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101.    A blanket protects ice from melting. How?

Ans: Ice melts when heat from outside reaches it. Under open air, heat easily reaches them and so they melt easily. However when a blanket is used, heat is prevented to a great deal because blankets are poor conductors of heat. If an ice block is covered with blankets, it does not allow heat to pass, from outside, into the ice block according to the relation . Thus, ice is protected from melting.


102. Why are cloudy nights warmer than clear nights ?

Ans: At night heat energy that the earth gained at the day time is flowing out of (being lost from) the earth. On clear nights, they are taken away by the air present by radiation and convection mainly as well as some conduction (infinitesimally). So they are much colder. However, clouds are bad conductors of heat. When they are present, the conduction part is prevented. Due to their presence, convection is also reduced a great deal leaving only the radiation part. This is more like blocking the heat radiated by the earth’s surface. Therefore, cloudy nights become warmer than clear nights.


103. Two layered window pane of equal thickness keeps a room warmer than a single layered window pane of double thickness.

Ans: The main idea behind using two layers of window panes is trapping some air in between. The air trapped between those two layers act as insulator of heat, so double layered pane window becomes a bad conductor as compared to a single layer of double thickness. Besides, due to lack of space, the air cannot undergo convection, which erases any chance of leakage of heat from the room to outside. Thus, the room will be warmer.


104. A rough and blackened vessel is preferred for cooking. Why?

Ans: During cooking, heat energy flows from the heat source to the vessel and then to the materials inside. For that the vessel should have a capability to absorb heat. This is possible if the body is black because black substances have a higher capability to absorb heat. Besides when they are made rough, multiple reflections occur in those rough surfaces, absorbing some heat in each of the reflections. After many reflections, heat energy is almost absorbed all, which is then transferred to the food materials to be cooked.


105. In winter, metallic objects feel cooler than wooden ones. Why?

Ans: Metals are good conductors of heat, whereas wood is a poor one. When metallic objects are touched, they ‘conduct’ away heat from the body. When there is loss of heat, the body feels cold at the region of touch. However, when wood is touched, wood does not accept heat from the body, because it is not a conductor. So the body heat remains preserved (no loss) and so no feelings of coldness. When Due to the same reason, metallic objects appear hotter than wooden ones when kept in the sun.


106. Why do we prefer woollen clothes to cotton clothes in winter?

Ans: The main idea is the trapping of air. Although, both wool and cotton have nearly the same thermal conductivity, the woollen clothes can trap more air. Since air is a bad conductor, heat is preserved in the body. In case of cotton clothes, no air is trapped, so heat escapes out easily.


107. Birds puff up their feathers on a cold day. Why?

Ans: On a cold day, there is more chance of losing body heat to the outer environment due to large temperature differences. The natural mechanism for reducing this flow is inserting a medium of insulator medium in between. For this they spread their feathers, which increase the space there, causing air to rush into it. Since air is an insulator it decreases the conduction of heat to the outside and preserves their body heat from escaping out.


108. Why are ventilators placed near the ceiling of a room?

Ans: The function of a ventilator is throwing out of impure air (which is generally warmer than fresh air due to its composition), so that fresh air can enter the rooms. Warm air has a tendency to rise because of its low density. When it finds the ventilator windows, it escapes easily. The void space left by it is filled by fresh air (which is generally cold) entering through the doors & windows placed comparatively at lower height than ventilators. Thus, the room will obtain continuous fresh air.


109. Why is snow a better insulator than ice?

Ans: Ice is generally a single piece, whereas snow is made of a lot of small pieces of ice particles placed loosely. So it includes air particles in between. Since air is one of the poorest conductors, the overall thermal conductivity decreases. So snow is a better insulator than ice.


110. Wood piece is a better conductor than saw dust of the same wood. Why?

Ans: Sawdust is made of wood particles with air molecules in between. Wood is a poor conductor, but air is poorer yet. Air being bad conductor of heat increases the insulation. So, saw dust becomes an insulator naturally. So wood will be a better conductor than its dust.


111. At what condition will a piece of metal and a piece of wood feel equally hot to the touch?

Ans: Since metal is a good conductor of heat gains or loses heat at higher rate than wood. Therefore, metals are found to be hotter at higher temperature and colder at lower temperature than a piece of wood at the same temperatures.


When the temperature of metal and wood is equal to the body temperature, there will be no net heat transfer between human body and metal piece or wood. Therefore, they would feel equally hot to the touch.


112. The thermal conductivity of felt is greater than air but felt is used for thermal insulation. Why?

Ans: Air is a bad conductor, but being a fluid, it undergoes convection, which results in at least some loss of energy. Felt is also a bad conductor like air. Moreover, it is a solid. So there is no chance of convection and no heat is lost by convection. Thus, felt provides better insulation than air.


113. Why does a piece of paper wrapped tightly on a wooden rod burn quickly when held over aflame than a similar piece wrapped on a metal rod?

Ans: Before wood or any other substance burns, they should reach a certain temperature often called as the Ignition Temperature, for which there should be a proper storage of heat energy. Wood is a bad conductor of heat, so it does not conduct away the heat absorbed by paper, so paper quickly reaches up to the ignition temperature, so it starts to burn. But metal is a good conductor, so it conducts away the heat absorbed by paper. The paper will constantly be losing its heat energy. So, paper takes a longer time to reach the ignition temperature.


114. How can water be boiled in a paper cup, without burning the paper?


    How is boiling of water in a paper cup possible without burning the cup?

Ans: The boiling of water requires 100 0C whereas the burning of paper requires its ignition temperature which is greater than 100 0C. When water is heated in paper cup, heat goes to the paper as well as water, so the temperature of water as well as the cup rise to 100 0C first. Then water starts to boil. At that time the ignition temperature of paper is still not arrived yet, so it does not burn. As the process of boiling goes on, a condition is reached when water finishes off. Then only all the heat is directed to paper, raising its temperature heavily and ultimately burning it.


115. Although aluminium is a good conductor of heat, how can an aluminium foil with shiny surface be used to keep food hot for longer time?

Ans: The phenomenon of conduction starts once the heat enters the body, not before entering. When aluminium foils are used, they reflect most of the radiation falling into them, because of their shine. The reflected heat, when the foil is used around food materials, goes back to the food itself. So loss of heat from the food is prevented and the food would remain warm for long. The shiny surface reflects heat radiations falling on it, thus prevents the heat from food being lost and keeps the food hot for a long time.


116. Hot water pipes used in rooms are painted black. Why?

Ans: Black surface is a good absorber, and also a good radiator. In the hot water piping systems, they should be able to radiate heat easily and sufficiently once they get heated. So they are painted black to radiate heat to the room at a faster rate which keeps the room warm.


117. Why are white clothes more comfortable in summer than black ones?

Ans: In summer the weather is hot which makes any gain of heat from outside very uncomfortable. So clothes are to be worn such that heat do not enter the body. This is achieved by wearing white clothes. White objects reflect all radiation falling on it. But, black objects absorb all radiation falling on it. Therefore, white clothes are suitable in summer than black ones. Due to the same reason, white umbrella is better than black ones on hot days.


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