The Total solution for NCERT class 6-12
Answer - 21 : -
(a) According toKirchh off’s law of black body radiations, good emitters are good absorbers andbad emitters are bad absorbers. A body with large reflectivity is a poorabsorber of heat and consequently, it is also a poor emitter.(b) Brass is a good conductor of heat, while wood is a bad conductor. When wetouch the brass tumbler on a chilly day, heat starts flowing from our body tothe tumbler and we feel it cold. However, when the wooden tray is touched, heatdoes not flow from our hands to the tray and we do not feel cold.(c) An optical pyrometer is based on the principle that the brightness of aglowing surface of a body depends upon its temperature. Therefore, if thetemperature of the body is less than 600°C, the image formed by the opticalpyrometer is not brilliant and we do not get the reliable result. It is forthis reason that the pyrometer gives a very low value for the temperature ofred hot iron in the open.(d) The lower layers of earth’s atmosphere reflect infrared radiations fromearth back to the surface of earth. Thus the heat radiation received by theearth from the sun during the day are kept trapped by the atmosphere. Ifatmosphere of earth were not there, its surface would become too cold to live.(e) Steam at 100°C possesses more heat than the same mass of water at 100°C.One gram of steam at 100°C possesses 540 calories of heat more than thatpossessed by 1 gm of water at 100°C. That is why heating systems based oncirculation of steam are more efficient than those based on circulation of hotwater.
Answer - 22 : -