks brighter than the surface of the coal, though their temperature is not higher than the coal surface, Why ?

Ans. The cavities in the hot coal behave like a black-body. Hence in the rate of radiation $\left(=\sigma e T^{4}\right)$ from these cavities the emissive power e is 1. For the surface of the coal the value of e is less than 1, its rate of radiation is less. So, cavities, being at the same temperature, look brighter.

Q.15 Black-body radiation is white. Comment on this statement.

Ans. White radiation is one which includes all possible wavelengths. Since, a black body absorbs all wavelengths, so on being heated to suitable

temperatures, it can also emit all wavelengths. Hence, black-body radiation is white.

Long Answer Type [5 Marks]

Q.16 What is meant by emissive power and absorptive power of a surface ? Describe an experiment to show that good absorbers are good emitters and bad absorbers are bad emitters.

Ans. Emissive Power : The emissive power of a body at a certain temperature is defined as the total amount of thermal energy emitted per unit time, per unit area of the body for all possible wavelengths.

It is represented by ‘e’ and for certain wavelength

range as ‘$e_{\lambda}$’ . Its value depends upon the nature of surface, its temperature and wavelength of radiation emitted.

Absorptive Power : The absorptive power of a body is defined as the ratio of the amount of thermal radiations absorbed by the body in a given time to the total amount of thermal radiations incident on the body in same time. It is represented by ‘a’ and for certain wavelength range it is$^{\prime} a_{\lambda}^{\prime}$

Its value depends upon the nature of surface colour of object, temperature and wavelength of incident radiations.

The experimental set up for showing good absorbers are good emitters and bad absorbers are bad emitters is shown in figure.

It consists of two cylindrical vessels Q & R fixed to two ends of a glass U-tube, which contains air. Between them is placed a third metallic vessal P mounted on a pillar which can be rotated about a vertical axis. The front and back faces of these vessels are flat and equal in area. The faces of D and B are blackened by coating lamp black and the faces C & A are polished. The U-tube is filled with coloured water.

Initially boiling water is filled in vessel P and it is found that no change in the levels of coloured water takes place. Now if the vessel P is rotated through 180°, then the level of coloured water starts moving from Q to R because in this position good absorber faces good emitter (black surface faces black surface) and bad absorber faces bad emitter (Polished surface faces polished surface).

Hence, from above experiment it is clear that good absorbers are good emitters and bad absorber are bad emitters.

 

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