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Q. Account for the following: By putting on a shirt, a person feels comfortable.

A. Cloth is a poor conductor of heat. It, therefore, prevents the body heat to escape in winter. During summer, external heat cannot reach the body for the same reason. Hence, a person wearing a shirt feels comfortable.

Q. Why is water from a hand pump warm in winter and cold in summer ?

A. In winter, outside temperature is low as compared to the temperature of water obtained from a hand pump. Hence, it feels warm. This is due to the fact that the upper layer of the Earth’s crust is exposed to the atmosphere and is at a lower temperature. Water, which is underground, is comparatively at a high temperature. In summer, the outside temperature is high and, hence, the water from a hand pump feels cold.

Q. A thick glass tumbler often cracks when a very hot liquid is poured into it. Why ?

A. The inner surface of the thick glass tumbler coming in contact with the hot liquid expands more in comparison to the outer surface which is at a relatively low temperature. The uneven expansion of inner and outer surface may produce cracks.

Q. Glass, when heated, cracks, while metal does not. Explain.

A. Glass is a poor conductor of heat. On heating, the heat is not transmitted quickly. This results in unequal expansion of the inner and outer surfaces of glass which may crack. On the other hand, metal is a good conductor of heat. Therefore, when heated, the heat is transmitted quickly and uniformly in all directions. The expansion produced is uniform and, therefore, cracking cannot take place.

Q. Ice wrapped in a blanket does not melt away quickly. Why ?

A. A woollen blanket is a bad conductor of heat. It does not allow the external heat rays to enter. Therefore, ice wrapped in a blanket does not melt for a considerable length of time.


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