Explain why : (a) A body with large reflectivity is a poor emitter. (b) A brass tumbler feels much colder then a wooden tray on a chilly day. (c ) an optical pyrometer (for measuring high temperatures) calibrated for an ideal black body correct value for the temperature when the same piece is in the furnace. (d) The earth without its atmosphere would be inshospitably cold. (e ) Heating system based on circulation of steam are more effecient in warming a building than those based on circulation of hot water.

Explanation

(a) This is bacause a body with large reflectivity is a poor absrber of heat, and poor absorbers of heat are poor emitters. (b) When we touch a brass tumbler on a chilly day , heat flows our body to the tumbler quickly (as thermal conductivity of brasss is very high) and as a result, it appears colder. On the other hand, as the wood is a bad conductor, heat does not flow to the wooden tray from out body, on touching it. (c ) When the red hot iron piece is in the oven, its temperature TK is given by the relation `E = sigma T^4` But if the red hot iron piece is in the open having the surrounding temperature `T_0` its energy is rediate3d ac cording to relation, `E' =sigma (T^4 - T_0^4)` As the working principle of optical pyrometer is based on the fact that the brightness of a glowing surface of a body depends upon its temperature. there fore, pyrometer gives too low a value for the temperature of red iron in the open. (d) The lower layers of erath's atmosphere reflact infrared radiation from earth back to the surface of earth. Thus the heat rediation received by the earth from the sun during the day are kept trapped by the atmosphere. If atmosphere fo earth were not there, its surface would becomes too cold to live. (e ) Steam at `100^@C` possesses more heat then the same mass of water at `100^@C`. One gram of steam at `100^@C` possesses 540 calories of heat more then that possessed by 1 gm. of water at `100^C` That is why heating system based on circulation of steam are more efficient then those based on circulation of hot water.